Comparison of particle size characteristics of the...

Comparison of particle size characteristics of the Tertiary‘red clay’ and Pleistocene loess in the Chinese Loess Plateau:implications for origin and sources of the ‘red clay’

S. L. YANG and Z. L. DINGInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P. R. China(E-mail: [email protected])

ABSTRACT

The origin of the Tertiary ‘red clay’ underlying the Pleistocene loess in the

Chinese Loess Plateau remains controversial, although several lines of

evidence have suggested a wind-blown origin. This study examines the

particle-size parameters of the late Miocene and Pliocene ‘red clay’ by

comparing it with those of the late Pleistocene loess. The particle-size

distribution of a total of 15 339 loess and 6394 ‘red clay’ samples taken from

12 loess sections along a north–south transect and two ‘red clay’ sequences at

Lingtai and Jingchuan was systematically analysed. The median grain size,

skewness and kurtosis of the late Pleistocene loess all show a systematic

southward change and are principally influenced by distance from source

region. The spatial and temporal differentiation of dust deposits is expressed

in a skewness–kurtosis–median grain size ternary diagram, from which the

distance to the source region can be inferred. The particle-size characteristics

of the Tertiary ‘red clay’ sediments are very similar to those of the palaeosols

within the late Pleistocene loess deposits, suggesting an aeolian origin for the

‘red clay’. Based on the comparison of ‘red clay’ and loess in the ternary

diagrams, it is inferred that the source–sink distance was greater in the

Neogene than in the last and penultimate interglacials, and that the dust

source region in north-western China underwent a progressive expansion

during the period from at least 7Æ0 Ma to the present.

Keywords Aeolian deposits, Chinese loess, grain size, kurtosis, red clay,skewness.

INTRODUCTION

The loess deposits of the Chinese Loess Plateauwere derived from the arid and semi-arid regions ofnorth-western China, having been transported bythe East Asia winter monsoon and the westerlies(Liu, 1985). The loess–palaeosol sequences pro-vide a quasi-continuous record of regional andglobal climatic changes during the past 2Æ6 my(Heller&Liu, 1982; Liu, 1985;Kukla, 1987;Kukla&An, 1989; Rutter et al., 1990; Ding et al., 1994). Ingeneral, such complete sequences of loess occur inthe central and southern parts of the Loess Plateau,have a thickness ranging from 130 to 200 m andconsist of over 30 loess–soil couplets (Liu, 1985;

Kukla & An, 1989; Rutter et al., 1991; Ding et al.,1994, 1999a, 2001a). Beneath the oldest loess unit(L33), there is a set of reddish clay–silt depositswith a thickness of about 30–130 m, informallyknown as the ‘red clay’ formation (Liu, 1985).According to Zhu&Ding (1994), this formation hasa geographical distribution similar to that of thePleistocene loess in the Loess Plateau. Palaeomag-netic measurements at different sites on the LoessPlateau indicate that the contact between the ‘redclay’ and the overlying loess is around the Matuy-ama–Gauss reversal boundary (Heller & Liu, 1982;Liu et al., 1988a; Kukla & An, 1989; Rutter et al.,1990; Zheng et al., 1992; Sun et al., 1998a,b; Dinget al., 1998a,b, 2001a).

Sedimentology (2004) 51, 77–93 doi: 10.1046/j.1365-3091.2003.00612.x

� 2004 International Association of Sedimentologists 77

In recent years, the ‘red clay’ sequence hasattracted much attention (Liu et al., 1988b; Zhenget al., 1992; Ding et al., 1998a,b, 1999a, 2001a,b;Sun et al., 1998a,b; Guo et al., 2001, 2002; Luet al., 2001; Qiang et al., 2001). One importantconclusion reached after investigation of the ‘redclay’ is that it may be wind blown in origin; thisopinion is based on anisotropy of magneticsusceptibility (Liu et al., 1988b), field character-istics (Zheng et al., 1992; Sun et al., 1998a,b),grain-size distributions (Ding et al., 1998a; Luet al., 2001) and geochemical concentrations(Ding et al., 1998a, 2001b). Magnetostratigraphicand pedostratigraphic studies of two thick loess–‘red clay’ sequences at Lingtai and Jingchuan(Fig. 1) have suggested that the relatively con-tinuous aeolian record in the Chinese LoessPlateau may well extend back in time from 2Æ6to 7Æ0–7Æ7 Ma (Sun et al., 1998a; Ding et al.,1998b, 2001a). However, debate continues onthe origin of the ‘red clay’ formation (Mo &Derbyshire, 1991; Zhang & Xue, 1996; Guo et al.,2001). Some authors suspected that hydraulicfactors may have played a significant transporta-tional role, especially in the formation of thelower part of this sequence (Guo et al., 2001). Todate, the most important evidence for a wind-blown origin comes from comparison of grain-size data of the ‘red clay’ and the overlying loess(Ding et al., 1998a; Lu et al., 2001), although

several aspects of these data require furtheranalysis.A striking difference between the loess and the

‘red clay’ lies in the fact that the Pleistocene loessconsists of alternating reddish soil and yellowishloess layers, whereas no typical loess horizons areseen within the ‘red clay’ (Ding et al., 1999a). Thealternation of loess and soils documents large-scale oscillations between glacial and interglacialconditions during the Pleistocene (Liu, 1985;Kukla, 1987). In this context, the ‘red clay’ mayhave developed under a generally warm andhumid climate (Ding et al., 1999a). Geologicalrecords (Sun, J.M. et al., 1998) have shown that,during glacial periods, the deserts in northernChina expanded significantly towards the south-east as a result of the growth of continental icesheets and an intensification of the winter mon-soon, with deserts retreating north-westwards assummer monsoonal rainfall increased duringinterglacials. Such dramatic changes in desertextent point to a changeable environment in thedust source regions. If, as has been proposed, the‘red clay’ sequence is truly of aeolian origin, thereis thus a need to investigate the possible extent ofthe dust source region during the late Mioceneand Pliocene.Using grain-size analysis, a north–south tran-

sect of loess deposits for the past two glacial–interglacial cycles and two thick loess–‘red clay’

Fig. 1. Schematic map showing thezonation of loess in the ChineseLoess Plateau: zone I, sandy loess;zone II, loess; and zone III, clayeyloess (adapted from Liu, 1985). Thearrow indicates the dominantdirection of the winter monsoonalwinds, coinciding with the observeddecrease in grain size and thicknessof the loess. Also shown are the MuUs desert (dotted) and mountains(black) around and within thePlateau. The study localities areindicated.

78 S. L. Yang & Z. L. Ding

� 2004 International Association of Sedimentologists, Sedimentology, 51, 77–93

sequences developed in the last 7Æ0–7Æ7 my is firstcharacterized. Then, on the basis of comparisonof the loess with the ‘red clay’, we consider theimplications for changes in the extent of the dustsource region in north-western China since thelate Miocene are considered.

SITE LOCATIONS ANDLITHOSTRATIGRAPHY

The north–south loess transect runs from Hong-de about 88 km south of the Mu Us desertmargin to Yangling in the southernmost part ofthe Loess Plateau, which is about 360 km awayfrom the southern margin of the Mu Us Desert.Twelve loess sections, located at Hongde,Huanxian, Mubo, Quzi, Qingyang, Baimapu,Xifeng, Ningxian, Binxianbei, Binxian, Yongshouand Yangling (Fig. 1), were logged and sampled.At present, the mean annual temperature atHongde is 8Æ0 �C and the annual precipitation400 mm, the values for Yangling being 12Æ9 �Cand 667 mm. Both mean annual temperaturesand rainfall increase southwards along the loesstransect. All loess sections except Quzi consist

of the loess (L)–soil (S) sequence S0, L1, S1, L2and S2. In the Quzi section, only loess unitsabove S1 are exposed. An obvious advantage ofthis loess transect over the previously studiedYulin–Weinan transect (Ding et al., 2000;Rokosh et al., 2002) (Fig. 1) lies in the fact thatthe loess sections along the Hongde–Yanglingtransect are closer together and more evenlyspaced.The Holocene soil S0 is dark in colour because

of its relatively high organic matter content. Theupper part of the Holocene soil has been partlyeroded or disturbed by agricultural activities atsome sites along the transect. The loess units L1and L2 were deposited during the last andpenultimate glacial periods respectively. BothL1 and L2 are yellowish in colour and massivein structure, ranging in thickness from over 20 min the north to several metres in the south (Fig. 2).The L1 loess unit can generally be subdividedinto five subunits, termed L1-1, L1-2, L1-3, L1-4and L1-5 (Fig. 2). L1-2 and L1-4 are weaklydeveloped soils, and the others are typical loesshorizons. Previous studies (Kukla, 1987; Kuklaet al., 1988) have shown that L1-1 is correlatedwith ocean oxygen isotope stage 2, L1-5 with

Fig. 2. Median grain-size (Md) and magnetic susceptibility (SUS) records for the 12 sections along the north–southloess transect. Subdivision of the loess–soil sequences is indicated. The stratigraphic positions for the selectedsamples in Figs 4, 7 and 8 are marked by solid triangles in each section. The axes for the grain-size curves are theobverse of the SUS curves. Note that the depth scale varies from one section to another. The shaded zones indicateinterglacials.

Comparison of particle size characteristics of Chinese ‘red clay’ and loess 79


stage 4, and L1-2, L1-3 and L1-4 together withstage 3. The fivefold subdivision of L1 is clearlyexpressed in the grain-size and magnetic suscep-tibility curves shown in Fig. 2. The L2 loess unitis also composed of three typical loess layers(L2-1, L2-3 and L2-5) and two weakly developedsoils (L2-2 and L2-4), as suggested by the grain-size and magnetic susceptibility curves (Fig. 2).The L2 loess unit is correlative with marineoxygen isotopic stage 6. The soil units S1 and S2developed in the last and penultimate interglacialperiods and correlate with marine oxygen isotopestages 5 and 7 respectively (Kukla, 1987; Kuklaet al., 1988). Both are brownish or reddish incolour and have an A-Bw-C or A-Bt-C horizonsequence. The thickness of S1 at Hongde is 6Æ5 m,and two thin loess layers are present within it. AtYangling, the southernmost site in the transect,the thickness of S1 decreases to 1Æ8 m, and noloess unit is visible. Soil unit S2 is composed oftwo soils (S2-1 and S2-2) and a thin interveningloess horizon. Only the upper soil (S2-1) wassampled in this study.The Lingtai and Jingchuan loess–‘red clay’

sections (Fig. 1) are situated in the central partof the Loess Plateau, with mean annual tempera-ture and precipitation values of 9–10 �C and 550–600 mm respectively. The Lingtai loess–‘red clay’sequence has a thickness of 305 m and consists of175 m of Pleistocene loess and 130 m of Tertiary‘red clay’ (Fig. 3). The Jingchuan loess–‘red clay’sediment, with a thickness of 325 m, is composedof 199 m of loess and 126 m of ‘red clay’ (Fig. 3).In the Pleistocene loess of both the Lingtai andJingchuan sections, each of the S0–L33 loess–soilunits (Rutter et al., 1991) is readily identified inthe field. Within the Tertiary ‘red clay’, over 100reddish soil horizons are distinguishable at bothLingtai and Jingchuan (Ding et al., 1999a, 2001a).Palaeomagnetic studies (Ding et al., 1998b,2001a) suggest a basal age of 7Æ05 Ma for theLingtai section and 7Æ7 Ma for the Jingchuansection (Fig. 3). Pedostratigraphic and magneto-stratigraphic correlation between the two sectionssuggests that sedimentation of the ‘red clay’ wassemi-continuous at both sites (Ding et al., 2001a).For the 12 loess sections along the north–south

transect, a total of 6694 samples were collected at5–10 cm intervals. In the Lingtai and Jingchuansections, sample spacing was 3–5 cm, a total of15 039 samples being taken (Ding et al., 1999a,2001a). Bulk magnetic susceptibility and grainsize were measured for all samples with a Bart-ington MS2 susceptibility meter and a SALD-3001 laser diffraction particle analyser. The

particle analytical procedures were as detailedby Ding et al. (1999b).The susceptibility and grain-size data are

shown in Fig. 2 for the loess transect and inFig. 3 for the Lingtai and Jingchuan sections.Palaeosols are characterized consistently byhigher susceptibility values and finer particlesizes compared with the loess horizons above andbelow them. The two ‘red clay’ sequences havevery fine grain-size distributions, with a mediangrain size centred around 4–8 lm (8–7 F).

SPATIAL CHANGES IN PARTICLE-SIZEPARAMETERS OF LOESS DEPOSITSALONG THE NORTH–SOUTH TRANSECT

The grain-size records from the loess transect(Fig. 2) clearly display an overall southwarddecrease in particle size in the case of both loessand soil horizons. From Hongde southwards toBaimapu, the median grain size decreases rapidlyfrom 50–60 lm (4Æ3–4Æ1 F) to 30–40 lm (5Æ1–4Æ6F) for the typical loess units within L1 andL2, then decreases relatively gradually from25–30 lm (5Æ3–5Æ1 F) at Xifeng to 15–20 lm(6Æ1–5Æ6 F) at Yangling. For the soil units S1 andS2-1, the median grain size shows a more gentlesouthward decrease along the north–south tran-sect, i.e. from 14–16 lm (6Æ2–6Æ0 F) at Hongde to8–9 lm (7–6Æ8 F) at Yangling.Figure 4 illustrates the grain-size distributions

of some representative samples fromhorizons L1-1(loess), L1-4 (weak soil) and S1 (soil) in the 12 loesssections. Almost all the samples show a bimodalcharacter, with a majority falling between 3Æ5 and6Æ5 F (88–11 lm), and a minority between 11 and12 F (0Æ5–0Æ25 lm). For loess unit L1-1, the prin-cipal mode becomes broader and flatter fromHongde to Yangling, whereas it is consistentlybroad and flat in the L1-4 and S1 soils. Also, theprincipal mode gradually shifts to the finer part ofthe curve from north to south. The minority modeinvariably lies between 11 and 12F (0Æ5–0Æ25 lm).At present, the sedimentological significance ofthis population is unclear. A possible reason for itsoccurrence is the peculiar function of the SALD-3001 laser particle analyser itself. Grain-sizeanalyses using this equipment were conducted at0Æ25 F intervals in the range 0Æ25 lm (12 F) to2000 lm ()1F). It appears that particles finer than0Æ25 lm cannot be detected by the granulometerand are automatically added to the componentbetween 11 and 12 F, a process that may explainthe nature of the minority mode in the size



distribution diagrams. Accordingly, specific signi-ficance to the finest portions of the grading curvescan be attributed.All the loess and palaeosol samples from the

transect show a positively skewed distribution

(Fig. 4), a typical characteristic of dust deposits inthe Loess Plateau (Liu, 1966). Kurtosis displaysmore complicated features. The frequency distri-bution curves for samples from the L1-1 loess unitall show a leptokurtic pattern, especially those

Fig. 3. Magnetic susceptibility (SUS) and median grain-size (Md) records of the ‘red clay’–loess sequences at Lingtaiand Jingchuan, together with the magnetic reversal polarities (modified from Ding et al., 1998b, 2001a). Most of thesoil units (Si) and major loess beds (Li) are indicated. The contacts between the loess and the ‘red clay’ at Lingtai andJingchuan are at a depth of about 175 m and 199 m, respectively, as marked by the dashed lines. The axes for thegrain-size curves are the obverse of the SUS curves.



from the northern part of the Loess Plateau. Onthe other hand, the weakly developed soil L1-4and the S1 soil unit are platykurtic.Skewness and kurtosis were calculated for all

samples from the loess transect (Fig. 5). In each ofthe sections, the loess units consistently exhibithigher skewness and kurtosis values than the soilunits. Even within loess units L1 and L2, thetypical loess layers such as L1-1, L1-5, L2-1, L2-3and L2-5 are clearly indicated by the skewnessand kurtosis peaks. Along the loess transect, thevalues of skewness and kurtosis show an overallsouthward decrease for both loess and soil units.Both values decrease more rapidly southwards in

the loess units compared with the palaeosols. Forexample, the skewness and kurtosis values of L1-1decrease rapidly from about 3Æ0 and 15 at Hongdeto only 1Æ0 and 3Æ1 at Yangling respectively. Forthe S1 soil unit, however, the two values decreasegradually from about 1Æ0 and 3Æ0 at Hongde to 0Æ6and 2Æ3 at Yangling.Both the spatial and temporal changes in

median grain size, skewness and kurtosis of theloess deposits closely parallel each other (Fig. 5).To test this relationship further, correlation coef-ficients for the three parameters down the loesstransect were calculated (Fig. 6). Both the skew-ness and kurtosis show a good inverse correlation

Fig. 4. Grain-size distributions of some representative samples from loess unit L1-1, from the weakly developed soilunit at L1-4 and from palaeosol unit S1 of the loess transect. The stratigraphic positions of the selected samples areindicated in Fig. 2.



with the median grain size (phi scale) (R2 ¼ 0Æ932and 0Æ889 respectively) (Fig. 6A and B), indica-ting that the coarser the grain size, the higher theskewness and kurtosis values. There is a strongpositive correlation between skewness andkurtosis (R2 ¼ 0Æ973; Fig. 6C).The incorporation of skewness, kurtosis and

median grain size in a ternary diagram provides a

means of assessing spatial and temporal changesin the particle-size characteristics of these aeolianmaterials. The two end-members of each param-eter must be fixed at 0 and 100 as a preconditionin constructing such a diagram; skewness, kurto-sis and median grain size were normalizedaccordingly. It should be noted that this type ofternary diagram is quite different from those

Fig. 5. Skewness (phi scale), kurtosis (phi scale) and median grain-size (lm) records of the 12 sections along theloess transect. The skewness and kurtosis are calculated using the method of moments (Friedman & Sanders, 1978).The subdivisions of the loess–soil sequences are clearly expressed. Note that the depth scales along the transect arevariable.

Fig. 6. Median grain size (phi scale), skewness (phi scale) and kurtosis (phi scale) coplots for all samples from thenorth–south loess transect.



conventionally used in the study of sedimentaryrocks, a particular advantage of the former type ofdiagram being that the changes in all threeparameters can be displayed.In order to examine variations in these param-

eters along the dust transport pathway, fouradjacent samples were selected from typical loesslayers (L1-1, L1-5, L2-1 and L2-3), weakly devel-oped soil units (L1-4 and L2-4) and palaeosolunits (S1 and S2-1). The basis of this sampleselection was the median grain-size record shownin Fig. 2. For typical loess units, samples wereconsistently selected from units containing thecoarsest grain sizes, whereas samples from soilunits were taken from the finest grain-size units(marked in Fig. 2). The three parameters of thefour adjacent samples in each unit were thenaveraged before plotting on the ternary diagrams.It is assumed that this set of samples wasdeposited at approximately the same time in eachof the loess and soil horizons.Three features are clearly visible in the dia-

gram for each of the selected horizons (Fig. 7).First, spatial changes in the three parameters all

form a narrow arc-like band for each horizon.Secondly, for a specific loess or soil horizon,the samples from the northern part of thetransect are generally located in the lowerportion of the arc-like band relative to thesamples from the southern part. This is consis-tent with downwind sorting of dust particles.Thirdly, soil horizons clearly lie higher in thediagrams than the loess horizons. In fact, thereis an upward progression from the coarsestloess horizons (L2-3, L2-1) to the finer loesshorizons (L1-1, L1-5), then the weakly devel-oped soils (L1-4, L2-4) and, finally, the well-developed soils (S1, S2-1) (Fig. 7). Thus, itappears that this type of ternary diagram effect-ively represents the spatial and temporal differ-entiation of loess deposits.

POTENTIAL FACTORS INFLUENCINGTHE PARTICLE SIZE OF CHINESE LOESS

The spatial and temporal changes in particle-sizecharacteristics, as illustrated above, may be

Fig. 7. Skewness (Sk)–kurtosis (Kr)–median grain size (F50) ternary diagrams showing spatial changes in particle-size characteristics in loess units (L2-3, L2-1, L1-1 and L1-5), weakly developed soil units (L1-4 and L2-4), andpalaeosol units (S1 and S2-1) along the loess transect. The stratigraphic positions of the selected samples areindicated in Fig. 2.



caused by several factors such as source–sinkdistance, transporting wind intensity, post-depo-sitional weathering and particle aggregation.Clearly, the principal factor needs to be identifiedbefore the implications of loess and ‘red clay’grain-size distributions can be assessed.

Source–sink distance and wind intensity

In earlier work, the grain size of Chinese loess wasfirst used as a proxy indicator for the intensity ofthe winter monsoon (An et al., 1991; Xiao et al.,1995), but was later regarded as reflecting both thewinter monsoon intensity and the source–sinkdistance (Ding et al., 1999c). In fact, it is hard toevaluate wind intensity changes in the geologicalpast, because the dust deposits are not the productof a single wind event but reflect a combined andaveraged history of many and varied wind situa-tions. In this study, the median grain size, skew-ness and kurtosis all show a consistent southwardchange along the loess transect, suggesting that thethree parameters may be closely related to the dusttransport distance.In order to determine the relationship between

distance and grain-size parameters during differ-ent periods, four adjacent samples were selectedfrom each of L1-1, L1-4, L1-5 and S1 (marked inFig. 2). The sample selection is the same as thatshown in Fig. 7. The sand content (> 63 lm%)and median grain size of the four adjacentsamples were then averaged. The grain-size chan-ges vs. the distance from the present desertmargin along the Hongde–Yangling transect areshown in Fig. 8.The sand content and median grain size of loess

units L1-1 and L1-5 decrease rapidly from20–32% to 0Æ5–2% (Fig. 8A), and from 40–52

to 10-14 lm (Fig. 8B), respectively, with theincrease in distance from � 90 to 360 km. Asfor the weakly developed soil L1-4 and soil unitS1, the two grain-size parameters show a gradualsouthward decrease, although the gradient of L1-4is a little steeper than that of S1. For example, thesand content decreases from 5–7% to nearly zero(Fig. 8A) and from 14–16 lm to 6–9 lm (Fig. 8B)for median grain size in soil unit S1 between � 90and 360 km from the source. The spatial grada-tion in particle-size parameters of both loess andsoils indicates that southward sorting of aeoliandust during subaerial transport was a regionalprocess during both glacial and interglacial peri-ods and that local dust input from within theLoess Plateau may have been a minor component.It is presumed that the location of the south-

east margin of the Mu Us desert during the lastglacial maximum (L1-1 in the loess stratigraphy)was similar to that of the present (Sun, J.M. et al.(1998). Thus, a strong inverse relationshipbetween grain-size parameters and the minimumdistance from the desert margin during the lastglacial maximum can be obtained (R2 > 0Æ95;Fig. 8). For L1-4, L1-5 and S1, the distance tothe source region is unknown. However, it issignificant that grain-size parameter–distancecurves closely match the lines of best fit of L1-1if shifted to the right along the distance axis(Fig. 9). This implies that the difference in grain-size gradient among the four units is a result ofchanges in source–sink distance rather than windintensity.

Weathering effects

The process of weathering tends to produceprogressively finer grained particles, especially

Fig. 8. Sand content (> 63 lm%)and median grain size vs. distancefrom desert margin in L1-1, L1-4,L1-5 and S1 of the loess transect.The stratigraphic positions of theselected samples from the loesstransect are indicated in Fig. 2. Thedashed lines are the best fit curvesfor grain-size parameters vs.distance in L1-1.



clay minerals, at the expense of the coarsergrained material. However, there is good evidenceindicating that the Chinese loess–palaeosoldeposits, even the best-developed soil unit (S5),have experienced only the incipient and earlystages of chemical weathering characterized byacid leaching and carbonate dissolution, with thealteration of silicate minerals, in particular, beinglimited (Chen et al., 1998, 2001; Han et al., 1998).Furthermore, many of the clay minerals in loessand palaeosol units have been shown to bemainly of detrital, rather than pedogenic origin(Liu, 1985; Ji et al., 1999). Thus, it appears that

weathering processes have had a very restrictedeffect on the grain-size characteristics of theChinese loess.

Particle aggregation

Previous studies (e.g. Derbyshire et al., 1998)have shown that modern airfall dust and Chineseloess contain some silt-sized aggregates. Thediameters of these aggregates fall in the range10–50 lm, and most of them have diametersbetween 10 and 20 lm. It follows from this thatrelatively coarse particles (i.e. the sand fraction)

Fig. 9. Sand content (> 63 lm%)and median grain size vs. distancefrom desert margin curves for L1-4,L1-5 and S1. The three curves havebeen differentially moved to theright along the distance axis tofacilitate comparison with the linesof best fit of L1-1 (dashed lines, seeFig. 8).



in dust deposits may contain few aggregates. Asmentioned earlier, the sand content of all thesections along the Hongde–Yangling transectshows a consistent decrease with increase insource–sink distance for both glacials and inter-glacials (Fig. 8). The median grain-size data forthe transect present a southward-fining patternsimilar to the sand content. This leads to theconclusion that loess aggregation does not have ameasurable effect upon the spatial differentiationpattern in the Loess Plateau.Based on the discussion above, it is concluded

that the source–sink distance is a dominant factorinfluencing the grain-size characteristics ofdeposited loess. Such particle-size characteristicsof aeolian deposits, as mentioned earlier, can be

clearly shown in the skewness–kurtosis–mediangrain size ternary diagram. Moreover, source–sinkdistance changes can also be inferred from theternary diagram.

COMPARISON OF PARTICLE-SIZEPARAMETERS OF LOESS AND ‘REDCLAY’

The particle-size distributions of some represen-tative loess–palaeosol and ‘red clay’ samples fromthe Lingtai and Jingchuan sections are shown inFig. 10. Again, all show a bimodal pattern, witha principal mode in the coarse fraction anda secondary mode in the finer fraction. The

Fig. 10. Grain-size distributions of some representative loess–palaeosol and ‘red clay’ samples from the Lingtai andJingchuan sections.



secondary mode in the soil and ‘red clay’ samplesis more prominent than in most of the loesssamples from the two loess–palaeosol sequences(Fig. 10) and the north–south transect (Fig. 4),reflecting the fact that a higher percentage of thefinest particles is found in the soil and ‘red clay’samples. Overall, the ‘red clay’ from the Lingtaiand Jingchuan sections shows grain-size distri-butions very similar to the loess-derived soilssuch as S5, S26 and S32, all being broad curves,in contrast to the relatively sharp and peakedgrain-size distribution pattern in the loess sam-ples from the two sections. Therefore, the Tertiary‘red clay’ sediments may have experienced depo-sitional and post-depositional processes similarto those evident in the well-developed Pleisto-cene palaeosols.Figure 11 shows changes in skewness, kurtosis

and median grain size for the Lingtai and Jing-chuan loess–‘red clay’ sequences. In both, theskewness and kurtosis values are much higher forthe loess beds than for the palaeosols, and quiteclosely match the pattern shown by the mediangrain size. The highest values of skewness andkurtosis in the two sequences occur in sandyloess units, such as L9, L15 and L33. Theskewness and kurtosis of the two ‘red clay’sequences centre, respectively, on 0Æ2–0Æ6 and2Æ2–2Æ6, being similar to or slightly lower thanthose found in the loessic palaeosol units. It isalso evident that skewness and kurtosis are veryconsistent in the ‘red clay’ parts of the twosections. Averaged median grain size, skewnessand kurtosis all show a stepwise increase upsec-tion at both sites. As indicated by the verticaldashed lines in Fig. 11, changes in these param-eters can be roughly subdivided into three por-tions, i.e. the ‘red clay’, the lower Pleistoceneloess (S15–L33) and the upper Pleistocene loess(L15–S0).The ternary plots of the three parameters for the

Pleistocene loess, the loessic palaeosol and theTertiary ‘red clay’ from the Lingtai and Jingchuansections are shown in Fig. 12. They form narrow,long arc-like bands, the location of these on theplots showing a progression from loess at the base(Fig. 12A and E), through a palaeosol zone(Fig. 12B andF) to the ‘red clay’ at the top (Fig. 12Cand G). Plotting of all samples on a single ternarydiagramshows that the three distributions overlap,the central zone consisting of palaeosols, relativelyfine loess and relatively coarse ‘red clay’ samples(Fig. 12D and H). It is inferred from this that thelocation of the ‘red clay’ in the upper part of the arcindicates that the ‘red clay’ underwent sorting over

greater distances than was the case for the loess–palaeosol deposits, and that the three populationsmay have shared a common origin.To investigate further changes in source–sink

distances since the late Miocene, ternary diagramsfor the entire Lingtai and Jingchuan loess–‘redclay’ sequences have been constructed (Fig. 13).Both aeolian sequences were subdivided into sixintervals mainly on the basis of the median grain-size curves (Fig. 11), and the averaged values of thethree parameters were computed for each interval.The six selected intervals are S0–L5 (� 0Æ5–0 Ma),S5–L15 (� 1Æ2–0Æ5 Ma), S15–L24 (� 1Æ6–1Æ2 Ma),S24–L33 (� 2Æ6–1Æ6 Ma), the upper ‘red clay’(� 4Æ0–2Æ6 Ma) and the lower ‘red clay’(> 4Æ0 Ma). As shown in Fig. 13, the two lowestintervals of the Pleistocene loess (i.e. S15–L24 andS24–L33) in both sections occupy almost the sameposition in both diagrams. From the uppermostpart of the loess down to the lowermost ‘red clay’,the averaged values of the three particle-sizeparameters shift consistently upwards, suggestingthat, in the last 7Æ0–7Æ7 my, the dust source regionin north-western China expanded progressivelytowards the south and east.

DISCUSSION

The ‘red clay’ sequence at Lingtai and Jingchuanhas been subjected to relatively stronger pedo-genic processes than the overlying loess and canbe regarded as an extremely thick soil complex(Ding et al., 1999a). However, geochemical stud-ies indicate that the chemical weathering of the‘red clay’ materials mainly occurred in the sourcearea (Gu et al., 1997, 1999; Ding et al., 2001b).According to Ding et al. (1999a), even the best-developed portion of the ‘red clay’ at Lingtai haspedogenic characteristics similar to or slightlymore advanced than the palaeosol units above S8.A recent study (Han et al., 2002) has shown thatthe late Pliocene ‘red clay’ experienced onlymechanical translocation of clays, chemical alter-ation being rather weak. In this context, theweathering process in the ‘red clay’, like that inthe loess, may have played only a minor role inalteration of the particle-size distribution.The Tertiary ‘red clay’ sediments at Lingtai and

Jingchuan have particle-size characteristics sim-ilar to those found in the loess deposits of the lasttwo glacial–interglacial periods along the north–south transect across the Loess Plateau. Thisprovides strong circumstantial evidence foran aeolian origin for both ‘red clay’ sequences.



Fig. 11. Changes in skewness (phi scale), kurtosis (phi scale) and median grain size (lm) in the ‘red clay’–loesssequences at Lingtai and Jingchuan. The skewness and kurtosis were calculated using the method of moments(Friedman & Sanders, 1978). The major soil (Si) and loess (Li) units are indicated. The horizontal dashed linesindicate the contact between the loess and ‘red clay’ at Lingtai and Jingchuan. The vertical dashed lines indicate theaveraged values of these records in the ‘red clay’, the lower Pleistocene loess (L33–S15) and the upper Pleistoceneloess (L15–S0) respectively.



On the basis of studies at Xifeng and Jingle(Fig. 1), some authors concluded that the ‘redclay’ at both sites contains waterlain sediments(Mo & Derbyshire, 1991; Guo et al., 2001). Itappears that the accumulation and preservationof Tertiary aeolian deposits at a specific sitedepends largely on the local sedimentary envi-ronment. In general, aeolian sediments can accu-mulate continuously only on flat, broad-reliefunits with a relatively dense vegetation covercapable of acting as a dust trap. It has beenshown (Liu, 1985; Zhu & Ding, 1994) that, duringthe early Cenozoic, the Loess Plateau was anerosional environment that gave rise to severalflat, broad basin-like features. Subsequently, thesewere filled with aeolian Tertiary ‘red clay’ and

Quaternary loess forming huge loess ‘yuan’ (pla-teaus). However, in other areas within the LoessPlateau, aeolian accumulation was affected by thepalaeodrainage system, leading to ‘red clay’ sedi-ments of mixed origin. By way of comparison, it isknown that local reworking was also active duringthe accumulation of the Pleistocene loess. Forexample, the so-called secondary loess deposits,many of which contain coarse sands and beddingstructures, are seen in many places on the LoessPlateau. It follows from this that detailed sedi-mentological studies must be undertaken on allnew ‘red clay’ sections on the Plateau in order toverify their precise sedimentary origins.Systematic changes in the particle-size charac-

teristics of the aeolian deposits along the loess

Fig. 12. Skewness (Sk)–kurtosis (Kr)–median grain size (F50) ternary diagrams for samples of loess (A and E),palaeosol (B and F) and ‘red clay’ (C and G) and for all samples (D and H) at Lingtai and Jingchuan.



transect indicate that differential downwind sort-ing mainly reflects differences in the distancebetween source areas and depositional sites.However, two underlying assumptions must beaddressed in order to estimate changes in source–sink distance using grain-size parameters fromthe two aeolian sequences and the loess transect:(1) the regional wind intensity, on average, isrelatively constant between glacial and intergla-cial periods; and (2) the production of grains overall size ranges in the source region is relativelyconstant over the last 7Æ7 my.During the last glacial maximum, the minimum

distance from the desert margin to the Yanglingsection was about 360 km, whereas it was muchmore remote during the last interglacial. In gen-eral, the skewness, kurtosis and median grain-size values of the two ‘red clay’ sequences arelower than those for the soil units S1 and S2-1from the loess transect (Figs 5 and 11). In theternary diagrams (Figs 7 and 13), the two ‘redclay’ data points are lower down the skewnessaxis than those for soil units S1 and S2-1 alongthe Hongde–Yangling transect. This suggests thatthe source–sink distance in the case of theTertiary ‘red clay’ was greater than for the latePleistocene palaeosols.

CONCLUSIONS

The particle-size characteristics of the ‘red clay’at Lingtai and Jingchuan are similar to the

palaeosols within the overlying loess deposits,supporting the aeolian origin of the two ‘redclay’ sequences. Chronological and stratigraphi-cal studies have shown that there is closecorrelation between the Pleistocene loess andthe Tertiary ‘red clay’ successions at the twosites studied (Ding et al., 2001a), suggesting thatthe two ‘red clay’–loess sequences can be regar-ded as the most complete and continuousaeolian sedimentary series for the last 7Æ0–7Æ7 my in the Chinese Loess Plateau. Given thequality of the data on regional and globalclimate changes provided by the Pleistoceneloess in this region, it is reasonable to expectthat a longer and more detailed climatic historyfor eastern Asia will emerge from further studyof the ‘red clay’.The spatial changes in the particle size of the

loess transect suggest that distance to the sourceregion is the principal factor influencing thegrain-size characteristics of loess deposits. Thus,the particle-size parameters of aeolian dust pro-vide a valuable indicator of changes in distance tosource through time. On the basis of comparisonof ‘red clay’ and loess, it is inferred that thedistance to source was greater during the Neogenethan in the last and penultimate interglacials. Thedust transport distance was greatest for the ‘redclay’ and least for the loess, implying that therehas been progressive expansion of the dust sourceregion in north-western China since the lateMiocene.

Fig. 13. Skewness (Sk)–kurtosis (Kr)–median grain size (F50) ternary diagrams showing temporal changes inparticle-size characteristics of the ‘red clay’–loess sequences at Lingtai and Jingchuan.



ACKNOWLEDGEMENTS

This study is funded by the Chinese Academy ofSciences (KZCX2-SW-133), the National NaturalScience Foundation of China (grant 40202016)and the NKBRSF project (G1999043402). Theauthors wish to thank Professor E. Derbyshireand Dr C. D. Rokosh for their critical commentsand language improvements on an earlier versionof this paper. Thanks are also extended to S. S.Hou, X. Wang and Dr S. F. Xiong for fieldassistance, and to Drs J. T. Han, Z. Y. Gu andZ. T. Guo for valuable discussions.

REFERENCES

An, Z.S., Kukla, G., Porter, S.C. and Xiao, J.L. (1991) Late

Quaternary dust flow on the Chinese Loess Plateau. Catena,18, 125–132.

Cande, S.C. and Kent, D.V. (1995) Revised calibration of the

geomagnetic polarity timescale for the Late Cretaceous and

Cenozoic. J. Geophys. Res., 100, 6093–6095.Chen, J., Ji, J.F., Qiu, G. and Lu, H.Y. (1998) Geochemical

studies on the intensity of chemical weathering in Luoc-

huan loess-paleosol sequence, China. Sci. China (Series D),41, 235–241.

Chen, J., An, Z.S., Liu, L.W., Ji, J.F., Yang, J.D. and Chen, Y.(2001) Variations in chemical compositions of the eolian

dust in Chinese Loess Plateau over the past 2.5 Ma and

chemical weathering in the Asian inland. Sci. China (Series

D), 44, 403–413.Derbyshire, E., Meng, X. and Kemp, R.A. (1998) Provenance,

transport and characteristics of modern aeolian dust in

western Gansu Province, China, and interpretation of the

Quaternary loess record. J. Arid Environ., 39, 497–516.Ding, Z.L., Yu, Z.W., Rutter, N.W. and Liu, T.S. (1994)

Towards an orbital time scale for Chinese loess deposits.

Quatern. Sci. Rev., 13, 39–70.Ding, Z.L., Sun, J.M., Liu, T.S., Zhu, R.X., Yang, S.L. and Guo,

B. (1998a) Wind-blown origin of the Pliocene red clay for-

mation in the central Loess Plateau, China. Earth Planet.

Sci. Lett., 161, 135–143.Ding, Z.L., Sun, J.M., Yang, S.L. and Liu, T.S. (1998b) Pre-

liminary magnetostratigraphy of a thick eolian red clay-

loess sequence at Lingtai, the Chinese Loess Plateau.

Geophys. Res. Lett., 25, 1225–1228.Ding, Z.L., Xiong, S.F., Sun, J.M., Yang, S.L., Gu, Z.Y. and Liu,

T.S. (1999a) Pedostratigraphy and paleomagnetism of a

�7.0 Ma eolian loess-red clay sequence at Lingtai, Loess

Plateau, north-central China and the implications for

paleomonsoon evolution. Palaeogeogr. Palaeoclimatol.Palaeoecol., 152, 49–66.

Ding, Z.L., Ren, J.Z., Yang, S.L. and Liu, T.S. (1999b) Climate

instability during the penultimate glaciation: Evidence from

two high-resolution loess records, China. J. Geophys. Res.,104, 20123–20132.

Ding, Z., Sun, J., Rutter, N.W., Rokosh, D. and Liu, T. (1999c)Changes in sand content of loess deposits along a north–

south transect of the Chinese Loess Plateau and the impli-

cations for desert variations. Quatern. Res., 52, 56–62.

Ding, Z.L., Rutter, N.W., Sun, J.M., Yang, S.L. and Liu, T.S.(2000) Re-arrangement of atmospheric circulation at about

2.6 Ma over northern China: evidence from grain size

records of loess-palaeosol and red clay sequences. Quatern.Sci. Rev., 19, 547–558.

Ding, Z.L., Yang, S.L., Hou, S.S., Wang, X., Chen, Z. and Liu,T.S. (2001a) Magnetostratigraphy and sedimentology of the

Jingchuan red clay section and correlation of the Tertiary

eolian red clay sediments of the Chinese Loess Plateau.

J. Geophys. Res., 106, 6399–6407.Ding, Z.L., Sun, J.M., Yang, S.L. and Liu, T.S. (2001b) Geo-

chemistry of the Pliocene red clay formation in the Chinese

Loess Plateau and implications for its origin, source prov-

enance and paleoclimate change. Geochim. Cosmochim.

Acta, 65, 901–913.Friedman, G.M. and Sanders, J.E. (1978) Principles of Sedi-

mentology. John Wiley & Sons, New York, 792 pp.

Gu, Z.Y., Lal, D., Liu, T.S., Guo, Z.T., Southon, J. and

Caffee, M.W. (1997) Weathering histories of Chinese loess

deposits based on uranium and thorium series nuclides

and cosmogenic 10Be. Geochim. Cosmochim. Acta, 61,5221–5231.

Gu, Z.Y., Ding, Z.L., Xiong, S.F. and Liu, T.S. (1999) A seven

million geochemical record from Chinese red-clay and

loess-paleosol sequence: Weathering and erosion in north-

western China. Quatern. Sci., 19, 357–365. (in Chinese with

English abstract).

Guo, Z.T., Peng, S.Z., Hao, Q.Z., Biscaye, P.E. and Liu, T.S.(2001) Origin of the Miocene-Pliocene Red-Earth Formation

at Xifeng in Northern China and implications for paleoen-

vironments. Palaeogeogr. Palaeoclimatol. Palaeoecol., 170,11–26.

Guo, Z.T., Ruddiman, W.F., Hao, Q.Z., Wu, H.B., Qiao, Y.S.,Zhu, R.X., Peng, S.Z., Wei, J.J., Yuan, B.Y. and Liu, T.S.(2002) Onset of Asian desertification by 22 Myr ago inferred

from loess deposits in China. Nature, 416, 159–163.Han, J., Fyfe, W.S. and Longstaffe, F.J. (1998) Climatic

implications of the S5 paleosol complex on the southern-

most Chinese loess plateau. Quatern. Res., 50, 21–33.Han, J., Fyfe, W.S. and Gu, Z. (2002) Assessment of the pal-

aeoclimate during 3.0–2.6 Ma registered by transition of Red

Clay to loess-palaeosol sequence in central North China.

Palaeogeogr. Palaeoclimatol. Palaeoecol., 185, 355–368.Heller, F. and Liu, T.S. (1982) Magnetostratigraphic dating of

loess deposits in China. Nature, 300, 431–433.Ji, J.F., Chen, J. and Lu, H.Y. (1999) Origin of illite in the loess

from the Luochuan area, Loess Plateau, Central China. Clay

Miner., 34, 525–532.Kukla, G. (1987) Loess stratigraphy in central China. Quatern.

Sci. Rev., 6, 191–219.Kukla, G. and An, Z.S. (1989) Loess stratigraphy in central

China. Palaeogeogr. Palaeoclimatol. Palaeoecol., 72, 203–

225.

Kukla, G., Heller, F., Liu, X.M., Xu, T.C., Liu, T.S. and

An, Z.S. (1988) Pleistocene climates in China dated by

magnetic susceptibility. Geology, 16, 811–814.Liu, T.S. (1966) Composition and Texture of Loess. Science

Press, Beijing, 132 pp. (in Chinese).

Liu, T.S. (1985) Loess and the Environment. China Ocean

Press, Beijing, 251 pp.

Liu, X.M., Liu, T.S., Xu, T.C., Liu, C. and Chen, M.Y. (1988a)The Chinese loess in Xifeng. I. The primary study on mag-

netostratigraphy of a loess profile in Xifeng area, Gansu

Province. Geophys. J., 92, 345–348.



Liu, X.M., Xu, T.C. and Liu, T.S. (1988b) The Chinese loess in

Xifeng. II. A study of anisotropy of magnetic susceptibility

of loess from Xifeng. Geophys. J., 92, 349–353.Lu, H.Y., Vandenberghe, J. and An, Z.S. (2001) Aeolian origin

and palaeoclimatic implications of the ‘Red Clay’ (north

China) as evidenced by grain-size distribution. J. Quatern.

Sci., 16, 89–97.Mo, D.W. and Derbyshire, E. (1991) The depositional envi-

ronment of the late Pliocene ‘red clay’, Jing-Le Basin,

Shanxi Province, China. Sed. Geol., 70, 33–40.Qiang, X.K., Li, Z.X., Powell, C.McA. and Zheng, H.B. (2001)

Magnetostratigraphic record of the Late Miocene onset of

the East Asian monsoon, and Pliocene uplift of northern

Tibet. Earth Planet. Sci. Lett., 187, 83–93.Rokosh, C.D., Rutter, N.W., Ding, Z. and Sun, J. (2002)

Regional lithofacies and pedofacies variations along a north

to south climatic gradient during the Last Glacial period in

the central Loess Plateau, China. Quatern. Sci. Rev., 21,811–817.

Rutter, N.W., Ding, Z.L., Evans, M.E. and Wang, Y.C. (1990)Magnetostratigraphy of the Baoji loess-paleosol section in

the north-central China Loess Plateau. Quatern. Int., 7–8,97–102.

Rutter, N.W., Ding, Z.L., Evans, M.E. and Liu, T.S. (1991)

Baoji-type pedostratigraphic section, Loess Plateau, north-

central China. Quatern. Sci. Rev., 10, 1–22.Sun, D.H., Shaw, J., An, Z.S., Chen, M.Y. and Yue, L.P.

(1998a) Magnetostratigraphy and paleoclimatic interpret-

ation of a continuous 7.2Ma Late Cenozoic eolian sediments

from the Chinese Loess Plateau. Geophys. Res. Lett., 25,85–88.

Sun, D.H., An, Z.S., Shaw, J., Bloemendal, J. and Sun, Y.B.(1998b) Magnetostratigraphy and palaeoclimatic signifi-

cance of Late Tertiary aeolian sequences in the Chinese

Loess Plateau. Geophys. J. Int., 134, 207–212.Sun, J.M., Ding, Z.L. and Liu, T.S. (1998) Desert distributions

during the glacial maximum and climatic optimum: exam-

ple of China. Episodes, 21, 28–31.Xiao, J.L., Porter, S.C., An, Z.S., Kumai, H. and Yoshikawa, S.

(1995) Grain size of quartz as an indicator of winter mon-

soon strength on the loess plateau of central China during

the last 130,000 yr. Quatern. Res., 43, 22–29.Zhang, Y.X. and Xue, X.X. (1996) Taphonomic characteristics

of Longjiagou Hipparion fauna and genesis of ‘Hipparionred clay’ in Wudu County, Gansu Province. Chin. Sci. Bull.,

41, 238–241.Zheng, H.B., An, Z.S. and Shaw, J. (1992) New contributions

to Chinese Plio-Pleistocene magnetostratigraphy. Phys.Earth Planet. Inter., 70, 146–153.

Zhu, Z.Y. and Ding, Z.L. (1994) The Climatic and Tectonic

Evolution in the Loess Plateau of China During the Qua-ternary. Geological Publishing House, Beijing, 226 pp.

(in Chinese).

Manuscript received 28 May 2002;revision accepted 7 October 2003.



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