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Stratigraphy and paleosols in the Sale terrace loess section, northwestern China
CATENA
vol. 17, p. 357-367
Cremlingen 1990 [
STRATIGRAPHY A N D PALEOSOLS IN THE SALE TERRACE LOESS SECTION, NORTHWESTERN CHINA W.C. Mahaney, North York, ILG.V. Hancock, Toronto, Linyuan Zhang, Lanzhou Summary Two paleosols in the Sale terrace loess sequence were investigated to determine if radiocarbon ages for each paleosol could assist in reconstructing the Holocene loess succession as well as the paleoenvironmental record. The upper loess, about 3 m in thickness, carries no surface soil and is still being emplaced. The middie loess was emplaced during the middle Holocene, whereas the lower loess was emplaced prior to 7500 yr BP. Relative weathering in the two buried paleosols is approximately the same with minor differences in clay mineral and elemental composition indicating a slightly wetter paleoclimate in the lower unit becoming drier upwards toward the present. Because there is evidence for considerable chemical homogeneity downwards in the section, with some minor leaching of the mobile elements Ca and Na, there is every reason to believe the 14C dates are uncontaminated and represent true ages.
ISSN 0341-8162 (~)1990 by CATENA VERLAG, D-3302 Cremlingen-Destedt, W. Germany 0341-8162/90/5011851/US$ 2.00 + 0.25
1
Introduction
Two paleosols and three loess units (aeolian above 1.5 m; alluvial loess below) were sampled at the Sale terrace section (fig. 1) in northwestern China. In this section three loess units are separated by two paleosols, which represent significant Holocene disconformities, 2500 years apart. This section was exposed following a 1983 landslide which killed 200 people after the sliding mass fell 300 m to the Baxie River in less than 2 minutes (ZHANG & SHIWEI 1983, ZHANG & ZHANG 1989). We sampied the three loesses and two paleosols to determine if paleosol characteristics could be used to estimate paleoenvironmental changes during the Holocene.
2
Methods
Samples were described in the field using the nomenclature of the SOIL SURVEY STAFF (1951, 1975) and BIRKELAND (1984). Colors are given in the moist and dry states following OYAMA & TAKEHARA (1970). Samples were fractionated for particle size analysis using the methods of DAY (1965) and BOUYO U C O S (1962). Particle size divisions
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Fig. 1: Location of Sale terrace loess section, Northwestern China. follow the Wentworth Scale of FOLK (1968) for sand-silt breaks (63 #m) and the SOIL SURVEY STAFF (1975) for silt-clay breaks (2 #m). Parent material designations follow BLOOM (1978). The pH was measured by electrode and total salts were determined by electrical conductivity (BOWER & WILCOX 1965). Mineralogical analyses were completed on the <2 #m fractions by X-ray diffraction using CuK 0t radiation generated by a Toshiba ADG-301H X-ray diffractometer. Samples were oriented on ceramic tiles by centrifugation and minerals were identified following procedures outlined by WHITTIG (1965) and MAHANEY (1981). The <2 mm fractions (0-2000 #m) of loessic sediments and paleosols were subsampled for neutron activation analysis at the SLOWPOKE Reactor Facility in
the Univesity of Toronto. Seven dried samples were taken for chemical analysis. They were chosen to range in mass from 700 to 850 mg in an attempt 1;o guarantee representative samples. T~J~ samples were analyzed in Olympic tic flip-top polyvials. For elements such as U, Dy, Ba, Ti, Mg, Na, V, A1, Mn, Ca and CI, which produce short-lived radioisotopes, the samples were irradiated serially for 1 minute at a neutron flux of 1.0xl011 n cm-2. s-1 in the SLOWPOKE nuclear reactor at the University of Toronto. After a waiting time of 19 minutes to allow the very short-lived 28A1 to decay to acceptable levels, each sample was assayed using 5 minute counts with on-site gammaray spectrometers, as described by HANCOCK (1984). Appropriate gamma-ray peak areas were measured and the clu~-
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Stratigraphy and Paleosolsin Loess, N-W China
359
SALE TERRACE LOESS SECTION 0
5000+ llO yr a 33 ~ MIDDLE
LOESS
:':':.i'::..:.".:.' ". : ::::..' ..":i;.'. . ..?; : : . ...'.;":.' . . . . . ..'.."'i" . ~" .!.'):.:. ....... ".".." "...: ....:.........:.......-.-.: •. . . . " . . . " , ".'.:. • "..-.......... 4.4 •
m m~
4,6
ii:ii!!!ii: !F
LOWERLOESS
753o+_oyr
~
[]
SAMPLE
[]TN AEOLIAN LOESS ALLUVIAL LOESS PALEOSOL
a 14C dotes determined from organic corbon in pdleosoIs. Age tlletormir~tia~ mode at Radiocarbon Laboratory, Lonzhou University, t?1~ of Chino
Fig. 2:
Stratigraphyat Sale terraceloesssection,NorthwesternChina.
ical concentrations calculated using the comparator method. The samples were serially recounted after 20-24 hours to measure the K, Eu (and remeasure the Na) concentrations,
construction. Climatic parameters for the Sale Shan area are not known with precision. However, the climate in the Lanzhou area is dry with mean annual precipitation (MAP) of 545 mm. Evapotranspiration is greater than MAP (ZHANG 1989). 3 Field a r e a As in other areas of the Loess Plateau, attempts are being made to introduce The Sale Loess Section (fig. 2) located in stands of poplar (Populus spp.) trees to the alluvial beds of the second terrace in assist in reducing mass wastage. Howthe Baxie River Valley (ZHANG 1986, ever, in the area around Sale Moun1989) is representative of Holocene al- tain only a few trees have been planted luvial loess sections on the westernmost to date; the lack of farmland and the part of the Loess Plateau of northwest- growth of population may make it imern China. Buried soils in the section possible to reforest the area. provide the opportunity for radiocarbon dating and for paleoenvironmental reCATENA--An Interdisciplinary Journal of SOIL 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 G Y
Qz
CROSS-VALLEY
1"2
PROFILE
- -- - .....
___ _-_.. . . . . . . . .
T~
- .
of
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...........
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.-__ _ - _ _ - - - - - ~ _ _ - -. -
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@
s
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.
BAXIE
.
_.. ~
I T2 J
.
RIVER
.
.
.
.
~I - - - - - - - - - - - - - - -
LANDSLIDE
--.--------
Fig, 3: Cross-valley profile .from the Saie Shah ridge to the Baxie River.
N2 (Red beds; Linxia Zu Fro)
-----;----. ------_ _
~II~~---__:_--Z~_"1"4~ - - - - - - - - - ~ Z - Z - - - - - - ' - - _ - - ~ -
AREA
LOESS Jincluding paleosols
in S A L E S H A N
.
loess ALLUVIAL gravel
2
loess ridge
(modiFied from Z~,'~J, 1983.)
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LOEssSLOPEWASH-ALLUVI Q ( 2Ql)3 * Q¢) ALLoESS
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manmade channel i
I GEOMORPHIC LOCATION] / of LOESS PROFILE t I / abandoned channel floodplain i
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= 2200~4__,
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Stratigraphy and Paleosols in Loess, N-W China
4 4.1
Results and discussion Stratigra0hy and paleosols
The loess stratigraphy and paleosols are shown in figs. 2 and 3. The lower alluvial unit appears to have a higher bulk density than the middle and upper units (hence the different degrees of shading in fig. 2), but otherwise they all have similar bulk densities (1.0-1.2 g/cm-3), All alluvial loess belongs to the T2 surface (fig. 3) of Holocene age that overlies Linzia Zu Formation of Pliocene age (ZHANG 1989). Color is a nearly uniform pale yellow (approximately 2.5Y 6/4 to 7/4 on a dry basis) across each alluvial and aeolian unit. There is littie textural variation between the loesses and no mottling was observed in the field. Radiocarbon dates in the lower paleosol indicate it was buried approximately 7530___160 yr BE The upper paleosol yielded a 14C date of 5000+110 yr BP. The ages of these two paleosols are based on dates obtained from the organic carbon fraction of the paleosols. The depth relationships show that all loess units are rather thin to the order of 1.0 to 3.0 m in thickness (fig. 2). The paleosols have colors that range from brownish gray (upper) (10YR 4/1) to black (lower) (10YR 2/1) under field conditions. After drying in the laboratory these colors lose only one value and one or two chroma becoming brownish black and grayish yellow brown. The lower paleosol has the darkest colors suggesting a higher organic carbon content, which was confirmed by weight loss on ignition (estimated at ,,~4.0 percent). Because these paleosols are covered with fresh unconsolidated alluvial and aeolian loess beds that lack root systems, we believe they are not contaminated with
361
younger organic carbon. Hence, the 14C dates are likely reliable giving true times of burial. 4.2
Particle size
Particle size ratios (tab. l) were analyzed to determine if differences exist among the three parent materials and the two paleosols. In all, sand is low but remarkably uniform across the sequence. Silt distributions vary somewhat, and with the exception of the lower unit, tend to increase upwards in the sequence. However, there is little overall appreciable difference in silt content between loess beds in the sequence with the exception that the lower and upper units contain 10 to 15% higher amounts than the middle unit and the paleosols. With respect to percnt clay, the lower paleosol has the highest amount relative to the underlying parent material, indicating that it may have formed in sito under a slightly wetter paleoclimate than today or over a longer time interval. The upper paleosol actually contained less clay than that found in its parent material. While it is highly possible that some clay was translocated into the underlying parent material, it is more likely that insufficient time had elapsed for its genesis. Because the middle unit is so thin, it is likely that some of its clay content is inherited from erosion of the lower paleosol during the alluvial event that emplaced it. In general, the particle size data tend to support the stratigraphy observed in the field and the available radiocarbon dates. 4.3
Mineralogy
The mineralogy (tab. 2) of the Sale terrace section was analyzed to determine if
CATENA--An Interdiseip/inary Journal of SOIL S C I E N C E ~ Y D R O L O G Y - X i E O M O R P H O L O G Y
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Unit
Sand (%)
Silt (%)
Clay (%)
(2 m m - 6 3 / ~ m )
(63-2 #m)
( < 2#m)
2.6 1.8 1.4 0.8 1.6 t.9 1.1
79.1 75.9 71.4 64.1 64.4 69.3 79.2
18.3 22.3 27.2 35.1 34.0 28.8 19.7
Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
Tab. 1 : Particle size ratios for paleosols and
Unit Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol Lower Paleosol (a) L o w e r Loess
Tab. 2:
parent materials in Sale Terrace Section.
Depth (m)
K
H
I
I-S
V
Chl
Q
O
P
CALC
k0 2.0 3.1 3.8 4.5 4.5 5.0
XX tr X X X X X
-tr tr X X X
XX X XX X XX X XX
X tr tr tr tr tr tr
tr
X tr X tr tr tr tr
XX XX XX X XX XX XX
XX -XX XX XX XX XX
tr
-------
X tr X --
tr
X
tr X
tr
tr
tr
--....
tr
Mineralogy of the Sale Loess Sequence, Northwestern China.
Mineral abundance is based on peak height: nil(--), minor amount (tr), small amount (X), moderate amount (XX), abundant. (XXX). Clay minerals are kaolinite (K), halloysite/metahalloysite (H), illite (I), illite-smectite (I-S), vermiculite (V), chlorite (Chl).
Primary minerals are quartz (Q), orthoclase (O), plagioclase (P) and calcite (Calc).
differences could be detected between the loess units or among the paleosols and parent matrials. Overall, the trends are for an upward increase in kaolinite, chlorite, and illite-smectite among the clay minerals studied. Plagioclase feldspars increase slightly upward as does calcite, which may reflect the stratigraphy in the source area of the loesses, With respect to weathering trends (paleosols vs. parent materials) and aeolian input in the paleosols, it is important to note that metahalloysite increases in the lower paleosol relative to the underlying parent material. This trend may result from increased moisture under wetter paleoclimatic conditions or from weather-
ing over a longer time period as in other dry areas (MAHANEY 1978). However, higher amounts of metahalloysite in the middle loess unit probably are related to the incorporation of preweathered sediments following aeolian transport and suggest that metahalloysite may not have formed in situ. Higher illite in the lower and upper paleosols (fig. 2) may result from aeolian input, which is known to occur in other areas (MAH A N E Y & FAHEY 1980, M A H A N E Y & HALVORSON 1986). The degree to which expandable clay minerals form in the environment is not known with precision, but the presence of illite-smectite in buried paleosols sug-
CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
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Stratigraphy and Paleosols in Loess, N-W China
gests it may have been an important weathering product in middle Holocene paleoenvironments. The origin of vermiculites in the upper unit is obscure, but appears to be the result of airfall influx rather than of weathering in situ. Chlorites in the sequence are diochtahedral (Fe chlorites), as indicated by the 001 peaks on X-ray diffractograms (MAHANEY 1978). Because chlorite increases upwards in the sequence, it likely reflects the source area of loess. The upper paleosol was likely exposed for too short a time, under too dry a climate, to chloritize iUite, and illite does not decrease with increasing chlorite. Increasing amounts of calcite upwards in the younger units may signal drier paleoclimatic conditions, perhaps resulting from the removal of forests by local inhabitants (see MAHANEY & ZHANG 1990).
363
ied. The trend shown on tab. 3 indicates a depletion of salts downward in the sequence with somewhat lower values in the paleosols. This might suggest removal of minor amounts of salt during the time each paleosol was exposed to subaerial weathering. The distributions of the 13 elements shown in tab. 4 show an overall similarity among the three loesses and two paleosols. The only major break occurs between the lower paleosol and underlying loess where CI increases downwards and this is probably an environmental rather than a geochemical effect. Slight decreases in Ca in the two paleosols and middle loess may be the result of minor leaching effects. Ratios of Na/A1, Ca/AI and Mn/A1 were analyzed (tab. 5) to determine if they provide information on relative rates of removal of mobile elements relative to immobile ones. Sodium is only 4.4 Chemistry slightly responsive showing differences among the two paleosols relative to the The dry colors (tab. 3) of the indi- loesses. The ratio Ca/AI gives somewhat vidual units range from dull yellowish lower quotients in the paleosols, indicatbrown, grayish yellow brown to dull ing a slight tendency towards Ca depleyellow orange. In general, the pale- tion during the time of paleosol formaosols are slightly darker (10YR 4/3; tion. The relative movement of Mn in5/2) compared with the parent materi- dicates slightly enriched Mn in the lower als. Buried paleosols are actually fos- and upper paleosol as well as in the midsil ochric epipedons (SOIL SURVEY die loess. These data indicate also that STAFF 1975). The parent material col- the middle loess contains some preweathors indicate a considerable proportion ered material from the lower paleosol, of preweathered sediments were incor- and agrees with the mineralogical analyporated in the deposits following eo- sis. lian/alluvial transport. The pH (tab. 3) was measured to de- 4.5 Correlation termine if differences existed between the paleosols and parent materials. Even The nearest sites to the Sale terrace though the paleosols are slightly less al- section with comparable Holocene sekaline, the differences (~0.5 pH unit) are quences are at Longxi, about 125 km to relatively minor. Total salts also show the southeast, and at Luochuan 300 km few differences among the samples stud- east (fig. 1). CATENA--An Interdisciplinary Journal of SOIL S C 1 E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
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u nit
Color
Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
10YR 10YR 10YR 10YR 10YR 10YR 10YR
6/3 6/3 5//2, 5/'3 4/"3 4/3 4/3, 5/3 6/4
(1:5)
(mmhos/25°C) (1:5)
9.0 9.0 8.6 8.5 8.7 8.8 9.0
.54 .47 .13 .19 .14 .10 .11
Tab. 3: Dry color, pH and electrical conductivities of parent materials and paleosols
in the Sale Loess Section. Na (%)
Mg (%]
A1 (%)
CI
K (%)
Ca (%)
Ti
V
Mn
Ba
Eu
Dy
U
Sample Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
1.28 1.30 1.19 1.17 1.14 1.22 1.28
0.9 1.0 0.9 1.2 1.2 1.0 1.2
6.0 6.1 6.2 6.4 6.6 6.5 6.1
<200 <230 _<220 _<180 _<200 _<190 600
1.9 1.9 2.1 2.0 1.9 2.1 1.8
5.2 5.9 4.0 4.2 4.1 3.5 5.4
3130 3250 3670 3350 4070 3400 3270
68 70 72 86 75 75 66
567 583 689 743 722 739 613
500 480 670 600 490 510 450
0,78 0,88 0,88 0.90 0.86 029 0.86
2.4 2.6 3.1 3.t 3.3 3.7 3.3
2.1 3.6 2.9 2.9 2.9 3.6 3.3
Tab. 4: Elemental chemistry* of the Sale Terrace Section, northwestern China. * Elemental concentrations in ppm unless otherwise indicated.
Sample Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
Na/A1
Ca/A1
Mn/A1
Caxl00 Ti
Naxl000 Ti
0.21 0.21 0.19 0.18 0.17 0.19 0.21
0.87 0.97 0.65 0.66 0.62 0.54 0.89
95 96 111 116 109 113 100
0.17 0.18 0.11 0.13 0.10 0.10 0.17
0.41 0.40 0.32 0.35 0.28 0.36 0.39
Tab. 5: Elemental ratios .from the Sale Terrace Section, northwestern China.
At Luochuan, Holocene paleosols (fig. 4A) w i t h d e p t h s o f 0.5 to 1.5 m in a e o l i a n loess h a v e b e e n s t u d i e d for p o l l e n a n d fossil snail c o n t e n t ( L I U et al. 1985). T h e t h r e e p a l e o s o l s s h o w a pollen succession downward that trends f r o m s u b h u m i d (Gramineae) a n d semiarid (Artemisia) to h u m i d (Betula, Pi-
(A1ENA
nus) e n v i r o n m e n t s . T h e fossil snail d a t a p r o v i d e s o m e w h a t less r e l i a b l e p a l e o c l i m a t i c i n f o r m a t i o n ( L I U et al. 1985). H o w e v e r , Metodontia beresowskii indicates a w a r m e r a n d w e t t e r c l i m a t e t h a n p r e s e n t a n d Cathaica pulveraticula indicates d r i e r a n d c o l d e r c o n d i t i o n s . Bec a u s e o n l y t h e l o w e r p a l e o s o l at Lu-
An Interdisciplinary Journal of SOIL SCIENCE- HYDROLOGY
(iEOMORPHOLOGY
365
Stratigraphy and Paleosols in Loess, N - W China
A.
POLLEN
FOSSIL SNAILS
0O.5-
~,
I o-
,,:~-:,,!:.!.,"..~:.::,-:::... Grom/neoe • ; :?..~.......
;':'~:;;~:~:'<":":
.
'
2o- ~
( Moel lendorf f )
Artem/s/a sp.
:.~-,~.~-~, ~,~ Betula sp LS-
Metodontiaberesowski/ Metodonha hausa/ensis
(Crosse)
Cathoicapu/vera/?cula
P/nus sp.
(Martens)
LOESS !:i:i
B.
14C
Alluvial loess Aeolian loess
POLLEN
,..,.........-..;.......-...,:.v....,
::::::::::::::::::::::::::::::::::::::::: .:.:.:-:.:.:.:.:,:.:.:.:.:.:.:.:.:.:,:.:.:. :::::::::::::::::::::::::::::::::::::::::
t- ....,.......,.-:,.,-.v.......,.....v .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:. Befula sp Carpinu$ sp. :':'.,:'. .:.i:.i:::'!.:::;': :,'i::?..i::.:i:i::;:,:i ~ ~ 7~,o*_a~o Pinussp. z- !":??"-'.::-.~:'.?':.??i:i:"i:i:! Artemisia sp. Polygonoceae asso*_3oo Chenopodioceoe 3-
~
ochuan is dated (8100-t-220 yr BP), it is possible to tentatively correlate it with the oldest paleosol at Longxi which dates from 85504-300 yr BP. If the two upper paleosols at Luochuan are coeval with the two paleosols from the Sale terrace section, the paleoclimatic trends appear to be out of phase. Clearly additional investigations are required to adequately reconstruct paleoclimate in this part of the Loess Plateau. At Longxi, the Holocene sequence (LIU et al. 1985) of paleosols (fig. 4B) formed in aeolian (lower) and alluvial (upper) loesses give somewhat different ages (WEN et al. 1982). The lower paleosol, ]4C dated at 85504-300 yr BP, formed in a drier climate (e.g. Artemisia pollen) after the cessation of aeolian loess deposition. While no correlative paleosol is known at the Sale
Fig. 4: Loess stratigraphy o f northwestern China. A: Holocenesequenceat Luochuan,northwesternChina B: Holocenesequenceat Longxi,northwesternChina
terrace section, the upper paleosol at Longxi (ZHANG & HU 1989), dated at 73604-250 yr BP appears to be equivalent to the older paleosol in the Sale terrace section. Indeed, one standard deviation of the upper Longxi paleosol would inelude the 7530+160 yr BP date of the lower Sale terrace paleosol. While the mineralogy, soil chemistry and geochemistry are not available for the Longxi paleosol sequence, the presence of Pinus pollen in the upper Longxi paleosol suggests a somewhat wetter climate.
5
Conclusions
The results of this study suggest that detailed analyses of episodic alluvial and aeolian loess deposits and paleosols may yield important information on age of paleosols and paleoclimates that per-
CATENA--An Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
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sisted during the time of their development. At the Sale Terrace Section two paleosols with similar characteristics are shown to have slightly different mineralogic and chemical characteristics after closer laboratory study. The lower paleosol, as indicated by stratigraphic position and a 14C date, is the older, Laboratory analyses confirm it may have formed under a wetter paleoclimate (not supported by pH and total salt content) that may have allowed the formation ofmetahalloysite. Because the genesis of metahalloysite is slow and requires a humid climate, the magnitude of paleoclimatic change required is likely higher than can be substantiated from the available pedological data. Therefore, the metahalloysite is likely preweathered. The tendency for calcite to increase upward in the sequence suggests that climate has become drier over the last few millennia or since the middle Holocene. Neutron activation data reveal overall chemical homogeneity among the loesses and paleosols studied. The paleosols reveal only slight removal of Ca and Na and minor enrichment of Mn (all relative to A1) during paleosol-forming episodes. The overall conclusion that some preweathered sediment (formed in the lower paleosol) was redistributed by aeolian activity into the middle loess follows from the elemental trends,
Acknowledgements We thank the U.N, Development Programme in China for financial support. Field work was carried out with assistance from graduate students at Lanzhou University, People's Rep. of China. Laboratory work was completed in the Geomorphology and Pedology Laboratory, York University, Atkinson College (with
the assistance of David Hinbest) and the Radiocarbon Laboratory of the Geography Department, Lanzhou University. Additional funding from the President's NSERC fund at York Univ. is Gratefully acknowledged. Janet Allin drafted the illustrations.
References BIRKELAND, P.W. (1984): Soils and Geomorphology. Oxford, N.Y., 372 p. BLOOM, A.L. (1978): Geomorphology: A SystematicAnalysis of late Cenozoic Landforms. Prentice Hall, Englewood Cliffs, N.J., 510 p.
BOU¥0UCOS,
G.J. (1962):
Hydrometer
method improved for making particle size analyses. Agron. Jour. 54, 464-465.
BOWER, C.A. & WILCOX, L.V. (1965): Soluble salts. In: C.A. Black (ed.), Methods of Soil Analysis: Part 2. Amer. Soc. Agron. Madison,
Wisc., 933-951. DAY,P. (1965): Particlefractionation and particle size analysis. In: C.A. Black (ed.), Methods
of Soil Analysis. Amer. Soc. Agron,, Madison,
Wisc., 545-567.
FOLK, R.L. (1968): Petrology of Sedimentary Rocks, Hemphill Press, Austin, Texas, 170 p.
HANCOCK,R.G.V. (1~4): On the source of clay used for Cologne Roman pottery.
chaeometry26, 210-217.
Ar-
LIU, D. et al. (1985): Loess and Environment,
SciencePress, p. 92. MAHANEY, W.C. (1978):
Late Quaternary stratigraphy and soils in the Wind River Mounrains, western Wyoming. In: W.C. Mahaney (ed.), Quaternary Soils. Geoabstracts, Norwich, U.K., 223-264.
MAHANEY, W.C. (1981): Paleoelimate reconstructed from paleosots: evidence from the Rocky Mountains and East Africa. In: W.C. Mahaney (ed.), Geoabstracts. Norwich, U.K.
227-248. MAHANEY, W.C. & HALVORSON, D,L. (1986):Rates of mineral weathering in R.ocky Mountainglacial sequences: evidence from western Wyoming. In: S.M, Colman (ed.), Rates of Chemical Weathering of Rocks and Minerals.
AcademicPress, N.Y., 147-167.
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MAHANEY, W.C. & FAHEY, B.D. (1980): Morphology, composition, and age of a buried paleosol on Niwot Ridge, Front Range, Colorado, USA. Geoderma 25, 209-218. MAHANEY, W.C. & ZHANG, L. (1990): Removal of local alpine vegetation ( Hamamelis mollis) and overgrazing in the DaLiJia Shan, Northwestern China, Mountain Research and Development, in press. OYAMA, M. & TAKEHARA, H. (1970): Standard Soil Color Charts, Japan Research Council for Agriculture, Forestry and Fisheries. SOIL SURVEY STAFF ('1951): Soil Survey Manual. Washington U.S. Gov't. Printing Office, 503 p. SOIL SURVEY STAFF (1975): Soil Taxonomy. Agriculture Handbook 436, Washington, U.S,D.A., 754 p. WEN, Q., ZHENG, H., HAN, J., WANG, J., SHAOMONG, L., QIAO, Y., WEI, L. & DIAO, G. (1982): Loess of the Longxi Basin in Gansu Province. Scientia Geographica Sinica 2(3), 202-209. WHI'I"rlG, L,D. (1965): X-ray diffraction techniques for mineral identification and mineralogical composition. In: C.A. Black (ed.), Methods of Soil Analyses. Amer. Soc. Agronomy, Madison, Wise., 671--696. ZHANG, L. (1986): Paleoenvironment factors and development process of Saleshan Landslide area. Landslide Symposium No. 6, Railroad Press (In Chinese). ZHANG, L. (1989): The landslide history and late Cenozoic environmental factors in Saleshan area, Dongxiang County, Gansu. Internat. Field Workshop on Loess geomorphological Process and Hazards, May 23-June 5, 1989. Journal of Lanzhou Univ. ZHANG, L. & SHIWEI, Z. (19113): The landslide of Saleshan Ridge in the County of Dongxiang, Gansu Province, China. Nature Journal 6(12), 918-923. ZHANG, W. & HU, S. (1989): On the age and processes of formation of Heilusol (Cambisol) in the Loess Plateau Area in Eastern Gansu Province, Kexiu Tongbao (SCIENCE BULLETIN), No. 16, 1251-55. ZHANG, L. & ZHANG, D. (1989): A comparative study of paleoenviromnental factors between both catastrophic Saleshan and Chana landslides. Proc. Japan-China Symposium on Landslides and Debris Flows, 1989. Niigata, Tokyo, Japan.
Addresses of authors: William C. Mahaney Department of Geography Geomorphology and Pedology Laboratory Atkinson College York University 4700 Keele Street North York, Ontario Canada M3J 1P3 R.G.V. Hancock SLOWPOKE Reactor Facility and Department of Chemical Engineering and Applied Chemistry University of Toronto Toronto, Ontario Canada M5S 1A4 Linyuan Zlmng Department of Geography Lanzhou University Lanzhou, Peoples Republic of China
CATENA--An Interdisciplinary Journal of SOIL S C I E N C E ~ H Y D R O L O G Y ~ E O M O R P H O L O O Y
vol. 17, p. 357-367
Cremlingen 1990 [
STRATIGRAPHY A N D PALEOSOLS IN THE SALE TERRACE LOESS SECTION, NORTHWESTERN CHINA W.C. Mahaney, North York, ILG.V. Hancock, Toronto, Linyuan Zhang, Lanzhou Summary Two paleosols in the Sale terrace loess sequence were investigated to determine if radiocarbon ages for each paleosol could assist in reconstructing the Holocene loess succession as well as the paleoenvironmental record. The upper loess, about 3 m in thickness, carries no surface soil and is still being emplaced. The middie loess was emplaced during the middle Holocene, whereas the lower loess was emplaced prior to 7500 yr BP. Relative weathering in the two buried paleosols is approximately the same with minor differences in clay mineral and elemental composition indicating a slightly wetter paleoclimate in the lower unit becoming drier upwards toward the present. Because there is evidence for considerable chemical homogeneity downwards in the section, with some minor leaching of the mobile elements Ca and Na, there is every reason to believe the 14C dates are uncontaminated and represent true ages.
ISSN 0341-8162 (~)1990 by CATENA VERLAG, D-3302 Cremlingen-Destedt, W. Germany 0341-8162/90/5011851/US$ 2.00 + 0.25
1
Introduction
Two paleosols and three loess units (aeolian above 1.5 m; alluvial loess below) were sampled at the Sale terrace section (fig. 1) in northwestern China. In this section three loess units are separated by two paleosols, which represent significant Holocene disconformities, 2500 years apart. This section was exposed following a 1983 landslide which killed 200 people after the sliding mass fell 300 m to the Baxie River in less than 2 minutes (ZHANG & SHIWEI 1983, ZHANG & ZHANG 1989). We sampied the three loesses and two paleosols to determine if paleosol characteristics could be used to estimate paleoenvironmental changes during the Holocene.
2
Methods
Samples were described in the field using the nomenclature of the SOIL SURVEY STAFF (1951, 1975) and BIRKELAND (1984). Colors are given in the moist and dry states following OYAMA & TAKEHARA (1970). Samples were fractionated for particle size analysis using the methods of DAY (1965) and BOUYO U C O S (1962). Particle size divisions
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Fig. 1: Location of Sale terrace loess section, Northwestern China. follow the Wentworth Scale of FOLK (1968) for sand-silt breaks (63 #m) and the SOIL SURVEY STAFF (1975) for silt-clay breaks (2 #m). Parent material designations follow BLOOM (1978). The pH was measured by electrode and total salts were determined by electrical conductivity (BOWER & WILCOX 1965). Mineralogical analyses were completed on the <2 #m fractions by X-ray diffraction using CuK 0t radiation generated by a Toshiba ADG-301H X-ray diffractometer. Samples were oriented on ceramic tiles by centrifugation and minerals were identified following procedures outlined by WHITTIG (1965) and MAHANEY (1981). The <2 mm fractions (0-2000 #m) of loessic sediments and paleosols were subsampled for neutron activation analysis at the SLOWPOKE Reactor Facility in
the Univesity of Toronto. Seven dried samples were taken for chemical analysis. They were chosen to range in mass from 700 to 850 mg in an attempt 1;o guarantee representative samples. T~J~ samples were analyzed in Olympic tic flip-top polyvials. For elements such as U, Dy, Ba, Ti, Mg, Na, V, A1, Mn, Ca and CI, which produce short-lived radioisotopes, the samples were irradiated serially for 1 minute at a neutron flux of 1.0xl011 n cm-2. s-1 in the SLOWPOKE nuclear reactor at the University of Toronto. After a waiting time of 19 minutes to allow the very short-lived 28A1 to decay to acceptable levels, each sample was assayed using 5 minute counts with on-site gammaray spectrometers, as described by HANCOCK (1984). Appropriate gamma-ray peak areas were measured and the clu~-
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Stratigraphy and Paleosolsin Loess, N-W China
359
SALE TERRACE LOESS SECTION 0
5000+ llO yr a 33 ~ MIDDLE
LOESS
:':':.i'::..:.".:.' ". : ::::..' ..":i;.'. . ..?; : : . ...'.;":.' . . . . . ..'.."'i" . ~" .!.'):.:. ....... ".".." "...: ....:.........:.......-.-.: •. . . . " . . . " , ".'.:. • "..-.......... 4.4 •
m m~
4,6
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LOWERLOESS
753o+_oyr
~
[]
SAMPLE
[]TN AEOLIAN LOESS ALLUVIAL LOESS PALEOSOL
a 14C dotes determined from organic corbon in pdleosoIs. Age tlletormir~tia~ mode at Radiocarbon Laboratory, Lonzhou University, t?1~ of Chino
Fig. 2:
Stratigraphyat Sale terraceloesssection,NorthwesternChina.
ical concentrations calculated using the comparator method. The samples were serially recounted after 20-24 hours to measure the K, Eu (and remeasure the Na) concentrations,
construction. Climatic parameters for the Sale Shan area are not known with precision. However, the climate in the Lanzhou area is dry with mean annual precipitation (MAP) of 545 mm. Evapotranspiration is greater than MAP (ZHANG 1989). 3 Field a r e a As in other areas of the Loess Plateau, attempts are being made to introduce The Sale Loess Section (fig. 2) located in stands of poplar (Populus spp.) trees to the alluvial beds of the second terrace in assist in reducing mass wastage. Howthe Baxie River Valley (ZHANG 1986, ever, in the area around Sale Moun1989) is representative of Holocene al- tain only a few trees have been planted luvial loess sections on the westernmost to date; the lack of farmland and the part of the Loess Plateau of northwest- growth of population may make it imern China. Buried soils in the section possible to reforest the area. provide the opportunity for radiocarbon dating and for paleoenvironmental reCATENA--An Interdisciplinary Journal of SOIL 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 G Y
Qz
CROSS-VALLEY
1"2
PROFILE
- -- - .....
___ _-_.. . . . . . . . .
T~
- .
of
the
...........
T3
.-__ _ - _ _ - - - - - ~ _ _ - -. -
........ .---_-Z-" . . . . . . . . --------" "-------"" ~ .......... -
@
s
S
I
.
BAXIE
.
_.. ~
I T2 J
.
RIVER
.
.
.
.
~I - - - - - - - - - - - - - - -
LANDSLIDE
--.--------
Fig, 3: Cross-valley profile .from the Saie Shah ridge to the Baxie River.
N2 (Red beds; Linxia Zu Fro)
-----;----. ------_ _
~II~~---__:_--Z~_"1"4~ - - - - - - - - - ~ Z - Z - - - - - - ' - - _ - - ~ -
AREA
LOESS Jincluding paleosols
in S A L E S H A N
.
loess ALLUVIAL gravel
2
loess ridge
(modiFied from Z~,'~J, 1983.)
~,~ ~" - ~ Z - _ - - _ . ~---.~-----~.-- _- - " ~ ~ - - Z - C-----. . . .
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LOEssSLOPEWASH-ALLUVI Q ( 2Ql)3 * Q¢) ALLoESS
.
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Stratigraphy and Paleosols in Loess, N-W China
4 4.1
Results and discussion Stratigra0hy and paleosols
The loess stratigraphy and paleosols are shown in figs. 2 and 3. The lower alluvial unit appears to have a higher bulk density than the middle and upper units (hence the different degrees of shading in fig. 2), but otherwise they all have similar bulk densities (1.0-1.2 g/cm-3), All alluvial loess belongs to the T2 surface (fig. 3) of Holocene age that overlies Linzia Zu Formation of Pliocene age (ZHANG 1989). Color is a nearly uniform pale yellow (approximately 2.5Y 6/4 to 7/4 on a dry basis) across each alluvial and aeolian unit. There is littie textural variation between the loesses and no mottling was observed in the field. Radiocarbon dates in the lower paleosol indicate it was buried approximately 7530___160 yr BE The upper paleosol yielded a 14C date of 5000+110 yr BP. The ages of these two paleosols are based on dates obtained from the organic carbon fraction of the paleosols. The depth relationships show that all loess units are rather thin to the order of 1.0 to 3.0 m in thickness (fig. 2). The paleosols have colors that range from brownish gray (upper) (10YR 4/1) to black (lower) (10YR 2/1) under field conditions. After drying in the laboratory these colors lose only one value and one or two chroma becoming brownish black and grayish yellow brown. The lower paleosol has the darkest colors suggesting a higher organic carbon content, which was confirmed by weight loss on ignition (estimated at ,,~4.0 percent). Because these paleosols are covered with fresh unconsolidated alluvial and aeolian loess beds that lack root systems, we believe they are not contaminated with
361
younger organic carbon. Hence, the 14C dates are likely reliable giving true times of burial. 4.2
Particle size
Particle size ratios (tab. l) were analyzed to determine if differences exist among the three parent materials and the two paleosols. In all, sand is low but remarkably uniform across the sequence. Silt distributions vary somewhat, and with the exception of the lower unit, tend to increase upwards in the sequence. However, there is little overall appreciable difference in silt content between loess beds in the sequence with the exception that the lower and upper units contain 10 to 15% higher amounts than the middle unit and the paleosols. With respect to percnt clay, the lower paleosol has the highest amount relative to the underlying parent material, indicating that it may have formed in sito under a slightly wetter paleoclimate than today or over a longer time interval. The upper paleosol actually contained less clay than that found in its parent material. While it is highly possible that some clay was translocated into the underlying parent material, it is more likely that insufficient time had elapsed for its genesis. Because the middle unit is so thin, it is likely that some of its clay content is inherited from erosion of the lower paleosol during the alluvial event that emplaced it. In general, the particle size data tend to support the stratigraphy observed in the field and the available radiocarbon dates. 4.3
Mineralogy
The mineralogy (tab. 2) of the Sale terrace section was analyzed to determine if
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Unit
Sand (%)
Silt (%)
Clay (%)
(2 m m - 6 3 / ~ m )
(63-2 #m)
( < 2#m)
2.6 1.8 1.4 0.8 1.6 t.9 1.1
79.1 75.9 71.4 64.1 64.4 69.3 79.2
18.3 22.3 27.2 35.1 34.0 28.8 19.7
Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
Tab. 1 : Particle size ratios for paleosols and
Unit Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol Lower Paleosol (a) L o w e r Loess
Tab. 2:
parent materials in Sale Terrace Section.
Depth (m)
K
H
I
I-S
V
Chl
Q
O
P
CALC
k0 2.0 3.1 3.8 4.5 4.5 5.0
XX tr X X X X X
-tr tr X X X
XX X XX X XX X XX
X tr tr tr tr tr tr
tr
X tr X tr tr tr tr
XX XX XX X XX XX XX
XX -XX XX XX XX XX
tr
-------
X tr X --
tr
X
tr X
tr
tr
tr
--....
tr
Mineralogy of the Sale Loess Sequence, Northwestern China.
Mineral abundance is based on peak height: nil(--), minor amount (tr), small amount (X), moderate amount (XX), abundant. (XXX). Clay minerals are kaolinite (K), halloysite/metahalloysite (H), illite (I), illite-smectite (I-S), vermiculite (V), chlorite (Chl).
Primary minerals are quartz (Q), orthoclase (O), plagioclase (P) and calcite (Calc).
differences could be detected between the loess units or among the paleosols and parent matrials. Overall, the trends are for an upward increase in kaolinite, chlorite, and illite-smectite among the clay minerals studied. Plagioclase feldspars increase slightly upward as does calcite, which may reflect the stratigraphy in the source area of the loesses, With respect to weathering trends (paleosols vs. parent materials) and aeolian input in the paleosols, it is important to note that metahalloysite increases in the lower paleosol relative to the underlying parent material. This trend may result from increased moisture under wetter paleoclimatic conditions or from weather-
ing over a longer time period as in other dry areas (MAHANEY 1978). However, higher amounts of metahalloysite in the middle loess unit probably are related to the incorporation of preweathered sediments following aeolian transport and suggest that metahalloysite may not have formed in situ. Higher illite in the lower and upper paleosols (fig. 2) may result from aeolian input, which is known to occur in other areas (MAH A N E Y & FAHEY 1980, M A H A N E Y & HALVORSON 1986). The degree to which expandable clay minerals form in the environment is not known with precision, but the presence of illite-smectite in buried paleosols sug-
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gests it may have been an important weathering product in middle Holocene paleoenvironments. The origin of vermiculites in the upper unit is obscure, but appears to be the result of airfall influx rather than of weathering in situ. Chlorites in the sequence are diochtahedral (Fe chlorites), as indicated by the 001 peaks on X-ray diffractograms (MAHANEY 1978). Because chlorite increases upwards in the sequence, it likely reflects the source area of loess. The upper paleosol was likely exposed for too short a time, under too dry a climate, to chloritize iUite, and illite does not decrease with increasing chlorite. Increasing amounts of calcite upwards in the younger units may signal drier paleoclimatic conditions, perhaps resulting from the removal of forests by local inhabitants (see MAHANEY & ZHANG 1990).
363
ied. The trend shown on tab. 3 indicates a depletion of salts downward in the sequence with somewhat lower values in the paleosols. This might suggest removal of minor amounts of salt during the time each paleosol was exposed to subaerial weathering. The distributions of the 13 elements shown in tab. 4 show an overall similarity among the three loesses and two paleosols. The only major break occurs between the lower paleosol and underlying loess where CI increases downwards and this is probably an environmental rather than a geochemical effect. Slight decreases in Ca in the two paleosols and middle loess may be the result of minor leaching effects. Ratios of Na/A1, Ca/AI and Mn/A1 were analyzed (tab. 5) to determine if they provide information on relative rates of removal of mobile elements relative to immobile ones. Sodium is only 4.4 Chemistry slightly responsive showing differences among the two paleosols relative to the The dry colors (tab. 3) of the indi- loesses. The ratio Ca/AI gives somewhat vidual units range from dull yellowish lower quotients in the paleosols, indicatbrown, grayish yellow brown to dull ing a slight tendency towards Ca depleyellow orange. In general, the pale- tion during the time of paleosol formaosols are slightly darker (10YR 4/3; tion. The relative movement of Mn in5/2) compared with the parent materi- dicates slightly enriched Mn in the lower als. Buried paleosols are actually fos- and upper paleosol as well as in the midsil ochric epipedons (SOIL SURVEY die loess. These data indicate also that STAFF 1975). The parent material col- the middle loess contains some preweathors indicate a considerable proportion ered material from the lower paleosol, of preweathered sediments were incor- and agrees with the mineralogical analyporated in the deposits following eo- sis. lian/alluvial transport. The pH (tab. 3) was measured to de- 4.5 Correlation termine if differences existed between the paleosols and parent materials. Even The nearest sites to the Sale terrace though the paleosols are slightly less al- section with comparable Holocene sekaline, the differences (~0.5 pH unit) are quences are at Longxi, about 125 km to relatively minor. Total salts also show the southeast, and at Luochuan 300 km few differences among the samples stud- east (fig. 1). CATENA--An Interdisciplinary Journal of SOIL S C 1 E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
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u nit
Color
Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
10YR 10YR 10YR 10YR 10YR 10YR 10YR
6/3 6/3 5//2, 5/'3 4/"3 4/3 4/3, 5/3 6/4
(1:5)
(mmhos/25°C) (1:5)
9.0 9.0 8.6 8.5 8.7 8.8 9.0
.54 .47 .13 .19 .14 .10 .11
Tab. 3: Dry color, pH and electrical conductivities of parent materials and paleosols
in the Sale Loess Section. Na (%)
Mg (%]
A1 (%)
CI
K (%)
Ca (%)
Ti
V
Mn
Ba
Eu
Dy
U
Sample Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
1.28 1.30 1.19 1.17 1.14 1.22 1.28
0.9 1.0 0.9 1.2 1.2 1.0 1.2
6.0 6.1 6.2 6.4 6.6 6.5 6.1
<200 <230 _<220 _<180 _<200 _<190 600
1.9 1.9 2.1 2.0 1.9 2.1 1.8
5.2 5.9 4.0 4.2 4.1 3.5 5.4
3130 3250 3670 3350 4070 3400 3270
68 70 72 86 75 75 66
567 583 689 743 722 739 613
500 480 670 600 490 510 450
0,78 0,88 0,88 0.90 0.86 029 0.86
2.4 2.6 3.1 3.t 3.3 3.7 3.3
2.1 3.6 2.9 2.9 2.9 3.6 3.3
Tab. 4: Elemental chemistry* of the Sale Terrace Section, northwestern China. * Elemental concentrations in ppm unless otherwise indicated.
Sample Upper Loess (b) Upper Loess (a) Upper Paleosol Middle Loess Lower Paleosol (b) Lower Paleosol (a) Lower Loess
Na/A1
Ca/A1
Mn/A1
Caxl00 Ti
Naxl000 Ti
0.21 0.21 0.19 0.18 0.17 0.19 0.21
0.87 0.97 0.65 0.66 0.62 0.54 0.89
95 96 111 116 109 113 100
0.17 0.18 0.11 0.13 0.10 0.10 0.17
0.41 0.40 0.32 0.35 0.28 0.36 0.39
Tab. 5: Elemental ratios .from the Sale Terrace Section, northwestern China.
At Luochuan, Holocene paleosols (fig. 4A) w i t h d e p t h s o f 0.5 to 1.5 m in a e o l i a n loess h a v e b e e n s t u d i e d for p o l l e n a n d fossil snail c o n t e n t ( L I U et al. 1985). T h e t h r e e p a l e o s o l s s h o w a pollen succession downward that trends f r o m s u b h u m i d (Gramineae) a n d semiarid (Artemisia) to h u m i d (Betula, Pi-
(A1ENA
nus) e n v i r o n m e n t s . T h e fossil snail d a t a p r o v i d e s o m e w h a t less r e l i a b l e p a l e o c l i m a t i c i n f o r m a t i o n ( L I U et al. 1985). H o w e v e r , Metodontia beresowskii indicates a w a r m e r a n d w e t t e r c l i m a t e t h a n p r e s e n t a n d Cathaica pulveraticula indicates d r i e r a n d c o l d e r c o n d i t i o n s . Bec a u s e o n l y t h e l o w e r p a l e o s o l at Lu-
An Interdisciplinary Journal of SOIL SCIENCE- HYDROLOGY
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365
Stratigraphy and Paleosols in Loess, N - W China
A.
POLLEN
FOSSIL SNAILS
0O.5-
~,
I o-
,,:~-:,,!:.!.,"..~:.::,-:::... Grom/neoe • ; :?..~.......
;':'~:;;~:~:'<":":
.
'
2o- ~
( Moel lendorf f )
Artem/s/a sp.
:.~-,~.~-~, ~,~ Betula sp LS-
Metodontiaberesowski/ Metodonha hausa/ensis
(Crosse)
Cathoicapu/vera/?cula
P/nus sp.
(Martens)
LOESS !:i:i
B.
14C
Alluvial loess Aeolian loess
POLLEN
,..,.........-..;.......-...,:.v....,
::::::::::::::::::::::::::::::::::::::::: .:.:.:-:.:.:.:.:,:.:.:.:.:.:.:.:.:.:,:.:.:. :::::::::::::::::::::::::::::::::::::::::
t- ....,.......,.-:,.,-.v.......,.....v .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:. Befula sp Carpinu$ sp. :':'.,:'. .:.i:.i:::'!.:::;': :,'i::?..i::.:i:i::;:,:i ~ ~ 7~,o*_a~o Pinussp. z- !":??"-'.::-.~:'.?':.??i:i:"i:i:! Artemisia sp. Polygonoceae asso*_3oo Chenopodioceoe 3-
~
ochuan is dated (8100-t-220 yr BP), it is possible to tentatively correlate it with the oldest paleosol at Longxi which dates from 85504-300 yr BP. If the two upper paleosols at Luochuan are coeval with the two paleosols from the Sale terrace section, the paleoclimatic trends appear to be out of phase. Clearly additional investigations are required to adequately reconstruct paleoclimate in this part of the Loess Plateau. At Longxi, the Holocene sequence (LIU et al. 1985) of paleosols (fig. 4B) formed in aeolian (lower) and alluvial (upper) loesses give somewhat different ages (WEN et al. 1982). The lower paleosol, ]4C dated at 85504-300 yr BP, formed in a drier climate (e.g. Artemisia pollen) after the cessation of aeolian loess deposition. While no correlative paleosol is known at the Sale
Fig. 4: Loess stratigraphy o f northwestern China. A: Holocenesequenceat Luochuan,northwesternChina B: Holocenesequenceat Longxi,northwesternChina
terrace section, the upper paleosol at Longxi (ZHANG & HU 1989), dated at 73604-250 yr BP appears to be equivalent to the older paleosol in the Sale terrace section. Indeed, one standard deviation of the upper Longxi paleosol would inelude the 7530+160 yr BP date of the lower Sale terrace paleosol. While the mineralogy, soil chemistry and geochemistry are not available for the Longxi paleosol sequence, the presence of Pinus pollen in the upper Longxi paleosol suggests a somewhat wetter climate.
5
Conclusions
The results of this study suggest that detailed analyses of episodic alluvial and aeolian loess deposits and paleosols may yield important information on age of paleosols and paleoclimates that per-
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sisted during the time of their development. At the Sale Terrace Section two paleosols with similar characteristics are shown to have slightly different mineralogic and chemical characteristics after closer laboratory study. The lower paleosol, as indicated by stratigraphic position and a 14C date, is the older, Laboratory analyses confirm it may have formed under a wetter paleoclimate (not supported by pH and total salt content) that may have allowed the formation ofmetahalloysite. Because the genesis of metahalloysite is slow and requires a humid climate, the magnitude of paleoclimatic change required is likely higher than can be substantiated from the available pedological data. Therefore, the metahalloysite is likely preweathered. The tendency for calcite to increase upward in the sequence suggests that climate has become drier over the last few millennia or since the middle Holocene. Neutron activation data reveal overall chemical homogeneity among the loesses and paleosols studied. The paleosols reveal only slight removal of Ca and Na and minor enrichment of Mn (all relative to A1) during paleosol-forming episodes. The overall conclusion that some preweathered sediment (formed in the lower paleosol) was redistributed by aeolian activity into the middle loess follows from the elemental trends,
Acknowledgements We thank the U.N, Development Programme in China for financial support. Field work was carried out with assistance from graduate students at Lanzhou University, People's Rep. of China. Laboratory work was completed in the Geomorphology and Pedology Laboratory, York University, Atkinson College (with
the assistance of David Hinbest) and the Radiocarbon Laboratory of the Geography Department, Lanzhou University. Additional funding from the President's NSERC fund at York Univ. is Gratefully acknowledged. Janet Allin drafted the illustrations.
References BIRKELAND, P.W. (1984): Soils and Geomorphology. Oxford, N.Y., 372 p. BLOOM, A.L. (1978): Geomorphology: A SystematicAnalysis of late Cenozoic Landforms. Prentice Hall, Englewood Cliffs, N.J., 510 p.
BOU¥0UCOS,
G.J. (1962):
Hydrometer
method improved for making particle size analyses. Agron. Jour. 54, 464-465.
BOWER, C.A. & WILCOX, L.V. (1965): Soluble salts. In: C.A. Black (ed.), Methods of Soil Analysis: Part 2. Amer. Soc. Agron. Madison,
Wisc., 933-951. DAY,P. (1965): Particlefractionation and particle size analysis. In: C.A. Black (ed.), Methods
of Soil Analysis. Amer. Soc. Agron,, Madison,
Wisc., 545-567.
FOLK, R.L. (1968): Petrology of Sedimentary Rocks, Hemphill Press, Austin, Texas, 170 p.
HANCOCK,R.G.V. (1~4): On the source of clay used for Cologne Roman pottery.
chaeometry26, 210-217.
Ar-
LIU, D. et al. (1985): Loess and Environment,
SciencePress, p. 92. MAHANEY, W.C. (1978):
Late Quaternary stratigraphy and soils in the Wind River Mounrains, western Wyoming. In: W.C. Mahaney (ed.), Quaternary Soils. Geoabstracts, Norwich, U.K., 223-264.
MAHANEY, W.C. (1981): Paleoelimate reconstructed from paleosots: evidence from the Rocky Mountains and East Africa. In: W.C. Mahaney (ed.), Geoabstracts. Norwich, U.K.
227-248. MAHANEY, W.C. & HALVORSON, D,L. (1986):Rates of mineral weathering in R.ocky Mountainglacial sequences: evidence from western Wyoming. In: S.M, Colman (ed.), Rates of Chemical Weathering of Rocks and Minerals.
AcademicPress, N.Y., 147-167.
CATENA - A n Interdisciplinary J¢~utnal of SOIL SCIENCE~ HYDROLOGY
GEOMORPHOLOGY
Stratigraphy and Paleosols in Loess, N - W China
367
MAHANEY, W.C. & FAHEY, B.D. (1980): Morphology, composition, and age of a buried paleosol on Niwot Ridge, Front Range, Colorado, USA. Geoderma 25, 209-218. MAHANEY, W.C. & ZHANG, L. (1990): Removal of local alpine vegetation ( Hamamelis mollis) and overgrazing in the DaLiJia Shan, Northwestern China, Mountain Research and Development, in press. OYAMA, M. & TAKEHARA, H. (1970): Standard Soil Color Charts, Japan Research Council for Agriculture, Forestry and Fisheries. SOIL SURVEY STAFF ('1951): Soil Survey Manual. Washington U.S. Gov't. Printing Office, 503 p. SOIL SURVEY STAFF (1975): Soil Taxonomy. Agriculture Handbook 436, Washington, U.S,D.A., 754 p. WEN, Q., ZHENG, H., HAN, J., WANG, J., SHAOMONG, L., QIAO, Y., WEI, L. & DIAO, G. (1982): Loess of the Longxi Basin in Gansu Province. Scientia Geographica Sinica 2(3), 202-209. WHI'I"rlG, L,D. (1965): X-ray diffraction techniques for mineral identification and mineralogical composition. In: C.A. Black (ed.), Methods of Soil Analyses. Amer. Soc. Agronomy, Madison, Wise., 671--696. ZHANG, L. (1986): Paleoenvironment factors and development process of Saleshan Landslide area. Landslide Symposium No. 6, Railroad Press (In Chinese). ZHANG, L. (1989): The landslide history and late Cenozoic environmental factors in Saleshan area, Dongxiang County, Gansu. Internat. Field Workshop on Loess geomorphological Process and Hazards, May 23-June 5, 1989. Journal of Lanzhou Univ. ZHANG, L. & SHIWEI, Z. (19113): The landslide of Saleshan Ridge in the County of Dongxiang, Gansu Province, China. Nature Journal 6(12), 918-923. ZHANG, W. & HU, S. (1989): On the age and processes of formation of Heilusol (Cambisol) in the Loess Plateau Area in Eastern Gansu Province, Kexiu Tongbao (SCIENCE BULLETIN), No. 16, 1251-55. ZHANG, L. & ZHANG, D. (1989): A comparative study of paleoenviromnental factors between both catastrophic Saleshan and Chana landslides. Proc. Japan-China Symposium on Landslides and Debris Flows, 1989. Niigata, Tokyo, Japan.
Addresses of authors: William C. Mahaney Department of Geography Geomorphology and Pedology Laboratory Atkinson College York University 4700 Keele Street North York, Ontario Canada M3J 1P3 R.G.V. Hancock SLOWPOKE Reactor Facility and Department of Chemical Engineering and Applied Chemistry University of Toronto Toronto, Ontario Canada M5S 1A4 Linyuan Zlmng Department of Geography Lanzhou University Lanzhou, Peoples Republic of China
CATENA--An Interdisciplinary Journal of SOIL S C I E N C E ~ H Y D R O L O G Y ~ E O M O R P H O L O O Y