Journal of Southeast Asian Earth Sciences, Vol. 7, No. 4, pp. 215--221, 1992 0743-9547/92 $5.00 + 0.00 Printed in Great Britain (( 1993 Pergamon Press Ltd

Stratigraphy and geochemistry of the Jiuzhoutai loess succession, northwestern China

WILLIAM C. MAHANEY,* R. G . V. HANCOCK t a n d L. ZHANG~

*Geomorphology and Pedology Laboratory, Atkinson College, York University, 4700 Keele Street, North York, Canada, M3J 1P3; tSLOWPOKE Reactor Facility and Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada, M5S 1A4; :~Department of Geography, Lanzhou University,

Lanzhou, People's Republic of China

(Received 14 April 1992; accepted for publication 25 September 1992)

Abstract--Unweathered sediments and paleosols in the Jiuzhoutai loess sequence of northwestern China were studied to determine the degree of parent material uniformity and to attempt to establish, from geochemical and mineralogical changes in the paleosols, the range and variability of paleoclimates that existed during Quaternary soil-forming intervals. The primary chemical variability in this sample suite comes from differences in salt (NaCI) content. Because of these salt differences, the Na content of each sediment and soil sample must be estimated by correcting the measured Na content, using the CI content to establish the Na content associated with NaC1. Small excesses of Ca (probably as CaCO 3) have slightly diluted the concentrations of other elements in the affected paleosols. The close relationship between the composition of the loessic sediments and most of the identified paleosols shows that there may be only a few true weathering zones in the sequence. Indeed, on the basis of low Ca contents, only paleosols S1, $4, SI0, S12, S13, S14, S16 and S19 are identified as moderately weathered pedogenic zones, some of which are Camborthids. Using percent clay increase and geochemical indices as evidence for stronger weathering profiles S1, $2, $3, $4, $5, $6, $9, SI 1, S13, S16 and S18 may also be considered paleosols !e.g. Haplargids). All other identified paleosols do not show any increase in Ca, which might be expected from dry soil-forming intervals which mainly produce Aridisols (e.g. Calciorthids). Parent material uniformity is demonstrated by a number of inter-element ratios, indicating a chemically consistent source for the loessic sediments and paleosols.

INTRODUCTION

THE JIUZHOUTAI ( J Z T ) section (Figs 1 and 2) is one of the few nearly complete loess successions dating to the early Quaternary, beyond the Jaramillo subchron to + 1.4 Myr. It has 22 paleosols in it (Burbank and Li 1985) that are considered to record weathering events which occurred during times when loess deposition ceased, and/or loess deposition was significantly reduced and soil formation took place. These parent materials (loessic sediments) and paleosols in the sequence at Lanzhou (see Fig. 1) were studied to determine if clay mineralogy and geochemical composition (especially ratios of immobile elements) could be used to identify lithic discontinuities. In addition, an attempt was made to determine whether ratios of the concentrations of mobile to immobile elements could be used to recon- struct relative paleoclimatic intensities of soil-forming intervals. Lastly, we attempted to correlate findings at Lanzhou with the documented successions at Lochuan (Heller and Liu 1984) and at Baoji (Rutter et al. 1991).

REGIONAL OVERVIEW

The Jiuzhoutai section (see Fig. 2) is representative of the massive accumulation of Quaternary loess flanking the southern margins of the Tengger Desert in north- western China (Fig. 1). At Lanzhou, the highest fluvial terrace of the Huang He (Yellow River) at an elevation of 1749 m.a.s.1, is covered with ~ 330 m of loess (Fig. 2) which appears to be the thickest accumulation (the

surface of the loess profile is 2067 m.a.s.1.) in the immedi- ate area. The JZT section at Lanzhou is representative of the Chinese loess deposited across the arid and semi-arid regions comprising the Loess Plateau, an east-west trending area between 33" and 47 ° N and 127 ° and 75 ° E. Lanzhou is located on the western edge of this great loess belt which covers an area of 300 000 km 2 along the Huang He (Heller and Liu 1984). The climate in the study area is dry with a mean annual precipitation (MAP) of 357mm and a mean annual temperature (MAT) of 5.9°C.

To the south of the Loess Plateau (Fig. 1), the thickest successions reach over 300 m; thicknesses in the central and southern areas are to the order of 150 m, reflecting increasing distance from the source areas in the deserts to the north and northwest. Owing to less erosion and wetter paleoclimate during the Quaternary, paleosols in loess are better preserved in the southern part of the Loess Plateau. As a result, the Baoji and Xian sections are considered to contain a complete record of the Quaternary deposits and paleosols (Rutter et al. 1991).

Recently, several investigations of the lithostratigra- phy of the loess deposits and morphogenesis of paleosols have been reported (Liu 1985, 1987). These workers subdivided the loess deposits into three subunits called, from oldest to youngest, Wucheng, Lishi and Malan. Wucheng Loess is the oldest group of deposits that overlie the red clay of Pliocene age. To the south, its thickness varies between 40 and 65 m (Rutter et al. 1991) and it is interbedded with paleosols. In the Jiuzhoutai section the thickness is greater (145 m in unit Q1 on Fig. 2). The Lishi Loess, of middle Pleistocene age,

215

216 W.C. MAHANEY et al.

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ranges from 70 to 85 m in thickness in the south to more than 120 m in the west (unit Q2 on Fig. 2). Overlying the Lishi Loess is the Malan Loess of late Pleistocene age. It is rather thin, usually with depths of up to 10 m in the south and + 60 m at the JZT Section (unit Q3 on Fig. 2). Unit Q4 contains the Holocene soil of the surface.

M E T H O D S

The samples were collected in the field as bulk material (200 g) and described according to the nomenclature of the Soil Survey Staff (1975) and Birkeland (1984). Colors follow the system of Oyama and Takehara (1970) and are essentially dry. During the course of the field work it became apparent that paleosol boundaries (Burbank and Li 1985) had been drawn on the basis of slight changes in color that could be caused by diagenesis (rather than pedogenesis). Particle size separations follow the Wentworth scale for silt/sand (63 #m) (Folk 1968) and for clay/silt (2 #m) (Soil Survey Staff 1975). The sands were separated by wet sieving and individual fractions by dry sieving (Day 1965). The clay plus silt slurries were analyzed using a hydrometer (Bouyoucos 1962). The ~<2#m fraction was prepared on oriented mounts and analyzed on a Toshiba ADG-301H X-ray diffractometer using Cu K~t radiation to obtain the mineral composition. For a more detailed discussion of the methods employed, see Mahaney (1990).

The < 2 mm fraction of each sample was analyzed by instrumental neutron activation analysis (INAA). Forty- one sediment and paleosol samples were selected to weigh between 700 and 800 mg to ensure representative results. The analytical samples were stored and analyzed in Olympic Plastic Co flip-top polyvials.

Elements such as U, Dy, Ba, Ti, Mg, Na, V, A1, Mn, Ca and K, which produce short-lived radioisotopes, were determined from samples irradiated serially for 5 min at a neutron flux of 1.0 x 1011 n cm -2 s -I in the SLOWPOKE nuclear reactor at the University of Toronto. After waiting about 19 rain, until the very short lived 2SA1 had decayed to acceptable levels, each sample was assayed using 5 min counts with an on-site gamma- ray spectrometer, as described by Hancock (1984) and Hancock et al. (1988). Appropriate gamma-ray peak areas were measured, and chemical concentrations were calculated using the comparator method.

The samples were later batch irradiated for 16 hr at a neutron flux of 2.5 x l0 II n cm 2 s-l, and after a waiting time of approximately 7 days the concentrations of Sin, U, Yb, La, As, Sb, Br, Sc, Fe and Na were determined, with Na used as a cross-check with the first analysis. After a 2-week delay, the samples were recounted and the concentrations of Nd, Ce, Lu, Ba, Th, Cr, Hf, Sr, Cs, Ni, Tb, Sc, Rb, Fe, Co, Ta and Eu were measured. Once again, Sc and Fe were used to cross-check the second phase of the analysis.

RESULTS AND DISCUSSION

Strat igraphy

The 330 m thick loess sequence at Jiuzhoutai shown in Fig. 2 is considered the oldest in the Lanzhou area (Burbank and Li 1985, Zhang et al. 1990). The paleo- magnetic record shown in Fig. 2 places the Brunhes/ Matuyama boundary 150 m above the base, and separ- ates the Matuyama Chron into three magnetozones: one normal (Jaramillo) and two reversed. The mean mag- netic orientation at each site was statistically evaluated

Jiuzhoutai loess succession, NW China 217

- 1 5 0

LITHOLOGY

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Fig. 2. Lithology, paleosols and the pale•magnetic record from the Lanzhou loess sequence. The pale•magnetic record is based on the data of Burbank and Li (1985). The revised age of the basal loess is based on fission-track dates of ~ 1.5 Myr. Loess-accumulation rates give mean values of 26 cm/kyr over the last million years (L. Zhang, pers. commun. 1990). A number of paleosols are present in the QI (early Pleistocene) and Q2 (middle Pleistocene) deposits. No paleosols are known

in the Q3 (late Pleistocene) beds. One pale•sol is recognized in the Holocene loess (Q4).

and ranked as either class I with Fisher k >~ 10 or class II with Fisher k < 10 (Fisher 1953), and discussed by Burbank and Li (1985).

All the parent materials and paleosols were studied to determine if particle size, clay mineral and geochemical variations would reveal differences that might be related to Quaternary soil-forming intervals.

Across the Loess Plateau of northwestern China, the most documented loess sequences are in Shaanxi Province where section thicknesses range from 80

to 140m. Recent magnetostratigraphical studies at Lochuan (Heller and Liu 1982, 1984) and at Baoji (Rutter et al. 1991), show that the loess sequence is older than 1.4 Myr and probably stretches back to the Matuyama/Gauss boundary (2.5 Myr).

Particle size

The particle size trends for sand, silt and clay in the parent materials (Table 1) were analyzed to determine

218 W.C. MAHANEY et al.

Table 1. Particle size data for parent materials in the Jiuzhoutai section, Lanzhou, northwestern China

Sand (%) Silt (%) Clay (%) Loess unit Age (63-2000pm) (2~3pm) (<2#m)

Q4 Upper Holocene 1.1 83.8 15.l Q4 Middle 1.1 79.7 19.2 Q4 Lower 5.4 81.9 12.7 Q3 Upper 2.3 82.9 14.8 Q3 Middle 1.2 83.7 15.1 Q3 Lower 2.0 82.9 15.1 Q3 Lower 3.8 85.1 11.I Q2 Upper 0.7 70.4 28.9 Q2 Upper 0.7 67.1 32.2 Q1 Upper 0.4 63.9 35.7 Q1 Lower 0.3 80.0 19.7

late Pleistocene

middle Pleistocene

early Pleistocene

the degree of variability throughout the section. Silt ranges from 63.9 to 85.1%, but most samples are approximately 80%. Sand, as expected, is low and generally reaches only 5%. Clay is generally less than 35% and the highest values are found in early to middle Pleistocene beds, possibly as a result of preweathering under the influence of Pleistocene interglacial or pre- Pleistocene warmer/wetter climates. Pleistocene and Holocene beds yield clay amounts of < 19.2%.

The ranges of sand, silt and clay as reported in Table 2 were analyzed to determine similarities and differences with the parent materials and to elicit information on weathering during those intervals when loess deposition ceased and soil formation occurred. As with the parent materials, sand is generally low ( < 2.3%); only one soil reached 4.0%. Silt is on the whole somewhat lower than in the parent materials giving a mean value of 69.6%

Table 2. Particle size data for paleosols in the Jiuzhoutai section, Lanzhou, northwestern China

Sand (%) Silt (%) Clay (%) Paleosol Age (63-2000 #m) (2~53 #m) (<2 #m)

S1a Slb Slc S2a S2b S3a S3b S4a S4b S5a S5b $5c $6 S7a S7b $8 $9 S10 Sll S12 S13 S14a S14b S15 S16a S16b S17 S18 S19 $20 $21 $22

middle Pleistocene

early Pleistocene

1.1 71.1 27.8 0.5 75.6 23.9 0.7 70.4 28.9 2.3 75.6 22.1 0.7 67.1 32.2 2.1 87.7 10.2 0.2 69.2 30.6 0.5 65.1 34.4 0.3 56.0 43.7 0.5 71.9 27.6 1.2 63.8 35.0 1.3 70.2 28.5 0.3 63.6 36.1 0.3 66.1 33.6 0.7 75.4 23.9 0.7 70.1 29.2 0.4 62.2 37.4 1.2 64.9 33.9 0.8 64.2 35.0 0.1 65.2 34.7 4.0 57.8 38.2 0.3 62.9 36.8 0.3 66.7 33.0 0.6 76.4 23.0 0.4 74.2 25.4 0.3 67.5 32.2 1.5 74.5 24.0 0.6 77.8 21.6 0.7 74.2 25.1 0.1 73.6 26.3 0.5 67.3 32.2 0.7 78.8 20.5

with a range of 56.0% to 87.7%. Clay percentages, which should yield the most information about pedoge- nesis show trends within paleosols with multiple hor- izons of slight downward increases in fine material. For example, in profiles S1, $2, $3, $4, $5, and S16, a case could be made for clay genesis in situ and downward movement. Most of the profiles with only one horizon fall within the parent material limits shown in Table 1. Only profiles $6, $9 and S13, have slightly higher clay percentages compared with the parent materials. The particle size data suggest only minor clay formation and/or movement in paleosols of the JZT section, of which a few could qualify as Camborthids (Soil Survey Staff 1975).

Clay mineralogy

The mineralogy of the < 2 pm fraction was analyzed to determine differences between the parent materials and paleosols. Both the paleosols and the parent ma- terials show a remarkable uniformity both with respect to primary and secondary minerals. Among the primary minerals, quartz and orthoclase dominate in moderate amounts, followed by slightly lower amounts of calcite and plagioclase feldspar. Even calcite varies by only _+ 15% and the other primary minerals by less than 10% (based on relative peak heights; Mahaney 1981). Within the clay minerals the ratios of illite to illite-smectite (1:0.8) and kaolinite to chlorite (2:1) are remarkably consistent (variation of about 20%). No vermiculite and smectite were detected within the suite of samples stud- ied. Given the dry climate, the absence of vermiculite is predicted but the absence of smectite is unexpected. On the whole, the mineralogy is somewhat similar to the Sale Shan Terrace loess described by Mahaney et al. (1990), about 40km southwest of the JZT section (Lanzhou). The mineralogy reported for the Sale Shan profile contains less chlorite, illite smectite, and calcite. From the mineralogical data, it appears that the loess originated in a mineralogically homogeneous area, and is derived principally from one source. Also, it is worth noting that there is relatively little change in clay min- erals within any of the Pleistocene paleosols which suggests that soil-forming intervals were not wet enough to alter the clay and primary mineralogy to any great extent, even though clay-size material was redistributed in a few paleosols.

This is contrary to the conclusions reached by Liu et al. (1980) at Lochuan (500 km to the east--see Fig. 1). They reported pronounced color changes as a result of advanced weathering, loss of CaCO3, and increases in clay content. The severe depletion of CaCO3 and enrich- ment of clay minerals that they reported did not occur at Lanzhou. Because the Lochuan loess beds contain cold-loving gastropod fauna, and the paleosols contain a warm-loving assemblage of Metodontia, Chen et al. (1979) argued that paleoenvironmental changes led to the development of braunerde and parabraunerde pale- osols (e.g. Inceptisols and Alfisols; Soil Survey Staff 1975) during interglacial periods. We did not find

Jiuzhoutai loess succession, NW China 219

evidence for such advanced weathering at Lanzhou, which may result from elevation differences between the two sites. Lanzhou is ~ 900 m higher, resulting in a high magnitude pedogenic gradient between the two sites.

At Baoji, 250 km southwest of Lochuan and 400 km southeast of Lanzhou, Rutter et al. (1991) described a pedostratigraphic section consisting of 37 paleosols dated largely on the basis of magnetostratigraphy. The morphology of the paleosols suggests colder and drier climates. As with the Lochuan paleosols, the presence of paleosols with Bt horizons (quite different than at Lanzhou) suggests a forest paleoenvironment.

Geochern&try

The parent materials (eight samples from five loesses) and paleosols (33 samples from 24 paleosols, labeled SO to $23 in Table 3) in the sequence show major fluctu- ations only in their C1 (and Br) contents, with associated minor fluctuations in the Na contents as the CI contents increase, presumably with increasing salt (NaC1) con- tent. The salt content rise in the paleosols is slight, precluding the formation of Salorthids (Soil Survey Staff 1975). The Na content of each loess and paleosol sample was calculated by subtracting the Na content

Table 3. Mean elemental concentrations for the major Quaternary loess units and the paleosol sequence in the Jiuzhoutai section near Lanzhou, northwestern China. Standard deviations are presented at the 95% confidence

level. All concentrations are in ppm unless specified

Loess units Paleosol sequence 8 samples 33 samples

Element from 5 units from 24 units

Natotat °/o 1.41 + 0.30 1.59 + 0.42 Nane t % 1.34 ___ 0.20 1.36 + 0.24 Mg% 1.5 + 0.2 1.5 + 0.3 Al% 6.1 +0.3 6 .2+0.8 K% 1.9+0.3 2.1 +0.5 Ca% 6.4 + 0.6 6.0 + 2.5 C1% 0.11 +0.10 0.35 +0.58 Sc 11.2+0.8 11.6+ 1.6 Ti 3300 ___ 400 3400 ___ 800 V 7 7 + 5 81 _+ 16 Cr 76 ___ 8 79 + 12 Mn 660 + 80 690 + 140 Fe% 3.1 ___ 0.4 3.4 + 0.5 Co I 1.5 + 1.8 12.9 -+_ 2.0 Ni ~< 20 ~< 25 As 14.6 + 2.0 16.7 + 3.2 Br 2.4 ___ 1.2 2.3 + 3.0 Rb 112-t- 15 116+20 Sr 290 + 70 240 + 70 Sb 1.19 + 0.14 1.36 + 0.22 Cs 7.2 ___ 1.2 8.9 _ 2.4 Ba 540 ___ 70 550 ___ 1 I0 La 34.7 ___ 1.5 34.3 _+ 6.2 Ce 62.5 + 6.5 65.3 __. 9.8 Nd 32 ___ 6 30 ___ 8 Sm 5.6 __+ 0.5 5.5 ___ 0.7 Eu 1.05 _ 0.10 1.14 __+ 0.16 Tb 0.82 _+ 0.20 0.85 _+ 0.22 Dy 5.6 -+ 1.2 5.0 __+ 1.8 Yb 2.9 _+ 0.5 2.9 -+_ 0.4 Lu 0.40 ___ 0.04 0.42 _+ 0.06 Hf 6.3 -+_ 1.1 5.8 + 1.0 Ta 1.13 +0.18 1.18 +0.18 Th 12.5+ 1.4 13.0_+2.1 U 3.3 __+ 0.3 3.4 + 0.7

associated with the NaC1 from the total measured Na content (%Na = %Natota 1 - %C1 + 1.54). Since all other elements have relatively uniform concentrations throughout the section, the analytical data are presented in Table 3, as concentration means ___ group standard deviations at the 95% confidence level (+2tr). Because the +__2a precision of the data varies from ___5% to +__ 20% for most elements, in both the loess and paleosol samples, it is clear that this material is geochemically rather homogeneous. The broader distribution of the geochemical data for the paleosols, relative to the loesses, is probably caused mainly by the variation in the Ca content in different paleosols, by the natural addition of more or less CaCO 3 than the average. The Ca contents in the paleosols range from 4.3 to 9.5% and this variation is enough to spread the associated elemental data by up to 10% relative to the loess data.

Pearson correlation coefficient calculations of the total data set showed that even this relatively uniform data could yield interesting chemical correlation information. Sodiumtota I and C1 were highly correlated, indicating the probable addition of NaC1 to the Na in the sediments. Calcium and Sr were weakly correlated, and both were negatively correlated with all of the other elements. This finding indicates that the majority of the Ca is present as CaCOa in the sediments. Of the other elements, Fe, Sc, Co, Cr, Cs, As and the REEs were quite strongly correlated, and this group of elements was also corre- lated with A1, K, Rb, Cs, V, Mn, Sb, U and Th. The remaining elements tended to correlate weakly with one or more of these elements. From the inter-element correlation data, it appears that the major geological constituents of the sediments include loess, NaC1 and CaCO3.

Although there is some variability in the individual rare earth element (REE) concentrations, all samples (loesses and paleosols) produce chondrite normalized REE plots with the same shape. There is an enrichment of the light rare earths, a small negative Eu anomaly and a relatively fiat heavy rare earth profile. Figure 3 shows the minimum, average and maximum ranges for the chondrite normalized REEs analyzed in this study.

Parent material uniformity is of considerable import- ance in assessing the degree of paleosol development in

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220 W . C . MAHANEY et al,

Table 4. Inter-element ratios for the Jiuzhoutai sequence

Unit Loess Na.~t/Ti Nan~t/Sc Ca/Ti Ca/So Mn/Ti Mn/Sc Hf/Sc

Holocene 4.0 0.13 18 0.59 0.20 65 6.6 Malan (upper) 4.0 0.14 17 0.58 0.19 665.8 Malan (lower) 4.2 0.13 21 0.65 0.21 665.8 Q4 (upper) 3.8 0.10 19 0.53 0.20 53 5.0 Q4 (lower) 4.1 0.12 21 0.61 0.20 58 5.9 Q3 (upper) 4.2 0.12 20 0.57 0.20 58 4.4 Q3 (lower) 4.3 0.14 20 0.63 0.19 60 5.9 Q1 3.8 0.11 18 0.53 0.19 57 5.3

Average Paleosol 4 . 2 + 0 . 6 0 .13+0 .03 1 9 _ _ + 3 0 .53+0 .07 0 .20+0 .02 5 9 + 9 5 .6+ 1.2

SO 3.5 0.16 31 1.03 0.20 66 4.9 Sla 4.1 0.14 16 0.53 0.17 57 5.1 Slb 3.9 0.12 13 0.39 0.20 62 5.1 Sic 4.3 0.12 28 0.73 0.18 49 5.0 S2a 4.2 0.13 28 0.74 0.24 64 5.0 S2b 3.9 0.10 17 0.42 0.25 60 4.7 S3a . . . . . . . S3b 4.4 0.12 25 0.63 0.23 59 4.9 S4a 4.0 0.12 12 0.38 0.21 60 4.6 S4b 3.9 0.11 13 0.35 0.22 61 4.5 S5a 4.3 0.12 17 0.47 0.20 57 4.7 S5b 3.6 0.13 14 0.47 0.16 54 4.9 $5c 4.3 0.12 18 0.50 0.22 62 4.6 $6 3.5 0.10 17 0.47 0.19 55 4.4 S7a 4.8 0.12 22 0.57 , 0.22 57 4.6 S7b 4.0 0.13 17 0.53 0.19 59 5.0 $8 4.2 0.13 19 0.62 0.21 67 4.7 $9 3.4 0.11 16 0.52 0.19 63 5.1 S10 3.9 0.11 13 0.36 0.20 56 4.6 S11 4.0 0.13 18 0.56 0.16 51 4.7 S12 3.7 0.11 12 0.35 0.22 65 4.9 S13 4.0 0.11 12 0.34 0.23 66 5.0 S14a 3.9 0.11 13 0.36 0.20 56 4.7 S14b 4.0 0.13 13 0.56 0.16 51 5.1 S15 4.6 0.13 20 0.55 0.20 55 5.9 Sl6a 3.4 0.12 10 0.34 0.18 61 5.0 S16b 5.1 0.12 16 0.44 0.20 56 5.3 S 17 4.0 0.11 16 0.44 0.20 57 4.6 S18 3.3 0.I 1 17 0.57 0.16 54 4.9 S19 4.2 0. I 1 14 0.41 0.24 64 5.0 $20 3.2 0.10 19 0.57 0.22 66 5.0 $21 4.7 0.14 19 0.57 0.23 68 5.7 $22 5.0 0. I 1 36 0.82 0.26 60 5.5 $23 3.6 0.11 16 0.48 0.18 54 6.7

Average 4 . 7 + 1 . 4 0 .12+0 .02 1 8 + 1 1 0 .52+0 .03 0 .20+0 .05 5 9 + 1 0 5 . 0 + 1 . 0

a geological sequence (Birkeland 1984). In most cases, various workers rely on the relative distributions of primary and secondary minerals to demonstrate the absence or presence of lithic discontinuities (Mahaney 1978), but there is always the question of the degree and amount of chemical inhomogeneity (Hancock and Mahaney 1991). To assess parent material uniformity, a number of inter-element ratios of mobile and immobile elements were calculated (see Table 4). The results show relatively uniform ratios in the loessic material.

In other loess sections of northwestern China, the ratio of mobile to immobile elements have been used to determine degrees of leaching in Holocene paleosols (Mahaney et al. 1990). To assess the mobility of Na, Ca and Mn in the paleosols, their ratios against Ti and Sc were calculated, and then compared with those of the parent materials. The ratios displayed in Table 4 are relatively uniform throughout the paleosol sequence, and correlate well with the parent material sequence. The Ca/Ti ratios in the parent materials range from 17 to 21, while in the paleosols they range from 12 to 31. The Ca/Sc ratios in the

parent materials range from 0.53 to 0.65, while in the paleosols they range from 0.34 to 1.03. The samples with high Ca/Ti and Ca/Sc ratios correspond to those containing extra CaCO 3 . On the other hand, if one assumes that lower ratios signal some movement in the paleosols, then profiles S1, $4, S10, S12, S13, S14, S16 and S19 all appear to have lost some Ca during pedogenesis.

The parent materials (Table 4) exhibit a range of 0.19-O.21 for Mn/Ti and a range of 53~6 for Mn/Sc. Assuming that more Mn is removed during pedogenesis (in a wetter climate), only paleosols S1, $5, S11, S14, and S18 show trivial evidence for Mn depletion. Since the Mn "depletions" are of the order of only 10% below the "minimum", this tends to rule against a strong pedogenetic effect.

CONCLUSIONS

Loessic parent materials, and paleosols formed in loesses, in the Jiuzhoutai sequence appear to be relatively

Jiuzhoutai loess succession, NW China 221

uniform, based on mineralogy, on the concentrations of 34 major, minor and trace elements, and on selected inter-element ratios. All of the paleosols correlate closely with the parent material geochemistry.

The mobility of Mn in the paleosols is slight. Only Ca varies enough to allow one to propose possible Ca depletions in paleosols S1, $4, S10, S12, S13, S14, S16 and S19. Because these Ca depletions are small, they may represent CaCO3 loss during slightly wetter paleoclimatic intervals. This still leaves one with a dry climate even during interglacial periods. Manganese depletions, based on Mn/Sc, allow one to add $5, S l l and S18, even though the depletions are slight. When compared with particle size evidence for pedo- genesis, profiles $2, $3 and $5 could be added as true paleosols. Thus, it seems that there are three groups of paleosols present in the Jiuzhoutai section: Hap- largids with argillic (clay-rich) horizons above the calcic horizons; Calciorthids with CaCO3 added as dust; and Camborthids with brownish to reddish cambic horizons.

The remarkable chemical homogeneity throughout the sequence supports the notion of a uniform source for the loessic sediments. Considering the thickness (~ 330 m) of the sequence, this means that the bulk of the sediments was probably derived from a considerable thickness of chemically homogeneous and unconsoli- dated sediments from the deserts to the north.

The overall evidence for chemical homogeneity and lack of mobility of certain elements in these paleosols supports the hypothesis of relative dryness during most soil-forming intervals (interglaciations). In light of this evidence, further study is needed to show that units without clay and/or CaCO3 build up and/or geochemical mobility are indeed paleosols. This includes profiles $6, $7, $8, $9, S15, S17, S19, $21 and $22, which have a similar chemical and mineralogical composition to the loessic sediments. Moreover, differences in the degree of soil chemical weathering and pedogenesis at JZT with the Lochuan and Baoji sections need to be explained in terms of paleoclimatic differences across a pedoclimatic gradient of ~500km, and elevation differences of

900 m, but with large overall effects.

Acknowledgements--The work was carried out with funding from the U.N.D.P. in China (to WCM) and an infrastructure grant from the Natural Sciences and Engineering Research Council of Canada to the SLOWPOKE Reactor Facility at the University of Toronto. Samples were collected in the field with the assistance of Professor L. Zhang, Lanzhou University, People's Republic of China. We are particularly indebted to Mr H. Zhang for many translations and for assistance in the field. D. Hinbest and G. Mahaney assisted with the laboratory

analyses in the Geomorphology and Pedology Laboratory in Atkinson College at York University. J. Allin drafted the illustrations.

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