Pedostratigraphy of Chinese loess deposits and climatic cycles in the last 2.5 Myr

CATENA

vol. 20, p. 73-91

Cremlingen 1993

P e d o s t r a t i g r a p h y of Chinese Loess D e p o s i t s and Climatic Cycles in the Last 2.5 M y r D i n g Zhongli, N. R u t t e r & Liu T u n g s h e n g

Summary The pedostratigraphy and magnetostratigraphy of several sections in the Loess Plateau of north-central China have been investigated and correlated. The most complete and distinct loess-paleosol sequence occurs in the Baoji section of the southern part of the Plateau. Thirty seven pedostratigraphic units are identifiable in the Baoji section and correlated with those in other parts of the Plateau. The sequence represents thirty seven major cold to warm climatic cycles in the last 2.5 Myr. At about the time of the M a t u y a m a - G a u s s magnetic reversal an abrupt climatic event occurred in north China, represented by the shift of the well weathered Red Clay formation below the M / G boundary to the alternating loess and paleosol units above, marking the beginning of a new epoch in the course of climatic evolution. In addition there are two other major shifts of dominant periods of climatic cycles recorded in the loess-paleosol sequences, one occurring at about 1.5 Myr and another at about 0.8 Myr. The pedostratigraphy of loess deposits in north-central China provides an independent measure of the world-wide climatic fluctuations and is now the most complete and reliISSN 0341-8162 @1993 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0341-8162/93/5011851/US$ 2.00 + 0.25

CATENA---

An

Inter,tisciplln~ty

J,:,Ut'ILal

of

SOIL

able record of the major climatic cycles in the last 2.5 Myr.

1

Introduction

Ill roughly the past decade the most significant advances in understanding Quaternary paleoclimates have been made in interpreting multiple glacial sequences. However, no glacial stratigraphic successions include evidence for all the major climatic cycles of the Quaternary. Recently paleoclimatologists have turned their atteution to records outside of glaciated areas, particularly on deep-sea sediments and loess-paleosol sequences. Beginning with the pioneering work of Emiliani (1955, 1958, 1966), numerous deep-sea cores have been investigated. Shackleton and Opdyke (1973, 1976) divided the oxygen-isotope curves above the Jaramillo subchron into 22 stages. However, few deep-sea cores have supplied oxygen-isotope records below the Jaramillo subchrou with satisfactory resuits, and therefore it is not clear how many major climatic cycles there were in the Quaternary. The loess-paleosol sequence in the Loess Plateau of north-central China provides an independent measure of world-wide climatic changes during the Quaternary. In the last thirty years the Luochuan loess-paleosol sequence has served as the type section (Liu Tung-

S(~'II~NCE

HYDROL()GY

(;]~OMOR.

PHOLOt-;Y

74

Ding, Rutter & Liu

sheng 1965, 1966, 1985, Sasajima et al. ferent geographical zones of the Loess 1984). Fourteen paleosols in the upper Plateau have been carried out by us. part of the section with the time in- Three new sections with clearly-defined terval approximately equivalent to the paleosols have been identified. These last 1.2 Myr have been identified (Liu are the Baoji, Xian and Weinan secTungsheng 1985). However, the pale- tions in the southernmost part of the osols in the lower part of Luochuan sec- Plateau. Magnetostratigraphy has been tion as well as in the recently investi- determined for each. gated Xifeng section (Liu Xiuming et al. This paper presents the pedostrati1987) are not as easy to identify as those graphy and magnetostratigraphy of loess in the upper part. in different areas of the Plateau and a A major problem of the loess-paleosol discussion on problems associated with sequence in recording climatic oscilla- climatic cycles during the Quaternary. tions is the continuity of loess. It is commonly observed that the upper part of Pedostratigraphic some soils have been truncated. This, 2 of course, will hamper the reliability of Succession in the Loess climatic reconstruction. In fact, Pecsi Plateau (1987) has stressed that it is difficult to imagine that a subaerial loess-paleosol In the southern Loess Plateau where the series from 50 to 200 m thicknesses climate is fairly wet and warm, most of should not have hiatuses. To evalu- the loess beds have undergone considerate the continuity of loess in record- able pedological weathering. Weakly deing climatic cycles it is first necessary veloped pedogenic horizons occur within to investigate and correlate pedostrati- thick loess units as interlayers. These graphic successions in different regions have been identified as either loess or of the Plateau. soil, depending o11 the investigator. To Initially, three problems in the pedo- avoid this confusion it is necessary to stratigraphic study of the loess should standardize the numbering system of be addressed. They are as follows: the pedostratigraphic sequence and to 1. Determine if the pedostratigraphic give precise definitions to the pedostratisuccessions of the upper part of the graphic units. In our work a pedostratiLuochuan section is representative graphic unit is defined as a buried, traceable, three-dimensional body of rock of the entire Loess Plateau. that consists of one or more differen2. Determine the number of pedo- tiated pedological horizons with simistratigraphic units in the lower part lar morphology and appearance to the of the succession. zonal Holocene soil at the site where the pedostratigraphy is being studied. 3. Determine if the loess-paleosol seAccording to this definition only paquence in the Plateau records conleosols that have undergone pedogenic tinuous climatic fluctuations of the processes similar to or stronger than Quaternary. the zonal Holocene soil can be identified In the past three years field investi- as pedostratigraphic units. The upper gations of the pedostratigraphy in dif- boundary of a unit is the top of the pa-

Pedostratigraphy of Chinese loess

I

I

I15"

110"

75

~ I 5"

_ ~

• .'-o'. .'.'

ROCKY DESERT

ROCKY

"

t

l

100"

95"

DESERT

i[:,,,_,,,

i:; . . . .

".'.''''.'1_ •

~ohhor

..;.:.:.:..

'.'DESERT'.

QINGHAI-XIZANG

'.

PLATEAU

-35"

0i

100 200 300 = , J

Y/bI V ] , Riw. ~J re/-

C~_

~

~. .~

' OUNTAINS~"~.

"

kilornetres 115"

IlO"

/

105"

I

I

100"

I

Fig. 1: Map of part of north-central China including the geographical divisions of the Loess Plateau. leosol and the lower boundary is the bottom of the underlying loess. 2.1

Central Loess Plateau

The central Loess Plateau, the Shaanbei Plateau, comprises the main body of loess. It extends from the Yellow River in the east to the Liupan Mountains in the west, and from just north of the Weihe River valley in the south to the deserts in the north (fig. 1). 2.1.1

Luochuan section

The loess in Luochuan, about 130 m thick, is subdivided from top to bottom into four formations: Malan Loess, Upper Lishi Loess, Lower Lishi Loess and Wucheng Loess. The Malan Loess is covered by a Holocene soil about 1 to

CATENA

An

InterdJsc]p]hi~ty

JOUEZI~I o f S O I L

2 m thick, and the Wucheng Loess is underlain by the Red Clay formation of Late Pliocene age (fig. 2; Liu Tungsheng 1985). The Holocene soil is designated So. The Malan Loess, designated as L1, accumulated during the last glaciation with a thickness of about 7 m. The Upper Lishi Loess consists of four soils, $1 to $4, and four loess members, L2 to L6. Paleosol $2 is twofold, separated by a thin loess bed about 30 cm thick. The Lower Lishi Loess has ten paleosols from $5 to $14 and ten loess beds from L6 to L15. Both $5 and So are soil complexes. $5 is composed of three soils, and $9 of two soils with clearly-defined physical boundaries. $5 is a good marker layer with the upper soil being very well developed. L0 and Lis are thick loess

S(JIEN(?E--

HYDI2. OLOGY

--GEOMORPHOLOGY

Ding, Rutter & Liu

76

B_ M

o

I~S(, = g

()

g

i IIIIIIII

N

less than 2 m thick. From Lls to the contact between loess and the Red Clay formation is the Wucheng Loess. In the Wucheng Loess the upper and lower boundaries of the soils are not as easily defined as those in the Lishi Loess, mainly because of indistinct transitional zones. This loess is subdivided into three soil groupings ( W s - 1 to W s - a ) and three loess beds (WL-1 to WL-3). All three loess beds are about 7 m thick and are much weathered. The Red Clay below the Wucheng Loess is reddish brown, with moderate to strong subangular blocky structure, and abundant carbonate nodules and some clay and Fe-Mn coatings. Paleomagnetic dating of Luochuan section has been performed by many investigators with similar results (Liu & An 1984, Heller & Liu 1982, 1984, Sasajima et al. 1984). The BrunhesM a t u y a m a boundary is in the upper part of Ss or the base of Ls. The Jaramillo subchron lies between the upper part of L10 and the base of Sll, whereas the Olduvai subchron almost overlaps the W s - 2 soil grouping. The Matuyama-Oauss magnetic reversal occurs about 2 m below the contact between the Wucheng Loess and the Red Clay formation.

Fig. 2: Luochuan Loess Section.

(Modified from Liu lg85).

beds with much coarser texture than the other loess beds and are important marker beds. In L7 there is a weakly developed soil, labelled as S~ by Sasajima et al. (1984). From $1 to L9 most of the soils are underlain by loess beds about 3-8 m thick. Below L9 soils are closely spaced, and most of the loess beds are

2.1.2

Xifeng section

The Xifeng section is at the town of Xifeng, about 170 km west of Luochuan (fig. 1). It is about 170 m thick and has the same general lithological structures as the Luochuan section (fig. 3). Liu, X.M. et al. (1987) identified fourteen paleosols in Lishi Loess. The soil units $2, $5 and S0 are soil complexes similar to Luochuan, while the intervening loess

CATI~NA--An hxtetdisciplill~.t-y Jourll~.[ of SOIL SCII~3N('TE--HYDI~.OLO(;Y--GEOMORPHOLOC,;Y

Pedostratigraphy of Chinese loess

77

~[ ~ t~ Loess-Paleosoll~" Susceptibility

Jo. ~ ~" Seouen o ///////b

z

sl

-

Inclination ] Declination

150 ~ 300-90

90 180 270 3609C t

~

"~

f

w

_

Q! ~ ~ I

--S7!

1

C'.':.'::I L9~

(

I,-,-,-,-,-~#--sm ~

z_.__.

l:~LI2s12~

~

Q1 ~o~1140L_

} V~L2U

~

, "

,~

l

~

~

,

ws.

I

>,

N2

I

g:

[

]

I~"~ Paleosol ~

Silt

I~T'~ RedClay

Fig. 3: Xifeng Loess Section. (Modified from Liu et al. 1987).

CATBNA--Aa

InterdlsclplhL~ry

J o u r n ~ l of S O I L S C I E N C B - - H Y D R O L O G Y - - G E O M O R P I I O L O G Y

Ding, Rutter & Liu

78

is a bit thicker. In the Wucheng Loess four soil groupings, designated as W s - 1 , W s - 2 , W s - 3 and W s - 4 , have been distinguished and are separated by loess beds WL-1, WL-2, WL-a and WL-4. Below the Wucheng Loess is the Red Clay formation. Closely-spaced paleomagnetic samples were taken from the Xifeng section by Liu X.M. et al. (1987; fig. 3). The Brunhes-Matuyama magnetic reversal is at the base of Ls. The aaramillo subchron, which is not equivocally defined, is interpreted to be situated between $10 and S12 (Liu et al. 1987), whereas the Olduvai subchron is identified from the middle part of W L - 3 to the base of W L - 2 . The Matuyama-Gauss boundary lies 3 m below the Wucheng Loess and the Red Clay contact. The magnetostratigraphy of the Xifeng section is compatible with Luochuan section, but there are some differences. For example, the exact positions of the Jaramillo and Olduvial subchrons are not quite the same. In addition, the magnetization of the Lower Lishi Loess and the middle part of the Wucheng Loess is not stable enough in the Xifeng section to obtain reliable results. Luochuan and Xifeng have similar climates at present. The average annual temperature is about 9°C and the precipitation around 600 m m per year. The zonal Holocene soil in both regions is a Black Loam in the Chinese soil classification. Black Loam is the transitional soil between a Chernozem and a Drab or Cinnamon soil (Nanjing Soil Institute of Chinese Academy of Sciences 1980). Its dominant pedogenic processes are moderate humus accumulation and weak carbonate leachhlg. The A horizon is diagnostic and is characterized by rich humus and abundant carbonate

CATENA--An

pseudomycels. The carbonate illuviation horizon below the A horizon is not apparent. Most of the paleosols in the Lishi Loess of Luochuan and Xifeng sections consist of Bw and Cca horizons. The Bw horizon is bright brown (7.5YR5/6) with medium subangular blocky structure and moderate to strong carbonate leaching. The Cca horizon, with the thickness varying from 20 to 50 cm, contains irregular carbonate nodules. The prevailing pedogenic process is carbonate eluviation and illuviation, indicating that their development is substantially stronger than the Holocene soil. S~-i in Lishi Loess is an exceptionally well developed soil with a Bt horizon. The primary carbonate ill Ss-1 has been leached out. It contains abundant reddish clay and black Fe-Mn coatings on structure faces. The diagnostic pedogenic process of S s - i is clay translocation. The so-called S~ in L7 contains a few carbonate pseudomycels and is somewhat redder than the loess groundmass. Its development is considerably weaker than the zonal Holocene soil, so it is not a pedostratigraphic unit according to our definition. Therefore, eighteen pedostratigraphic units in the Lishi Loess are identifiable in Luochuan and Xifeng sections. The soils of the Wucheng Loess in Luochuan and Xifeng also contain Bw and Cca horizons. The upper part of most of the Bw horizons have been truncated and mixed with loess. Black Fe-Mn speckles are visible in the Bw horizons, whereas Cca horizons are composed of carbonate nodules, most of which are cemented together. The pedogenic horizons suggest that the climate during the development of the soils in the Wucheng

lltterdisc~ipllno, ly Jouvn~,l of SOIL

SCIENCE

HYDROLOGY--GEOMORPHOLOGY

Pedostratigraphy of Chinese loess

79

Loess was warmer than the Holocene. However, the contrast between them and the loess beds is not as distinct as those in the Lishi Loess. Their pedological structures are weak, as well as having indistinct upper and lower boundaries. It is therefore difficult to distinguish them from loess beds and to estimate the nunlber of soils in the Wucheng Loess of the Luochuan and Xifeng sections. 2.2

Southern Loess Plateau

The Weihe River valley is the southernmost part of the Loess Plateau, and it forms the Guanzhong Basin with the Qinling Mountains as the southern boundary. The basin consists of river terraces, alluvial fans and piedmonts increasing in age toward tile Qinling Mountains. The thickest and most complete loess sections are preserved on piedmonts resting on the Red Clay formarion. Since 1987 three new sections with clearly defined paleosols have been identified and investigated (Ding et al. 1990, Rutter et al. 1991a, Liu et al. 1989). 2.2.1

Baoji section

Baoji, about 200 km west of Xian and 250 km south of Xifeng, is in the westernmost part of the Guangzhong Basin. The Baoji loess section at Lingfei village about 5 kin north of Baoji is easily accessible. The loess is about 160 m thick and underlain by the Red Clay formation. All loess and soil units are well exposed and easily traceable along natural outcrops (fig. 4). A detailed description for the soil and loess horizons is given in Rutter et al. (1991a). The Malan Loess in the Baoji section is about 6 m thick, is strongly weath-

CATI~NA

An

Intelcilsciplln~vy

J(,uln~l

,:,f S O I L

ered and can be subdivided into four Chernozem-type soils, with the two middle soils better developed than the upper and lower ones. The lithological structure of the Lishi Loess in the Baoji section coincides with the Luochuan section. A notable characteristic in the Lishi Loess is that there are some weakly developed soils within thick loess beds, for instance, in L2, L4, L7 and L9. The Wucheng Loess in Baoji section contains eighteen paleosols. We designate them as $15 (youngest) to $32 and the intervening loess beds as L1G to L33. The Wucheng Loess can be lithologically subdivided into four units. The upper units, $15 to L24, consist of nine closely spaced soils. Most of the intervening loess beds are moderately weathered with thicknesses less than 2 m. L24 is about 6 m thick, dotted with many carbonate nodules. The second unit is fronl $24 to L27, with three moderately well developed soils and three loess beds. S2G is an exceptionally well developed soil like $5. S~.s is composed of two soils, whereas the lower one is less developed than the Holocene soil. The loess nlembets L25 and L2G are strongly weathered with a thickness of abou~ 3 m. L27 is about 7 m thick. The third unit conrains five closely-spaced soils from $27 to $31 and five loess members from L2s to L32. $30 and S3t are much more developed than $27, S2s and $29. L32 is a thick loess bed about 8 m, with which two weakly developed soils are identifiable. The loess members L2s to L31 are all less than 1 m thick. $32 and L33 comprise the fourth unit of the Wucheng Loess. $32 is a well developed soil and L33, about 2 m thick, is a typical loess bed. The Red Clay in the Baoji section is

SCII~NI3E

HYDROLO(iY-

(~/~OMOI~.PHOLOGY

Ding, Rutter & Liu

80

Inclination

Polarity

Thickness (rn) Lithology

-90

0--

+90

0

so L1 __

Sl

L2 $2-I $2-|! $3

L3

(.

L4 $4 L5 $5-! S5-11 S5-111 L6

50--

$6 $7 ,?,8

L7 L8

L9 $9-1 $9-11 L10 $10 Lll $12 L13 $13 $14 L14 $15 $16

100--

$19 S21 $23 $24 S25

L15 L16 L19 L20 [~22

5-

?

L24 L25 L26

$26 L27

150 - -

$29

1_29

S31

L31

$32

1-32 L33

Fig. 4: Baoji Loess Sec-

tion. (Modified from Liu et al. 1989).

characterized by thick, continuous clay and Fe-Mn coatings as well as a great many fairly small carbonate nodules. The magnetostratigraphy of Baoji section has been determined twice (Liu et al. 1989, Rutter et al. 1991b). The results of laboratory measurement show that the magnetization of Baoji loesspaleosol sequence is stable. The positions of the main magnetic events can be seen in fig. 4. The Brunhes-Matuyama magnetic reversal appeared at the base of Ls. the Jaramillo subchron lies between L10 and L12, and the Olduvai sub-

CAT~NA--An

chron occurred from the uppermost part of L27 to the middle part of L25. The Matuyama-Gauss boundary is located at the base of L33. 2.2.2

Xian section

The Xian section is situated in Duanjia village about 20 km east of Xian. The loess-soil sequence is approximately 140 m thick and overlies the Red Clay formation (fig. 5). The pedostratigraphy of the Xian section is compatible with the Baoji section although it is not as

|nterdlsclpllnary Journa,] of S O I L S C I E N C ~ - - H Y D R O L O G Y - - G I ~ O M O R P H O L O G Y

Pedostratigraphy of Chinese loess

Baoji

81

Xian

Weinan

,I

I

!

J-

i,

i

,=l

:~

M

well exposed as the Baoji section, largely because it is masked by slope-wash. 2.2.3

Weinan section

The Weinan section is in Miaogou gully about 20 km south of Weinan city with the foothills of the Qinling Mountains less than 5 km away (fig. 5). The section is about 90 m thick, overlying the Red Clay formation which is about 10 m thick, which is in turn underlain by 20 m of gravel. The Lishi Loess in the Weinan section is incomplete, with the part above Ss missing. The marker beds L9 and L15 axe easily recognized, but only five soils can be recognized between them. The Wucheng Loess is clearly exposed, and the pedostratigraphy is the same as the Baoji section. However, the marker beds L24, L~7 and L32 are thinCAT~NA--.An

n.

Gravel ~ . ~

Fig. 5: Baoji, Xian and Weinan Loess Sections.

ner than those in the Baoji section. Paleosol $26 is much redder than the others and similar to the underlying Red Clay. The average annual temperature in the Guangzhong Basin today is 13°C to 14°C, and the average annual precipitation varies from 600 to 700 mm. The zonal Holocene soil there is a Drab soil, characterized by strong carbonate leaching and weak clay translocation. The Holocene soil in Baoji, however, has a typical Bt horizon containing nearly continuous clay coatings on structure faces. Therefore, it seems that the Holocene soil in Guanzhong Basin has been developed in a forest or steppeforest environment, or perhaps both. The paleosol above L9 in Guangzhong Basin consists mainly of A and Bt horizons. Most of the A horizons are brown, with moderate to granular or subangular blocky structures, and with common

Interdls¢ipHn&ry Jourztal of S O I L S C I e N C e - - H Y D R O L O G Y - - G E O M O R P H O L O ( ; Y

Ding, Rutter & Liu

82

carbonate pseudomycels. The Bt horizons are reddish brown, and have strong subangular blocky structure and continuous clay coatings. Most of the soils between L0 and L15 have three pedogenic horizons - - A, Bt and Cca. The A and Bt horizons are similar to those above Lg. The Cca horizons are about 20 cm thick and contain some carbonate nodules. The paleosols of the Wucheng Loess are mostly composed of Bt and K horizons. The A horizons in most soils have been truncated and the Bt horizons are somewhat redder than those in the Lishi Loess. Most of the K horizons are 30 to 50 cm thick with the nodules cemented together by secondary carbonate. Therefore, in the loess deposits in Guangzhong Basin, 36 paleosols are identified and are comparable to, or better developed than the zonal Holocene soil. 2.3

Eastern Loess Plateau

The region between the Huanghe (Yellow) Rive and the Taiheng Mountains is called the Shanxi Plateau. Complete loess-paleosol sequences have not been found, probably because of active neotectonism during the Late Pliocene and Pleistocene. However, the loess stratigraphy has been investigated here for a long time. The type localities of the Wucheng and Lishi Loess are in this area.

2.3.1

tinguished (fig. 6). We judge that the Wucheng section contains the complete Lishi Loess sequence, whereas the section below SuG is missing. 2.3.2

Lishi section

The Lishi section occurs at the town of Lishi, about 150 km north of the Wucheng section. We recently observed it resting on a river terrace, with lithology similar to the type locality (Liu Tungsheng 1965; fig. 6). The loess is around 80 m thick, with 21 paleosols recognized. The marker beds Ss, L0 and L15 indicate that the Lishi Loess is complete, but only three paleosols in the Wueheng Loess are present. No magnetostratigraphy or pedostratigraphy has been established for these sections. 2.4

Western Loess Plateau

The region west of Liupan Mountains is known as the Longxi Plateau, and there the loess is up to 300 m thick. The arid climate pedogenesis is weak and so it is difficult to distinguish paleosols from loess. Consequently, only a little pedostratigraphic work has been done. Two loess sections have been investigated, one in Jiuzhoutai near Lanzhou city and another in Biacaoyuan in Fuining county. 2.4.1

Wucheng section

The loess in Wucheng section is about 120 m thick, overlying Tertiary gravels. The exposure of the Wucheng section is not very distinct. About thirty paleosols in the Wucheng section have been dis-

(JATI~NA

An

InteI,lis(iplin~ly

,I

ut]~;,I

,,t

Lanzhou section

The loess section in Jiuzhoutai, just northwest of Lanzhou city, rests on the highest terrace of the Huanghe (Yellow) River. At least 20 m at the base of the section is reworked alluvial loess, with the thickness of eolian loess reaching about 300 m (fig. 7).

~4()IL

S(2IEN(:E

ItYDR()LO(gY

(;EOMORPHOLO(;Y

Pedostratigraphy of Chinese loess

(m) O--

10

Lishi . ~

83

Wucheng so

(m)

SO

I

15 S2-1

$2-I

20

$2-11

30

S2-[I

m 30

m

45 S5-I $5-II S5-111

m

40

60

m m

m m

s~

m

50

$9-1 $9-11

n

n

60

75

/ i

m

m

m $15

90

m m

70

S9

S20

80

105

u m m

i n l n

mmmm

m

S25

120

/

~ml Gravel

90

n

$15

m : : ; :.'.'. ......

Alluvia

6: Lishi and Wucheng Loess Sections. Fig.

$17

There is still a question on the geochronology of the loess in Jiuzhoutai. Early magnetostratigraphic studies by Wang & Yue (1978) and Wang (1982), using alternating feld demagnetization, suggested a basal date of 1.1 Myr. Burbank & Li (1985), using alternating field and stepwise-thermal demagnetization, gave a basal date of about 1.3 Myr. Derbyshire et al. (1987) extended the magnetostratigraphic basal age back to 2.2 Myr and assumed that a large propor-

CATI~NA--An

Interdisciplinary

Journa[

of S O I L

tion of the M a t u y a m a is missing. A recent study by Zhao Jiuxiu et al. (1988) supported the conclusion of Burbank & Li (1985). In the field we found that L9 and L15 are easily recognizable, the first at a depth between 200 and 220 m and the other at 270 to 280 m. Between L9 and L15, seven paleosols are readily observed, coinciding with the Luochuan and Baoji sections. From the top down to L9 some parts are mantled by re-

SCIENC~--HYDROLOf;Y--GEOMORPHOLOGY

Ding, Rutter & Liu

84

Baicaoyuan

O--

Lanzhou

so

(m) --

so

0

m 50

50

~

S2-1 S2-ll

$2-1 S2-1I

-100

I

,ss-i ss-u ' 85-111

S5-11 $5-111

150

100

- - 200 S9-I J [ S,9-II i

I

150

~

S9-1 $9-[[

250

$15

-300

$15

I

200

Fig. 7: Lanzhou and Baicaoyuan Loess Sections. (Lanzhou section is modified from Zhao et al. 1988; Baicaoyuan Loess section is provided by Liu, X. 1989).

Alluvia

$20

Gravel

250 - -

worked loessic colluvium, but most of the soil units can still be identified. The Lanzhou loess section rests on the highest terrace of the Huanghe (Yellow) River, which correlates with the highest terrace of other rivers throughout the Loess Plateau. Twenty to twentythree paleosols are commonly observed. From the magnetostratigraphy of the Luochuan and Baoji sections it seems that the highest terrace was uplifted around 1.3 Myr ago. Therefore, CATENA--AIt

Interdiscipii]tary

Journ~.]

the conclusion of Burbank & Li (1985), that the basal age is about 1.3 Myr, appears reasonable. 2.4.2

Baicaoyuan section

Liu Xiuming and others investigated a natural exposure in Baicaoyuan of eastern Gansu province in 1987. The loess section (fig. 7) is about 220 m thick, containing some 25 paleosols (Liu, M. 1989). Liu Xiuming concluded that the

of S O I L

SCIENCI~--HYDROLOGY

(;~OMORPHOLOGY

Pedostratigraphy of Chinese loess

85

Lishi Loess in Baicaoyuan is complete, whereas most of the Wucheng Loess is missing. The lithological structure of Baicaoyuan section appears to be similar to that of the Lanzhou section. Closely-spaced paleomagnetic samples were taken and analyzed by Liu Xiuming and Heller. Unfortunately, the laboratory results are inconclusive, and no stable polarity has been obtained (personnal communication). The paleosols in Longxi Plateau are light brown, with a few carbonate pseudomycels and massive structure, formed in an arid environment.

3

Correlation of sections

The annum temperature and precipitation of the Loess Plateau decrease from the south-east to the north-west. The paleosols appear to be zonally distributed. Recently Kukla & An (1989) have successfully correlated the loesssoil sequence between Luochuan and Xifeng in the central part of the Plateau, utilizing magnetic susceptibility. Magnetic susceptibility change in the Baoji section has been measured at 10 cm intervals. However, it shows significant differences in character from those in the Luochuan and Xifeng sections. For example, the well-developed soils $6, $7 and Ss have very small values of susceptibility, whereas the susceptibility in the best-developed $5 is less than Si, S2-i, $3 and $4. We attribute this phenomenon to pedological modification of iron minerals, such as pseudogleization by seasonal waterloggh~g. Therefore, we question the use of magnetic susceptibility signals in regional pedostratigraphic correlation throughout the Loess Plateau. In the field some well established stratigraphic markers CATI~NA

Air h l t e r d i s c i p H n a r y

.],*uln~l

,,f S O I L

such as the well-developed soil Ss and the thick, coarser loess units Lg, Li5, L24(WL-1), L27(WL-2) and La2(WL-3) are readily recognizable, which, integrated with magnetic reversals, allow an unambiguous correlation for the succession throughout the Plateau. Fig. 8 shows the correlation of the pedostratigraphy and magnetostratigraphy of the loess in different parts of the Plateau. 3.1

M a l a n Loess

In the Guangzhong Basin Malan Loess can be divided into four Chernozemtype soils, with the two middle soils better developed than the upper and lower. In Luochuan and Xifeng the middle part of the Malan Loess appears to be weathered slightly stronger than the upper and lower parts. In Longxi Plateau it is difficult to subdivide the soils. 3.2

U p p e r Lishi L o e s s

The Upper Lishi Loess extends from the top of St to the bottom of L5. In the Luochuan and Baoji sections, Si and $3 each appear to be joined by a second soil whereas in Longxi Plateau, both are separated by a thin loess layer. The intervening loess in $2 thickens from east to west. $4 is the best developed soil in the Upper Lishi Loess. L2, L3, L4 and L5 are thick loess beds. 3.3

L o w e r Lishi Loess

In this unit $5, L9 and Li5 are marker beds for correlation. Between $5 and L9 three loess-soil units are clearly defined in each part of the Plateau. In the Guangzhong Basin seven soils between L9 and Lis, with Si3 the best developed

SCII~NCE

HYDROLOIIY

t;EOMOII.

FHOLOGY

Ding, Rutter & Liu

86

Lanzhou m

SO

==~

$1

Baicaoyuan

Xifeng

Luochuan

Baoji

Xian

Weinan

=

m n

m m

a

~

B

~

M

L9

--,, ~ j

L9

L15

=

-

j

/

kl L24

~,-

AI 1_27

Gravel L32

Fig. 8: Representative sections in the Loess Plateau (data sources: Luochuan, Lin

1985; Xifeng, Liu, X. et al. 1987; Baicaoyuan, Liu, X. 1989; Baoji, Liu et al. 1989; Lanzhou, Zhao et al. 1988; others, this study). soil, and 89_11 and 814 the least developed, are well-expressed and correlative among the three sections. This series also correlates with other sections, particularly with Luochuan and Xifeng. 3.4

Wucheng Loess

As stated above, the soils in Luochuan and Xifeng are not well defined, and they are designated as WS groupings. Three soil groupings are recognized in Luochuan and four in Xifeng (Liu Tungsheng 1985, Liu Xiuming et al. 1987). Eighteen paleosols, labelled $15 to $32, in the Wucheng Loess of Gnangzhong Basin are easily identified. As seen in fig. 8, the pedostratigraphy in Guangzhong Basin can be correlated, utilizing magnetic control and three thick loess beds, with Luochuan and Xifeng as follows:

CATENA--An

Interdiscipllna.ry

Journal

of S O I L

1. $15 to $23 in Guangzhong Basin to W.,-I ill Luochuan and W s - 1 to W s - 2 in Xifeng. 2. L24 in Guangzhong Basin with WL-1 in Luochuan and WL-2 in Xifeng. 3. $24 to $20 in Guangzhong Basin with W s - 2 in Luochuan and W s - 3 in Xifeng. 4. L27 in Guangzhong Basin with WL-2 in Luochuan and WL-3 in Xifeng. 5. $27 to $31 in Guangzhong Basin with W s - 3 in Luochuan and W s _ 4 in Xifeng. 6. L32 in Guangzhong Basin with WL-3 in Luochuan and W L _ 4 in Xifeng.

SCIENCP~--HYDIIOLOGY--G]~OMORPFIOLOGY

Pedostratigraphy of Chinese loess

INC.

Baoji m

-90

0

SUS.SI(×I0 s)

0 +900

87

IN C

Xian

200 400

-90

S U S lSI

0 +90 0

,

SUS SI (xl0-5) INC. 0 +90 ) 150 300

(xIO-51 Weinan

150 300

-90

I

I i

lb

, L

b

I

¥

I I

t t

I I I I I I

t E

I •

i

t

~

I t t t I t

--.._.. I I I

I

10

12

I

l Loess

I

Paleosol

~

Red Clay

~

Mixed Layer

Fig. 9: G / M boundary in the Baofi, Xian and Weinan Sections.

4

Basal age of the Loess Plateau deposits

As the Matuyama-Gauss boundary occurred 2 to 3 m below the contact of the Wucheng Loess and the Red Clay in Luochuan section, the basal age of Chinese loess was inferred to be around 2.4 Myr (Lin T.S. 1985, Liu X.M. et al. 1987). In Liujiapo section near Xian Sun et al. (1987) identified the M / G boundary about 7 m above the base of the loess and suggested a basal date of about 2.66 Myr. We determined the M / G boundary in the oldest loess layer - - L33 in the Baoji section (Liu et al. 1989). To check these results paleomagnetic samples were taken from the base of the Wucheng Loess and the upper part of the Red Clay formation in the Xian sec-

C A T E N A

All

Interlllsclplhla[y

,,o~irnal

of

SOIL

tion at Duanjia village and the Weinan section at Miaogou Gully. Laboratory analysis shows the same results as the Baoji section (fig. 9). In Luochuan and Xifeng the deposits corresponding to $22 and L33 in Guangzhong Basin appear to be mixed into the upper part of the Red Clay according to a micromorphological study (Bronger & Heinkele 1989) and susceptibility data (Kukla & An 1989). The Liujiapo section, about 5 km west of Duanjia village, contahls a complete section of Lishi Loess, but here only 12 to 13 paleosols in the Wucheng Loess, fewer than at Baoji and other sections. In addition, the 6-8 m thick deposit above the Red Clay is masked by thick reworked loessial colluvium, so the reliability of 2.66 Myr as the basal age of the loess remains to be tested. Assuming 20 to

S C I B N C E - - - H Y D R O L O G Y - - G E O M O R P H O L O G Y

Ding, Ratter & Liu

88

150 cm of loess being deposited from the M / G boundary down to Red Clay in Guangzhong Basin, the basal age of Chinese loess is judged to be 2.5 Myr. In the Loess Plateau, the Baoji section is clearly exposed from top to bottom, and every loess bed and soil is well defined. Therefore, it can be used as the type section on which the pedostratigraphy of Chinese loess is divided and numbered. In the Lishi Loess the soil complexes S:, $5 and $9 represent multiple-climatic cycles, and so they are separated into pedostratigraphic divisions named $ 2 - I , S 2 - H , $ 5 - / , S5-x/, S ~ - I ~ , S g - l , and S o - H . Therefore, eighteen pedostratigraphic units are present in the Lishi Loess. Also, there are eighteen pedostratigraphic units, from Sis to Sa2, in the Wucheng Loess. Malan Loess and the Holocene soil constitute another pedostratigraphic unit, designated as SO. Therefore, thirty seven pedostratigraphic units are identified in the Chinese loess for the last 2.5 Myr.

5

P e d o s t r a t i g r a p h y of C h i n e s e loess and climatic cycles in the last 2-5 M y r

As seen in fig. 8, each pedostratigraphic unit in the Loess Plateau can be correlated over long distances in at least six sections for Lishi Loess and three sections for Wucheng Loess. In addition, the Wucheng Loess in Luochua~l and Xifeng are lithologically comparable with those in Guangzhong Basin, demonstrating the continuity of loess in recording major climatic fluctuations for the last 2.5 Myr. Based on the pedostratigraphy of the

(:ATI~NA---An

lntetdisciplin~ry

,loallll~l

Loess Plateau, two major characteristics of climatic cycles during the last 2.5 Myr have been recognized. First, around 2.5 Myr, loess began to be deposited oi1 a large scale in northcentral China as evidenced by the striking contrast between the oldest loess, L33, and the underlying Red Clay, which is actually a thick soil formed under forest. This break represents an abrupt change in climate in north China, and it corresponds with changes observed in other records throughout the world, such as the earliest glacial conditions in the northern henlispt:Lere as revealed in DSDP552A by Shackeleton et al. (1984), the complete change of vegetational types in Holland (Zagwijn 1974), and the deposition of the earliest loess in Europe (Kohl 1986). The evidence suggests that there was then an abrupt climatic shift at around 2.5 Myr, at least in the northern hemisphere. It appears that in the Gauss the climate was fairly warm continuously punctuated with short oscillations, and then at about the time of the M a t u y a m a - G a u s s reversal, the climate began to fluctuate frequently and widely, and it continues to do so today. Second, over the last 2.5 Myr, the orbital periods of 100, 40 and 20 ka are documented in the pedostratigraphy of the loess as a result of the response of climatic systems to the rhythmic variations in insolation. The relative importance of the periods is a function of the time interval considered, and in turn, the amplitude of climatic cycles is determined by the periodicities. From the base of L33 to the top of $23 the paleosols are recognized within a time span of about; 1.0 Myr, assessed from the paleomagnetic stratigraphy (about 2.5 to 1.5 Myr). During

of SOIL

SCIENCI~

tfYDRC)LOGY

GEOMORPItOLOGY

Pedostratigraphy of Chinese loess

89

this time, the dominant periodicity of climate is assumed to be 100 ka, with the amplitudes varying widely. Two weakly-weathered, fairly thick loess beds L33 and L32 and two strongly developed soils, $32 and $31 represent the two earliest climatic cycles, displaying large amplitudes. The four cycles from L31 to L27, represented by moderately developed soils and thinner loess beds, have smaller amplitudes. In the four cycles recorded between Ls7 and Ss3, L27-Sz6 and L24-Sz3 have larger amplitudes than those of Lz6-$25 and L25-$24. The time span from the bottom of L~_3 to the top of $9 is about 0.7 Myr (from about 1.5 Myr to 0.8 Myr), containing fifteen pedostratigraphic units. The loess beds are thin and comparatively strongly-weathered except L15 and L13, which show small climatic cycle amplitudes. The periods in this time span are dominated by 40 ka cycles except for a few 100 and 20 ka cycles. There are twelve climatic cycles fronl 0.8 Myr to the present, represented by L0 to So. Most of the loess beds are thick and weakly weathered whereas the soils are well developed, showhlg large climatic cycle amplitudes. The periods in the last 0.8 Myr are dominated by 100 ka cycles with a few of 40 and 20 ka cycles. Therefore, there are two shifts of dominant periods of climatic cycles in the last 2.5 Myr, one occurring about 1.5 Myr and another around 0.8 Myr. In addition, the amplitude of climatic fluctuations is largely determined by their periodicities. Cycles with the period 100 ka are commonly accompanied by large amplitudes, whereas those with 40 and 20 ka periods have smaller amplitudes. As above, the most complete record of CATBNA

-An

Ii1terdlsclpliaa~y

Journal

r,f S O I L

the climatic fluctuations in the Quaternary is provided by the oxygen-isotope stratigraphy in deep-sea cores and the loess-paleosol sequence in north-central China. Pedostratigraphy ill the Loess Plateau reveals that there were thirtyseven climatic cycles in the last 2.5 Myr. Oxygen-isotope stratigraphy in deepsea cores of the Brunhes Epoch has been successfully correlated with loess- paleosol sequences in north-central China by many workers (Liu & Yuan 1982, Liu 1985, Liu et al. 1987, Kukla 1987). For the oxygen-isotopic records in the Matuyama Epoch only ,~ few cores, for example, V28-239, V16-205, DSDP552A and DSDP site 607 and 609, have been investigated. Shackleton & Opdyke (1976) did not divide core V28-239 below stage 22 because the oxygen-isotopic variations in the Matuyama Epoch are not as obvious as in the Brunhes. Donk (1976) analyzed the North Atlantic core V16205 and extended the numbering system from stage 19 to stage 41. According to his results, 21 glacial and interglacial oscillations can be identified in the last 2.5 Myr. North Atlantic Core DSDP552A covers the late Pliocene and the entire Pleistocene (Shackleton et al. 1984). Unfortunately, the signals of oxygen-isotopic variation between 1.26 Myr and 1.45 Myr are missing. DSDP site 607, 609 are the only cores reported so far that contain a continuous oxygen-isotopic record from the Holocene down to the Gauss-Matuyama boundary (Ruddiman et al. 1986). The oxygen-isotopic changes above stage 19 in the two cores clearly match the cores V28-238 and V28-239. Between the B/M boundary and the top of the O1dural subchron, Ruddiman et al. (1986) named the oxygen-isotopic variations from stage 19 to stage 63. From stage

SCIENCE--HYDROLOGY

GEOMORPHOLOGY

Ding, Rutter & Liu

90

63 to the M / G b o u n d a r y , however, num e r o u s a m p l i t u d e s show m i s m a t c h e s between site 607 and 609. T h e r e are still problems for deep-sea core s t u d y in the M a t u y a m a because of l i m i t a t i o n of coring and other techniques. Therefore, we conclude t h a t the p e d o s t r a t i g r a p h y of the Loess P l a t e a u is p r o b a b l y the m o s t c o m p l e t e and reliable r e c o r d of the m a j o r climatic cycles in the last 2.5 Myr. Acknowledgements We are i n d e b t e d to Dr. Liu Xiuming of the I n s t i t u t e of G e o l o g y of the Chinese A c a d e m y of Sciences for kindly making the p e d o s t r a t i g r a p h y of Baicaoyuaal section available, and to Mrs. Jennifer Ko wal for c o m p u t e r entry. This is a cont r i b u t i o n of the C h i n a Young Scientist F o u n d a t i o n . T h e a u t h o r s acknowledge the financial s u p p o r t of the N a t u r a l Sciences and E n g i n e e r i n g Research Comlcil of C a n a d a . References B R O N G E R , A. & H E I N K E L E , TH. (1989): Micromorphology and genesis of paleosols in the Luochuan loess section, China: pedostratigraphic and environmental implications. Geoderma 45, 123-143. BURBANK, D . W . & LI, J.J. (1985): Age and paleoclimatic significance of the loess of Lanzhou, north China. Nature 316, London, 429-431.

D E R B Y S H I R E , E., W A N G , L., S H A W , •] & R O L P H , T. (1987): Interim results of studies of the sedimentology and remnant magnetization of the loess succession at Jiuzhoutai, Lanzhou, China. In: Liu, T.S. (ed.), Aspects of Loess Research. Beijing: China Ocean Press, 175-191. D I N G , Z.L., LIU, T.S., LIU, X.M., C H E N , M.Y. & AN, Z.A. (1990): The thirty-seven climatic cycles in the last 2.5 Myr. Chinese Science Bulletin 35. 668671.

CATENA--An

Interdisciplinary

Journ~.l

D O N K , J.V. (1976): O is record of the Atlantic Ocean for the entire Pleistocene Epoch. Geological Society of America, Memoir 145, 147-163. E M I L I A N I , C. (1955): Pleistocene temperature. Journal of Geology 63, 538-578. E M I L I A N I , C. (1958): Paleotemperature analysis of Core 280 and Pleistocene correlation. JournM of Geology 66, 264-257. E M I L I A N I , C. (1966): Paleotemperature analysis of the Caribbean Core P6304-8 and a generalized temperature curve for the past 425,000 years. Journal of Geology 14, 109124. H E L L E R , F. & LIU, T.S. (1982): Magnetostratigraphic dating of loess deposits in China. Nature 300, London, 431-433 H E L L E R , F. & LIU, T.S. (1984): Magnetism of Chinese loess deposits (astr.). Geophysical Journal of Research 77, 125-141. KOHL, H. (1986): Pleistocene glaciations in Austria. Quatern,~ry Science Reviews 5, 421-428. K U K L A , G. (1987): Loess stratigraphy in central China. Quaternary Science Reviews 6, 191-219. K U K L A , G. & AN. Z. (1989): Loess stratigraphy in central China. Paleogeography, Paleoclimatology, Paleoecology 6, 191219. LIU, Tungsheng (Ed.) (1965): The loess deposits in China. Science Press, Beijing, 244 pp. (in Chinese). LIU, Tungsheng (Ed.) (1966): The Composition and Texture of Loess. Science Press, Beijing, 132 pp. (in Chinese). LIU, Tungsheng (1985): Loess and the Environment. China Ocean Press, Beijing, 251 pp. LIU, T.S. & AN, Z.S. (1984): A preliminary magnetostratigraphic study of the Beihanzhai loess section. Geochimica 2, 134137 (in Chinese). LIU, T.S., D I N G , Z.L., C H E N , M.Y. & AN, Z.S. (1989): The global surface energy system and the geological role of wind stress. Quaternary International 2, 43-55. LIU, T.S. & Y U A N , B.Y. (1982): Quaternary climatic fluctuation - - A correlation of records in loess with that of the deep-sea core V28-238. In: Research on Geology I, Institute of Geology, Academia Sinica, 113-121 (in Chinese).

of S O I L

SCIENCE--HYDROLOGY--GI~OMOFLPHOLO(gy

Pedostratigraphy of Chinese loess

91

LIU, X.M.

(1989): Magnetostratigraphy and climatostratigraphy of Chinese Loess. Ph.D. thesis, Institute of Geology, Academia sinica.

LIU, X . M . , LIU, T.S., X U , T . C . , LIU, C. & C H E N ~ M . Y . (1987); A preliminary study on magnetostratigraphy of a loess profile in Xifeng area, Gansu province. In: Liu, T.S. (ed.), Aspects of Loess Research. China Ocean Press, 164-174. NANJING SOIL INSTITUTE OF CHEINESE ACADEMY OF SCIE N C E S (1980): Soils in China. Science Press, Bejing, 578-582. P E C S I , M . (198'/): International symposium on loess research, Xian, China: Interpretation of loess-like formations, paleosols and red clays in loess research. In: Liu, T.S. (Ed.), Aspects of Loess Research. China Ocean Press, 85-106. R U D D I M A N , W . F . , R A Y M O , M. & McINTYRE, A. (1986): Matuyama 41,000 years cycles: North Atlantic Ocean and northern hemisphere ice sheets. Earth and Planetary Science Letters 80, 117-129.

RUTTER, N., DING, Z.L., EVANS, M . E . & LIU, T.S. (1991a): Baoji-type pedostratigraphic section, Loess Plateau, north-central China. Quaternary Science Reviews 10, 1-22.

S H A C K L E T O N , N . J . et al. (1984): Oxygen isotope calibration of the onset of ice-raftlng and history of glaciation in the North Atlantic region. Nature 307, London, 620-623. SUN, J.Z., Z H A O , J.B., W E I , M . J . , LI, H . M . & (1987): Loess is ever older. ogy and Quaternary Geology Chinese).

SUN, X.Y., Z H O U , Z.B. Marine Geol7, 106-112 (in

W A N G , Y . Y . & Y U E , L.P. (1978): The date of formation of loess in Lanzhou on the base of the paleomagnetic analysis. Geol. Sci., and Geol. Tech. J. 4, 76-81 (in Chinese). W A N G , Y.Y. (1982): Loess and Quaternary Geology, 1976-1980. People's Press, Shaanxi, 20-47 (in Chinese). Z A G W I J N , W. (19~'4): The PliocenePleistocene bundary in western and southern Europe. Boreas 3, 75-97. Z H A O , J . X . , X I U , Q.Z., Z H A N G , Y.T. & C H E N , F . F . (1988): The loess-paleosol sequence in Jiuzhoutai, Lanzhou and the environmental evolution. Bulletin of Lanzhou University 24, 119-129 (in Chinese).

R U T T E R ~ N., D I N G , Z.L., E V A N S , M . E . & W A N G , Y. (1991b): Magentostratigraphy of the Baoji loess-paleosol section, Loess Plateau, north-central China. Quaternary International 7/8, 97-102. S A S A J I M A , S., W A N G , Y., T O R R I , M., N I S H I D A , J & M A E N A K A , K. (1984): Magneto- and chronostratigraphy revealed from the Luochuan loess sequence in China and it's relevance to the Quaternary climatic change. In: Sasajima, S. & Wang, J. (eds.), The recent research of loess in China. Kyoto: Kyoto Institute of Natural History, 221-232. SHACKLETON, N.J. & OPDYKE, N . D . (19~r3): Oxygen isotope and paleomagnetic stratigraphy of Equatorial Pacific Core V28-238. Quaternary Research 3, 3955. SHACKELETON, N.J. & OPDYKE, N . D . (1976): Oxygen isotope and paleomagnetic stratigraphy of Pacific Core V28239. Geological Society of America, Memoir 145, 449-464.

CATENA--AIt

Interdisciplinary

Journal

.f

SOIL

SCI~NC~

A d d r e s s e s of a u t h o r s : Dr. D i n g Z h o n g l i Institute of Geology, Academia Sinica P.O. Box 634 Beijing, China P r o f . Dr. N a t R u t t e r Department of Geology University of Alberta Edmonton, Alberta T6G 2E3 Canada Dr. Liu T u n g s h e n g Institute of Geology, Academia Siniea P.O. Box 634 Beijing and Xian Laboratory of Loess and Quaternary Geology Academia Sinica Xian, China

HYDROLOGY---GI~OMORPHOLOGY