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Loess stratigraphy in central China
Quaternary Science Reviews, Vol. 6, pp. 191-219, 1987.
Printed in Great Britain. All rights reserved.
LOESS STRATIGRAPHY
I)277-3791/87 $(I.(111+ .511 Copyright © 1987 Pergamon Journals Ltd.
IN CENTRAL CHINA
G e o r g e Kukla Lamont-Doherty Geological Observatory, Palisades, New York 10964, U.S.A.
The loess deposits in central China record world-wide climate changes of the last 2.5 Ma. Numerous climatic oscillations are marked by alternating loess and soil units with continuous coverage of the Matuyama and Brunhes Epochs. Magnetic susceptibility of the deposits correlates closely with the oxygen isotope record of the deep-sea sediments and provides an independent measure of climate and time. The key marker bed of the chinese loess sequence is the paleosol $5, the time equivalent of the mid-Brunhes oxygen isotope stages 13, 14 and 15. It marks a prolonged interval of warm humid climate lasting from approximately 615 to 470 thousand years ago. Several episodes of river downcutting coincide with the deposition of the exceptionally thick loess units LI (oceanic oxygen isotope stages 2 to 4) L2 (stage 6), L5 (stage 12) L6, (stage 16) L9 (stage 22) L15 (stage 38 about 1.15 Ma) and WS4 (about 2.3 Ma). These erosional events are interpreted as a result of episodic uplift of the Loess Plateau. The deposition of the earliest loess layers between 2.5 and 2.3 Ma ago marks a first order shift from warm and humid environments toward harsh continental steppes comparable to those of the Middle and Late Pleistocene. Little, if any lithologic or paleontologic changes were noted within or above the Olduvai magnetozone, so that the proposed Plio/Pleistocene boundary at approximately 1.65 Ma has no lithostratigraphic and biostratigraphic representation in the chinese loess series. Comparison with the Deep Sea Drilling Program core 552A in the North Atlantic and with the Santerno River section near Bologna shows that the occurrence of the earliest loess in China coincides with the timing of the first significant ice rafting in the North Atlantic and with the appearance of cold water foraminifcrs in the marine deposits of northern Italy. An extension of the stage system of the oxygen isotope signal extended back to the Gauss-Matuyama boundary is proposed.
INTRODUCTION The last twenty years witnessed major advances in the understanding of paleoclimatic history of the Quaternary. The bulk of the new information has come from ocean sediments. Equally important, but lesser known findings, have been made on the continents in the loess sequences. Loess is a silt, transported and deposited by wind, loosely cemented by a fine syngenetic carbonate incrustation, formed in semi-arid continental climates. Loess deposits of Pleistocene age cover large areas in the middle latitudes of the Northern Hemisphere, stretching in a discontinuous belt from southern Britain across central Europe to eastern Asia, and from Alaska and eastern Washington State into the Mississippi and Ohio valleys. Loess also occurs in New Zealand and South America. The most extensive deposits comprise the Loess Plateau of central China (Fig. 1) where the most important discoveries in loess stratigraphy and geochronology have been made in recent decades, and which are reviewed here. Loess covers over 440,000 km 2, or approximately 40% of the arable land in China (Liu, T.S. et al., 1985). In the Loess Plateau between the Huanghe (Yellow) River and the Weihe River, loess with subhorizontal interbedded soils is over 150 m thick. Ever since the pioneering study of Richthofen (1877) many sections in the Loess Plateau have been described in varying degrees of detail (Wang et al., 1984a; Liu, T.S. et al., 1965, 1966, 1985, 1985a; Burbanks and Li, 1985). The
discussion here concentrates on sites in the vicinity of Luochuan, Xifeng and Xian (Fig. 2), for which the best climato-stratigraphic and time stratigraphic information exists and which currently represents the stratigraphy of the Loess Plateau of Central China. The key data of stratigraphic importance presented were described in detail in the recent review of Sasajima and Wang (1984) and Liu, T.S. et al. (1985), and in the study of the magnetic susceptibility of the Xifeng section of Liu, X.M. et al. (1985).
LITHOSTRATIGRAPHY The three key stratigraphic sections are: Xifeng, the Heimugou Valley at Luochuan, borehole QUN 22 near Luochuan and the Liujiapo section near Xian (Figs 4, 5 and 6). Xifeng is about 200 km and Xian about 160 km away from Luochuan, but all three localities have a similar lithostratigraphy. Individual units are subhorizontal and can be followed in natural exposures, in the walls of gullies, for long distances. A summary of the chinese loess stratigraphy is shown in Fig. 3. The sequences were subdivided into four major stratigraphic units. From the bottom to the top these are: Wucheng Loess, Lower Lishi Loess, Upper Lishi Loess and Malan Loess. The Wucheng Loess is underlain by the Red Clay formation of the upper Pliocene age. Dark grey humus-rich biogenic soil, locally interbedded or overlain by redeposited loess (the Black Loam formation) formed on top of the Pleistocene sediments in the Holocene.
192
G. Kukla i
[
j'
I
Yinsnln Mr5
~---- .....
H
J,uq
100
105
110
115
t-IZO
FIG. 1. Mean thickness of primary loess with interbedded soils in central China. Total thickness tess than 50 m in diagonal hatching, between 50 and 100 m vertical hatching, between 100 and 200 m horizontal hatching and over 200 m crosshatched. After Wang e t a l . , 1984.
FIG. 2. Localities discussed in the text.
The units are briefly described as follows: (1) 'Red Clays': This is the underlying formation of the loess (5 to 40 m thick) increasing in thickness toward the northwest. The Red Clays are Late Pliocene in age and their basal sand and gravel rests uncomformably on pre-Tertiary bedrock. In Xifeng they date from the Gilbert and Gauss magnetic epochs. The formation consists of red-brown silty noncalcareous clays with a high proportion of kaolinite and illite with carbonate nodules. Their Munsell color ranges around 5 YR 4/6 when moist (Kyuma et al., 1984). It is supposed to be of partly fluviolacustrine origin. In its upper part there is frequent evidence of pedogenesis and steppe rodents. The boundary between the Red Clays and the overlying loess is transitional, with loess and loess-like sediments appearing in the upper part of the Red Clays with red-brown, deeply weathered, and possibly partly hydromorphic soils, in the lowest part of the Wucheng loess. Subhorizontal carbonate concretions in the transitional zone (the 'Mixed Bed' of Liu, T.S. et al.,
1985) probably show an oscillating but shallow ground water level at the time of deposition in an intermittently flooded basin. (2) Wucheng Loess: is the oldest loess formation in central China. It is up to about 50 m thick, is composed of yellowish and reddish-brown compact loess, and is interbedded with reddish-brown soils and subhorizontal beds of carbonate concretions. The mineralogic composition of the soils is similar to that of the Red Clays. In the Wucheng formation the loess units differ from the soils principally by a lighter color, a high proportion of calcium carbonate, and by a higher content of silt and fine sand. It is, relatively, dark colored, 7.5 YR 5/6 of the Munseil scale when moist (Kyuma et al., 1984). The Wucheng Formation can be distinguished from the overlying Lishi Loess by agenerally darker color, a higher density, and frequent carbonate concretions, occurring partly in the form of subhorizontal beds. The formation has been subdivided into four soil groups
Loess Stratigraphy in China
~K
I""1 '1
~4
o a a
u
o
gl
i N
1
FIG. 3. Comparison of the loess sequence in the Loess Plateau with the chronostratigraphic subdivisions as used by different authors.
(WS1 to WS4) and four loess members (WL1 to WL4). Compared with the younger sediments, this is an arbitrary and general subdivision. In the soil groups, several soils, each one polygenetic in origin, dominate the loess: each loess member contains several soils. Some of the units may be subdivided into subzones each identified by a suffix (Figs 20, 21 and 22). The Wucheng loess spans the interval from about 1.15 to about 2.3 Ma. Its base is placed at the top of the Mixed Zone. The most recent subdivision of Wucheng Formation, which is followed here, was proposed by Liu, X.M. et al. (1985) in Xifeng. It differs from the earlier proposals (Liu, T.S. et al., 1985) in which the two soil-loess couplets on top of the Olduvai event (WS1, WL1, WS2 and WL2) were recognized as a single pair marked WS1 and WL1 (Fig. 3). (3) Lower Lishi Loess: the Lower Lishi formation is composed of light yellowish-brown and brown compact loess interbedded with sharply delimited dark brown paleosols. The color of the wet loess is 10 YR 6/4 to 8.75 YR 5/6 of Munsell scale, and the soils vary from 5
193
YR 4/4 (in $5) to 6.25 YR 5/6. The fossil soils and their carbonate horizons contain relatively frequent and large carbonate concretions. They are labeled from top to bottom as $5 to S14, with the youngest paleosol $5 exceptionally strongly developed as a marker bed of reddish polygenetic soil about 4 to 6 m thick. Three loess members are exceptionally thick and have a higher content o f coarse silt' and primary carbonate. They are from top to bottom L6, L9 and L15. Loess units L9 and L15 are sometimes referred to as the First (L9) and the Second (L15) Sandy Loess. The base of the Second Sandy Loess forms the base of the Lower Lishi. The formation is about 40 to 50 m thick and covers the Lower Brunhes and Upper Matuyama magnetozones, including the Jaramillo Event. Paleosol $5 is by far the strongest developed and thickest in the whole loess series. The loess members L l l to L14 are poorly expressed and relatively dark colored, so that the interval between S10 and L14 appears to be dominated by soil development, and as such analogous to the soil groups of the Wucheng formation. (4) The Upper Lishi Formation: this formation, about 20-30 m thick, includes four paleosols and four loess members. The loess units are labeled L2 to L5 from the top to the bottom. The loess is relatively porous and rich in carbonate with Munsell color of 8.75 YR 6/4. L3 and L4 are darker in color and lower in carbonate than the remaining two. The soils are similar in color to those of the Lower Lishi Loess, and are more strongly developed than the Holocene soil, but less so than the $5 soil. Carbonate concretions are relatively rare and small. Paleosol $2 is twofold, separated by a layer of a light colored weathered loess (S2b). The Upper Lishi formation is of Upper Brunhes age. (5) Malan Loess Formation: (L1) of Late Pleistocene age, is up to about 10 m thick. It is greyish-yellow and porous, loosely cemented and in its upper part intensely bioturbated. Over most of the Loess Plateau it has no recognizable interlayers, whereas around Xian and in the Biejing area (Fig. 7) it is subdivided by a weakly developed steppe soil (LS1). The best studied and dated sections of Malan Loess are in its type area in the Zhaitang Basin near Beijing (An and Lu, 1984; Lu et al., in press). (6) Black Loam Formation: sediments and soils of latest Pleistocene and Holocene age are poorly represented in the Plateau partly because of their young age and small thickness, and partly due to the artificial reworking and removal of topsoil by agricultural activities. On most sites of the Loess Plateau, the Holocene soil, labeled SO is a grey, calcareous, humus rich, artificially reworked topsoil at most about 1 to 2 m thick. In rare sheltered depressions, atypical in the Plateau environment, two to three dark grey to black, humusrich, calcareous zones about 2 m thick are interbedded and overlain by a light colored loess-like sediment with a relatively high carbonate content. The soils are of the
194
G. Kukla
XI~ENG Thickness [In1
h'tholoLy
Interpreted Polarity
VGP La t i t u d e -90
0
.....
-45
0
Log stlsceptibiJiO, [SI- u~its] +45
+90
-5
-4
--
50
100
......
WS 2
WS 3 150
- - - -
YS 4
2OO
~a
R~
FIG. 4. Stratigraphiccolumnof the Xifenglocality.(1) unweatheredloess, (2) weatheredloess (3) soilsor soil groups (4) normal polarity magnetozones,VGP: Virtual geomagneticpole position. Log magneticsusceptibilityin SI units from Liu, X.M. et al., 1985. tschernosem family and at the locality Nuanquangou were t4C dated to 7-11 ka BP(Fig. 8). Little information has been published on the light colored silt separating and overlying the black soils, except on its particle size and mineralogic composition which are similar to that of the Malan Loess; its maximum thickness is about 1 to 3 m. It is difficult to estimate how much of the loess in the Black Loam formation is a secondary waterlain deposit on slopes, and on alluvial plains; and how much is primarily wind blown loess. Both types are probably present. However, given the existence of numerous ancient settlements in the Loess Plateau, and the destructive impact of cultivation and grazing on surface stability, it is possible that part of the Holocene loess is locally redeposited. Indeed the Holocene soil and loess are rare and thin on the Loess Plateau around Luochuan, Xifeng and Xian.
Chinese Loess and Soil Terminology Up to four types of primary loess have been recognized by local researchers (Table 1). They are 'unweathered', 'weakly weathered', 'moderately weathered' and 'strongly weathered' loess. 'Strongly weathered' loess is the lithology transitional to the 'dark loessial soil', which is the least developed stage of a paleosol. The weathered loesses have a lower carbonate content, lower porosity, and darker color, because of a higher proportion of limonitic impregnations. Such a subdivision of the loess is subjective because of imprecise lithological definition. But sometimes, reference is made to a 'lithic loess', usually a higher density sediment with subhorizontal carbonate impregnations and concretions. The soils are labeled in order of increasing degree of development as the 'dark loessial soil', 'calcareous drab soil', 'drab soil', 'leached drab soil' (or luvic drab soil),
195
Loess Stratigraphy in China
[
i
.r
g.
Q Z
..J
,mc
:E
IE
z cA
20 --
~
20
--
"~
40 -40
--
z
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60 60 -o ..i o
80
I0
-
-
100
100
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120
~
120
-
--
and 'drab-brown soil' (or burosem). In some recent publications 'cinnamon' may be substituted for 'drab'. All soil types except for the dark loessial soil, were found to have a clay content higher than 25% (Kyuma et al., 1984); whereas all loess types have less than 25% clay. Because all the soils are reworked and/or truncated, the terminology of recent soils is inappropriate. Kubiena's system of classification of paleosols would be more useful. The 'drab soils' seem to correspond roughly to the 'Braunerde' of Kubiena's (1964) as used in Europe. The 'drab brown soil' and the 'leached drab soil' with strong clay illuviation is comparable to a 'Parabraunerde'. The burosem or the 'drab-brown soil' is similar to Kubiena's 'Braunlehm'. The carbonate content decreases from the calcareous 'drab soil' to the 'drab-brown soil' (Table 1). The Holocene soil 'Dark Loessial Soil' or 'Black Loam' (Hei-lu to), has a thick and dark calcareous horizon with an organic content between 1 and 1.5%, and relates to the less developed stages of the tschernosem group. This soil has less than 25% clay, similar to the loesses. While a voluminous literature refers to the chemical composition and mineralogy of soils (cf. An and Wei, 1980; Han, 1982; Kyuma et al., 1984), relatively little work has been done on the micromorphology of the paleosols (An and Wei, 1978). Nevertheless it is obvious from a macroscopic examination of the sequences, that most of the soil horizons are polygenetic and have suffered strong weathering which has partly obliterated their original character. Aligned carbonate nodules, and subvertically oriented concretions, formed along rootcasts, are common, especially within the Lishi formations. Also common are the illuvination argillans, filling voids in the leached cinnamon soils, and the calcans in the calcareous soils.
FIG. 5. Stratigraphic columns of the Heimugou exposure (A), and QUN22 borehole in Luochuan (B). Lithoiogie symbols same as in Fig. 4. After Liu et al., 1985.
"-"--~.
0
metres
I
I
I
LI !i
50
I
I
I
EDC~Y
FIG. 6. Liujiapo outcrop at Xian. Symbols same as in Fig. 4.
196
G. Kukla O~
[L age (xlO00 yr BP)
SO
on fine
grains
L1-3
~18t2
on COarSe grains ~
20÷2,5
~--28±5 ~32±3
'*'--3627
L1-S .c
5.~41±4
L1-1 *.--5124
"*--52±5
*'--b6±7
*-'84±9
10"
"*'-'84~10
$1
FIG. 7. Lithostratigraphy, ~4Cand thermoluminescence(TL) dating of the Malan Loesssection near Beijing. Modifiedfrom Lu, Y.C., in press.
TABLE 1. Comparison of soil terminologyequivalents as used in China Wang et al., 1984
Liu et al., 1985
Approx. Kubiena's Equivalents (1953)
Dark Loessial Soil Calcareous Drab S o i l Drab Soil Leached Drab Soil Drab Brown Soil
Black Loam CarbonateCinnamon Soil Cinnamon Soil Leached (luvic) Cinnamon Soil Brown Cinnamon Soil
Braunerdc Para Braunerde Braunlehm
Depth Layer (ml Column -1 I
I I,, I'
CaCO 3 (%) 20
10
' \
14C a g e s B.P.
Organic Carbon
,,.,
]
Carbonates
"
1,11]1 2
2.5
3
3.3
4
3.8
N I1t' ,;
l
7360±250 21200±1000 19960~1000 8550z300 11280±510
21750Z1000
FIG. 8. Black Loam stratigraphy in the middle reaches of Huanghe River. Soil hatched, loess dotted. Also shown carbonate content and 14C ages on organic carbon (C) and on the carbonates (CaCO3) in years BP. Modified from Liu et al., 1985.
LOESS OVERLYING RIVER TERRACES AGE OF THE GULLIES
AND THE
Relatively few observations exist on the relation of the loess series to the river terraces. Such information is essential for inferring times of accelerated crustal movements (Kukla, 1975). Figure 9 is a schematic and
composite model, based on regional mapping in the Luohe and Weihe River valleys in the Luochuan and Xian areas as reported by Wang et al., 1984a; and Liu et al., 1985. The depicted schematic sequence was never observed at any single locality. Terrace TI: is the lowest terrace defined by Wang et al., (1984a) on the Weihe River. It includes contemporary floodplain sediments but also gravels partly covered by loess or redeposited loess younger than paleosol S1. It probably corresponds to Terraces 1 and 2 described on Luohe River near Luochuan by Liu et al. (t985). The terrace is overlain by loessial deposits in places, which are probably retransported. It is possible, however, that some of the oldest deposits are primary loess, of Malan age, blown onto the water bodies or swampy intermittently flooded river beds. The base of terrace T1 is close to the recent river floor. Terrace 7"2: corresponds to T3 of Liu and co-workers in the Luohe River. It is covered by the loess L2, soil S1 and loess L1. Its base is around 2 m, and its surface is about 10 m above the river. Terrace T3: corresponds to the T4 of Liu and coworkers. The loess covering the terrace contains four
197
Loess stratigraphy in China
T1 FIG. 9. Schematic cross section showing the relation of the soils (S) and loess (L) in the Weihe and Luohe River areas to the terraces. Gravel in circles, reworked loess in wavy lines. Constructed from the information published by Wang et al., 1984a; and Liu et al., 1985.
paleosols (S1 to $4). At Luohe, as at the Weihe River, paleosol $5 is thick and steeply (20°) inclined toward the T3 terrace cliffs, whereas soil $6 and the older soils, undercut by the terrace, are nearly horizontal. This sequence is interpreted as the result of two erosional episodes: one preceding the formation of $5; the other during, or shortly after, the development of paleosol $5. Alternatively, accelerated downcutting may have started during the L6 stage and continued through the early stage of L5. T3 is the oldest recognized terrace of Brunhes age. Terrace T4: is covered by loess L9 and by the subhorizonal Lishi and Malan loess with paleosols $1 to $8. This terrace is the youngest of Matuyama age. In the Luochuan area the step separating T4 from T3 is 80 m high. Older terraces: Terrace T5 is overlain by loess L15 and by the younger members of the loess series which enclose paleosols $1 to S14. Two additional terraces developed contemporaneously with the L9 soil, one between the Jaramillo event and Brunhes chron, the other, during L15 time, between the Jaramillo and the Olduvai events. The relation of the Wucheng loess to the river network is unclear. Liu et al. (1985) suggest that the earliest downcutting of the Luohe River into the Red Clay basin started at about 2.4 Ma ago. But it seems that the principal changes in the evolution of the river network are related to the deposition of the exceptionally thick loess members L1, L2, L5, L6, L9 and L15, and with the last major erosional step, about 80 m, of the Luohe River, which occurred in MidBrunhes time, and before, or shortly after, the formation of paleosol $5. Except in the immediate vicinity of the rivers the paleosols within the Loess Plateau are more or less horizontal, continuous and uninterrupted by gullies. This contrasts with the current relief of the Plateau which is dissected by an intricate system of deep gullies with steep wails. The erosion which took place after the
formation of the early Black Loam has no analog in the Pleistocene strata. The volume of loess removed from the Loess Plateau during Holocene is much greater than at any time in the past. Jing and Chen (1983) have calculated that, about 1.6 billion tons of silt are carried away from the Plateau by the Huanghe River every year, which is sufficient enough to remove all the loess in under 60 thousand years. This contrasts with earlier interglacials, which were times of accumulation, rather than of erosion. The causes of the exceptional erosion in Holocene time may be multiple: (1) The Holocene climate is considerably drier than the earlier Pleistocene interglacials. This is evident from the difference between the carbonate content and structure of the Black Loam compared with earlier soils of Pleistocene age. A dry climate, combined with occasional torrential rains, accelerated erosion of loosely vegetated surfaces. (2) The natural vegetation, especially the sod, was destroyed by cultivation. Unattended, frequently shifting wagon trails also resulted in accelerated erosion. Natural grass sod and the root systems of low shrubs provide an efficient barrier to soil erosion even in otherwise unfavorable climates. These, however, were severely decimated by agriculture, and by goat and sheep grazing which, in Central China, has been common for several millennia. (3) In at least parts of the Plateau an accelerated uplift is taking place; and a relative subsidence of the river valleys of considerable magnitude has been determined in engineering surveys along the Huanghe, Weihe and Fenhe Rivers (Fig. 10). TIME CONTROL The dating of the loess sequence was at first based on vertebrate paleontology, later on radiocarbon, and then on thermoluminescence. It has been improved greatly in recent years by the application of paleomagnetic techniques.
198
G. Kukla
Mu Us Desert YINCHUAN
0 !
lOOkm I
FIG. 10. Major subsiding areas in the vicinity of the Loess Plateau dotted. From regional surveys. Modified after Liu et al., 1985.
Vertebrate Paleontology The basal Red Clays, also called Hipparion clays, contain typical elements of the Late Pliocene Hipparion assemblage and are considered an equivalent of the Early Villafranchian faunas in Europe (Xue, 1984). After the characteristic rodent Prosiphneus intermedius, which marks the evolutionary transition from the rooted Prosiphneus to the rootless Myosphalax, the upper part of the Red Clays is assigned to the final Prosiphneus zone. The Wucheng Formation is characterized by the voles of the Myosphalax fingi group with Myosphalax fontanieri being the most common species. Also present in the uppermost Wucheng WS1 layer is thc archaic hamster Kowalskia yunanica. The fauna of the middle part of the Wucheng Formation is the equivalent of the Nihowan fadna in northern China (Xue, 1984), which corresponds to the Late Villafranchian of Europe. Mierovertebrates in the Lower Lishi Loess enable a general correlation with the Biharian of Europe; whereas those in the Upper Lishi and Malan Formations contain microvertebrates whose evolutionary stages are typical of the Middle and Late Pleistocene. Recent reviews and faunal lists of vertebrate remains in the loess sequences were published, among others, by Xue (1984) and Liu et al. (1982).
Radiocarbon Dates Finite radiocarbon dates were obtained from the organic fraction of the Holocene soils and from the carbonate extracted from the Malan Loess. Because part of the carbonate may be detrital the latter determinations are likely to be biased toward an older age.
The biogenic steppe soils from sheltered sites apparently undisturbed by cultivation yielded ~4C ages between 11,280 + 510 and 1,935 + 130 BP. Out of 17 published determinations, 14 fall between 2.5 ka and 8.5 ka BP, with clusters around 3 ka, 5 ka and 7.5 ka to 8 ka BP. Carbonate in the Malan Loess of Luochuan yielded ages between 11.5 ka and 19 ka BP. It must be borne in mind, however, that the ~4C determinations in the two early Holocene black soils in Nuanquangou gave ages from 7,360 to 11,280 BP on organic carbon, but between 19,960 to 21,750 BP on the carbonate (Fig.
8). Thermoluminescence Ages Published thermoluminescence (TL) dates are shown in Table 2. Disagreement between the ages obtained by various researchers at different times illustrates the progress made throughout the world in the past few years in TL dating. Figure 11 shows how the published TL determinations of the age of some key loess and soil horizons have progressively been refined during the past decade. Currently any TL age greater than 100 ka is considered unreliable; and older sediments have been found to have TL dates biased towards underestimated ages. In the early TL determinations this was in part due to calibration against an incorrectly located MatuyamaBrunhes paleomagnetic reversal, which was originally placed within the basal loess L15 of the Lower Lishi (Figs 11 and 12). This layer is now known, however, to be over 1 Ma in age. The early TL dates were in general agreement with similarly dated long loess records in Hungary (Pecsi, 1984) and in Tadzhikistan (Dodonov, 1986), which showed very high apparent sedimentation rates in the last 150 to 200 ka, contrasting with very low
Loess Sti.atigraphy in China
199
TABLE 2. Examples of thermoluminescence ages (ka BP) as determined by different analysts by different methods at different times Layer
TL Age
L1 base S1 L2 top L5 base $5 base $8 base S14
31 41 71 178 212 316 666
SI $2 $3
110 210 250
L1 various depth
+ + + + + + +
Reference
3 3 5 24 15.8 10.3 47
Lu, Lu, Lu, Lu, Lu,
65 I00 140 460 590
Nishimura et al., 1984 Nishimura et al., 1984 Nishimura et al., 1984
19 45 103 148 357 696 876 1040
+ + + + + + + +
2 4 10 16 31 60 91 120
Dates on samples from 1980-1982 Liu et al., 1985
10-73
Liu Liu Liu Liu Liu Liu Liu Liu
20 40-50 70 140 435 730 860 2300
et et et et et et et et
AGE 103y
al., al., al., al., al., al., al., al.,
1985 1985 1985 1985 1985 1985 1985 1985
PUBLICATION DATE 1977 (cf.1981)
0
1981 1981 1981 1981 1981
1000
7 to 135
L1 top center bottom L2 center L5 top L8 LI0 WL4
Y.C., Y.C., Y.C., Y.C., Y.C.,
Approx. True Age
1984
1985
1987+
m
+ 41is
L! i////////51
"--,..
14tl
100 - -
Sl t
200 - 300 400-500 - -
17a±14 L5 SS \
~..
212211
+
llo 52
\
\\ T 2. 83 ~7.,'777.,'~$9 \ + 6is±ll L 9 \ \ \ \ X ',, . . . . X \ B/M in
LI5
X\
I
500 - -
X
~57±61 L5
_J ~/f/~..
z
X
B~M in L9
\ X \
700 - -
Authors:
b_-I- Io3±lo L1 ,, 4- ~ , S l
Lu Y.C.
Niskimura etnl.
\
- - i 6064"60 777~'/~. L8 88 Li H.H. in Liu et nl
L,u Y.i~. It nl lin prmtl
FIG. 11. Comparison of the thermoluminescence dating of several key loess layers in Luochuan plotted as a function of time of the publication. Earlier TL determinations were much too young. Also shown the changing determination of the Matuyama-Brunhes boundary. First authors of the determinations given, same as in the list of references.
sedimentation rates in the lower Brunhes. The ultimate causes of the young bias of the TL determinations of the Middle Pleistocene loess are physical, and were discussed in some depth in several recent papers (Wintle et al., 1984; Debenham, 1985). Thus far, no method has been devised to give reliable age determinations from loess layers older than about 100 ka. Magnetic Reversal Stratigraphy
The Matuyama-Brunhes reversal is located in the upper part of the loess layer L8 (Table 2). This was first
discovered by Liu Tung Sheng and An Zhisheng (1984) and later by Nishida et al. (1984) in Luochuan, and then confirmed by Heller and Liu, T.S. (1984, 1986). It was also reported from other localities by Liu, S.M. et al. (1985) and Liu, X.M. et al. (1985). Earlier interpretations placed the M - B boundary within loess L15, loess L7 (Li and Wang, 1982), or in the loess L9 (Heller and Liu, 1982). There are several reasons why this lithostratigraphic position of the reversal was determined incorrectly. The low intensity of the magnetic field preceding and accompanying a reversal results in a
200
G. Kukla weathering horizon below $6 was counted as an independent soil horizon, or as an interlayer of the loess unit L7. The Gauss-Matuyama boundary was located in the uppermost part of the Red Clay formation of the Loess Plateau (Fig. 13). The Jaramillo magnetozone was identified within a relatively intensively weathered series of soils and loesses bracketed by the marker beds L9 and L15. Figure 14 shows a comparison of different published polarity interpretations. To delimit the normal polarity zone in this interval is complicated by a relatively strong post-depositional remagnetization and by the difficulties in recognizing individual soils within the strongly weathered sequence. The Olduvai magnetozone was discovered by all investigators at the base of WL2, and in the soil group WS3 (labeled WS2 in the earlier work of Liu's team; cf. Liu et al., 1985; Heller and Liu, t986). Additional normal polarity subzones within Matuyama, and reversed polarity intervals within Brunhes were proposed, but their lithostratigraphic position was not verified in multiple exposures. Because of the possibility of unrecognized mechanical disturbances and secondary remagnetizations, the existence of these additional magnetozones in the loess series is questionable (Kukla and Nakagawa, 1977).
The Magnetic Susceptibility Time Scale It was shown earlier that the thickness-to-time ratio of loess and soil deposits, derived from the position of the reversal planes, shows only minor variations within the last 2.5 Ma (Fig. 15). This enabled approximate estimates of the age of key stratigraphic horizons of the loess sequence based on the assumption of a constant deposition rate between the reversal planes, and on the depth below the surface of individual layers, corrected for density. These estimates are given in Table 5. It was also obvious, however, that within each loess~a ~b ~c ld soil couplet, the soil accumulated at a much slower ~e ~f ~g lh average rate than the loess. This was confirmed by the accumulation rates of 1°Be analyzed in the Luochuan FIG. 12. Carbonate content of loesses and soils in Luochuan section by Shen (1986). Major differences between the (modified from Lu, 1981). Depth below the top of the plateau in deposition rates of the soil and the loess have also been metres (A). CaCO3 content in bulk samples in percentage. Column B: (a) weaklyweathered loess; (b) moderatelyweathered loess; (c) observed in radiometrically dated loess sequences in stronglyweathered loess; (d) dark loessial; (e) calcareousdrab soil; central Europe (Klima et al., 1961). (f) drab soil; (g) leacheddrab soil; (h) drab brown soil. ColumnD, Because the low field magnetic susceptibility is paleomagnetism. greater in the soils than in the loess, and because it apparently increases as a function of the degree of soil weathering, Kukla et al. (in press) proposed a weighting for the thickness of each layer by its magnetic relatively weak primary magnetization of the sediments susceptibility. This index, interpolated within the of the uppermost Matuyama age (Clement and Kent, framework of paleomagnetic reversals, is used as a 1986; Davis et al., 1977). These layers then acquire a measure of relative time. The susceptibility time series produced in this relatively strong normal overprint, whose separation is difficult, and needs multiple testing on sensitive mag- manner shows remarkable parallels with the oxygen netometers. Such instruments were not previously isotope record of the deep-sea cores, both in the available in China. Further, the numbering of the loess amplitude of individual peaks as well as in their timing. and soil layers in the sequences studied was not Figure 16 shows the comparison of the SPECMAP standardized. For instance, loess L8 was labeled by oceanic average oxygen isotope record of the Brunhes some as L9, depending on whether a poorly developed Epoch (Imbrie et al., I984) with the upper portion of
201
LOess Stratigraphy in China
.=J
Red cloy Loess nodules
--Red ctey
S27
~ $ 2 8
FIG. 13. Lithostratigraphic position of the Gauss-Matuyama boundary at key localities.
Chronozone
8runhes
• . ° • . . °. °o .'o,.
daramiU.o
,..'
Cob MT ( ? )
• .: .* ..°
FIG. 14. Same for the Matuyama-Brunhes Boundary and Jaramillo. Symbols same as in Fig. 4.
202
G. Kukla O L
50 --[1---
nM I U.LISI-ll T
LOWEll LISHI
So_
~ •
I )0 (m) /
I00 I
= - - ~ - -
1
WIJCH'I~I~G' IR.C.
-/- a-~,
-~s~ s _
RUNHES
"~.S, S'~',.Ss $9
l
-z 's3)
I•_v,Ws
A~
,k~LWES -3 La.
~I~GAUSS
Assemblage
FIG. 15. Time/depth ratio of the Luochuan sequence. Depth from the top of the Plateau (m), reversal chronology showing only magnetozones established in the sequence. After Liu et al., 1985.
OCEAN
LOESS ATFENG
SPECMAP 84 TIME
d180 RECORD
[kyl"l
LOG SUSCEPTIBILITy {Sl-u*~itsl
2.0
-3.0 -5
-3
500
s5
i
/1 FIG. 16. Comparison of the (A) Xifeng magnetic susceptibility record plotted as a function of time (log SI units) with the (B) SPECMAP 84 age model of the averaged oxygen isotope stratigraphy of the Brunhes epoch. Delta oxygen record tuned to astronomic frequencies, susceptibility record plotted on an independent time scale derived from the thickness weighted by susceptibility. More details in text (cf. Kukla et al., in press).
203
Loess Sti'atigraphy in China
T A B L E 3. A g e determinations of ihe loess and soil units in Xifeng from the susceptibility time scale model. Susceptibility thresholds: 1000 SI for SO through $5; 700 SI for older units; 600 for L9 and $9. A g e of the oxygen isotope stages 1-20 from Imbrie et al., 1984. Ocean
Loess
Oxygeu S t a g e
Formation
Unit
Age Base
Holocene
SO
10
10
Malan
L1
68
58
Upper Lishi
SI L2
128 174
60 46
S2a S2b S2c
218 230 247
44 12 17
L3
271
24
S3a S3b $3c
292 293 328
21 I 35
L4 $4
357 428
29 71
L5
467
39
S5a SSb $5c S5d S5e
549 558 584 592 614
82 9 26 8 22
L6 $6
652 672
38 20
£7a L7b L7c S7
678 690 698 726
6 12 8 28
L8 $8
737 801
|1 64
L9 $9
834 843
33 9
42
LI0 $10
870 904
27 34
63
Llia LIIb Lllc SII
907 929 935 987
3 22 6 52
83
L12 SI2
996 1013
9 17
26
L13 SI3
1039 1090
26 51
77
L14 S14
III0 1135
20 25
45
LI5
1172
37
WSla WS1
(1205) 1316
(33) I11
WLI WS2
1434 1566
118 132
~..2
1695
129
WS3 WL3 W$4 WL4
1939 2102 2247 2342
244 163 145 95
RL! RL2
2435 2.514
Lower L i s h l
Wucheng
Red Clay i
Duration
Duration
I
II
II
4
71
62
5 6
128 186
55 58
220 233 245
34 13 12
303 339
58 36
94
362 423
23 61
84
12
478
55
13 14
524 565
46 41
15
620
55
16
659
39
112
17 18 19
689 726 736
30 37 10
75
20 21
I0
118
73
119
7a 7b 7c
57
81
8 9
100
147
Age Base
186
II
117
59
142
117
197
58
54
31
70 14~
69 47
116
204
G. Kukla
the susceptibility record in Xifeng measured by Liu et al. (1985). The frequency analysis of the two time series, shows periods of 100 ka and 40 ka in each. However while the SPECMAP 84 oxygen isotope series of Imbrie et al. (1984) was tuned to astronomic frequencies, the Xifeng series was not. The cause of the high magnetic susceptibility of the fossil soils in the chinese loess is still unclear. "l'hcrnlomagnetic investigations and studies of ferromagnetic concentrates did not reveal any noticeable differences in the composition of magnetic minerals in the loess and the soils (Sasajima et al., 1984; Kukla et al., in press). The only systematic difference found, was a higher concentration of the magnetic fraction in the paleosols. The soils contain a higher proportion of the ferromagnetic minerals, principally of titanohematite, titanomagnetite and titanomaghemite, than the loess, and are possibly composed of finer grain size. Potential sources of the ferromagnetic component are fine airborne detritus from the deserts northwest of the accumulation area (Sasajima et al., 1984; Liu et al., 1985), volcanic ash of distant origin, micrometeorites (Hilton, 1987), and detritus transported by wind from within the Loess Plateau and from the local river floodplains. Although more detailed studies are required to explain the susceptibility variations, the currently available evidence favors depositional rather than pedogenetic processes as controls. Weighting by magnetic susceptibility equalizes the differences in sedimentation rates of individual layers, and enables close correlation of a single glacial cycle of the loess sequence with the deep-sea sediments. It also provides a relative time scale against which the tuned oxygen isotope chronostratigraphy of the deep-sea sediments can be tested.
CLIMATE INDICATORS
The alternation of loess and soils document oscillations of the environment between a bare or poorly vegetated semi-desert on the one hand, and grasslands, woody grasslands, and woodlands on the other. Thus, in essence, the primary contrast of the lithology of the loess series is the absence or presence of vegetational cover and its density. In this respect the chinese loess sequences are similar to the loess areas elsewhere. Information on the environment and climate of the loess sequence is obtained from the lithology, from its gastropod fauna and its vertebrates and pollen. Lithology A gross assessment of climatic conditions can be inferred from lithology. As the environment repeatedly oscillated between the two extremes of a semidesert, with meager patches of grass and scattered shrubs, on the one hand, and a wooded savana on the other, the deposits alternated between the light colored calcareous loess and the dark colored decalcified soils.
As for the influx of eolian dust, this occurs during both the cold and the warm intervals. Dust storms were mentioned as early as in 1150 B.C. in the Bamboo Slip Annals (Zhang, 1984), and were observed by Richthofen (1882) and Obruchev (t958), who both held past dust storms responsible for the deposition of Pleistocene loess. Several recent events have been described in detail, partly with the help of satellite imagery (Zhang, 1984; Liu, T.S. et al., 1981, 1982). The dust was composed predominantly of silt-sized grains of quartz, feldspar and carbonates similar to the Pleistocene loess. Turbulent mixing activated by the upper air jet stream, lifted the dust from the deserts in Mongolia and northern China. The strong upper winds transported the dust at an altitude of up to 10 km over the distances of 1,500 km or more eastward from the source areas. Dust storms are today most common in spring, especially in April. They are more frequent after dry winters. Although in exceptional circumstances a single dust storm may deposit several centimeters of silt, the average dust fall in China deposits only about 1 ton km -2 day -t (Liu, T.S. et al., 1981). At these rates the pedofauna and the flora can incorporate the newly deposited dust into existing soils so that no new loess layer is formed. During the glacials, however, the dust storms were, in all probability, much more frequent, and the strong winds in the Loess Plateau transported fine sand and silt from local sources. With reduced vegetation and an impoverished pedofauna, the sediment formed discrete loess units. Gastropods Fossil gastropods of the Plio-Pleistocene sequence in the Loess Plateau have been studied by Pei and Zhou (1958), Chia and Wei (1976) and Chen et al. (1982). The Luochuan sequence was studied in most detail. Snail shells were most abundant in the Malan and Upper Lishi Formations and least abundant in the Wucheng Formation. They do not occur in the decalcified horizons of the interglacial soils where the carbonate shells are leached during pedogenesis. It is probable, therefore, that snails of interglacial peaks were largely destroyed, and are thus not represented in the relatively limited faunal lists published to date. It seems that no attempts have been made hitherto to concentrate shells by washing, to identify broken shells, or to search for interglacial assemblages in favorable pockets for preservation of calcareous early interglacial sediments (Lozek, 1964). The two dominant species present throughout the sequence, both in the loess as in the soils, are Cathaica pulveratrix (Martens) and Metodontia hausaiensis (Crosse). Both are common even in the Holocene soil and still live at the sites. The fossil distribution of Cathaica is almost identical with its present one, whereas the present distribution of Metodontia extends beyond its fossil range. M. hausaiensis is clearly more frequent in the soils than in the loess, while C. pulveratrix is more frequent in the loess.
LoeSsSti~itigraphy in China In the Malan Loess, M. hausaiensis is most abundant at the base and least abundant at the top, while the drought and cold resistant Pupilla aeoli (Hiber), Valo Ionia costata (Muller) and Cathaica lutuosa (Crosse et Deshayes) increase in abundance toward the top. This is interpreted as a sign of progressive change of the loess environment from relatively mild and humid towards harsher and colder conditions. The fossils found in the Lishi and Wucheng Formations are too limited to enable meaningful reconstructions of paleoenvironmental changes. Several species only occur in sediments younger than $6. They include Vallonia costata (Muller), V. tenera (Reinhard), Cathaica fasciola (Drap) C. pulveratricula (Martens), C. teilhardi (Yen) and Succinea erythrophana (Ancey). On the other hand, Cathaica shikauens/s (Yen) is relatively common in the Wucheng and Lower Lishi Formations up to the soil S10, but is unknown from any younger layer. Cathaica subrugos only occurs in the Wucheng Formation. The climatic implications of these local disappearances are as yet unknown.
Vertebrates Fossil vertebrate bones are relatively rare, and their stratigraphic position is not always accurately known because of the transitional nature of the upper soil boundaries, and because rodent bones are sometimes found in burrows (krotowinas) reaching deep beneath the stratigraphic level at which the animal lived. From the basal loess WS4 of the Wucheng Formation in Luochuan the rodent Myospalax omegodon marks a shift from a previously relatively balmy environment of Red Clays time, to a dry, and seasonally cold, harsh continental climate. From the middle and upper part of the Wucheng Formation the rodents Myospalax chaoyatseni, M. tingi, M. arvicolinus, Sinocricetus zdanskyi, and of Ochotonoides complicidens have been discovered; together with bear Ursus etruscus, Eustenoceros, Cervus sp., Boris sp. and GazeUa sp. (Kawamura and Xue, 1984; Liu and Yuan, 1982; Xue, 1982). This fauna shows alternations between cold continental steppes, comparable to those of northern China today, and wooded steppes, or open woodlands, developing under a relatively mild and humid climate. The Lower Lishi Formation yielded vertebrates of mixed climatic requirements. Rodents of the Myospalax and Microtus families, known to live in cold, dry continental steppes, are frequent. Also present, however, are woodland species. In the basal loess of the lower Lishi sediments in Lantien elephant, monkey (Megamacaca), fossil panda, and woodland rhinoceros (Dicerorhinus) have been collected (Zhou et al., 1965). From the lower part of paleosol $5 (or from the top of $6) in Luochuan giant deer (Megaloceros luochuanensis) and Apodemus agrarius, a rodent of savanas and light woodlands, occur (Xue, 1982). The vertebrate fauna of the Lower Lishi Formation shows extreme oscillations, between cold and dry continental climates, comparable to the interiors of northern Asia, and the
205
relatively moderate and moist climates supporting tall grasslands and woodlands comparable to those in southern China today. The Upper Lishi and Malan Loess Formations yielded relatively few fossils, the majority of which belong to steppe elements. The common rodent being Myospalax fontanieri.
Flora Pollen from the loess and fossil soils represent decimated residuals of original pollen rain assemblages which survived burial in the adverse environment of carbonate rich, well aerated sediments with an active bacterial population (Zhu, 1984). An additional complication in interpreting the pollen record in the loess is the potential of long distance transport by strong winds up to several hundred kilometers into an area with low autochtonous pollen production. It is difficult, if not impossible, to distinguish such distant from local pollen. A further serious problem is the small amount of pollen recovered from most of the Wucheng and Lower Lishi Formations. Only about 20 grains or less were recovered from these units in Luochuan, while at Xian average recovery was about twice as abundant (Figs 17 and 18). Thus, only general conclusions can be drawn from the two sections which have been studied in some detail, one in Liujiapo near Xian, and the other in the Heimugou Valley near Luochuan (Wang et al., 1984; Liu et al., 1985). At both localities herbaceous pollen dominate over tree pollen. Especially frequent is Artemisia. Because this pollen has hard shells favoring preservation, its relative abundance may be the result of selective preservation. The relative abundance of tree pollen is higher in the soils and lower in the loess. The pollen of Artemisia, Chenopodiacae, Pinus and Selaginella (one grain of each) found in the base of the Wucheng formation in Xian indicate an arid steppe or desert, but no environmental reconstruction can be made from such meagre evidence. The WS3 in Luochuan and the corresponding middle part of the Wucheng Formation in Xian show tree pollen to comprise almost one third of the total count. These are mostly broadleaf species. Artemisia and grasses are the other main components. Typha orbicularis is also present. The top of the WS3 unit is the oldest layer in which the pollen are relatively abundant. Because pollen is generally rare in the loess, a relative abundance in WS3 is believed to show a humid, and probably warm, episode with open woodlands near the sites (Wang et al., 1984). Higher in the Wucheng Formation, in a layer probably correlative with WL2, pollen grains are again rare. The highest layer of the Wucheng Formation WSla contains a pollen assemblage similar to WS3, with dominance of broadleaf species over pine. Interesting is the rare presence of southern species Carpinus, Pterocarya and Comus. Loess layers of the Lishi Formation are poor in pollen, represented mostly by grasses, Artemisia and
206
O. Kukla
Wood
Pollen
Herb
LI m
n m
.! ,J
WS4
i
i
i~
w
J
m
WL4
F
I
Wood
pollen
• Herb
pollen
~
~
~
~ ~
~o o~~ • ~~
o.
~
. ~
.~ ~
"
~o . . . ~. . 7.
~
~
u
,~
FIG. 17. Number and types of pollen grains recovered in the sequence at Xian (modified from Wang et al., 1984).
Chemopodiacae, which indicates a loess steppe environment. Commencing with L9 the assemblage includes Ephedra. In soils $9 to S14 of the Lower Lishi Formation: that is, in a layer which includes the Jaramillo event, woody plants account for about one-third of the total count. Anacardiacae dominates in Luochuan, and pine dominates in Xian. Broad leaved species include the temperate element Castanea. Celtis is also present. The climate was probably marked by relatively warm episodes. In Luochuan, soils $8 to $6, from before and after the M-B boundary, contain relatively rare pollen, among others with Juglans and Typha angustifolia the latter an indicator of a humid environment. The pollen assemblage of marker bed $5 has frequent tree elements at both localities, with Anacardiacae dominant in Luochuan and pine in Xian. Juglans shows a rather temperate climate. The appearance of Ericaceae, which grow in acidic soils, ~hows deep decalcification of the $5 surface, probably in a relatively humid climate, without a prolonged dry
season, which may have occurred during part of the $5 development. Loess layers L4 and L3 of the Upper Lishi Formation are pollen impoverished, but have a considerable proportion of tree species. Especially interesting is oak, hornbeam and beech in loess L3. In loess L2 a small proportion of tree pollen includes pine and, for the first time in the Loess Plateau, spruce. Artemisia and Chenopodiaceae are common in Xian; Ephedra is also present. Paleosols $3 and $4 in Luochuan, in contrast to the older layers contain a relatively rich pollen assemblage with Anacardiacae and Thalicrum. Grasses and Artemisia are rare. Compared to the soils of the Lower Lishi Formation, the surface may have been wetter. Paleosol $2 shows dominant pine and birch in both Luochuan and Xian, with elm in Luochuan. Rare Gleditschia, Albizzia and Fraxinus found in Xian, suggest a proximity of temperate forest, possibly in the nearby mountains. Percentages of Artemisia, however, are high. Paleosol S1 yielded pine pollen with some oak and
207
Loess Stratigraphy in China
Number of Wood Herb Pollens
&
Wood
Pollen
LI
Lz
-IllILI!IllIl!II LIIlli
m~-', m -
o
"=-- rfl t[I rl
rr lrl i
~,lllllrllr[lllll
lIIl[l!I
g~ ~u~.m. ~ , ....... ,-.----.
Illllltllll!llll
II Illlt!lII
m
n
rnm
~ ~
IllllllllIllllll Wood
Pollen
Herb
Pollen
FIG. 18. Same for Luochuan.
birch in Xian, and Anacardiaceae, pine and oak in Luochuan. Typha is also present. In the Malan Loess tree pollen are rare except for the top layer, where pine, birch and elm occur in Luochuan and pine in Xian. It is possible that the uppermost layer of the Malan Loess marks the climatic amelioration of the Late-glacial. A dominance of Artemisia, however, shows a xeric steppe landscape. The Holocene Black Loam (SO) is rich in pollen; herbs dominate; pine is most common in Xian followed by birch and oak; elm is present in Luochuan. Artemisia, Chenopodiaceae, Graminae and Compositae appear together with frequent Selaginella. The assemblage is representative of a forest-steppe landscape, developing under a moderately temperate and humid climate. There is a disagreement between the pollen assemblage of the SO unit, which shows a partially wooded landscape, and the soil, which is high in carbonate, and probably developed under an open grassland (Bronger, 1976). This seriously limits the validity of any paleoclimatic reconstructions based on the fossil spectra of the Loess Plateau sequence. Thus far only the most general conclusions about this area can be made: (1) The pollen in the loess members shows landscapes with a highly restricted vegetational cover of grass, shrubs and weeds with few, if any, trees.
(2) The pollen of the soil members is compatible with a model of a relatively dense vegetation during some developmental phases of these polygenetic soils. Although differential preservation is likely to have altered significantly the ratios of the pollen, it is probable that woods were present in or near the Loess Plateau. (3) Climatically demanding species, in paleosols WS3, WS1, $9 to $14, $5 and $2 may show a wetter and more humid climate in the Plateau and its vicinity thanduring the formation of the rest of the soils.
Carbonate Content Carbonate content is high in the loess and low in the paleosols. As proposed by Lu (1981), it can be used for a first order approximation of the weathering regimes prevailing over the Plateau (probably also in the deflation areas) during the time of deposition of individual members of the loess series. As can be seen in Fig. 12 and Table 1, the mean carbonate content in the Luochuan Formation progressively decreases from 12% in the weakly weathered loess to 0.1% in the soil $5. The loess units L9, L6, L5, L2 and L1 show the lowest weathering intensity, while the loess L7, L8, L10 and L l l are the strongest weathered within the Lishi and Malan Formations. Within the upper half of Brunhes time, L3 and L4 have the lowest carbonate content, and could, therefore, be interpreted as show-
Loess Stratigraphy in China susceptibility signal is due to the high level transport of airborne dust from distant sources. Such sources may include deserts in Mongolia or the Pacific volcanoes. On a scale of several millenia the changes of the influx rate of such distant components are small and depend little on climate. In contrast the silt from local and regional sources has low susceptibility and its accumulation rate is closely controlled by climate. During cold and dry intervals the deposition from local sources (deserts of Northern Shaanxi, floodplains of braided rivers and the bare Plateau surface) dominates, and the resulting deposits have low susceptibility. The opposite holds for the relatively warm and humid intervals during which surface of the Plateau supports a closed vegetation, the valleys are forested and the rivers carry little silt. At these times accumulation is dominated by fine grained dust from distant sources and the resulting deposits have high susceptibility. If the model is correct in principle, then the susceptibility would be inversely related to the regional density of vegetation cover. As yet, no results are at hand which would either support or refute this hypothesis. Within the range of variability of the oxygen isotope record, however, the susceptibility fluctuations in Luochuan (Heller and Liu, 1984) and in Xifeng (Kukla et al., in press) correlate closely with the climatic signal from deep-sea cores. This allows the following, tentative, conclusions: (1) The susceptibility signal is indirectly related to changes of ice volume. (2) The close agreement of the oxygen isotope SPECMAP time scale of Imbrie et al. (1984), tuned to astronomic chronology, with the susceptibility-age model, supports the general validity of the SPECMAP tuning procedure as well as of the susceptibility-age model. The Silt to Clay Ratio
It was observed earlier that the ratio of silt to clay changes between the soil and the loess (Lozek and Kukla, 1959; Liu et al., 1985). The silt to clay ratio can then be used to differentiate a wind blown loess from a weathered paleosol. It is commonly referred to in the chinese loess literature as the K d index. It is the ratio of silt fraction in the 0.05 to 0.01 mm category (S) to the clay fraction smaller than 0.005 mm (C), computed after the formula: Kd=
S C
It was hypothesized that the high K d index points to a dominant mechanical weathering in the source (and deposition) areas of the loess, whereas the low K d index shows an increased production of clay size particles in a relatively humid and mild climate. The Iron Oxide Ratio
It was observed in the Luochuan section that the content of Fe203 is inversely correlated with carbonate.
209
The more advanced the degree of decalcification, the higher the FezO3/FeO (or the lower the FeO/Fe203) ratio of corresponding layer (Wen et al., 1984). In deeply weathered paleosols, the absolute concentration of iron and aluminum increases. Fe203/FeO record of the upper 1 Ma interval in the loess section in Heimugou Valley near Luochuan was successfully compared by Liu et al. (1985) with other proxy climate indices.
CLIMATES OF THE LAST 2.5 Ma IN CHINA COMPARED WITH THE DEEP-SEA STRATIGRAPHY
The climate history of the chinese Loess Plateau closely parallels the oxygen isotope record in the ocean. Correlation of the loess stratigraphy with the oxygen isotope stages 1 to 22 was proposed by several authors (Liu et al., 1985a; Sasajma et al., 1984; Heiler and Liu, 1986). In Fig. 16 the magnetic susceptibility of the Xifeng outcrop is compared with the average oxygen isotope record of the 12 deep-sea cores covering the Brunhes Epoch selected by Imbrie et al. (1984) and dated by tuning to orbital periodicities. The Xifeng record is plotted against the susceptibility time scale which has been interpolated between the base of soil S1 and the Matuyama-Brunhes boundary. Soil S1 was dated by thermoluminescence to 110 ka (Nishimura et al., 1984), and its base is taken to be 128 ka, the age of termination II (Broecker and Van Donk, 1970; Imbrie et al., 1984). It is reasonable to expect that the start of the soil development in the Loess Plateau was roughly synchronous with the oceanic terminations, since such time relationships occur in the case of the Holocene Black Soil SO, which is 11 thousand years old (Liu et al., 1985). Figure 19 compares the entire Xifeng susceptibility record, plotted against the susceptibility time scale, with the oxygen isotope record of the Deep Sea Drilling Program core 552A (Shackleton et al., 1984; Zimmerman et al., 1984). It is assumed that the sedimentation rate between the reversals is homogenous. DSDP 552A is one of the few analyzed cores which covers the interval of interest in detail. Both records are interpolated between the same reversal time lines which are: The top of Matuyama at 730 ka, the top of the Jaramillo at 910 ka, the top of the Olduvai at 1660 ka, the top of Gauss at 2470 ka, the top of Mammoth at 2920 ka, and the top of the Gilbert at 3400 ka. No orbital tuning was applied to either of the records. The Gauss-Matuyama boundary in the DSDP 552A was drawn at the base of the interval with transitional polarity rather than at its top, as done earlier in the original references. It is in core 9, section 4 at a depth of 26 cm. Comparison of Figs 16 and 20 demonstrates the limitations of the oxygen isotope record, which varies widely in detail from site to site and in which only the gross features of climate signal can be extracted from
210
(;. Kukla Xifeno I
2
DSDP 552A 3
4
-5
-4
-3
-2
...n~ ~7----=..
1400
r-
a~
24130
FIG. 19. Comparison of the magnetic susceptibility record in Xifeng as measured by Liu, X.M. et al. (1985), with the oxygen isotope record of DSDP 552A in central North Atlantic measured by Shaekleton et al., 1984. The Xifeng record plotted on a time scale derived from thickness weighted by susceptibility, the deep-sea core record linearly interpolated between the reversal planes. More information in text.
any single core (cf. Shackleton and Opdyke, 1976; Imbrie et al., 1984). On grounds of expediency the following discussion refers to the high susceptibility layers as 'warm' and to the low susceptibility peaks as 'cold' even though the temperature may have had little to do with the fluctuating density of the vegetational cover, controlled principally by moisture. The following information is obtained from the comparison of the two records. Holocene
In line with many other paleoclimatic indicators, the susceptibility record of Holocene soils and sediments is
disturbed by the artificial land use and soil erosion. This has no analogs during the past 3 Ma of Loess Plateau history. In contrast, previous interglacials were times of net accumulation, when soil erosion by rivers was modest and locally restricted. Cultivation by tillage commenced on the Loess Plateau some four thousand years ago (Liu et al., 1985), but vegetation clearance as well as grazing and severe disturbance of the sod has much earlier origins. It is difficult to judge what the vegetational cover and soils would be today without human interference. It is possible that the Holocene environment would have developed differently from previous interglacials, because the biogenic, humus rich
211
Loess Stratigraphy in China Xifeng Loess SusceptibiLity SI units I '1"~*
0
oI
DSDP 552A ~le 0
~ ~'~
2 I
..~ I LI
Sl~
4 I
-6 0
-4
-5
-30xy
r...:.,
IO
+~e'"
4
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.__..,.~
2O
1.2 --~
200--
*
6
$2
20C
S3
~
30 ~.,.,~ 0; -
5OO
--
9
•
x:,
4O 4OO
50
5OO
g~
.-.--~. $6
...v~.,...~
~'~"+--+ ~
16
"7OO 7G
21 8OO 8C
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9C -
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_:Z=~ ~'~....____
~
~oo
L.,2 s,2 1-13
' ~
.~. ~--~
LI4
32 33 34
I I00
1200
1200
FIG. 20. Same as Fig. 19, but for the first 1.1 Ma of record. Also showing proposed correlation of the Chinese loess and soil units with the oxygen isotope stages (OXY) in the DSDP 552A. Depth in meters from the top of the Plateau and from the top of the core. Age scale in thousand of years. The OXY stages 22 to 65 from Ruddiman et al., 1986.
Black Loams of the Late-glacial and early Holocene have no known earlier analogs. The pollen assemblages of early Holocene soils, however, are similar to those of earlier interglacials. The Malan and the Upper Lishi Formations The susceptibility record of the last glacial cycle ($1 and L1) in Xifeng shows a typical sawtooth pattern with a tripartite soil S1 corresponding to Oxygen Isotope Stage 5. Rapid loess deposition at the base of the Malan Loess (L1) is contemporaneous with the Oxygen Isotope Stage 4, and is followed by a slower rate of loess deposition, interpreted as being caused by a denser vegetational cover during Oxygen Isotope Stage 3. Another high rate dust deposition interval correlates with the Isotope Stage 2. As seen in Fig. 16, the magnetic susceptibility in S1 decreases moderately during Substages 5b and 5d, thus indicating a higher influx of eolian silt. Because of intensive bioturbation, however, the record of the peak climate of Substage 5d is smoothed. The Xifeng sampling does not allow detailed reconstruction of environmental fluctuations within Oxygen Isotope Stage 5. It is, therefore, unclear whether wooded steppe, the presence of which is indicated by
the pollen of the $1 soil in Xian and Luochuan, was present throughout all of Stage 5 or only during some of its Substages. Soil S1 is the youngest of the four Pleistocene polygenetic paleosols of Late Brunhes age; thermoluminescence dating shows it to be 110 ka in age (Nishimura et al., 1984). The susceptibility time scale shows that it developed over approximately 55,000 years, and represents the Last Interglacial as well as the Early Glacial. Soil $2 is markedly different from $1. It is subdivided by a well defined layer of weathered loess, which correlates with Oxygen Isotope Substage 7b. This separation has no analog in the paleosol S1. In China the soil correlative with Substage 7a lasted longer (some 30,000 years) and apparently developed a denser vegetational cover than that present during Substage 7c.
Soil $3 is the correlative of the Oxygen Isotope Stage 9. It shows a pronounced twofold subdivision in its susceptibility record, (Fig. 16 from Xifeng), and in its carbonate content (Fig. 12 from Luochuan); but its bipartite nature has not yet (so far as is known) been observed in field. The two warm peaks of $3 are approximately 40,000 years apart. About 40 millennia
212
G. Kukla
also separate the late warm peak of $3 from the early warm peak of $2. The intervening modulations at the wavelength of about 20 millennia, well expressed in the oxygen isotope record, are only weakly indicated in Xifeng. Soil $4, which is correlated with Oxygen Isotope Stage 11, resembles a polygenetic soil S1, but its development apparently lasted some 10,000 years longer. It shows only few signs of climatic deterioration in its upper part. In this respect the oceanic record of Oxygen Isotope Stage 11 is similar to the $4 in China. The 'cold' susceptibility peaks of all four loess units of the Upper Lishi Formation reach the values recorded during the last glacial cycle; but the apparent duration of the cold spells, as derived from the susceptibility time scale, differs considerably. The L1 took about 60,000 years to be deposited, the L2 about 50,000 years, L5 about 40,000 years and L4 approximately 30,000 years. L3 has an apparent duration of only about 25,000 years. Loess L3 and L4 have a relatively low CaCO 3 content (Fig. 12), in agreement with the relatively poorly expressed Oxygen Isotope Peaks of Stages 8 and 10. Loess L5 was deposited during Oxygen Isotope Stage 12, and is contemporaneous with the downcutting episode at the Luohe and Weihe Rivers. Similarly, loess L2 is time equivalent to Oxygen Stage 6 and is contemporaneous with the downcutting of the rivers into the T1 terrace level. The Soils and Loesses o f the Lower Lishi Formation
The soils of the Lower Lishi Formation differ considerably from the younger ones. The upper part of the polygenetic paleosol $5 reaches extreme susceptibility values and has the lowest carbonate content of the whole sequence. The polygenetic soil took over 70,000 years to develop, and reached the highest degree of pedogenetic weathering attained in the last 2.5 Ma. All the older paleosols, including the lower segment of $5 have considerably lower magnetic susceptibilities than the soils of upper Brunhes time. With the decrease in the amplitude of the susceptibility oscillations, the frequency increased, showing the dominance of the 20,000 to 40,000 year periods. This general character of the susceptibility record is maintained throughout the 2 million year long interval covering lower Brunhes time and the Matuyama Epoch. The upper part of $5 correlates with the Oxygen Isotope Stage 13. In the oceanic record this stage barely reaches two-thirds of the isotopic values of the younger warm stages. Its continental equivalent in China, however, marks an exceptional interval of prolonged warmth and humidity during which the surface became deeply decalcified, and exotic southern trees flourished in the vicinity of the Loess Plateau. Judging from both the susceptibility readings and the pollen content, the landscape was probably less densely vegetated during the Early Brunhes and uppermost Matuyama than in the younger interval. The eolian
deposition rate was higher and the decalcification of the soil horizons less complete. Some of the pollen grains, however, do indicate relatively warm and humid conditions in the vicinity of the studied sites. The polygenetic paleosois indicative of longer duration and/or intensity of weathering are the $6, $8, S10, SII, $13 and $14 soils. The loess units L6, L9, L13 and L15 are relatively unweathered, and show low susceptibility comparable to the upper part of the record. Several deep-sea cores were studied covering the time interval of the Lower Lishi Formation. Only two of them, however, have sufficiently high deposition rates for the oxygen isotope signal to give sufficient detail for meaningful comparisons with the chinese susceptibility record. These are the DSDP 607 (Ruddiman et al., 1986) and DSDP 552A (Shackleton et al.. 1984; Zimmerman et al., 1984), both in the central North Atlantic. In DSDP 607, Ruddiman extended the numbering of the oxygen isotope stages below stage 22 (Fig. 21) following the system proposed by Emiliani (1966). This is now extended further toward the Matuyama-Gauss boundary in core DSDP 552A (Fig. 22). The prefix OXY is used for stages in the oxygen isotope stratigraphy to differentiate them from other systems. Piston core V28-239 is considered to be a reference section for OXY 22 to OXY 65. DSDP 552A is the reference sequence for OXY stages 66 to 90. The numbering of the oxygen isotope record below stage 22 must be considered tentative until a sufficient number of sequences are studied and cross-correlated to recognize spurious signals. It would appear that this proposed tentative extension of designated stages in oxygen isotope stratigraphy to the Matuyama-Gauss boundary is both useful and justified because of the close parallels observed between the oxygen isotope and the chinese susceptibility records. A tentative correlation of the Xifeng susceptibility record with the Oxygen Isotope Stages is proposed in Figs 20, 21 and 22 and in Table 5. It shows the following common features: (1) A pronounced lengthy cold spell during the L6 and the Oxygen Isotope Stage 16 (OXY 16). (2) High frequency, low amplitude fluctuations arc found in the lowermost Brunhes and in the Upper Matuyama, which contrast with the low frequency and high amplitude oscillations in the Upper Brunhes (cf. Ruddiman et al., 1986). (3) A major warm-cold couplet of about 80,000 years duration immediately precedes the MatuyamaBrunhes reversal (OXY stages 20 and 21; soil $8 and loess L8 in China). (4) A generally warm interval of rapidly oscillating climate occurred during Jaramillo time. (5) A pronounced and lengthy cold period occurred between approximately 790 and 850 ka. (Loess units L10 and L9, OXY Stages 22 and 24.) (6) Three pronounced cold spells occurred between 1000 and 1150 ka (OXY 34, 36, 38). The earliest (L15 in China and OXY 38), which started at about 1160 ka
213
Loess Stratigraphy in China
80
Xifeng Loess Susceptibitity $I units 0 I 2 8001 I I
DSDP 552A
LithoLogic LIO
A"o
-5
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-4
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35
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units 90
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~=o
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•
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%
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o •
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54 55 Q
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130
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56 57
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63 1700
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1800
IOJO0
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69
~6--71
.~ ;'-,, ]
73
2000
FIG. 21. Same as Fig. 20 but for the interval 0.8 to 2.0 million years. Also shown correlation with the D S D P 607 for the interval missing in D S D P 552A core, taken after R u d d i m a n et al., 1986.
was the first to reach the oxygen isotope values of the Middle and Late Pleistocene glaciations since the Olduvai Event. Important contrasting features are: (1) The position of the Matuyama-Brunhes reversal lies within a cold climate deposit (loess L8) in China [this is also true in other continents, and in sediments other than loess] but within the warm climate interval (OXY 19) in the deep sea cores. The difference may be due to the 'lock-in' delay of the magnetic signal in unconsolidated near bottom ocean sediments (Clement and Kent, 1984). (2) The relative warmth of the Jaramillo interval in the chinese loess, contrasts with relative cold inferred from the deep-sea record (weathered loess units L12 and L l l and their probable deep-sea equivalents OXY 32 and 30). It is not yet clear to what degree these variations are caused by regional differences in climatic forcing, or to what degree they are an artefact of sampling intervals, or local anomalies. The limitations of oxygen isotope stratigraphy from any single core have been discussed by Imbrie et al. (1984) and Shackleton and Opdyke (1976).
Loess and Soils of the Wucheng Formation The uppermost soil complex WS1 of the Wucheng Formation shows high frequency oscillations with an apparent average length of about 25,000 years. This contrasts with the 40,000 to 60,000 years recurrence of susceptibility oscillations throughout most of the record. Similar observations have been made from the high resolution DSDP 607 core from the North Atlantic which covers the gap in the 552A record (Fig. 21). Based on the susceptibility measurements, the subdivision of the WS1 to WS2 interval into two soil groups and one loess unit is arbitrary, because the WS1-WL1 boundary is poorly expressed. The WS2 soil, however, marks a prolonged interval of relatively mild climate during which the surface of the Loess Plateau was covered by a wooded steppe. Similar episodes of stable, apparently warm and relatively humid climate are indicated by the WS3 and WS4 soils. They lasted from 1.7 Ma to about 1.9 Ma, and from about 2.1 Ma to 2.25 Ma. This is evident in Fig. 22, which compares the Xifeng susceptibility record with the oxygen isotope stratigraphy of the DSDP 552A. The loess record is dated by the susceptibility age model, whereas the oxygen isotope record of
214
G. Kukla Xifenq: susceptibility 0 1600
I
2 I
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3 l
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I
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""t .~,÷
(691 71
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PotorRy I ~ Sondstones Erosio~L boundary 1800
1900
i900 4
2O00 -- , . . ~
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! +>iT,;_
~ i60
-5
WL2 1700 --
150
Sant.erno
DSDP 5 5 2 A : /~eO 4
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75
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5
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FIG. 22. Same as Fig. 20, for the interval 1.6 to 2.7 million years B P. Also shown proposed correlation with the Santerno sequence in Italy, where the clam Arctica (Cyprina) islandica occurs in the paleomagneticallystudied sequence between the Olduvai and Gauss magnetozones (Modified from Kukla et al., 1980). Note that the first appearance of the molluscArctica islandica marks the base of the Calabrian stage and consequentlyof Quaternary, accordingto the originaldefinitionof Gignoux in 1913. The OXY stages 66 to 90 informallyproposed in this work.
DSDP 552A is interpolated between the base of the Olduvai and the top of the Gauss on an assumption of constant, sedimentation, This assumption is only approximately valid, when no major changes in productivity, carbonate disolution, and detritic sediment impact occur through time. This is not the case for the DSDP 552A. Where any disagreement occurs, the susceptibility time scale should be given preference. Although the oxygen record is sampled at lower intervals than the susceptibility record, so that some OXY peaks are likely to be missed, the general parallels are close enough to allow correlation. OXY stages tentatively proposed for core 552A range in length from approximately 10,000 to approximately 80,000 years, in accordance with the original definition of the OXY Stages 1-17 by Emiliani (1966). The equivalent of the WS4 warm episode is clear in the oxygen isotope stratigraphy of the DSDP 552A; but the cool spells preceding and following the WS3 zone in China, however, are not seen in the ocean record, where the interval from 1.5 Ma to about 2 Ma shows high frequency oscillations of suppressed amplitude, but no prolonged episodes of dominant low or high oxygen isotope ratios. Because of its relevance to the Plio-Pleistocene boundary, the interval between the OIduvai and the Upper Gauss, shown in Fig. 22 in the Xifeng susceptibility record should also be compared with the San-terno section in Italy, where the base of the Calabrian, the lowermost stage of the Quaternary as defined by Gignoux in 1913, is located in a continuous sequence.
The susceptibility signal shows a gradual increase from Gilbert to the Upper Gauss, then a drop at about 2.5 Ma in the first loess-like sediment interlayer of the Mixed Zone. This unit is still considered a part of the Red Clay formation and is labeled Red Clay Loess 1 or RL1. The next similar interlayer RL2, of lesser amplitude is about 2.4 Ma old. The susceptibility signal reaches a peak shortly before 2.4 Ma in the top of the Red Clay formation. This horizon is subaerially weathered and marks a prolonged interval of pedogenesis. Approximately 2.3 Ma the susceptibility values dropped sharply to the levels characteristic of the Middle Pleistocene glaciations, at the base of the loess unit WL4. The low susceptibility signal in the Wucheng Loess WL4 is tripartite, showing cold peaks in WL4b, WL4c and WL4e. The oxygen isotope record of DSDP 552A is similar, including the tripartite subdivision of the isotopic high between 2.2 Ma and 2.4 Ma, Also recorded in the core is the cooling at approximately 2.4 Ma which is correlated with RL2. The cooling peak correlated with RL1 is poorly expressed. The Pliocene-Pleistocene Boundary and the Loess Plateau Sequence
Chinese researchers invariably draw the PlioPleistocene boundary at the top of the Red Clay formation and the base of Wucheng Formation, which lies about 2.4 Ma. This contrasts with the boundary position placed at 1.65 Ma as proposed by others based
215
Loess Stratigraphy in China TABLE 5. Age models of loess sequence
Oxygen Isotope Stages Time/Weight Ratios Luoehuan Liu et al., 1985, page 176 page 163
Unit
(A) lmbrie et al.(1984)
Susceptibility Age Model, Xifeng Kukla et al. (in press)
astro-tuned (B) Shackleton et al. (1984)
(A) SO L1 S1 L2 $2 L3 S3 L4 $4 L5 $5 L6 $6 L7 $7
Erosion
Erosion Erosion
10 95 140 200 250 300 330 375 410 460 555 642 660 700 720
145 244 (335) 425 561 660 722
10 68 128 174 247 271 328 357 428 467 614 652 672 698 726
11 128 161 245 339 423 478 620 659 726 736
(B) $8 $8 L9 $9 L10 SI0 LII S 11 L12 S12 LI3 S13 L14 S14 L15 WS1 WL1 WS2 WL2 WS3 WL3 WS4 WL4
Erosion
1135 1172
763 792 809 843 856 872 970 992 1018 1025 1052 1079 1093 1129 1152
Erosion
1316 1434 1566 1695 1939 2102 2247 2342
1573 1695 1921 2125 (2270) (2357)
Erosion
742 770 870 900 920 940 960 970
970
737 801 834 843 870 904 935 987
1033
1013
1096
1090
776 898 945
1482 1870 2261 2435
on the study of sections in southern Italy (Aguirre and Pasini, 1985), and also at variance with the original definition of the base of the Quaternary by Gignoux
(1913). Reasons for the chinese usage are clear: (1) The base of Wucheng Formation is an easily mappable horizon of a major change in lithology, recognizable throughout the Loess Plateau, and traceable into continental deposits of different facies elsewhere in China. (2) It is a level at which the environment of the Loess Plateau shifted into the mode maintained throughout the Quaternary. (3) It is a level at which elsewhere in China vertebrates considered typical of the Quaternary appear. This includes the horse in the Nihowan fauna and the Bovidae in Luochuan (Xu, 1984). (4) It is a horizon marked elsewhere in China by the appearance of cold water marine fauna (An, 1984).
The question, therefore, is: should the chinese usage be revised and regional chronostratigraphic systems be reclassified so as to show the Pliocene-Pleistocene boundary at the base of the Wucheng unit WL2, which would roughly coincide with the proposition of Aguirre and Pasini (1985)? This is possible, but before it is done, the appropriateness of the 1.65 Ma boundary should be critically reviewed. The Tertiary-Quaternary boundary was originally defined by Gignoux (1913) at several localities in northern and southern Italy at a level of the first appearance of cold water molluscs, the so called northern guests. The principal species of the northern guests assemblage is the bivalve Arctica (Cyprina) islandica. The stratigraphic position of the first appearance level of Arctica islandica was determined in several exposures in northern Italy. The key section is at the gorge of the Santerno River which drains the northern
216
G. Kukla
slopes of the Appenines near Bologna. The cold water clams Arctica islandica, Chlamys septemradiata and Panopea islandica were found within a continuous marine depositional sequence about half way between the Gauss-Matuyama boundary and the Olduvai Event in a normal polarity subzone correlated with the Reunion Event (Kukla et al., 1980). It is thus safely established, at least in this area, that the conventional Plio-Pleistocene boundary, as originally defined by Gignoux, is older than the Olduvai event. It was also found in these sections that the appearances of the cold water species are episodic and probably respond to a climatic rhythm. A rare Arctica islandica locally appears at a deeper level than its first abundant presence in Stirone, but temporarily retreats during a warm phase of the climatic cycle (Pelosio et al., 1980). The same behavior was also observed with floral taxa •(Bertolini et al., 1979). In Calabria, southern Italy, where the name (but not the definition) of the Calabrian stage comes from, the Arctica islandica datum has not, hitherto, been found within a continuous sequence. The substitute locality of Vrica was studied and a zone marked by an appearance of the cold water nanoplankton on top of a normal polarity magnetozone was proposed for a stratotype of the Plio-Pleistocene boundary (Selli et al., 1977). Because the sequence is truncated, the interpretation of the reversal stratigraphy is ambiguous (Tauxe et al., 1983; Backman, 1979). Two age models were proposed: One correlates the normal magnetozone at the proposed boundary with the Reunion Event. This would make the Vrica boundary about 2 Ma and coeval with the first abundant appearance of Arctica islandica in Santerno. It would, however, disagree with the nanoplankton appearance in the cores from the open ocean, which are dated to the top of the OIduvai Event (Backman, 1979). The second alternative, favored by Tauxe and her coauthors, correlates the magnetozone in question with the Olduvai Event, placing the boundary at about 1.65 Ma, a level at which the corresponding nanoplankton occur in Atlantic deep-sea cores. In this respect Fig. 22 showing the proposed correlation of the Santerno sequence with the chinese loess record is of interest. In Santerno the marine silts and marlstones are exposed in an accumulated thickness of 2.5 km. In this section, where the Arctica islandica datum is within a magnetozone interpreted as the Reunion Event, Hyalinea baltica appears about one-third down between the Olduvai and the Reunion Events, whereas Globorotalia bulloides and frequent Bolivia catensis with cold water preference occur at the base of the Globorotalia inflata zone in the lowermost Matuyama sediments. It is presumed that the Hyalinea baltica appearance in Santerno correlates with WL3a, the youngest subunit of WL3. The susceptibility peak of WL3 is well expressed. The relatively negative ratio of OXY 74 in the DSDP 552A, which is of the same apparent age, conflicts with this correlation, but it may be the result
of a low sampling interval, or of core disturbance. The first appearance of Arctica islandica in Santerno seems to correlate with OXY 78 and with the subzone WL3e in China. In this unit, at the base of WL3, a normal polarity zone was detected in Liujiapo near Xian (Wang et al., 1984a) and was interpreted as thc Reunion Event. Thus, paleomagentic as well as climatostratigraphic criteria, used in the comparison of the two records, would allow the base of the Calabrian at Santerno to be placed at about 2.1 Ma. The three pronounced cold susceptibility peaks of WL4, namely WL4a, WL4c, and WL4e, correlate with OXY 82, 84 and 86, and with the appearance of Globorotalia buUoides and Bolivia catensis in Santerno. The Chinese researchers draw the Plio-Pliostocene boundary at this level which is about 250,000 years earlier than the base of the Calabrian stage in Italy. There is no need at present to revise the chronostratigraphic system of Chinese scientists to place the base of the Quaternary on top of the Olduvai Event because: (1) The base of the Calabrian Stage, in the sense of the original definition by Gignoux, based on molluscs, is older than the Olduvai Event. It probably corresponds to the base of WL3 in China, but by no means to the WL2. (2) The currently preferred age model of the Vrica stratotype is ambiguous and may be in serious error. (3) The appearance of pelagic species with cold water affinities as observed in northern Italy is at the base of the Matuyama at a chronostratigraphic level conspicuously close to the base of the Wucheng Formation in China. (4) There is no doubt that a major build-up of ice in the northern hemisphere took place in the lowermost Matuyama, accompanied by world-wide first order environmental shifts. This makes the vicinity of the Gauss-Matuyama boundary much more convenient for long distance correlations than the top of the Oiduvai Event.
The Chinese Loess Record and the Pleistocene Climate
Cycles There is an ongoing discussion about the nature and cause of the prominent periodicity in the Pleistocene climate records whose length is approximately 100,000 years (Pisias and Moore, 1981; Birchfield et al., 1981). The 100 ka power is strong in the last 730 ka above the Matuyama-Brunhes boundary (Imbrie and Imbrie, 1979). Currently there are two principal opinions about the dominance of the 100 ka cycle, both based on frequency analysis of the deep-sea records. One maintains that the cycle occurred abruptly about 0.8 to 0.9 Ma ago (Pisias and Moore, 1981; Prell, 1983), and replaced the earlier dominant frequency of about 40 ka. The other holds that the last 100 ka cycle developed gradually since 1.1 Ma and replaced the 40 ka cycle which was dominant earlier in the Matuyama (Ruddiman et al., 1986, 1986a).
Loess Stratigraphy in China
Frequency analysis of the Xifeng susceptibility record is not yet available, except for the youngermost part of the record (Kukla et al., in press). Visual examination, however, tends to support a model of episodic increases and decreases of the 100 ka power over the entire 2.5 Ma length of the loess record. Although the quasi 100 ka periodicity is most pronounced during the last 450,000 years, earlier equivalents of the 100 ka glacial cycle do occur between 800 ka and 900 ka, 1 to 1.1 Ma, and 2.3 to 2.4 Ma. It is interesting to note that the early appearances of the 100 ka cycle coincided with the times of the deposition of the exceptionally thick and coarse grained loess units L5-L6, L9, L15 and WL4, which are synchronous with river downcutting of the Loess Plateau and with episodes of accelerated uplift. If, as is proposed by some, the episodes of increased tectonic activity are globally synchronous (Hsu, 1982), then the changing power of the 100 ka cycle may indeed be related to the tectonic modulation of the land surface and ocean floor as proposed by Ruddiman et al. (1986a). Perhaps the uplift in the Tibetan Plateau at about 2.5 Ma (Liu et al., 1985) may have been paralleled by mountain uplift in high latitudes of the northern hemisphere, and with changes in the morphology of oceanic sills separating the Arctic and the North Atlantic deep ocean circulations (Vigdorchik, 1980), setting the geomorphic stage favoring glaciations (Emiliani and Geiss, 1959). These suggestions, however, are still highly hypothetical and more work is needed before the causes of Pleistocene climate periodicities are clarified. ACKNOWLEDGEMENTS My thanks go first to Dr Liu Xiu Ming who made this report possible by kindly making his original susceptibility measurements from the Xifeng section available. I am also grateful to Dr Lu Yang Chon for his still unpublished TL determinations and Professors Liu Tung Sheng and D.Q. Bowen, Fridrich Heller and An ZhiSheng for numerous suggested improvements of the manuscript, to Joyce Gavin for the computations of Xifeng time scale, Ellen Went for careful typing of the manuscript and the staff of Quaternary Science Reviews and University of London, UK for their assistance with the figures. For the critical reading of the manuscript my thanks go to Drs W. Ruddiman, N. Shackleton, R. Fairbanks, J. Morley and W.S. Broecker. The work was supported by NSF grant ATM 870317 and is a Lamont-Doherty Geological Observatory Contribution.
REFERENCES Aguirre, E. and Pasini, G. (1985). The Pliocene-Pleistocene Boundary. Episodes, 8(2), 116-120. An, Z.S. (1984). A study on the lower boundary of Quaternary in North China - - stratigraphic significance of the Matuyama/Gauss Boundary. In: 27th International Geological Congress, Moscow, Science Press, Beijing, pp. 149-156. An, Z.S. and Wei, L.Y. (1980). The fifth layer paleosoi in the Lishi Loess and their Paleoclimatic significance. Acta Pedologica Sinica, 17, 1-10. An, Z.S. and Lu, Y.C. (1984). A climatostratigraphic subdivision of Late Pleistocene strata named by Malan formation in North China. Kexue Tongbao, 29(9), 1239-1242.
217
An, Z.S. and Wei, L.Y. (1978). The illuviation ferri-argillans and their genetic inference. Kexue Tongbao, 24(8), 356-359. Backman, J. (1979). Pliocene biostratigraphy of DSDP Sites 111 and 116 from the North Atlantic Ocean and the age of the Northern Hemisphere Glaciation. Acta University Stockholm, 32, 115 pp. Bertolani, M., Accorsi, C.A., Pelosio, G. and Raffi, S. (1979). Palynology and stratigraphy of the Plio-Pleistocene sequence of the Stirone River (Northern Italy). Pollen et Spores, 21,149-167. Birchfield, G.E., Weertman, J. and Lunde, A. (1981). A paleoclimate model of Northern Hemisphere ice sheets. Quaternary Research, 15, 126 pp. Broecker, W.S. and Van Donk, J. (1970). Insolation changes, ice volumes, and the 0 TM record in deep-sea cores. Review of Geophysics and Space Physics, 8, 169-198. Bronger, A. (1976). Zur quartaren Klima-und Landschaftsentwicklung des Karpatenbeckens auf (palao-) pedologischer und Bodengeographischer Grundlage. Kieler Geographische Schriften, 45,268 PP. Burbanks, D.W. and Li, J.J. (1985). Age and paleoclimatic significance of the loess of Lanzhou, north China. Nature, 316, 429-431. Chen, D., Lu, Y.C. and An, Z.S. (1982). Snail assemblages in loess strata and their environmental implication. In: Proceedings of the 3rd National Quaternary Conference, China, pp. 7-15. Science Press, Beijing. Chia, L. and Wei, C. (1976). A Palaeolithic site at Hsu-Chia Yao in Yangkao county, Shansi Province. Kaogu Suebao, 2, 97-114. Clement, B.M. and Kent, D.V. (1984). A detailed record of the lower Jaramillo polarity transition from a southern hemisphere deep-sea sediment core. Journal of Geophysical Research, 89, 1049-1058. Clement, B.M. and Kent, D.V. (1986). Short polarity intervals within the Matuyama: transitional field records from hydraulic piston cored sediments from the North Atlantic. Earth and Planetary Science Letters, 81,253-264. Davis, P., Smith, J., Kukla, G.J. and Opdykc, N.D. (1977). Paleomagnetic study at a nuclear power plant site near Bakersfield, California. Quaternary Research, 7, 380-397. Debenham, N.C. (1985). Comments on extrapolation methods of dating sediments by TL. Ancient TL, 3(2), 17-26. Dodonov, A.E. (1986). Antropogen Yuzhnogo Tadzhikistana, (Anthropogene of South Tajikistan). Transaction, Vol. 409, Academy of Sciences of the USSR, Nauka, Moscow, 166 pp. Emiliani, C. (1966). Paleotemperature analysis of the Carribean cores P6304-8 and P6304-9, and a generalized temperature curve for the past 425,000 years. Journal of Geology, 74, 109-124. Emiliani, C. and Geiss, J. (1959). On glaciations and their causes. Geology Rdsch., 46, 576-601. Gignoux, M. (1913). Les formations marines pliocenes et quaternaires de l'Italie duSud et de la Cicile. Ann. University of Lyon, N.S., 36, 693 pp. Han, J.M. (1982). A preliminary study on the clay mineralogy of the loess at Luochuan section. In: Quaternary Geology and Environment of China, pp. 67-72, China Ocean Press, Beijing. Heller, F. and Liu, T.S. (1982). Magnetostratigraphical dating of loess deposits in China. Nature, 300, 431-433. Heller, F. and Liu, T.S. (1984). Magnetism of Chinese loess deposits (astr.). Geophysical Journal of Research, 77, 125-141. Heller, F. and Liu, T.S. (1986). Paleoclimatic and sedimentary history from magnetic susceptibility of loess in China. Geophysical Research Letters, 13, 1169-1172. Hilton, J. (1987). A simple model for the interpretation of magnetic records in Lacustrine and ocean sediments. Quaternary Research, 27, 160-166. Hsu, K.J. (1982). Deep structure and recent dynamics in the Alps. In: Hsu, K.J. (ed.), Mountain Building Processes, pp. 179-228. Academic Press. Imbrie, J. and Imbrie, K.P. (1979). Ice Ages: Solving the Mystery. MacMillan, London. Imbrie, J., Hays, J.D., Martinson, D.G., Mclntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., Prell, W.L. and Shackleton, N.J. (1984). The orbital theory of Pleistocene climate: support from a revised chronology of the marine delta 1~O record. In: Berger, A.L. and others (eds), Milankovitch and Climate, Part 1. pp. 169-305. Reidel Publishing Co., Boston. Kawamura, Y. and Xue, X. (1984). Palaeontological results of the field survey in Luochuan area, Shaanxi Province, China in 1983. In: Sasajima, S. and Wang, Y. (eds), The Recent Research of Loess in China, pp. 160-169. Kyoto Institute of Natural History. Klima, B., Kukla, G., Lozek, V. and DeVries, H. (1962).
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l'leistocenc climate: a time series approach. Earth Planetary Science Letters, 52, 450-458. Prell, W.L. (19831. Oxygen and carbon isotope stratigraphy for the Quaternary of Hole 502B: evidence for two modes of isotopic variability, b~: Prell, W.L., Gardner, J.V. et al, (cds), h#tial Rqmrts DSDP 68: US Government Printing Office, Washington, D.C. Richthofen, F. (18771. China, 1. Bcrlia. Richothofen, F. (1882). On the mode of origin of the loess. Geological Magazine, 9(2), 293-305. Ruddiman, W.F., Raymo, M. and Mclntyrc, A. (19861. Matuyama 41,000 year cycles: North Atlantic ocean and northern hemisphere icc sheets. Earth and Planetary Science Letters, 80, 117-129. Ruddiman, W.F., Shackleton, N.J. and Mclntyre, A. (1986a). North Atlantic sea-surface temperatures for the last 1. I million years. In: Summerhayes, C.P. and Shackleton, N.J. (eds), North Atlantic Palaeoceanography, Geological Society Special Publication 21, pp. 155-173. Sasajima, S., Wang, Y., Torii, M., Nishida. J. and Maenaka, K. (19841. Magneto- and chronostratigraphy revealed from the Luochuan Loess sequence in China and its relevance to the Quaternary climatic change. In: Sasajima, S. and Wang, Y. (eds). The Recent Research of Loess in China, pp. 211-232. Kyoto Institute of Natural History. Sasajima, S. and Wang, T. (cds) (19841. The Recent Research ol Loess in Ch#ta. Kyoto Institute of Natural History, pp. 21 t-232. Selli, R., Accorsi, C.A., Bandini, M., Bertolani, M.D., Bigazzi, G., Bonadonna, F.P., Borsetti, A.M., Cati, F., Colalongo, M.L., D'Onofrio, S., Landini, W., Menesini, E., Mezzetti, R., Pasini, G. Savclli, C'. and Tampieri, R. (1977). The Vrica Section (Calabria, Italy). Giornale di Geologia, 2(42), 181-204. Shaekleton, N.J., Backman, J., Zimmerman, H., Kent, D.V., Hall, M.A., Roberts, D.G., Schnitker, D., Baldauf, J.G., Desprairics, A., Homrighausen, R., Huddlestun, P., Keenc, J.B., Kaltcnback, A.J., Krumsiek, K.A.O., Morton, A.C., Murray, J.W. and Westberg-Smith, J. ( 19841. Oxygen isotope calibration of the onset of icerafting in DSDP Site 552A: history of glaciation in the North Atlantic region. Nature, 307, 620-633. Shaekleton, N.J. and Opdyke. N.D. (19761. Oxygen isotope and paleomagnetic stratigraphy of Pacific core V28-239. Late Pliocenc to Latest Pleistocene. Geological Society of American Memoirs, 145, 449-464. Sun, J.Z., Zhao, J.B., Stm, X.Y.. Wci, M.J., Li, H.M. and Zhou. Z.B. (1985). Loess in even older. Liujiapo. In: Proceedings of the International Symposium on Loess Research, Oct. 5-16, Xian, 14 PP. Tauxe, L., Opdyke, N.D., Pasini, G. and Ehni, C. (19831. Age of the Pliocene-Pleistocene Boundary in the Vrica section, southern Italy. Nature, 304, 125-129. Torii, M., Yue, L.P., Hayashida, A., Maenaka, K., Yokoyama, T. and Sasajima, S. (19841. Natural remanent magnetization of Ioess-paleosol deposits in Luochnan area. In: Sasajima, S. and Wang, Y.Y. (eds), The Recent Research of Loess in China, pp. 32-41. Kyoto Inst. Nat. History, Kyoto. Vigdorchik, M.E. (1980). Westview Special Studies in Earth Sciences. ln: Vigdorchik, M.E. (ed.), Arctic Pleistocene History and the Development of Submarine Permafrost. Westview Press, Boulder, CO. 286 pp. Wang, Y.Y., Sasajima, S., Teng, Z.H., Lei, X.Y. and Sun, W. (1984a). Loess in China and its stratigraphic sequence. In: Sasajima, S. and Wang, Y. (cds) The Re~ent Research of Loe,~ m China, pp. 1-31. Kyoto Institute of Natural History, Wang, Y.Y., Ishida, S. and Zhao, J.P. (1984). The variations of the Quaternary paleoclimate as reflected by the sporo-pollens in the Loess of China. In: Sasajima, S. and Wang, Y. (eds), The Recent Research of Loess in China, pp. 170-t84. Kyoto Institute of Natural History. Wen, Q.Z., Sun, F.Q., Diao, G.Y. and Yu, S.H. (1981). Ratios of oxides and relative values of weathering leaching or accumulation in Luochuan loess section, Shaanxi Province and their geological significance. Geochimica, 4, 381-387. Wintle, A.G., Shackleton, N.J. and Lautridou, J.P. (19841. Thermoluminescence dating of periods of loess deposition and soil formation in Normandy. Nature, 310, 491-493. Xue, X.X. (19821. Notes on Megaloceros luochuanensis (sp. nov..) from Hei-mugou, Luochuan, Shaanxi Province (in addition, the other mammalian fossils from the same district). Vertebrate Palos, 20, 228-235. Xue, X.X. (19841. The Quaternary mammalian fossils in the Loess area of China. In: Sasajima, S. and Wang, Y. (eds), The Recent
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Zhu, X.M. (1984). On the process of primary soil formation. Scientia Sinica, 27(10), 1035-1045. Zimmerman, H.B., Shackleton, N.J., Backman, J., Kent, D.V., Baldauf, J.G., Kaltenback, A.J. and Morton, A.C. (1984). History of Piio-Pleistocene climate in the northeastern Atlantic, Deep Sea Drilling Project Hole 552A. In: Roberts, D.G., Schnitker, D. et al. (eds), Initial Reports of the Deep Sea Drilling Project, 81, 861875.
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LOESS STRATIGRAPHY
I)277-3791/87 $(I.(111+ .511 Copyright © 1987 Pergamon Journals Ltd.
IN CENTRAL CHINA
G e o r g e Kukla Lamont-Doherty Geological Observatory, Palisades, New York 10964, U.S.A.
The loess deposits in central China record world-wide climate changes of the last 2.5 Ma. Numerous climatic oscillations are marked by alternating loess and soil units with continuous coverage of the Matuyama and Brunhes Epochs. Magnetic susceptibility of the deposits correlates closely with the oxygen isotope record of the deep-sea sediments and provides an independent measure of climate and time. The key marker bed of the chinese loess sequence is the paleosol $5, the time equivalent of the mid-Brunhes oxygen isotope stages 13, 14 and 15. It marks a prolonged interval of warm humid climate lasting from approximately 615 to 470 thousand years ago. Several episodes of river downcutting coincide with the deposition of the exceptionally thick loess units LI (oceanic oxygen isotope stages 2 to 4) L2 (stage 6), L5 (stage 12) L6, (stage 16) L9 (stage 22) L15 (stage 38 about 1.15 Ma) and WS4 (about 2.3 Ma). These erosional events are interpreted as a result of episodic uplift of the Loess Plateau. The deposition of the earliest loess layers between 2.5 and 2.3 Ma ago marks a first order shift from warm and humid environments toward harsh continental steppes comparable to those of the Middle and Late Pleistocene. Little, if any lithologic or paleontologic changes were noted within or above the Olduvai magnetozone, so that the proposed Plio/Pleistocene boundary at approximately 1.65 Ma has no lithostratigraphic and biostratigraphic representation in the chinese loess series. Comparison with the Deep Sea Drilling Program core 552A in the North Atlantic and with the Santerno River section near Bologna shows that the occurrence of the earliest loess in China coincides with the timing of the first significant ice rafting in the North Atlantic and with the appearance of cold water foraminifcrs in the marine deposits of northern Italy. An extension of the stage system of the oxygen isotope signal extended back to the Gauss-Matuyama boundary is proposed.
INTRODUCTION The last twenty years witnessed major advances in the understanding of paleoclimatic history of the Quaternary. The bulk of the new information has come from ocean sediments. Equally important, but lesser known findings, have been made on the continents in the loess sequences. Loess is a silt, transported and deposited by wind, loosely cemented by a fine syngenetic carbonate incrustation, formed in semi-arid continental climates. Loess deposits of Pleistocene age cover large areas in the middle latitudes of the Northern Hemisphere, stretching in a discontinuous belt from southern Britain across central Europe to eastern Asia, and from Alaska and eastern Washington State into the Mississippi and Ohio valleys. Loess also occurs in New Zealand and South America. The most extensive deposits comprise the Loess Plateau of central China (Fig. 1) where the most important discoveries in loess stratigraphy and geochronology have been made in recent decades, and which are reviewed here. Loess covers over 440,000 km 2, or approximately 40% of the arable land in China (Liu, T.S. et al., 1985). In the Loess Plateau between the Huanghe (Yellow) River and the Weihe River, loess with subhorizontal interbedded soils is over 150 m thick. Ever since the pioneering study of Richthofen (1877) many sections in the Loess Plateau have been described in varying degrees of detail (Wang et al., 1984a; Liu, T.S. et al., 1965, 1966, 1985, 1985a; Burbanks and Li, 1985). The
discussion here concentrates on sites in the vicinity of Luochuan, Xifeng and Xian (Fig. 2), for which the best climato-stratigraphic and time stratigraphic information exists and which currently represents the stratigraphy of the Loess Plateau of Central China. The key data of stratigraphic importance presented were described in detail in the recent review of Sasajima and Wang (1984) and Liu, T.S. et al. (1985), and in the study of the magnetic susceptibility of the Xifeng section of Liu, X.M. et al. (1985).
LITHOSTRATIGRAPHY The three key stratigraphic sections are: Xifeng, the Heimugou Valley at Luochuan, borehole QUN 22 near Luochuan and the Liujiapo section near Xian (Figs 4, 5 and 6). Xifeng is about 200 km and Xian about 160 km away from Luochuan, but all three localities have a similar lithostratigraphy. Individual units are subhorizontal and can be followed in natural exposures, in the walls of gullies, for long distances. A summary of the chinese loess stratigraphy is shown in Fig. 3. The sequences were subdivided into four major stratigraphic units. From the bottom to the top these are: Wucheng Loess, Lower Lishi Loess, Upper Lishi Loess and Malan Loess. The Wucheng Loess is underlain by the Red Clay formation of the upper Pliocene age. Dark grey humus-rich biogenic soil, locally interbedded or overlain by redeposited loess (the Black Loam formation) formed on top of the Pleistocene sediments in the Holocene.
192
G. Kukla i
[
j'
I
Yinsnln Mr5
~---- .....
H
J,uq
100
105
110
115
t-IZO
FIG. 1. Mean thickness of primary loess with interbedded soils in central China. Total thickness tess than 50 m in diagonal hatching, between 50 and 100 m vertical hatching, between 100 and 200 m horizontal hatching and over 200 m crosshatched. After Wang e t a l . , 1984.
FIG. 2. Localities discussed in the text.
The units are briefly described as follows: (1) 'Red Clays': This is the underlying formation of the loess (5 to 40 m thick) increasing in thickness toward the northwest. The Red Clays are Late Pliocene in age and their basal sand and gravel rests uncomformably on pre-Tertiary bedrock. In Xifeng they date from the Gilbert and Gauss magnetic epochs. The formation consists of red-brown silty noncalcareous clays with a high proportion of kaolinite and illite with carbonate nodules. Their Munsell color ranges around 5 YR 4/6 when moist (Kyuma et al., 1984). It is supposed to be of partly fluviolacustrine origin. In its upper part there is frequent evidence of pedogenesis and steppe rodents. The boundary between the Red Clays and the overlying loess is transitional, with loess and loess-like sediments appearing in the upper part of the Red Clays with red-brown, deeply weathered, and possibly partly hydromorphic soils, in the lowest part of the Wucheng loess. Subhorizontal carbonate concretions in the transitional zone (the 'Mixed Bed' of Liu, T.S. et al.,
1985) probably show an oscillating but shallow ground water level at the time of deposition in an intermittently flooded basin. (2) Wucheng Loess: is the oldest loess formation in central China. It is up to about 50 m thick, is composed of yellowish and reddish-brown compact loess, and is interbedded with reddish-brown soils and subhorizontal beds of carbonate concretions. The mineralogic composition of the soils is similar to that of the Red Clays. In the Wucheng formation the loess units differ from the soils principally by a lighter color, a high proportion of calcium carbonate, and by a higher content of silt and fine sand. It is, relatively, dark colored, 7.5 YR 5/6 of the Munseil scale when moist (Kyuma et al., 1984). The Wucheng Formation can be distinguished from the overlying Lishi Loess by agenerally darker color, a higher density, and frequent carbonate concretions, occurring partly in the form of subhorizontal beds. The formation has been subdivided into four soil groups
Loess Stratigraphy in China
~K
I""1 '1
~4
o a a
u
o
gl
i N
1
FIG. 3. Comparison of the loess sequence in the Loess Plateau with the chronostratigraphic subdivisions as used by different authors.
(WS1 to WS4) and four loess members (WL1 to WL4). Compared with the younger sediments, this is an arbitrary and general subdivision. In the soil groups, several soils, each one polygenetic in origin, dominate the loess: each loess member contains several soils. Some of the units may be subdivided into subzones each identified by a suffix (Figs 20, 21 and 22). The Wucheng loess spans the interval from about 1.15 to about 2.3 Ma. Its base is placed at the top of the Mixed Zone. The most recent subdivision of Wucheng Formation, which is followed here, was proposed by Liu, X.M. et al. (1985) in Xifeng. It differs from the earlier proposals (Liu, T.S. et al., 1985) in which the two soil-loess couplets on top of the Olduvai event (WS1, WL1, WS2 and WL2) were recognized as a single pair marked WS1 and WL1 (Fig. 3). (3) Lower Lishi Loess: the Lower Lishi formation is composed of light yellowish-brown and brown compact loess interbedded with sharply delimited dark brown paleosols. The color of the wet loess is 10 YR 6/4 to 8.75 YR 5/6 of Munsell scale, and the soils vary from 5
193
YR 4/4 (in $5) to 6.25 YR 5/6. The fossil soils and their carbonate horizons contain relatively frequent and large carbonate concretions. They are labeled from top to bottom as $5 to S14, with the youngest paleosol $5 exceptionally strongly developed as a marker bed of reddish polygenetic soil about 4 to 6 m thick. Three loess members are exceptionally thick and have a higher content o f coarse silt' and primary carbonate. They are from top to bottom L6, L9 and L15. Loess units L9 and L15 are sometimes referred to as the First (L9) and the Second (L15) Sandy Loess. The base of the Second Sandy Loess forms the base of the Lower Lishi. The formation is about 40 to 50 m thick and covers the Lower Brunhes and Upper Matuyama magnetozones, including the Jaramillo Event. Paleosol $5 is by far the strongest developed and thickest in the whole loess series. The loess members L l l to L14 are poorly expressed and relatively dark colored, so that the interval between S10 and L14 appears to be dominated by soil development, and as such analogous to the soil groups of the Wucheng formation. (4) The Upper Lishi Formation: this formation, about 20-30 m thick, includes four paleosols and four loess members. The loess units are labeled L2 to L5 from the top to the bottom. The loess is relatively porous and rich in carbonate with Munsell color of 8.75 YR 6/4. L3 and L4 are darker in color and lower in carbonate than the remaining two. The soils are similar in color to those of the Lower Lishi Loess, and are more strongly developed than the Holocene soil, but less so than the $5 soil. Carbonate concretions are relatively rare and small. Paleosol $2 is twofold, separated by a layer of a light colored weathered loess (S2b). The Upper Lishi formation is of Upper Brunhes age. (5) Malan Loess Formation: (L1) of Late Pleistocene age, is up to about 10 m thick. It is greyish-yellow and porous, loosely cemented and in its upper part intensely bioturbated. Over most of the Loess Plateau it has no recognizable interlayers, whereas around Xian and in the Biejing area (Fig. 7) it is subdivided by a weakly developed steppe soil (LS1). The best studied and dated sections of Malan Loess are in its type area in the Zhaitang Basin near Beijing (An and Lu, 1984; Lu et al., in press). (6) Black Loam Formation: sediments and soils of latest Pleistocene and Holocene age are poorly represented in the Plateau partly because of their young age and small thickness, and partly due to the artificial reworking and removal of topsoil by agricultural activities. On most sites of the Loess Plateau, the Holocene soil, labeled SO is a grey, calcareous, humus rich, artificially reworked topsoil at most about 1 to 2 m thick. In rare sheltered depressions, atypical in the Plateau environment, two to three dark grey to black, humusrich, calcareous zones about 2 m thick are interbedded and overlain by a light colored loess-like sediment with a relatively high carbonate content. The soils are of the
194
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XI~ENG Thickness [In1
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~a
R~
FIG. 4. Stratigraphiccolumnof the Xifenglocality.(1) unweatheredloess, (2) weatheredloess (3) soilsor soil groups (4) normal polarity magnetozones,VGP: Virtual geomagneticpole position. Log magneticsusceptibilityin SI units from Liu, X.M. et al., 1985. tschernosem family and at the locality Nuanquangou were t4C dated to 7-11 ka BP(Fig. 8). Little information has been published on the light colored silt separating and overlying the black soils, except on its particle size and mineralogic composition which are similar to that of the Malan Loess; its maximum thickness is about 1 to 3 m. It is difficult to estimate how much of the loess in the Black Loam formation is a secondary waterlain deposit on slopes, and on alluvial plains; and how much is primarily wind blown loess. Both types are probably present. However, given the existence of numerous ancient settlements in the Loess Plateau, and the destructive impact of cultivation and grazing on surface stability, it is possible that part of the Holocene loess is locally redeposited. Indeed the Holocene soil and loess are rare and thin on the Loess Plateau around Luochuan, Xifeng and Xian.
Chinese Loess and Soil Terminology Up to four types of primary loess have been recognized by local researchers (Table 1). They are 'unweathered', 'weakly weathered', 'moderately weathered' and 'strongly weathered' loess. 'Strongly weathered' loess is the lithology transitional to the 'dark loessial soil', which is the least developed stage of a paleosol. The weathered loesses have a lower carbonate content, lower porosity, and darker color, because of a higher proportion of limonitic impregnations. Such a subdivision of the loess is subjective because of imprecise lithological definition. But sometimes, reference is made to a 'lithic loess', usually a higher density sediment with subhorizontal carbonate impregnations and concretions. The soils are labeled in order of increasing degree of development as the 'dark loessial soil', 'calcareous drab soil', 'drab soil', 'leached drab soil' (or luvic drab soil),
195
Loess Stratigraphy in China
[
i
.r
g.
Q Z
..J
,mc
:E
IE
z cA
20 --
~
20
--
"~
40 -40
--
z
..i
60 60 -o ..i o
80
I0
-
-
100
100
- -
--
~,
-
w
g -
120
~
120
-
--
and 'drab-brown soil' (or burosem). In some recent publications 'cinnamon' may be substituted for 'drab'. All soil types except for the dark loessial soil, were found to have a clay content higher than 25% (Kyuma et al., 1984); whereas all loess types have less than 25% clay. Because all the soils are reworked and/or truncated, the terminology of recent soils is inappropriate. Kubiena's system of classification of paleosols would be more useful. The 'drab soils' seem to correspond roughly to the 'Braunerde' of Kubiena's (1964) as used in Europe. The 'drab brown soil' and the 'leached drab soil' with strong clay illuviation is comparable to a 'Parabraunerde'. The burosem or the 'drab-brown soil' is similar to Kubiena's 'Braunlehm'. The carbonate content decreases from the calcareous 'drab soil' to the 'drab-brown soil' (Table 1). The Holocene soil 'Dark Loessial Soil' or 'Black Loam' (Hei-lu to), has a thick and dark calcareous horizon with an organic content between 1 and 1.5%, and relates to the less developed stages of the tschernosem group. This soil has less than 25% clay, similar to the loesses. While a voluminous literature refers to the chemical composition and mineralogy of soils (cf. An and Wei, 1980; Han, 1982; Kyuma et al., 1984), relatively little work has been done on the micromorphology of the paleosols (An and Wei, 1978). Nevertheless it is obvious from a macroscopic examination of the sequences, that most of the soil horizons are polygenetic and have suffered strong weathering which has partly obliterated their original character. Aligned carbonate nodules, and subvertically oriented concretions, formed along rootcasts, are common, especially within the Lishi formations. Also common are the illuvination argillans, filling voids in the leached cinnamon soils, and the calcans in the calcareous soils.
FIG. 5. Stratigraphic columns of the Heimugou exposure (A), and QUN22 borehole in Luochuan (B). Lithoiogie symbols same as in Fig. 4. After Liu et al., 1985.
"-"--~.
0
metres
I
I
I
LI !i
50
I
I
I
EDC~Y
FIG. 6. Liujiapo outcrop at Xian. Symbols same as in Fig. 4.
196
G. Kukla O~
[L age (xlO00 yr BP)
SO
on fine
grains
L1-3
~18t2
on COarSe grains ~
20÷2,5
~--28±5 ~32±3
'*'--3627
L1-S .c
5.~41±4
L1-1 *.--5124
"*--52±5
*'--b6±7
*-'84±9
10"
"*'-'84~10
$1
FIG. 7. Lithostratigraphy, ~4Cand thermoluminescence(TL) dating of the Malan Loesssection near Beijing. Modifiedfrom Lu, Y.C., in press.
TABLE 1. Comparison of soil terminologyequivalents as used in China Wang et al., 1984
Liu et al., 1985
Approx. Kubiena's Equivalents (1953)
Dark Loessial Soil Calcareous Drab S o i l Drab Soil Leached Drab Soil Drab Brown Soil
Black Loam CarbonateCinnamon Soil Cinnamon Soil Leached (luvic) Cinnamon Soil Brown Cinnamon Soil
Braunerdc Para Braunerde Braunlehm
Depth Layer (ml Column -1 I
I I,, I'
CaCO 3 (%) 20
10
' \
14C a g e s B.P.
Organic Carbon
,,.,
]
Carbonates
"
1,11]1 2
2.5
3
3.3
4
3.8
N I1t' ,;
l
7360±250 21200±1000 19960~1000 8550z300 11280±510
21750Z1000
FIG. 8. Black Loam stratigraphy in the middle reaches of Huanghe River. Soil hatched, loess dotted. Also shown carbonate content and 14C ages on organic carbon (C) and on the carbonates (CaCO3) in years BP. Modified from Liu et al., 1985.
LOESS OVERLYING RIVER TERRACES AGE OF THE GULLIES
AND THE
Relatively few observations exist on the relation of the loess series to the river terraces. Such information is essential for inferring times of accelerated crustal movements (Kukla, 1975). Figure 9 is a schematic and
composite model, based on regional mapping in the Luohe and Weihe River valleys in the Luochuan and Xian areas as reported by Wang et al., 1984a; and Liu et al., 1985. The depicted schematic sequence was never observed at any single locality. Terrace TI: is the lowest terrace defined by Wang et al., (1984a) on the Weihe River. It includes contemporary floodplain sediments but also gravels partly covered by loess or redeposited loess younger than paleosol S1. It probably corresponds to Terraces 1 and 2 described on Luohe River near Luochuan by Liu et al. (t985). The terrace is overlain by loessial deposits in places, which are probably retransported. It is possible, however, that some of the oldest deposits are primary loess, of Malan age, blown onto the water bodies or swampy intermittently flooded river beds. The base of terrace T1 is close to the recent river floor. Terrace 7"2: corresponds to T3 of Liu and co-workers in the Luohe River. It is covered by the loess L2, soil S1 and loess L1. Its base is around 2 m, and its surface is about 10 m above the river. Terrace T3: corresponds to the T4 of Liu and coworkers. The loess covering the terrace contains four
197
Loess stratigraphy in China
T1 FIG. 9. Schematic cross section showing the relation of the soils (S) and loess (L) in the Weihe and Luohe River areas to the terraces. Gravel in circles, reworked loess in wavy lines. Constructed from the information published by Wang et al., 1984a; and Liu et al., 1985.
paleosols (S1 to $4). At Luohe, as at the Weihe River, paleosol $5 is thick and steeply (20°) inclined toward the T3 terrace cliffs, whereas soil $6 and the older soils, undercut by the terrace, are nearly horizontal. This sequence is interpreted as the result of two erosional episodes: one preceding the formation of $5; the other during, or shortly after, the development of paleosol $5. Alternatively, accelerated downcutting may have started during the L6 stage and continued through the early stage of L5. T3 is the oldest recognized terrace of Brunhes age. Terrace T4: is covered by loess L9 and by the subhorizonal Lishi and Malan loess with paleosols $1 to $8. This terrace is the youngest of Matuyama age. In the Luochuan area the step separating T4 from T3 is 80 m high. Older terraces: Terrace T5 is overlain by loess L15 and by the younger members of the loess series which enclose paleosols $1 to S14. Two additional terraces developed contemporaneously with the L9 soil, one between the Jaramillo event and Brunhes chron, the other, during L15 time, between the Jaramillo and the Olduvai events. The relation of the Wucheng loess to the river network is unclear. Liu et al. (1985) suggest that the earliest downcutting of the Luohe River into the Red Clay basin started at about 2.4 Ma ago. But it seems that the principal changes in the evolution of the river network are related to the deposition of the exceptionally thick loess members L1, L2, L5, L6, L9 and L15, and with the last major erosional step, about 80 m, of the Luohe River, which occurred in MidBrunhes time, and before, or shortly after, the formation of paleosol $5. Except in the immediate vicinity of the rivers the paleosols within the Loess Plateau are more or less horizontal, continuous and uninterrupted by gullies. This contrasts with the current relief of the Plateau which is dissected by an intricate system of deep gullies with steep wails. The erosion which took place after the
formation of the early Black Loam has no analog in the Pleistocene strata. The volume of loess removed from the Loess Plateau during Holocene is much greater than at any time in the past. Jing and Chen (1983) have calculated that, about 1.6 billion tons of silt are carried away from the Plateau by the Huanghe River every year, which is sufficient enough to remove all the loess in under 60 thousand years. This contrasts with earlier interglacials, which were times of accumulation, rather than of erosion. The causes of the exceptional erosion in Holocene time may be multiple: (1) The Holocene climate is considerably drier than the earlier Pleistocene interglacials. This is evident from the difference between the carbonate content and structure of the Black Loam compared with earlier soils of Pleistocene age. A dry climate, combined with occasional torrential rains, accelerated erosion of loosely vegetated surfaces. (2) The natural vegetation, especially the sod, was destroyed by cultivation. Unattended, frequently shifting wagon trails also resulted in accelerated erosion. Natural grass sod and the root systems of low shrubs provide an efficient barrier to soil erosion even in otherwise unfavorable climates. These, however, were severely decimated by agriculture, and by goat and sheep grazing which, in Central China, has been common for several millennia. (3) In at least parts of the Plateau an accelerated uplift is taking place; and a relative subsidence of the river valleys of considerable magnitude has been determined in engineering surveys along the Huanghe, Weihe and Fenhe Rivers (Fig. 10). TIME CONTROL The dating of the loess sequence was at first based on vertebrate paleontology, later on radiocarbon, and then on thermoluminescence. It has been improved greatly in recent years by the application of paleomagnetic techniques.
198
G. Kukla
Mu Us Desert YINCHUAN
0 !
lOOkm I
FIG. 10. Major subsiding areas in the vicinity of the Loess Plateau dotted. From regional surveys. Modified after Liu et al., 1985.
Vertebrate Paleontology The basal Red Clays, also called Hipparion clays, contain typical elements of the Late Pliocene Hipparion assemblage and are considered an equivalent of the Early Villafranchian faunas in Europe (Xue, 1984). After the characteristic rodent Prosiphneus intermedius, which marks the evolutionary transition from the rooted Prosiphneus to the rootless Myosphalax, the upper part of the Red Clays is assigned to the final Prosiphneus zone. The Wucheng Formation is characterized by the voles of the Myosphalax fingi group with Myosphalax fontanieri being the most common species. Also present in the uppermost Wucheng WS1 layer is thc archaic hamster Kowalskia yunanica. The fauna of the middle part of the Wucheng Formation is the equivalent of the Nihowan fadna in northern China (Xue, 1984), which corresponds to the Late Villafranchian of Europe. Mierovertebrates in the Lower Lishi Loess enable a general correlation with the Biharian of Europe; whereas those in the Upper Lishi and Malan Formations contain microvertebrates whose evolutionary stages are typical of the Middle and Late Pleistocene. Recent reviews and faunal lists of vertebrate remains in the loess sequences were published, among others, by Xue (1984) and Liu et al. (1982).
Radiocarbon Dates Finite radiocarbon dates were obtained from the organic fraction of the Holocene soils and from the carbonate extracted from the Malan Loess. Because part of the carbonate may be detrital the latter determinations are likely to be biased toward an older age.
The biogenic steppe soils from sheltered sites apparently undisturbed by cultivation yielded ~4C ages between 11,280 + 510 and 1,935 + 130 BP. Out of 17 published determinations, 14 fall between 2.5 ka and 8.5 ka BP, with clusters around 3 ka, 5 ka and 7.5 ka to 8 ka BP. Carbonate in the Malan Loess of Luochuan yielded ages between 11.5 ka and 19 ka BP. It must be borne in mind, however, that the ~4C determinations in the two early Holocene black soils in Nuanquangou gave ages from 7,360 to 11,280 BP on organic carbon, but between 19,960 to 21,750 BP on the carbonate (Fig.
8). Thermoluminescence Ages Published thermoluminescence (TL) dates are shown in Table 2. Disagreement between the ages obtained by various researchers at different times illustrates the progress made throughout the world in the past few years in TL dating. Figure 11 shows how the published TL determinations of the age of some key loess and soil horizons have progressively been refined during the past decade. Currently any TL age greater than 100 ka is considered unreliable; and older sediments have been found to have TL dates biased towards underestimated ages. In the early TL determinations this was in part due to calibration against an incorrectly located MatuyamaBrunhes paleomagnetic reversal, which was originally placed within the basal loess L15 of the Lower Lishi (Figs 11 and 12). This layer is now known, however, to be over 1 Ma in age. The early TL dates were in general agreement with similarly dated long loess records in Hungary (Pecsi, 1984) and in Tadzhikistan (Dodonov, 1986), which showed very high apparent sedimentation rates in the last 150 to 200 ka, contrasting with very low
Loess Sti.atigraphy in China
199
TABLE 2. Examples of thermoluminescence ages (ka BP) as determined by different analysts by different methods at different times Layer
TL Age
L1 base S1 L2 top L5 base $5 base $8 base S14
31 41 71 178 212 316 666
SI $2 $3
110 210 250
L1 various depth
+ + + + + + +
Reference
3 3 5 24 15.8 10.3 47
Lu, Lu, Lu, Lu, Lu,
65 I00 140 460 590
Nishimura et al., 1984 Nishimura et al., 1984 Nishimura et al., 1984
19 45 103 148 357 696 876 1040
+ + + + + + + +
2 4 10 16 31 60 91 120
Dates on samples from 1980-1982 Liu et al., 1985
10-73
Liu Liu Liu Liu Liu Liu Liu Liu
20 40-50 70 140 435 730 860 2300
et et et et et et et et
AGE 103y
al., al., al., al., al., al., al., al.,
1985 1985 1985 1985 1985 1985 1985 1985
PUBLICATION DATE 1977 (cf.1981)
0
1981 1981 1981 1981 1981
1000
7 to 135
L1 top center bottom L2 center L5 top L8 LI0 WL4
Y.C., Y.C., Y.C., Y.C., Y.C.,
Approx. True Age
1984
1985
1987+
m
+ 41is
L! i////////51
"--,..
14tl
100 - -
Sl t
200 - 300 400-500 - -
17a±14 L5 SS \
~..
212211
+
llo 52
\
\\ T 2. 83 ~7.,'777.,'~$9 \ + 6is±ll L 9 \ \ \ \ X ',, . . . . X \ B/M in
LI5
X\
I
500 - -
X
~57±61 L5
_J ~/f/~..
z
X
B~M in L9
\ X \
700 - -
Authors:
b_-I- Io3±lo L1 ,, 4- ~ , S l
Lu Y.C.
Niskimura etnl.
\
- - i 6064"60 777~'/~. L8 88 Li H.H. in Liu et nl
L,u Y.i~. It nl lin prmtl
FIG. 11. Comparison of the thermoluminescence dating of several key loess layers in Luochuan plotted as a function of time of the publication. Earlier TL determinations were much too young. Also shown the changing determination of the Matuyama-Brunhes boundary. First authors of the determinations given, same as in the list of references.
sedimentation rates in the lower Brunhes. The ultimate causes of the young bias of the TL determinations of the Middle Pleistocene loess are physical, and were discussed in some depth in several recent papers (Wintle et al., 1984; Debenham, 1985). Thus far, no method has been devised to give reliable age determinations from loess layers older than about 100 ka. Magnetic Reversal Stratigraphy
The Matuyama-Brunhes reversal is located in the upper part of the loess layer L8 (Table 2). This was first
discovered by Liu Tung Sheng and An Zhisheng (1984) and later by Nishida et al. (1984) in Luochuan, and then confirmed by Heller and Liu, T.S. (1984, 1986). It was also reported from other localities by Liu, S.M. et al. (1985) and Liu, X.M. et al. (1985). Earlier interpretations placed the M - B boundary within loess L15, loess L7 (Li and Wang, 1982), or in the loess L9 (Heller and Liu, 1982). There are several reasons why this lithostratigraphic position of the reversal was determined incorrectly. The low intensity of the magnetic field preceding and accompanying a reversal results in a
200
G. Kukla weathering horizon below $6 was counted as an independent soil horizon, or as an interlayer of the loess unit L7. The Gauss-Matuyama boundary was located in the uppermost part of the Red Clay formation of the Loess Plateau (Fig. 13). The Jaramillo magnetozone was identified within a relatively intensively weathered series of soils and loesses bracketed by the marker beds L9 and L15. Figure 14 shows a comparison of different published polarity interpretations. To delimit the normal polarity zone in this interval is complicated by a relatively strong post-depositional remagnetization and by the difficulties in recognizing individual soils within the strongly weathered sequence. The Olduvai magnetozone was discovered by all investigators at the base of WL2, and in the soil group WS3 (labeled WS2 in the earlier work of Liu's team; cf. Liu et al., 1985; Heller and Liu, t986). Additional normal polarity subzones within Matuyama, and reversed polarity intervals within Brunhes were proposed, but their lithostratigraphic position was not verified in multiple exposures. Because of the possibility of unrecognized mechanical disturbances and secondary remagnetizations, the existence of these additional magnetozones in the loess series is questionable (Kukla and Nakagawa, 1977).
The Magnetic Susceptibility Time Scale It was shown earlier that the thickness-to-time ratio of loess and soil deposits, derived from the position of the reversal planes, shows only minor variations within the last 2.5 Ma (Fig. 15). This enabled approximate estimates of the age of key stratigraphic horizons of the loess sequence based on the assumption of a constant deposition rate between the reversal planes, and on the depth below the surface of individual layers, corrected for density. These estimates are given in Table 5. It was also obvious, however, that within each loess~a ~b ~c ld soil couplet, the soil accumulated at a much slower ~e ~f ~g lh average rate than the loess. This was confirmed by the accumulation rates of 1°Be analyzed in the Luochuan FIG. 12. Carbonate content of loesses and soils in Luochuan section by Shen (1986). Major differences between the (modified from Lu, 1981). Depth below the top of the plateau in deposition rates of the soil and the loess have also been metres (A). CaCO3 content in bulk samples in percentage. Column B: (a) weaklyweathered loess; (b) moderatelyweathered loess; (c) observed in radiometrically dated loess sequences in stronglyweathered loess; (d) dark loessial; (e) calcareousdrab soil; central Europe (Klima et al., 1961). (f) drab soil; (g) leacheddrab soil; (h) drab brown soil. ColumnD, Because the low field magnetic susceptibility is paleomagnetism. greater in the soils than in the loess, and because it apparently increases as a function of the degree of soil weathering, Kukla et al. (in press) proposed a weighting for the thickness of each layer by its magnetic relatively weak primary magnetization of the sediments susceptibility. This index, interpolated within the of the uppermost Matuyama age (Clement and Kent, framework of paleomagnetic reversals, is used as a 1986; Davis et al., 1977). These layers then acquire a measure of relative time. The susceptibility time series produced in this relatively strong normal overprint, whose separation is difficult, and needs multiple testing on sensitive mag- manner shows remarkable parallels with the oxygen netometers. Such instruments were not previously isotope record of the deep-sea cores, both in the available in China. Further, the numbering of the loess amplitude of individual peaks as well as in their timing. and soil layers in the sequences studied was not Figure 16 shows the comparison of the SPECMAP standardized. For instance, loess L8 was labeled by oceanic average oxygen isotope record of the Brunhes some as L9, depending on whether a poorly developed Epoch (Imbrie et al., I984) with the upper portion of
201
LOess Stratigraphy in China
.=J
Red cloy Loess nodules
--Red ctey
S27
~ $ 2 8
FIG. 13. Lithostratigraphic position of the Gauss-Matuyama boundary at key localities.
Chronozone
8runhes
• . ° • . . °. °o .'o,.
daramiU.o
,..'
Cob MT ( ? )
• .: .* ..°
FIG. 14. Same for the Matuyama-Brunhes Boundary and Jaramillo. Symbols same as in Fig. 4.
202
G. Kukla O L
50 --[1---
nM I U.LISI-ll T
LOWEll LISHI
So_
~ •
I )0 (m) /
I00 I
= - - ~ - -
1
WIJCH'I~I~G' IR.C.
-/- a-~,
-~s~ s _
RUNHES
"~.S, S'~',.Ss $9
l
-z 's3)
I•_v,Ws
A~
,k~LWES -3 La.
~I~GAUSS
Assemblage
FIG. 15. Time/depth ratio of the Luochuan sequence. Depth from the top of the Plateau (m), reversal chronology showing only magnetozones established in the sequence. After Liu et al., 1985.
OCEAN
LOESS ATFENG
SPECMAP 84 TIME
d180 RECORD
[kyl"l
LOG SUSCEPTIBILITy {Sl-u*~itsl
2.0
-3.0 -5
-3
500
s5
i
/1 FIG. 16. Comparison of the (A) Xifeng magnetic susceptibility record plotted as a function of time (log SI units) with the (B) SPECMAP 84 age model of the averaged oxygen isotope stratigraphy of the Brunhes epoch. Delta oxygen record tuned to astronomic frequencies, susceptibility record plotted on an independent time scale derived from the thickness weighted by susceptibility. More details in text (cf. Kukla et al., in press).
203
Loess Sti'atigraphy in China
T A B L E 3. A g e determinations of ihe loess and soil units in Xifeng from the susceptibility time scale model. Susceptibility thresholds: 1000 SI for SO through $5; 700 SI for older units; 600 for L9 and $9. A g e of the oxygen isotope stages 1-20 from Imbrie et al., 1984. Ocean
Loess
Oxygeu S t a g e
Formation
Unit
Age Base
Holocene
SO
10
10
Malan
L1
68
58
Upper Lishi
SI L2
128 174
60 46
S2a S2b S2c
218 230 247
44 12 17
L3
271
24
S3a S3b $3c
292 293 328
21 I 35
L4 $4
357 428
29 71
L5
467
39
S5a SSb $5c S5d S5e
549 558 584 592 614
82 9 26 8 22
L6 $6
652 672
38 20
£7a L7b L7c S7
678 690 698 726
6 12 8 28
L8 $8
737 801
|1 64
L9 $9
834 843
33 9
42
LI0 $10
870 904
27 34
63
Llia LIIb Lllc SII
907 929 935 987
3 22 6 52
83
L12 SI2
996 1013
9 17
26
L13 SI3
1039 1090
26 51
77
L14 S14
III0 1135
20 25
45
LI5
1172
37
WSla WS1
(1205) 1316
(33) I11
WLI WS2
1434 1566
118 132
~..2
1695
129
WS3 WL3 W$4 WL4
1939 2102 2247 2342
244 163 145 95
RL! RL2
2435 2.514
Lower L i s h l
Wucheng
Red Clay i
Duration
Duration
I
II
II
4
71
62
5 6
128 186
55 58
220 233 245
34 13 12
303 339
58 36
94
362 423
23 61
84
12
478
55
13 14
524 565
46 41
15
620
55
16
659
39
112
17 18 19
689 726 736
30 37 10
75
20 21
I0
118
73
119
7a 7b 7c
57
81
8 9
100
147
Age Base
186
II
117
59
142
117
197
58
54
31
70 14~
69 47
116
204
G. Kukla
the susceptibility record in Xifeng measured by Liu et al. (1985). The frequency analysis of the two time series, shows periods of 100 ka and 40 ka in each. However while the SPECMAP 84 oxygen isotope series of Imbrie et al. (1984) was tuned to astronomic frequencies, the Xifeng series was not. The cause of the high magnetic susceptibility of the fossil soils in the chinese loess is still unclear. "l'hcrnlomagnetic investigations and studies of ferromagnetic concentrates did not reveal any noticeable differences in the composition of magnetic minerals in the loess and the soils (Sasajima et al., 1984; Kukla et al., in press). The only systematic difference found, was a higher concentration of the magnetic fraction in the paleosols. The soils contain a higher proportion of the ferromagnetic minerals, principally of titanohematite, titanomagnetite and titanomaghemite, than the loess, and are possibly composed of finer grain size. Potential sources of the ferromagnetic component are fine airborne detritus from the deserts northwest of the accumulation area (Sasajima et al., 1984; Liu et al., 1985), volcanic ash of distant origin, micrometeorites (Hilton, 1987), and detritus transported by wind from within the Loess Plateau and from the local river floodplains. Although more detailed studies are required to explain the susceptibility variations, the currently available evidence favors depositional rather than pedogenetic processes as controls. Weighting by magnetic susceptibility equalizes the differences in sedimentation rates of individual layers, and enables close correlation of a single glacial cycle of the loess sequence with the deep-sea sediments. It also provides a relative time scale against which the tuned oxygen isotope chronostratigraphy of the deep-sea sediments can be tested.
CLIMATE INDICATORS
The alternation of loess and soils document oscillations of the environment between a bare or poorly vegetated semi-desert on the one hand, and grasslands, woody grasslands, and woodlands on the other. Thus, in essence, the primary contrast of the lithology of the loess series is the absence or presence of vegetational cover and its density. In this respect the chinese loess sequences are similar to the loess areas elsewhere. Information on the environment and climate of the loess sequence is obtained from the lithology, from its gastropod fauna and its vertebrates and pollen. Lithology A gross assessment of climatic conditions can be inferred from lithology. As the environment repeatedly oscillated between the two extremes of a semidesert, with meager patches of grass and scattered shrubs, on the one hand, and a wooded savana on the other, the deposits alternated between the light colored calcareous loess and the dark colored decalcified soils.
As for the influx of eolian dust, this occurs during both the cold and the warm intervals. Dust storms were mentioned as early as in 1150 B.C. in the Bamboo Slip Annals (Zhang, 1984), and were observed by Richthofen (1882) and Obruchev (t958), who both held past dust storms responsible for the deposition of Pleistocene loess. Several recent events have been described in detail, partly with the help of satellite imagery (Zhang, 1984; Liu, T.S. et al., 1981, 1982). The dust was composed predominantly of silt-sized grains of quartz, feldspar and carbonates similar to the Pleistocene loess. Turbulent mixing activated by the upper air jet stream, lifted the dust from the deserts in Mongolia and northern China. The strong upper winds transported the dust at an altitude of up to 10 km over the distances of 1,500 km or more eastward from the source areas. Dust storms are today most common in spring, especially in April. They are more frequent after dry winters. Although in exceptional circumstances a single dust storm may deposit several centimeters of silt, the average dust fall in China deposits only about 1 ton km -2 day -t (Liu, T.S. et al., 1981). At these rates the pedofauna and the flora can incorporate the newly deposited dust into existing soils so that no new loess layer is formed. During the glacials, however, the dust storms were, in all probability, much more frequent, and the strong winds in the Loess Plateau transported fine sand and silt from local sources. With reduced vegetation and an impoverished pedofauna, the sediment formed discrete loess units. Gastropods Fossil gastropods of the Plio-Pleistocene sequence in the Loess Plateau have been studied by Pei and Zhou (1958), Chia and Wei (1976) and Chen et al. (1982). The Luochuan sequence was studied in most detail. Snail shells were most abundant in the Malan and Upper Lishi Formations and least abundant in the Wucheng Formation. They do not occur in the decalcified horizons of the interglacial soils where the carbonate shells are leached during pedogenesis. It is probable, therefore, that snails of interglacial peaks were largely destroyed, and are thus not represented in the relatively limited faunal lists published to date. It seems that no attempts have been made hitherto to concentrate shells by washing, to identify broken shells, or to search for interglacial assemblages in favorable pockets for preservation of calcareous early interglacial sediments (Lozek, 1964). The two dominant species present throughout the sequence, both in the loess as in the soils, are Cathaica pulveratrix (Martens) and Metodontia hausaiensis (Crosse). Both are common even in the Holocene soil and still live at the sites. The fossil distribution of Cathaica is almost identical with its present one, whereas the present distribution of Metodontia extends beyond its fossil range. M. hausaiensis is clearly more frequent in the soils than in the loess, while C. pulveratrix is more frequent in the loess.
LoeSsSti~itigraphy in China In the Malan Loess, M. hausaiensis is most abundant at the base and least abundant at the top, while the drought and cold resistant Pupilla aeoli (Hiber), Valo Ionia costata (Muller) and Cathaica lutuosa (Crosse et Deshayes) increase in abundance toward the top. This is interpreted as a sign of progressive change of the loess environment from relatively mild and humid towards harsher and colder conditions. The fossils found in the Lishi and Wucheng Formations are too limited to enable meaningful reconstructions of paleoenvironmental changes. Several species only occur in sediments younger than $6. They include Vallonia costata (Muller), V. tenera (Reinhard), Cathaica fasciola (Drap) C. pulveratricula (Martens), C. teilhardi (Yen) and Succinea erythrophana (Ancey). On the other hand, Cathaica shikauens/s (Yen) is relatively common in the Wucheng and Lower Lishi Formations up to the soil S10, but is unknown from any younger layer. Cathaica subrugos only occurs in the Wucheng Formation. The climatic implications of these local disappearances are as yet unknown.
Vertebrates Fossil vertebrate bones are relatively rare, and their stratigraphic position is not always accurately known because of the transitional nature of the upper soil boundaries, and because rodent bones are sometimes found in burrows (krotowinas) reaching deep beneath the stratigraphic level at which the animal lived. From the basal loess WS4 of the Wucheng Formation in Luochuan the rodent Myospalax omegodon marks a shift from a previously relatively balmy environment of Red Clays time, to a dry, and seasonally cold, harsh continental climate. From the middle and upper part of the Wucheng Formation the rodents Myospalax chaoyatseni, M. tingi, M. arvicolinus, Sinocricetus zdanskyi, and of Ochotonoides complicidens have been discovered; together with bear Ursus etruscus, Eustenoceros, Cervus sp., Boris sp. and GazeUa sp. (Kawamura and Xue, 1984; Liu and Yuan, 1982; Xue, 1982). This fauna shows alternations between cold continental steppes, comparable to those of northern China today, and wooded steppes, or open woodlands, developing under a relatively mild and humid climate. The Lower Lishi Formation yielded vertebrates of mixed climatic requirements. Rodents of the Myospalax and Microtus families, known to live in cold, dry continental steppes, are frequent. Also present, however, are woodland species. In the basal loess of the lower Lishi sediments in Lantien elephant, monkey (Megamacaca), fossil panda, and woodland rhinoceros (Dicerorhinus) have been collected (Zhou et al., 1965). From the lower part of paleosol $5 (or from the top of $6) in Luochuan giant deer (Megaloceros luochuanensis) and Apodemus agrarius, a rodent of savanas and light woodlands, occur (Xue, 1982). The vertebrate fauna of the Lower Lishi Formation shows extreme oscillations, between cold and dry continental climates, comparable to the interiors of northern Asia, and the
205
relatively moderate and moist climates supporting tall grasslands and woodlands comparable to those in southern China today. The Upper Lishi and Malan Loess Formations yielded relatively few fossils, the majority of which belong to steppe elements. The common rodent being Myospalax fontanieri.
Flora Pollen from the loess and fossil soils represent decimated residuals of original pollen rain assemblages which survived burial in the adverse environment of carbonate rich, well aerated sediments with an active bacterial population (Zhu, 1984). An additional complication in interpreting the pollen record in the loess is the potential of long distance transport by strong winds up to several hundred kilometers into an area with low autochtonous pollen production. It is difficult, if not impossible, to distinguish such distant from local pollen. A further serious problem is the small amount of pollen recovered from most of the Wucheng and Lower Lishi Formations. Only about 20 grains or less were recovered from these units in Luochuan, while at Xian average recovery was about twice as abundant (Figs 17 and 18). Thus, only general conclusions can be drawn from the two sections which have been studied in some detail, one in Liujiapo near Xian, and the other in the Heimugou Valley near Luochuan (Wang et al., 1984; Liu et al., 1985). At both localities herbaceous pollen dominate over tree pollen. Especially frequent is Artemisia. Because this pollen has hard shells favoring preservation, its relative abundance may be the result of selective preservation. The relative abundance of tree pollen is higher in the soils and lower in the loess. The pollen of Artemisia, Chenopodiacae, Pinus and Selaginella (one grain of each) found in the base of the Wucheng formation in Xian indicate an arid steppe or desert, but no environmental reconstruction can be made from such meagre evidence. The WS3 in Luochuan and the corresponding middle part of the Wucheng Formation in Xian show tree pollen to comprise almost one third of the total count. These are mostly broadleaf species. Artemisia and grasses are the other main components. Typha orbicularis is also present. The top of the WS3 unit is the oldest layer in which the pollen are relatively abundant. Because pollen is generally rare in the loess, a relative abundance in WS3 is believed to show a humid, and probably warm, episode with open woodlands near the sites (Wang et al., 1984). Higher in the Wucheng Formation, in a layer probably correlative with WL2, pollen grains are again rare. The highest layer of the Wucheng Formation WSla contains a pollen assemblage similar to WS3, with dominance of broadleaf species over pine. Interesting is the rare presence of southern species Carpinus, Pterocarya and Comus. Loess layers of the Lishi Formation are poor in pollen, represented mostly by grasses, Artemisia and
206
O. Kukla
Wood
Pollen
Herb
LI m
n m
.! ,J
WS4
i
i
i~
w
J
m
WL4
F
I
Wood
pollen
• Herb
pollen
~
~
~
~ ~
~o o~~ • ~~
o.
~
. ~
.~ ~
"
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~
~
u
,~
FIG. 17. Number and types of pollen grains recovered in the sequence at Xian (modified from Wang et al., 1984).
Chemopodiacae, which indicates a loess steppe environment. Commencing with L9 the assemblage includes Ephedra. In soils $9 to S14 of the Lower Lishi Formation: that is, in a layer which includes the Jaramillo event, woody plants account for about one-third of the total count. Anacardiacae dominates in Luochuan, and pine dominates in Xian. Broad leaved species include the temperate element Castanea. Celtis is also present. The climate was probably marked by relatively warm episodes. In Luochuan, soils $8 to $6, from before and after the M-B boundary, contain relatively rare pollen, among others with Juglans and Typha angustifolia the latter an indicator of a humid environment. The pollen assemblage of marker bed $5 has frequent tree elements at both localities, with Anacardiacae dominant in Luochuan and pine in Xian. Juglans shows a rather temperate climate. The appearance of Ericaceae, which grow in acidic soils, ~hows deep decalcification of the $5 surface, probably in a relatively humid climate, without a prolonged dry
season, which may have occurred during part of the $5 development. Loess layers L4 and L3 of the Upper Lishi Formation are pollen impoverished, but have a considerable proportion of tree species. Especially interesting is oak, hornbeam and beech in loess L3. In loess L2 a small proportion of tree pollen includes pine and, for the first time in the Loess Plateau, spruce. Artemisia and Chenopodiaceae are common in Xian; Ephedra is also present. Paleosols $3 and $4 in Luochuan, in contrast to the older layers contain a relatively rich pollen assemblage with Anacardiacae and Thalicrum. Grasses and Artemisia are rare. Compared to the soils of the Lower Lishi Formation, the surface may have been wetter. Paleosol $2 shows dominant pine and birch in both Luochuan and Xian, with elm in Luochuan. Rare Gleditschia, Albizzia and Fraxinus found in Xian, suggest a proximity of temperate forest, possibly in the nearby mountains. Percentages of Artemisia, however, are high. Paleosol S1 yielded pine pollen with some oak and
207
Loess Stratigraphy in China
Number of Wood Herb Pollens
&
Wood
Pollen
LI
Lz
-IllILI!IllIl!II LIIlli
m~-', m -
o
"=-- rfl t[I rl
rr lrl i
~,lllllrllr[lllll
lIIl[l!I
g~ ~u~.m. ~ , ....... ,-.----.
Illllltllll!llll
II Illlt!lII
m
n
rnm
~ ~
IllllllllIllllll Wood
Pollen
Herb
Pollen
FIG. 18. Same for Luochuan.
birch in Xian, and Anacardiaceae, pine and oak in Luochuan. Typha is also present. In the Malan Loess tree pollen are rare except for the top layer, where pine, birch and elm occur in Luochuan and pine in Xian. It is possible that the uppermost layer of the Malan Loess marks the climatic amelioration of the Late-glacial. A dominance of Artemisia, however, shows a xeric steppe landscape. The Holocene Black Loam (SO) is rich in pollen; herbs dominate; pine is most common in Xian followed by birch and oak; elm is present in Luochuan. Artemisia, Chenopodiaceae, Graminae and Compositae appear together with frequent Selaginella. The assemblage is representative of a forest-steppe landscape, developing under a moderately temperate and humid climate. There is a disagreement between the pollen assemblage of the SO unit, which shows a partially wooded landscape, and the soil, which is high in carbonate, and probably developed under an open grassland (Bronger, 1976). This seriously limits the validity of any paleoclimatic reconstructions based on the fossil spectra of the Loess Plateau sequence. Thus far only the most general conclusions about this area can be made: (1) The pollen in the loess members shows landscapes with a highly restricted vegetational cover of grass, shrubs and weeds with few, if any, trees.
(2) The pollen of the soil members is compatible with a model of a relatively dense vegetation during some developmental phases of these polygenetic soils. Although differential preservation is likely to have altered significantly the ratios of the pollen, it is probable that woods were present in or near the Loess Plateau. (3) Climatically demanding species, in paleosols WS3, WS1, $9 to $14, $5 and $2 may show a wetter and more humid climate in the Plateau and its vicinity thanduring the formation of the rest of the soils.
Carbonate Content Carbonate content is high in the loess and low in the paleosols. As proposed by Lu (1981), it can be used for a first order approximation of the weathering regimes prevailing over the Plateau (probably also in the deflation areas) during the time of deposition of individual members of the loess series. As can be seen in Fig. 12 and Table 1, the mean carbonate content in the Luochuan Formation progressively decreases from 12% in the weakly weathered loess to 0.1% in the soil $5. The loess units L9, L6, L5, L2 and L1 show the lowest weathering intensity, while the loess L7, L8, L10 and L l l are the strongest weathered within the Lishi and Malan Formations. Within the upper half of Brunhes time, L3 and L4 have the lowest carbonate content, and could, therefore, be interpreted as show-
Loess Stratigraphy in China susceptibility signal is due to the high level transport of airborne dust from distant sources. Such sources may include deserts in Mongolia or the Pacific volcanoes. On a scale of several millenia the changes of the influx rate of such distant components are small and depend little on climate. In contrast the silt from local and regional sources has low susceptibility and its accumulation rate is closely controlled by climate. During cold and dry intervals the deposition from local sources (deserts of Northern Shaanxi, floodplains of braided rivers and the bare Plateau surface) dominates, and the resulting deposits have low susceptibility. The opposite holds for the relatively warm and humid intervals during which surface of the Plateau supports a closed vegetation, the valleys are forested and the rivers carry little silt. At these times accumulation is dominated by fine grained dust from distant sources and the resulting deposits have high susceptibility. If the model is correct in principle, then the susceptibility would be inversely related to the regional density of vegetation cover. As yet, no results are at hand which would either support or refute this hypothesis. Within the range of variability of the oxygen isotope record, however, the susceptibility fluctuations in Luochuan (Heller and Liu, 1984) and in Xifeng (Kukla et al., in press) correlate closely with the climatic signal from deep-sea cores. This allows the following, tentative, conclusions: (1) The susceptibility signal is indirectly related to changes of ice volume. (2) The close agreement of the oxygen isotope SPECMAP time scale of Imbrie et al. (1984), tuned to astronomic chronology, with the susceptibility-age model, supports the general validity of the SPECMAP tuning procedure as well as of the susceptibility-age model. The Silt to Clay Ratio
It was observed earlier that the ratio of silt to clay changes between the soil and the loess (Lozek and Kukla, 1959; Liu et al., 1985). The silt to clay ratio can then be used to differentiate a wind blown loess from a weathered paleosol. It is commonly referred to in the chinese loess literature as the K d index. It is the ratio of silt fraction in the 0.05 to 0.01 mm category (S) to the clay fraction smaller than 0.005 mm (C), computed after the formula: Kd=
S C
It was hypothesized that the high K d index points to a dominant mechanical weathering in the source (and deposition) areas of the loess, whereas the low K d index shows an increased production of clay size particles in a relatively humid and mild climate. The Iron Oxide Ratio
It was observed in the Luochuan section that the content of Fe203 is inversely correlated with carbonate.
209
The more advanced the degree of decalcification, the higher the FezO3/FeO (or the lower the FeO/Fe203) ratio of corresponding layer (Wen et al., 1984). In deeply weathered paleosols, the absolute concentration of iron and aluminum increases. Fe203/FeO record of the upper 1 Ma interval in the loess section in Heimugou Valley near Luochuan was successfully compared by Liu et al. (1985) with other proxy climate indices.
CLIMATES OF THE LAST 2.5 Ma IN CHINA COMPARED WITH THE DEEP-SEA STRATIGRAPHY
The climate history of the chinese Loess Plateau closely parallels the oxygen isotope record in the ocean. Correlation of the loess stratigraphy with the oxygen isotope stages 1 to 22 was proposed by several authors (Liu et al., 1985a; Sasajma et al., 1984; Heiler and Liu, 1986). In Fig. 16 the magnetic susceptibility of the Xifeng outcrop is compared with the average oxygen isotope record of the 12 deep-sea cores covering the Brunhes Epoch selected by Imbrie et al. (1984) and dated by tuning to orbital periodicities. The Xifeng record is plotted against the susceptibility time scale which has been interpolated between the base of soil S1 and the Matuyama-Brunhes boundary. Soil S1 was dated by thermoluminescence to 110 ka (Nishimura et al., 1984), and its base is taken to be 128 ka, the age of termination II (Broecker and Van Donk, 1970; Imbrie et al., 1984). It is reasonable to expect that the start of the soil development in the Loess Plateau was roughly synchronous with the oceanic terminations, since such time relationships occur in the case of the Holocene Black Soil SO, which is 11 thousand years old (Liu et al., 1985). Figure 19 compares the entire Xifeng susceptibility record, plotted against the susceptibility time scale, with the oxygen isotope record of the Deep Sea Drilling Program core 552A (Shackleton et al., 1984; Zimmerman et al., 1984). It is assumed that the sedimentation rate between the reversals is homogenous. DSDP 552A is one of the few analyzed cores which covers the interval of interest in detail. Both records are interpolated between the same reversal time lines which are: The top of Matuyama at 730 ka, the top of the Jaramillo at 910 ka, the top of the Olduvai at 1660 ka, the top of Gauss at 2470 ka, the top of Mammoth at 2920 ka, and the top of the Gilbert at 3400 ka. No orbital tuning was applied to either of the records. The Gauss-Matuyama boundary in the DSDP 552A was drawn at the base of the interval with transitional polarity rather than at its top, as done earlier in the original references. It is in core 9, section 4 at a depth of 26 cm. Comparison of Figs 16 and 20 demonstrates the limitations of the oxygen isotope record, which varies widely in detail from site to site and in which only the gross features of climate signal can be extracted from
210
(;. Kukla Xifeno I
2
DSDP 552A 3
4
-5
-4
-3
-2
...n~ ~7----=..
1400
r-
a~
24130
FIG. 19. Comparison of the magnetic susceptibility record in Xifeng as measured by Liu, X.M. et al. (1985), with the oxygen isotope record of DSDP 552A in central North Atlantic measured by Shaekleton et al., 1984. The Xifeng record plotted on a time scale derived from thickness weighted by susceptibility, the deep-sea core record linearly interpolated between the reversal planes. More information in text.
any single core (cf. Shackleton and Opdyke, 1976; Imbrie et al., 1984). On grounds of expediency the following discussion refers to the high susceptibility layers as 'warm' and to the low susceptibility peaks as 'cold' even though the temperature may have had little to do with the fluctuating density of the vegetational cover, controlled principally by moisture. The following information is obtained from the comparison of the two records. Holocene
In line with many other paleoclimatic indicators, the susceptibility record of Holocene soils and sediments is
disturbed by the artificial land use and soil erosion. This has no analogs during the past 3 Ma of Loess Plateau history. In contrast, previous interglacials were times of net accumulation, when soil erosion by rivers was modest and locally restricted. Cultivation by tillage commenced on the Loess Plateau some four thousand years ago (Liu et al., 1985), but vegetation clearance as well as grazing and severe disturbance of the sod has much earlier origins. It is difficult to judge what the vegetational cover and soils would be today without human interference. It is possible that the Holocene environment would have developed differently from previous interglacials, because the biogenic, humus rich
211
Loess Stratigraphy in China Xifeng Loess SusceptibiLity SI units I '1"~*
0
oI
DSDP 552A ~le 0
~ ~'~
2 I
..~ I LI
Sl~
4 I
-6 0
-4
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4
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~
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9
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16
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' ~
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I I00
1200
1200
FIG. 20. Same as Fig. 19, but for the first 1.1 Ma of record. Also showing proposed correlation of the Chinese loess and soil units with the oxygen isotope stages (OXY) in the DSDP 552A. Depth in meters from the top of the Plateau and from the top of the core. Age scale in thousand of years. The OXY stages 22 to 65 from Ruddiman et al., 1986.
Black Loams of the Late-glacial and early Holocene have no known earlier analogs. The pollen assemblages of early Holocene soils, however, are similar to those of earlier interglacials. The Malan and the Upper Lishi Formations The susceptibility record of the last glacial cycle ($1 and L1) in Xifeng shows a typical sawtooth pattern with a tripartite soil S1 corresponding to Oxygen Isotope Stage 5. Rapid loess deposition at the base of the Malan Loess (L1) is contemporaneous with the Oxygen Isotope Stage 4, and is followed by a slower rate of loess deposition, interpreted as being caused by a denser vegetational cover during Oxygen Isotope Stage 3. Another high rate dust deposition interval correlates with the Isotope Stage 2. As seen in Fig. 16, the magnetic susceptibility in S1 decreases moderately during Substages 5b and 5d, thus indicating a higher influx of eolian silt. Because of intensive bioturbation, however, the record of the peak climate of Substage 5d is smoothed. The Xifeng sampling does not allow detailed reconstruction of environmental fluctuations within Oxygen Isotope Stage 5. It is, therefore, unclear whether wooded steppe, the presence of which is indicated by
the pollen of the $1 soil in Xian and Luochuan, was present throughout all of Stage 5 or only during some of its Substages. Soil S1 is the youngest of the four Pleistocene polygenetic paleosols of Late Brunhes age; thermoluminescence dating shows it to be 110 ka in age (Nishimura et al., 1984). The susceptibility time scale shows that it developed over approximately 55,000 years, and represents the Last Interglacial as well as the Early Glacial. Soil $2 is markedly different from $1. It is subdivided by a well defined layer of weathered loess, which correlates with Oxygen Isotope Substage 7b. This separation has no analog in the paleosol S1. In China the soil correlative with Substage 7a lasted longer (some 30,000 years) and apparently developed a denser vegetational cover than that present during Substage 7c.
Soil $3 is the correlative of the Oxygen Isotope Stage 9. It shows a pronounced twofold subdivision in its susceptibility record, (Fig. 16 from Xifeng), and in its carbonate content (Fig. 12 from Luochuan); but its bipartite nature has not yet (so far as is known) been observed in field. The two warm peaks of $3 are approximately 40,000 years apart. About 40 millennia
212
G. Kukla
also separate the late warm peak of $3 from the early warm peak of $2. The intervening modulations at the wavelength of about 20 millennia, well expressed in the oxygen isotope record, are only weakly indicated in Xifeng. Soil $4, which is correlated with Oxygen Isotope Stage 11, resembles a polygenetic soil S1, but its development apparently lasted some 10,000 years longer. It shows only few signs of climatic deterioration in its upper part. In this respect the oceanic record of Oxygen Isotope Stage 11 is similar to the $4 in China. The 'cold' susceptibility peaks of all four loess units of the Upper Lishi Formation reach the values recorded during the last glacial cycle; but the apparent duration of the cold spells, as derived from the susceptibility time scale, differs considerably. The L1 took about 60,000 years to be deposited, the L2 about 50,000 years, L5 about 40,000 years and L4 approximately 30,000 years. L3 has an apparent duration of only about 25,000 years. Loess L3 and L4 have a relatively low CaCO 3 content (Fig. 12), in agreement with the relatively poorly expressed Oxygen Isotope Peaks of Stages 8 and 10. Loess L5 was deposited during Oxygen Isotope Stage 12, and is contemporaneous with the downcutting episode at the Luohe and Weihe Rivers. Similarly, loess L2 is time equivalent to Oxygen Stage 6 and is contemporaneous with the downcutting of the rivers into the T1 terrace level. The Soils and Loesses o f the Lower Lishi Formation
The soils of the Lower Lishi Formation differ considerably from the younger ones. The upper part of the polygenetic paleosol $5 reaches extreme susceptibility values and has the lowest carbonate content of the whole sequence. The polygenetic soil took over 70,000 years to develop, and reached the highest degree of pedogenetic weathering attained in the last 2.5 Ma. All the older paleosols, including the lower segment of $5 have considerably lower magnetic susceptibilities than the soils of upper Brunhes time. With the decrease in the amplitude of the susceptibility oscillations, the frequency increased, showing the dominance of the 20,000 to 40,000 year periods. This general character of the susceptibility record is maintained throughout the 2 million year long interval covering lower Brunhes time and the Matuyama Epoch. The upper part of $5 correlates with the Oxygen Isotope Stage 13. In the oceanic record this stage barely reaches two-thirds of the isotopic values of the younger warm stages. Its continental equivalent in China, however, marks an exceptional interval of prolonged warmth and humidity during which the surface became deeply decalcified, and exotic southern trees flourished in the vicinity of the Loess Plateau. Judging from both the susceptibility readings and the pollen content, the landscape was probably less densely vegetated during the Early Brunhes and uppermost Matuyama than in the younger interval. The eolian
deposition rate was higher and the decalcification of the soil horizons less complete. Some of the pollen grains, however, do indicate relatively warm and humid conditions in the vicinity of the studied sites. The polygenetic paleosois indicative of longer duration and/or intensity of weathering are the $6, $8, S10, SII, $13 and $14 soils. The loess units L6, L9, L13 and L15 are relatively unweathered, and show low susceptibility comparable to the upper part of the record. Several deep-sea cores were studied covering the time interval of the Lower Lishi Formation. Only two of them, however, have sufficiently high deposition rates for the oxygen isotope signal to give sufficient detail for meaningful comparisons with the chinese susceptibility record. These are the DSDP 607 (Ruddiman et al., 1986) and DSDP 552A (Shackleton et al.. 1984; Zimmerman et al., 1984), both in the central North Atlantic. In DSDP 607, Ruddiman extended the numbering of the oxygen isotope stages below stage 22 (Fig. 21) following the system proposed by Emiliani (1966). This is now extended further toward the Matuyama-Gauss boundary in core DSDP 552A (Fig. 22). The prefix OXY is used for stages in the oxygen isotope stratigraphy to differentiate them from other systems. Piston core V28-239 is considered to be a reference section for OXY 22 to OXY 65. DSDP 552A is the reference sequence for OXY stages 66 to 90. The numbering of the oxygen isotope record below stage 22 must be considered tentative until a sufficient number of sequences are studied and cross-correlated to recognize spurious signals. It would appear that this proposed tentative extension of designated stages in oxygen isotope stratigraphy to the Matuyama-Gauss boundary is both useful and justified because of the close parallels observed between the oxygen isotope and the chinese susceptibility records. A tentative correlation of the Xifeng susceptibility record with the Oxygen Isotope Stages is proposed in Figs 20, 21 and 22 and in Table 5. It shows the following common features: (1) A pronounced lengthy cold spell during the L6 and the Oxygen Isotope Stage 16 (OXY 16). (2) High frequency, low amplitude fluctuations arc found in the lowermost Brunhes and in the Upper Matuyama, which contrast with the low frequency and high amplitude oscillations in the Upper Brunhes (cf. Ruddiman et al., 1986). (3) A major warm-cold couplet of about 80,000 years duration immediately precedes the MatuyamaBrunhes reversal (OXY stages 20 and 21; soil $8 and loess L8 in China). (4) A generally warm interval of rapidly oscillating climate occurred during Jaramillo time. (5) A pronounced and lengthy cold period occurred between approximately 790 and 850 ka. (Loess units L10 and L9, OXY Stages 22 and 24.) (6) Three pronounced cold spells occurred between 1000 and 1150 ka (OXY 34, 36, 38). The earliest (L15 in China and OXY 38), which started at about 1160 ka
213
Loess Stratigraphy in China
80
Xifeng Loess Susceptibitity $I units 0 I 2 8001 I I
DSDP 552A
LithoLogic LIO
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-6
-4
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4.5
35
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units 90
DSDP 6 0 7
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stoges 900
•
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%
~2001 wsI
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46 47
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o •
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54 55 Q
WS2
130
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56 57
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1800
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69
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73
2000
FIG. 21. Same as Fig. 20 but for the interval 0.8 to 2.0 million years. Also shown correlation with the D S D P 607 for the interval missing in D S D P 552A core, taken after R u d d i m a n et al., 1986.
was the first to reach the oxygen isotope values of the Middle and Late Pleistocene glaciations since the Olduvai Event. Important contrasting features are: (1) The position of the Matuyama-Brunhes reversal lies within a cold climate deposit (loess L8) in China [this is also true in other continents, and in sediments other than loess] but within the warm climate interval (OXY 19) in the deep sea cores. The difference may be due to the 'lock-in' delay of the magnetic signal in unconsolidated near bottom ocean sediments (Clement and Kent, 1984). (2) The relative warmth of the Jaramillo interval in the chinese loess, contrasts with relative cold inferred from the deep-sea record (weathered loess units L12 and L l l and their probable deep-sea equivalents OXY 32 and 30). It is not yet clear to what degree these variations are caused by regional differences in climatic forcing, or to what degree they are an artefact of sampling intervals, or local anomalies. The limitations of oxygen isotope stratigraphy from any single core have been discussed by Imbrie et al. (1984) and Shackleton and Opdyke (1976).
Loess and Soils of the Wucheng Formation The uppermost soil complex WS1 of the Wucheng Formation shows high frequency oscillations with an apparent average length of about 25,000 years. This contrasts with the 40,000 to 60,000 years recurrence of susceptibility oscillations throughout most of the record. Similar observations have been made from the high resolution DSDP 607 core from the North Atlantic which covers the gap in the 552A record (Fig. 21). Based on the susceptibility measurements, the subdivision of the WS1 to WS2 interval into two soil groups and one loess unit is arbitrary, because the WS1-WL1 boundary is poorly expressed. The WS2 soil, however, marks a prolonged interval of relatively mild climate during which the surface of the Loess Plateau was covered by a wooded steppe. Similar episodes of stable, apparently warm and relatively humid climate are indicated by the WS3 and WS4 soils. They lasted from 1.7 Ma to about 1.9 Ma, and from about 2.1 Ma to 2.25 Ma. This is evident in Fig. 22, which compares the Xifeng susceptibility record with the oxygen isotope stratigraphy of the DSDP 552A. The loess record is dated by the susceptibility age model, whereas the oxygen isotope record of
214
G. Kukla Xifenq: susceptibility 0 1600
I
2 I
140
3 l
-6
-4
~ 67
1800~--
WS3
-3
I
i 2.5 ~-
""t .~,÷
(691 71
+¢.~, ~./
PotorRy I ~ Sondstones Erosio~L boundary 1800
1900
i900 4
2O00 -- , . . ~
WL3b
! +>iT,;_
~ i60
-5
WL2 1700 --
150
Sant.erno
DSDP 5 5 2 A : /~eO 4
•~
i 2100~
2000
w--~<
75
__
WL3d
76
J~+
~__...,2~ ~,l...~
FirsL
i
4
5
"=~-Hvol/neo boltt¢o
2100 4 1.5-
Arct/co /slo/~t/¢o
22O0
r~ 170
g
2~oI 256Mo~
8687
+-----'-+~"-'<*;~-'-'-'-'~" _>4004
~J
25O0 \
--
t
Redclay
L~oO0
<~<'
O.5[-
~Bose
GLoborotoUo [nfloto zone
~Oose
GLoborotoOa crossoforrms zone
180 270O 2800--
2800
FIG. 22. Same as Fig. 20, for the interval 1.6 to 2.7 million years B P. Also shown proposed correlation with the Santerno sequence in Italy, where the clam Arctica (Cyprina) islandica occurs in the paleomagneticallystudied sequence between the Olduvai and Gauss magnetozones (Modified from Kukla et al., 1980). Note that the first appearance of the molluscArctica islandica marks the base of the Calabrian stage and consequentlyof Quaternary, accordingto the originaldefinitionof Gignoux in 1913. The OXY stages 66 to 90 informallyproposed in this work.
DSDP 552A is interpolated between the base of the Olduvai and the top of the Gauss on an assumption of constant, sedimentation, This assumption is only approximately valid, when no major changes in productivity, carbonate disolution, and detritic sediment impact occur through time. This is not the case for the DSDP 552A. Where any disagreement occurs, the susceptibility time scale should be given preference. Although the oxygen record is sampled at lower intervals than the susceptibility record, so that some OXY peaks are likely to be missed, the general parallels are close enough to allow correlation. OXY stages tentatively proposed for core 552A range in length from approximately 10,000 to approximately 80,000 years, in accordance with the original definition of the OXY Stages 1-17 by Emiliani (1966). The equivalent of the WS4 warm episode is clear in the oxygen isotope stratigraphy of the DSDP 552A; but the cool spells preceding and following the WS3 zone in China, however, are not seen in the ocean record, where the interval from 1.5 Ma to about 2 Ma shows high frequency oscillations of suppressed amplitude, but no prolonged episodes of dominant low or high oxygen isotope ratios. Because of its relevance to the Plio-Pleistocene boundary, the interval between the OIduvai and the Upper Gauss, shown in Fig. 22 in the Xifeng susceptibility record should also be compared with the San-terno section in Italy, where the base of the Calabrian, the lowermost stage of the Quaternary as defined by Gignoux in 1913, is located in a continuous sequence.
The susceptibility signal shows a gradual increase from Gilbert to the Upper Gauss, then a drop at about 2.5 Ma in the first loess-like sediment interlayer of the Mixed Zone. This unit is still considered a part of the Red Clay formation and is labeled Red Clay Loess 1 or RL1. The next similar interlayer RL2, of lesser amplitude is about 2.4 Ma old. The susceptibility signal reaches a peak shortly before 2.4 Ma in the top of the Red Clay formation. This horizon is subaerially weathered and marks a prolonged interval of pedogenesis. Approximately 2.3 Ma the susceptibility values dropped sharply to the levels characteristic of the Middle Pleistocene glaciations, at the base of the loess unit WL4. The low susceptibility signal in the Wucheng Loess WL4 is tripartite, showing cold peaks in WL4b, WL4c and WL4e. The oxygen isotope record of DSDP 552A is similar, including the tripartite subdivision of the isotopic high between 2.2 Ma and 2.4 Ma, Also recorded in the core is the cooling at approximately 2.4 Ma which is correlated with RL2. The cooling peak correlated with RL1 is poorly expressed. The Pliocene-Pleistocene Boundary and the Loess Plateau Sequence
Chinese researchers invariably draw the PlioPleistocene boundary at the top of the Red Clay formation and the base of Wucheng Formation, which lies about 2.4 Ma. This contrasts with the boundary position placed at 1.65 Ma as proposed by others based
215
Loess Stratigraphy in China TABLE 5. Age models of loess sequence
Oxygen Isotope Stages Time/Weight Ratios Luoehuan Liu et al., 1985, page 176 page 163
Unit
(A) lmbrie et al.(1984)
Susceptibility Age Model, Xifeng Kukla et al. (in press)
astro-tuned (B) Shackleton et al. (1984)
(A) SO L1 S1 L2 $2 L3 S3 L4 $4 L5 $5 L6 $6 L7 $7
Erosion
Erosion Erosion
10 95 140 200 250 300 330 375 410 460 555 642 660 700 720
145 244 (335) 425 561 660 722
10 68 128 174 247 271 328 357 428 467 614 652 672 698 726
11 128 161 245 339 423 478 620 659 726 736
(B) $8 $8 L9 $9 L10 SI0 LII S 11 L12 S12 LI3 S13 L14 S14 L15 WS1 WL1 WS2 WL2 WS3 WL3 WS4 WL4
Erosion
1135 1172
763 792 809 843 856 872 970 992 1018 1025 1052 1079 1093 1129 1152
Erosion
1316 1434 1566 1695 1939 2102 2247 2342
1573 1695 1921 2125 (2270) (2357)
Erosion
742 770 870 900 920 940 960 970
970
737 801 834 843 870 904 935 987
1033
1013
1096
1090
776 898 945
1482 1870 2261 2435
on the study of sections in southern Italy (Aguirre and Pasini, 1985), and also at variance with the original definition of the base of the Quaternary by Gignoux
(1913). Reasons for the chinese usage are clear: (1) The base of Wucheng Formation is an easily mappable horizon of a major change in lithology, recognizable throughout the Loess Plateau, and traceable into continental deposits of different facies elsewhere in China. (2) It is a level at which the environment of the Loess Plateau shifted into the mode maintained throughout the Quaternary. (3) It is a level at which elsewhere in China vertebrates considered typical of the Quaternary appear. This includes the horse in the Nihowan fauna and the Bovidae in Luochuan (Xu, 1984). (4) It is a horizon marked elsewhere in China by the appearance of cold water marine fauna (An, 1984).
The question, therefore, is: should the chinese usage be revised and regional chronostratigraphic systems be reclassified so as to show the Pliocene-Pleistocene boundary at the base of the Wucheng unit WL2, which would roughly coincide with the proposition of Aguirre and Pasini (1985)? This is possible, but before it is done, the appropriateness of the 1.65 Ma boundary should be critically reviewed. The Tertiary-Quaternary boundary was originally defined by Gignoux (1913) at several localities in northern and southern Italy at a level of the first appearance of cold water molluscs, the so called northern guests. The principal species of the northern guests assemblage is the bivalve Arctica (Cyprina) islandica. The stratigraphic position of the first appearance level of Arctica islandica was determined in several exposures in northern Italy. The key section is at the gorge of the Santerno River which drains the northern
216
G. Kukla
slopes of the Appenines near Bologna. The cold water clams Arctica islandica, Chlamys septemradiata and Panopea islandica were found within a continuous marine depositional sequence about half way between the Gauss-Matuyama boundary and the Olduvai Event in a normal polarity subzone correlated with the Reunion Event (Kukla et al., 1980). It is thus safely established, at least in this area, that the conventional Plio-Pleistocene boundary, as originally defined by Gignoux, is older than the Olduvai event. It was also found in these sections that the appearances of the cold water species are episodic and probably respond to a climatic rhythm. A rare Arctica islandica locally appears at a deeper level than its first abundant presence in Stirone, but temporarily retreats during a warm phase of the climatic cycle (Pelosio et al., 1980). The same behavior was also observed with floral taxa •(Bertolini et al., 1979). In Calabria, southern Italy, where the name (but not the definition) of the Calabrian stage comes from, the Arctica islandica datum has not, hitherto, been found within a continuous sequence. The substitute locality of Vrica was studied and a zone marked by an appearance of the cold water nanoplankton on top of a normal polarity magnetozone was proposed for a stratotype of the Plio-Pleistocene boundary (Selli et al., 1977). Because the sequence is truncated, the interpretation of the reversal stratigraphy is ambiguous (Tauxe et al., 1983; Backman, 1979). Two age models were proposed: One correlates the normal magnetozone at the proposed boundary with the Reunion Event. This would make the Vrica boundary about 2 Ma and coeval with the first abundant appearance of Arctica islandica in Santerno. It would, however, disagree with the nanoplankton appearance in the cores from the open ocean, which are dated to the top of the OIduvai Event (Backman, 1979). The second alternative, favored by Tauxe and her coauthors, correlates the magnetozone in question with the Olduvai Event, placing the boundary at about 1.65 Ma, a level at which the corresponding nanoplankton occur in Atlantic deep-sea cores. In this respect Fig. 22 showing the proposed correlation of the Santerno sequence with the chinese loess record is of interest. In Santerno the marine silts and marlstones are exposed in an accumulated thickness of 2.5 km. In this section, where the Arctica islandica datum is within a magnetozone interpreted as the Reunion Event, Hyalinea baltica appears about one-third down between the Olduvai and the Reunion Events, whereas Globorotalia bulloides and frequent Bolivia catensis with cold water preference occur at the base of the Globorotalia inflata zone in the lowermost Matuyama sediments. It is presumed that the Hyalinea baltica appearance in Santerno correlates with WL3a, the youngest subunit of WL3. The susceptibility peak of WL3 is well expressed. The relatively negative ratio of OXY 74 in the DSDP 552A, which is of the same apparent age, conflicts with this correlation, but it may be the result
of a low sampling interval, or of core disturbance. The first appearance of Arctica islandica in Santerno seems to correlate with OXY 78 and with the subzone WL3e in China. In this unit, at the base of WL3, a normal polarity zone was detected in Liujiapo near Xian (Wang et al., 1984a) and was interpreted as thc Reunion Event. Thus, paleomagentic as well as climatostratigraphic criteria, used in the comparison of the two records, would allow the base of the Calabrian at Santerno to be placed at about 2.1 Ma. The three pronounced cold susceptibility peaks of WL4, namely WL4a, WL4c, and WL4e, correlate with OXY 82, 84 and 86, and with the appearance of Globorotalia buUoides and Bolivia catensis in Santerno. The Chinese researchers draw the Plio-Pliostocene boundary at this level which is about 250,000 years earlier than the base of the Calabrian stage in Italy. There is no need at present to revise the chronostratigraphic system of Chinese scientists to place the base of the Quaternary on top of the Olduvai Event because: (1) The base of the Calabrian Stage, in the sense of the original definition by Gignoux, based on molluscs, is older than the Olduvai Event. It probably corresponds to the base of WL3 in China, but by no means to the WL2. (2) The currently preferred age model of the Vrica stratotype is ambiguous and may be in serious error. (3) The appearance of pelagic species with cold water affinities as observed in northern Italy is at the base of the Matuyama at a chronostratigraphic level conspicuously close to the base of the Wucheng Formation in China. (4) There is no doubt that a major build-up of ice in the northern hemisphere took place in the lowermost Matuyama, accompanied by world-wide first order environmental shifts. This makes the vicinity of the Gauss-Matuyama boundary much more convenient for long distance correlations than the top of the Oiduvai Event.
The Chinese Loess Record and the Pleistocene Climate
Cycles There is an ongoing discussion about the nature and cause of the prominent periodicity in the Pleistocene climate records whose length is approximately 100,000 years (Pisias and Moore, 1981; Birchfield et al., 1981). The 100 ka power is strong in the last 730 ka above the Matuyama-Brunhes boundary (Imbrie and Imbrie, 1979). Currently there are two principal opinions about the dominance of the 100 ka cycle, both based on frequency analysis of the deep-sea records. One maintains that the cycle occurred abruptly about 0.8 to 0.9 Ma ago (Pisias and Moore, 1981; Prell, 1983), and replaced the earlier dominant frequency of about 40 ka. The other holds that the last 100 ka cycle developed gradually since 1.1 Ma and replaced the 40 ka cycle which was dominant earlier in the Matuyama (Ruddiman et al., 1986, 1986a).
Loess Stratigraphy in China
Frequency analysis of the Xifeng susceptibility record is not yet available, except for the youngermost part of the record (Kukla et al., in press). Visual examination, however, tends to support a model of episodic increases and decreases of the 100 ka power over the entire 2.5 Ma length of the loess record. Although the quasi 100 ka periodicity is most pronounced during the last 450,000 years, earlier equivalents of the 100 ka glacial cycle do occur between 800 ka and 900 ka, 1 to 1.1 Ma, and 2.3 to 2.4 Ma. It is interesting to note that the early appearances of the 100 ka cycle coincided with the times of the deposition of the exceptionally thick and coarse grained loess units L5-L6, L9, L15 and WL4, which are synchronous with river downcutting of the Loess Plateau and with episodes of accelerated uplift. If, as is proposed by some, the episodes of increased tectonic activity are globally synchronous (Hsu, 1982), then the changing power of the 100 ka cycle may indeed be related to the tectonic modulation of the land surface and ocean floor as proposed by Ruddiman et al. (1986a). Perhaps the uplift in the Tibetan Plateau at about 2.5 Ma (Liu et al., 1985) may have been paralleled by mountain uplift in high latitudes of the northern hemisphere, and with changes in the morphology of oceanic sills separating the Arctic and the North Atlantic deep ocean circulations (Vigdorchik, 1980), setting the geomorphic stage favoring glaciations (Emiliani and Geiss, 1959). These suggestions, however, are still highly hypothetical and more work is needed before the causes of Pleistocene climate periodicities are clarified. ACKNOWLEDGEMENTS My thanks go first to Dr Liu Xiu Ming who made this report possible by kindly making his original susceptibility measurements from the Xifeng section available. I am also grateful to Dr Lu Yang Chon for his still unpublished TL determinations and Professors Liu Tung Sheng and D.Q. Bowen, Fridrich Heller and An ZhiSheng for numerous suggested improvements of the manuscript, to Joyce Gavin for the computations of Xifeng time scale, Ellen Went for careful typing of the manuscript and the staff of Quaternary Science Reviews and University of London, UK for their assistance with the figures. For the critical reading of the manuscript my thanks go to Drs W. Ruddiman, N. Shackleton, R. Fairbanks, J. Morley and W.S. Broecker. The work was supported by NSF grant ATM 870317 and is a Lamont-Doherty Geological Observatory Contribution.
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