Red Clay’ and Pleistocene Loess in the Chinese Loess Plateau: Implications for Origin and Sources of the ‘Red Clay’

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

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. DING Institute 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 Kukla & An, 1989; Rutter et al., 1991; Ding et al., 1994, 1999a, 2001a). Beneath the oldest loess unit The loess deposits of the Chinese Loess Plateau (L33), there is a set of reddish clay–silt deposits were derived from the arid and semi-arid regions of with a thickness of about 30–130 m, informally north-western China, having been transported by known as the ‘red clay’ formation (Liu, 1985). the East Asia winter monsoon and the westerlies According to Zhu & Ding (1994), this formation has (Liu, 1985). The loess–palaeosol sequences pro- a geographical distribution similar to that of the vide a quasi-continuous record of regional and Pleistocene loess in the Loess Plateau. Palaeomag- global climatic changes during the past 2Æ6my netic measurements at different sites on the Loess (Heller & Liu, 1982; Liu, 1985; Kukla, 1987; Kukla & Plateau indicate that the contact between the ‘red An, 1989; Rutter et al., 1990; Ding et al., 1994). In clay’ and the overlying loess is around the Matuy- general, such complete sequences of loess occur in ama–Gauss reversal boundary (Heller & Liu, 1982; the central and southern parts of the Loess Plateau, Liu et al., 1988a; Kukla & An, 1989; Rutter et al., have a thickness ranging from 130 to 200 m and 1990; Zheng et al., 1992; Sun et al., 1998a,b; Ding consist of over 30 loess–soil couplets (Liu, 1985; et al., 1998a,b, 2001a).

2004 International Association of Sedimentologists 77 78 S. L. Yang & Z. L. Ding

In recent years, the ‘red clay’ sequence has several aspects of these data require further attracted much attention (Liu et al., 1988b; Zheng analysis. et al., 1992; Ding et al., 1998a,b, 1999a, 2001a,b; A striking difference between the loess and the Sun et al., 1998a,b; Guo et al., 2001, 2002; Lu ‘red clay’ lies in the fact that the Pleistocene loess et al., 2001; Qiang et al., 2001). One important consists of alternating reddish soil and yellowish conclusion reached after investigation of the ‘red loess layers, whereas no typical loess horizons are clay’ is that it may be wind blown in origin; this seen within the ‘red clay’ (Ding et al., 1999a). The opinion is based on anisotropy of magnetic alternation of loess and soils documents large- susceptibility (Liu et al., 1988b), field character- scale oscillations between glacial and interglacial istics (Zheng et al., 1992; Sun et al., 1998a,b), conditions during the Pleistocene (Liu, 1985; grain-size distributions (Ding et al., 1998a; Lu Kukla, 1987). In this context, the ‘red clay’ may et al., 2001) and geochemical concentrations have developed under a generally warm and (Ding et al., 1998a, 2001b). Magnetostratigraphic humid climate (Ding et al., 1999a). Geological and pedostratigraphic studies of two thick loess– records (Sun, J.M. et al., 1998) have shown that, ‘red clay’ sequences at Lingtai and Jingchuan during glacial periods, the deserts in northern (Fig. 1) have suggested that the relatively con- China expanded significantly towards the south- tinuous aeolian record in the Chinese Loess east as a result of the growth of continental ice Plateau may well extend back in time from 2Æ6 sheets and an intensification of the winter mon- to 7Æ0–7Æ7 Ma (Sun et al., 1998a; Ding et al., soon, with deserts retreating north-westwards as 1998b, 2001a). However, debate continues on summer monsoonal rainfall increased during the origin of the ‘red clay’ formation (Mo & interglacials. Such dramatic changes in desert Derbyshire, 1991; Zhang & Xue, 1996; Guo et al., extent point to a changeable environment in the 2001). Some authors suspected that hydraulic dust source regions. If, as has been proposed, the factors may have played a significant transporta- ‘red clay’ sequence is truly of aeolian origin, there tional role, especially in the formation of the is thus a need to investigate the possible extent of lower part of this sequence (Guo et al., 2001). To the dust source region during the late Miocene date, the most important evidence for a wind- and Pliocene. blown origin comes from comparison of grain- Using grain-size analysis, a north–south tran- size data of the ‘red clay’ and the overlying loess sect of loess deposits for the past two glacial– (Ding et al., 1998a; Lu et al., 2001), although interglacial cycles and two thick loess–‘red clay’

Fig. 1. Schematic map showing the zonation of loess in the Chinese Loess Plateau: zone I, sandy loess; zone II, loess; and zone III, clayey loess (adapted from Liu, 1985). The arrow indicates the dominant direction of the winter monsoonal winds, coinciding with the observed decrease in grain size and thickness of the loess. Also shown are the Mu Us desert (dotted) and mountains (black) around and within the Plateau. The study localities are indicated.

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 79 sequences developed in the last 7Æ0–7Æ7 my is ﬁrst of the loess (L)–soil (S) sequence S0, L1, S1, L2 characterized. Then, on the basis of comparison and S2. In the Quzi section, only loess units of the loess with the ‘red clay’, we consider the above S1 are exposed. An obvious advantage of implications for changes in the extent of the dust this loess transect over the previously studied source region in north-western China since the Yulin–Weinan transect (Ding et al., 2000; late Miocene are considered. Rokosh et al., 2002) (Fig. 1) lies in the fact that the loess sections along the Hongde–Yangling transect are closer together and more evenly SITE LOCATIONS AND spaced. LITHOSTRATIGRAPHY The Holocene soil S0 is dark in colour because of its relatively high organic matter content. The The north–south loess transect runs from Hong- upper part of the Holocene soil has been partly de about 88 km south of the Mu Us desert eroded or disturbed by agricultural activities at margin to Yangling in the southernmost part of some sites along the transect. The loess units L1 the Loess Plateau, which is about 360 km away and L2 were deposited during the last and from the southern margin of the Mu Us Desert. penultimate glacial periods respectively. Both Twelve loess sections, located at Hongde, L1 and L2 are yellowish in colour and massive Huanxian, Mubo, Quzi, Qingyang, Baimapu, in structure, ranging in thickness from over 20 m Xifeng, Ningxian, Binxianbei, Binxian, Yongshou in the north to several metres in the south (Fig. 2). and Yangling (Fig. 1), were logged and sampled. The L1 loess unit can generally be subdivided At present, the mean annual temperature at into ﬁve subunits, termed L1-1, L1-2, L1-3, L1-4 Hongde is 8Æ0 C and the annual precipitation and L1-5 (Fig. 2). L1-2 and L1-4 are weakly 400 mm, the values for Yangling being 12Æ9 C developed soils, and the others are typical loess and 667 mm. Both mean annual temperatures horizons. Previous studies (Kukla, 1987; Kukla and rainfall increase southwards along the loess et al., 1988) have shown that L1-1 is correlated transect. All loess sections except Quzi consist with 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–south loess transect. Subdivision of the loess–soil sequences is indicated. The stratigraphic positions for the selected samples in Figs 4, 7 and 8 are marked by solid triangles in each section. The axes for the grain-size curves are the obverse of the SUS curves. Note that the depth scale varies from one section to another. The shaded zones indicate interglacials.

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 80 S. L. Yang & Z. L. Ding stage 4, and L1-2, L1-3 and L1-4 together with particle analytical procedures were as detailed stage 3. The fivefold subdivision of L1 is clearly by Ding et al. (1999b). expressed in the grain-size and magnetic suscep- The susceptibility and grain-size data are tibility curves shown in Fig. 2. The L2 loess unit shown in Fig. 2 for the loess transect and in is also composed of three typical loess layers Fig. 3 for the Lingtai and Jingchuan sections. (L2-1, L2-3 and L2-5) and two weakly developed Palaeosols are characterized consistently by soils (L2-2 and L2-4), as suggested by the grain- higher susceptibility values and finer particle size and magnetic susceptibility curves (Fig. 2). sizes compared with the loess horizons above and The L2 loess unit is correlative with marine below them. The two ‘red clay’ sequences have oxygen isotopic stage 6. The soil units S1 and S2 very fine grain-size distributions, with a median developed in the last and penultimate interglacial grain size centred around 4–8 lm (8–7 F). periods and correlate with marine oxygen isotope stages 5 and 7 respectively (Kukla, 1987; Kukla et al., 1988). Both are brownish or reddish in SPATIAL CHANGES IN PARTICLE-SIZE colour and have an A-Bw-C or A-Bt-C horizon PARAMETERS OF LOESS DEPOSITS sequence. The thickness of S1 at Hongde is 6Æ5m, ALONG THE NORTH–SOUTH TRANSECT and two thin loess layers are present within it. At Yangling, the southernmost site in the transect, The grain-size records from the loess transect the thickness of S1 decreases to 1Æ8 m, and no (Fig. 2) clearly display an overall southward loess unit is visible. Soil unit S2 is composed of decrease in particle size in the case of both loess two soils (S2-1 and S2-2) and a thin intervening and soil horizons. From Hongde southwards to loess horizon. Only the upper soil (S2-1) was Baimapu, the median grain size decreases rapidly sampled in this study. from 50–60 lm(4Æ3–4Æ1 F) to 30–40 lm(5Æ1–4Æ6 The Lingtai and Jingchuan loess–‘red clay’ F) for the typical loess units within L1 and sections (Fig. 1) are situated in the central part L2, then decreases relatively gradually from of the Loess Plateau, with mean annual tempera- 25–30 lm(5Æ3–5Æ1 F) at Xifeng to 15–20 lm ture and precipitation values of 9–10 C and 550– (6Æ1–5Æ6 F) at Yangling. For the soil units S1 and 600 mm respectively. The Lingtai loess–‘red clay’ S2-1, the median grain size shows a more gentle sequence has a thickness of 305 m and consists of southward decrease along the north–south tran- 175 m of Pleistocene loess and 130 m of Tertiary sect, i.e. from 14–16 lm(6Æ2–6Æ0 F) at Hongde to ‘red clay’ (Fig. 3). The Jingchuan loess–‘red clay’ 8–9 lm (7–6Æ8 F) at Yangling. sediment, with a thickness of 325 m, is composed Figure 4 illustrates the grain-size distributions of 199 m of loess and 126 m of ‘red clay’ (Fig. 3). of some representative samples from horizons L1-1 In the Pleistocene loess of both the Lingtai and (loess), L1-4 (weak soil) and S1 (soil) in the 12 loess Jingchuan sections, each of the S0–L33 loess–soil sections. Almost all the samples show a bimodal units (Rutter et al., 1991) is readily identified in character, with a majority falling between 3Æ5 and the field. Within the Tertiary ‘red clay’, over 100 6Æ5 F (88–11 lm), and a minority between 11 and reddish soil horizons are distinguishable at both 12 F (0Æ5–0Æ25 lm). For loess unit L1-1, the prin- Lingtai and Jingchuan (Ding et al., 1999a, 2001a). cipal mode becomes broader and flatter from Palaeomagnetic studies (Ding et al., 1998b, Hongde to Yangling, whereas it is consistently 2001a) suggest a basal age of 7Æ05 Ma for the broad and flat in the L1-4 and S1 soils. Also, the Lingtai section and 7Æ7 Ma for the Jingchuan principal mode gradually shifts to the finer part of section (Fig. 3). Pedostratigraphic and magneto- the curve from north to south. The minority mode stratigraphic correlation between the two sections invariably lies between 11 and 12 F (0Æ5–0Æ25 lm). suggests that sedimentation of the ‘red clay’ was At present, the sedimentological significance of semi-continuous at both sites (Ding et al., 2001a). this population is unclear. A possible reason for its For the 12 loess sections along the north–south occurrence is the peculiar function of the SALD- transect, a total of 6694 samples were collected at 3001 laser particle analyser itself. Grain-size 5–10 cm intervals. In the Lingtai and Jingchuan analyses using this equipment were conducted at sections, sample spacing was 3–5 cm, a total of 0Æ25 F intervals in the range 0Æ25 lm (12 F)to 15 039 samples being taken (Ding et al., 1999a, 2000 lm()1 F). It appears that particles finer than 2001a). Bulk magnetic susceptibility and grain 0Æ25 lm cannot be detected by the granulometer size were measured for all samples with a Bart- and are automatically added to the component ington MS2 susceptibility meter and a SALD- between 11 and 12 F, a process that may explain 3001 laser diffraction particle analyser. The the nature of the minority mode in the size

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 81

Fig. 3. Magnetic susceptibility (SUS) and median grain-size (Md) records of the ‘red clay’–loess sequences at Lingtai and Jingchuan, together with the magnetic reversal polarities (modified from Ding et al., 1998b, 2001a). Most of the soil units (Si) and major loess beds (Li) are indicated. The contacts between the loess and the ‘red clay’ at Lingtai and Jingchuan are at a depth of about 175 m and 199 m, respectively, as marked by the dashed lines. The axes for the grain-size curves are the obverse of the SUS curves. distribution diagrams. Accordingly, specific signi- (Fig. 4), a typical characteristic of dust deposits in ficance to the finest portions of the grading curves the Loess Plateau (Liu, 1966). Kurtosis displays can be attributed. more complicated features. The frequency distri- All the loess and palaeosol samples from the bution curves for samples from the L1-1 loess unit transect show a positively skewed distribution all show a leptokurtic pattern, especially those

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 82 S. L. Yang & Z. L. Ding

Fig. 4. Grain-size distributions of some representative samples from loess unit L1-1, from the weakly developed soil unit at L1-4 and from palaeosol unit S1 of the loess transect. The stratigraphic positions of the selected samples are indicated in Fig. 2. from the northern part of the Loess Plateau. On the loess units compared with the palaeosols. For the other hand, the weakly developed soil L1-4 example, the skewness and kurtosis values of L1-1 and the S1 soil unit are platykurtic. decrease rapidly from about 3Æ0 and 15 at Hongde Skewness and kurtosis were calculated for all to only 1Æ0and3Æ1 at Yangling respectively. For samples from the loess transect (Fig. 5). In each of the S1 soil unit, however, the two values decrease the sections, the loess units consistently exhibit gradually from about 1Æ0 and 3Æ0 at Hongde to 0Æ6 higher skewness and kurtosis values than the soil and 2Æ3 at Yangling. units. Even within loess units L1 and L2, the Both the spatial and temporal changes in typical loess layers such as L1-1, L1-5, L2-1, L2-3 median grain size, skewness and kurtosis of the and L2-5 are clearly indicated by the skewness loess deposits closely parallel each other (Fig. 5). and kurtosis peaks. Along the loess transect, the To test this relationship further, correlation coef- values of skewness and kurtosis show an overall ﬁcients for the three parameters down the loess southward decrease for both loess and soil units. transect were calculated (Fig. 6). Both the skew- Both values decrease more rapidly southwards in ness and kurtosis show a good inverse correlation

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 83

Fig. 5. Skewness (phi scale), kurtosis (phi scale) and median grain-size (lm) records of the 12 sections along the loess 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 are variable. with the median grain size (phi scale) (R2 ¼ 0Æ932 means of assessing spatial and temporal changes and 0Æ889 respectively) (Fig. 6A and B), indica- in the particle-size characteristics of these aeolian ting that the coarser the grain size, the higher the materials. The two end-members of each param- skewness and kurtosis values. There is a strong eter must be ﬁxed at 0 and 100 as a precondition positive correlation between skewness and in constructing such a diagram; skewness, kurto- kurtosis (R2 ¼ 0Æ973; Fig. 6C). sis and median grain size were normalized The incorporation of skewness, kurtosis and accordingly. It should be noted that this type of median grain size in a ternary diagram provides a ternary diagram is quite different from those

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

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 84 S. L. Yang & Z. L. Ding conventionally used in the study of sedimentary form a narrow arc-like band for each horizon. rocks, a particular advantage of the former type of Secondly, for a specific loess or soil horizon, diagram being that the changes in all three the samples from the northern part of the parameters can be displayed. transect are generally located in the lower In order to examine variations in these param- portion of the arc-like band relative to the eters along the dust transport pathway, four samples from the southern part. This is consis- adjacent samples were selected from typical loess tent with downwind sorting of dust particles. layers (L1-1, L1-5, L2-1 and L2-3), weakly devel- Thirdly, soil horizons clearly lie higher in the oped soil units (L1-4 and L2-4) and palaeosol diagrams than the loess horizons. In fact, there units (S1 and S2-1). The basis of this sample is an upward progression from the coarsest selection was the median grain-size record shown loess horizons (L2-3, L2-1) to the finer loess in Fig. 2. For typical loess units, samples were horizons (L1-1, L1-5), then the weakly devel- consistently selected from units containing the oped soils (L1-4, L2-4) and, finally, the well- coarsest grain sizes, whereas samples from soil developed soils (S1, S2-1) (Fig. 7). Thus, it units were taken from the finest grain-size units appears that this type of ternary diagram effect- (marked in Fig. 2). The three parameters of the ively represents the spatial and temporal differ- four adjacent samples in each unit were then entiation of loess deposits. averaged before plotting on the ternary diagrams. It is assumed that this set of samples was deposited at approximately the same time in each POTENTIAL FACTORS INFLUENCING of the loess and soil horizons. THE PARTICLE SIZE OF CHINESE LOESS Three features are clearly visible in the diagram for each of the selected horizons (Fig. 7). The spatial and temporal changes in particle-size First, spatial changes in the three parameters all characteristics, 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), and palaeosol units (S1 and S2-1) along the loess transect. The stratigraphic positions of the selected samples are indicated in Fig. 2.

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 85 caused by several factors such as source–sink to 10-14 lm (Fig. 8B), respectively, with the distance, transporting wind intensity, post-depo- increase in distance from 90 to 360 km. As sitional weathering and particle aggregation. for the weakly developed soil L1-4 and soil unit Clearly, the principal factor needs to be identified S1, the two grain-size parameters show a gradual before the implications of loess and ‘red clay’ southward decrease, although the gradient of L1-4 grain-size distributions can be assessed. is a little steeper than that of S1. For example, the sand content decreases from 5–7% to nearly zero (Fig. 8A) and from 14–16 lmto6–9lm (Fig. 8B) Source–sink distance and wind intensity for median grain size in soil unit S1 between 90 In earlier work, the grain size of Chinese loess was and 360 km from the source. The spatial grada- first used as a proxy indicator for the intensity of tion in particle-size parameters of both loess and the winter monsoon (An et al., 1991; Xiao et al., soils indicates that southward sorting of aeolian 1995), but was later regarded as reflecting both the dust during subaerial transport was a regional winter monsoon intensity and the source–sink process during both glacial and interglacial peri- distance (Ding et al., 1999c). In fact, it is hard to ods and that local dust input from within the evaluate wind intensity changes in the geological Loess Plateau may have been a minor component. past, because the dust deposits are not the product It is presumed that the location of the south- of a single wind event but reflect a combined and east margin of the Mu Us desert during the last averaged history of many and varied wind situa- glacial maximum (L1-1 in the loess stratigraphy) tions. In this study, the median grain size, skew- was similar to that of the present (Sun, J.M. et al. ness and kurtosis all show a consistent southward (1998). Thus, a strong inverse relationship change along the loess transect, suggesting that the between grain-size parameters and the minimum three parameters may be closely related to the dust distance from the desert margin during the last transport distance. glacial maximum can be obtained (R2 >0Æ95; In order to determine the relationship between Fig. 8). For L1-4, L1-5 and S1, the distance to distance and grain-size parameters during differ- the source region is unknown. However, it is ent periods, four adjacent samples were selected significant that grain-size parameter–distance from each of L1-1, L1-4, L1-5 and S1 (marked in curves closely match the lines of best fit of L1-1 Fig. 2). The sample selection is the same as that if shifted to the right along the distance axis shown in Fig. 7. The sand content (> 63 lm%) (Fig. 9). This implies that the difference in grain- and median grain size of the four adjacent size gradient among the four units is a result of samples were then averaged. The grain-size chan- changes in source–sink distance rather than wind ges vs. the distance from the present desert intensity. margin along the Hongde–Yangling transect are shown in Fig. 8. Weathering effects The sand content and median grain size of loess units L1-1 and L1-5 decrease rapidly from The process of weathering tends to produce 20–32% to 0Æ5–2% (Fig. 8A), and from 40–52 progressively finer grained particles, especially

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

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 86 S. L. Yang & Z. L. Ding

Fig. 9. Sand content (> 63 lm%)

and median grain size vs. distance from desert margin curves for L1-4, L1-5 and S1. The three curves have been differentially moved to the right along the distance axis to facilitate comparison with the lines of best ﬁt of L1-1 (dashed lines, see Fig. 8). clay minerals, at the expense of the coarser weathering processes have had a very restricted grained material. However, there is good evidence effect on the grain-size characteristics of the indicating that the Chinese loess–palaeosol Chinese loess. deposits, even the best-developed soil unit (S5), have experienced only the incipient and early Particle aggregation stages of chemical weathering characterized by acid leaching and carbonate dissolution, with the Previous studies (e.g. Derbyshire et al., 1998) alteration of silicate minerals, in particular, being have shown that modern airfall dust and Chinese limited (Chen et al., 1998, 2001; Han et al., 1998). loess contain some silt-sized aggregates. The Furthermore, many of the clay minerals in loess diameters of these aggregates fall in the range and palaeosol units have been shown to be 10–50 lm, and most of them have diameters mainly of detrital, rather than pedogenic origin between 10 and 20 lm. It follows from this that (Liu, 1985; Ji et al., 1999). Thus, it appears that relatively coarse particles (i.e. the sand fraction)

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 87 in dust deposits may contain few aggregates. As clearly shown in the skewness–kurtosis–median mentioned earlier, the sand content of all the grain size ternary diagram. Moreover, source–sink sections along the Hongde–Yangling transect distance changes can also be inferred from the shows a consistent decrease with increase in ternary diagram. source–sink distance for both glacials and interglacials (Fig. 8). The median grain-size data for the transect present a southward-fining pattern COMPARISON OF PARTICLE-SIZE similar to the sand content. This leads to the PARAMETERS OF LOESS AND ‘RED conclusion that loess aggregation does not have a CLAY’ measurable effect upon the spatial differentiation pattern in the Loess Plateau. The particle-size distributions of some represen- Based on the discussion above, it is concluded tative loess–palaeosol and ‘red clay’ samples from that the source–sink distance is a dominant factor the Lingtai and Jingchuan sections are shown in influencing the grain-size characteristics of Fig. 10. Again, all show a bimodal pattern, with deposited loess. Such particle-size characteristics a principal mode in the coarse fraction and of aeolian deposits, as mentioned earlier, can be a secondary mode in the finer fraction. The

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

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 88 S. L. Yang & Z. L. Ding secondary mode in the soil and ‘red clay’ samples greater distances than was the case for the loess– is more prominent than in most of the loess palaeosol deposits, and that the three populations samples from the two loess–palaeosol sequences may have shared a common origin. (Fig. 10) and the north–south transect (Fig. 4), To investigate further changes in source–sink reflecting the fact that a higher percentage of the distances since the late Miocene, ternary diagrams finest particles is found in the soil and ‘red clay’ for the entire Lingtai and Jingchuan loess–‘red samples. Overall, the ‘red clay’ from the Lingtai clay’ sequences have been constructed (Fig. 13). and Jingchuan sections shows grain-size distri- Both aeolian sequences were subdivided into six butions very similar to the loess-derived soils intervals mainly on the basis of the median grain- such as S5, S26 and S32, all being broad curves, size curves (Fig. 11), and the averaged values of the in contrast to the relatively sharp and peaked three parameters were computed for each interval. grain-size distribution pattern in the loess sam- The six selected intervals are S0–L5 ( 0Æ5–0 Ma), ples from the two sections. Therefore, the Tertiary S5–L15 ( 1Æ2–0Æ5 Ma), S15–L24 ( 1Æ6–1Æ2 Ma), ‘red clay’ sediments may have experienced depo- S24–L33 ( 2Æ6–1Æ6 Ma), the upper ‘red clay’ sitional and post-depositional processes similar ( 4Æ0–2Æ6 Ma) and the lower ‘red clay’ to those evident in the well-developed Pleisto- (> 4Æ0 Ma). As shown in Fig. 13, the two lowest cene palaeosols. intervals of the Pleistocene loess (i.e. S15–L24 and Figure 11 shows changes in skewness, kurtosis S24–L33) in both sections occupy almost the same and median grain size for the Lingtai and Jing- position in both diagrams. From the uppermost chuan loess–‘red clay’ sequences. In both, the part of the loess down to the lowermost ‘red clay’, skewness and kurtosis values are much higher for the averaged values of the three particle-size the loess beds than for the palaeosols, and quite parameters shift consistently upwards, suggesting closely match the pattern shown by the median that, in the last 7Æ0–7Æ7 my, the dust source region grain size. The highest values of skewness and in north-western China expanded progressively kurtosis in the two sequences occur in sandy towards the south and east. loess units, such as L9, L15 and L33. The skewness and kurtosis of the two ‘red clay’ sequences centre, respectively, on 0Æ2–0Æ6 and DISCUSSION 2Æ2–2Æ6, being similar to or slightly lower than those found in the loessic palaeosol units. It is The ‘red clay’ sequence at Lingtai and Jingchuan also evident that skewness and kurtosis are very has been subjected to relatively stronger pedo- consistent in the ‘red clay’ parts of the two genic processes than the overlying loess and can sections. Averaged median grain size, skewness be regarded as an extremely thick soil complex and kurtosis all show a stepwise increase upsec- (Ding et al., 1999a). However, geochemical stud- tion at both sites. As indicated by the vertical ies indicate that the chemical weathering of the dashed lines in Fig. 11, changes in these param- ‘red clay’ materials mainly occurred in the source eters can be roughly subdivided into three por- area (Gu et al., 1997, 1999; Ding et al., 2001b). tions, i.e. the ‘red clay’, the lower Pleistocene According to Ding et al. (1999a), even the best- loess (S15–L33) and the upper Pleistocene loess developed portion of the ‘red clay’ at Lingtai has (L15–S0). pedogenic characteristics similar to or slightly The ternary plots of the three parameters for the more advanced than the palaeosol units above S8. Pleistocene loess, the loessic palaeosol and the A recent study (Han et al., 2002) has shown that Tertiary ‘red clay’ from the Lingtai and Jingchuan the late Pliocene ‘red clay’ experienced only sections are shown in Fig. 12. They form narrow, mechanical translocation of clays, chemical alter- long arc-like bands, the location of these on the ation being rather weak. In this context, the plots showing a progression from loess at the base weathering process in the ‘red clay’, like that in (Fig. 12A and E), through a palaeosol zone the loess, may have played only a minor role in (Fig. 12B and F) to the ‘red clay’ at the top (Fig. 12C alteration of the particle-size distribution. and G). Plotting of all samples on a single ternary The Tertiary ‘red clay’ sediments at Lingtai and diagram shows that the three distributions overlap, Jingchuan have particle-size characteristics sim- the central zone consisting of palaeosols, relatively ilar to those found in the loess deposits of the last fine loess and relatively coarse ‘red clay’ samples two glacial–interglacial periods along the north– (Fig. 12D and H). It is inferred from this that the south transect across the Loess Plateau. This location of the ‘red clay’ in the upper part of the arc provides strong circumstantial evidence for indicates that the ‘red clay’ underwent sorting over an aeolian origin for both ‘red clay’ sequences.

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 89

Fig. 11. Changes in skewness (phi scale), kurtosis (phi scale) and median grain size (lm) in the ‘red clay’–loess sequences 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 lines indicate the contact between the loess and ‘red clay’ at Lingtai and Jingchuan. The vertical dashed lines indicate the averaged values of these records in the ‘red clay’, the lower Pleistocene loess (L33–S15) and the upper Pleistocene loess (L15–S0) respectively.

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 90 S. L. Yang & Z. L. Ding

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.

On the basis of studies at Xifeng and Jingle Quaternary loess forming huge loess ‘yuan’ (pla- (Fig. 1), some authors concluded that the ‘red teaus). However, in other areas within the Loess clay’ at both sites contains waterlain sediments Plateau, aeolian accumulation was affected by the (Mo & Derbyshire, 1991; Guo et al., 2001). It palaeodrainage system, leading to ‘red clay’ sedi- appears that the accumulation and preservation ments of mixed origin. By way of comparison, it is of Tertiary aeolian deposits at a specific site known that local reworking was also active during depends largely on the local sedimentary envi- the accumulation of the Pleistocene loess. For ronment. In general, aeolian sediments can accu- example, the so-called secondary loess deposits, mulate continuously only on flat, broad-relief many of which contain coarse sands and bedding units with a relatively dense vegetation cover structures, are seen in many places on the Loess capable of acting as a dust trap. It has been Plateau. It follows from this that detailed sedi- shown (Liu, 1985; Zhu & Ding, 1994) that, during mentological studies must be undertaken on all the early Cenozoic, the Loess Plateau was an new ‘red clay’ sections on the Plateau in order to erosional environment that gave rise to several verify their precise sedimentary origins. flat, broad basin-like features. Subsequently, these Systematic changes in the particle-size charac- were filled with aeolian Tertiary ‘red clay’ and teristics of the aeolian deposits along the loess

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 Comparison of particle size characteristics of Chinese ‘red clay’ and loess 91

Fig. 13. Skewness (Sk)–kurtosis (Kr)–median grain size (F50) ternary diagrams showing temporal changes in particle-size characteristics of the ‘red clay’–loess sequences at Lingtai and Jingchuan. transect indicate that differential downwind sort- palaeosols within the overlying loess deposits, ing mainly reﬂects differences in the distance supporting the aeolian origin of the two ‘red between source areas and depositional sites. clay’ sequences. Chronological and stratigraphi- However, two underlying assumptions must be cal studies have shown that there is close addressed in order to estimate changes in source– correlation between the Pleistocene loess and sink distance using grain-size parameters from the Tertiary ‘red clay’ successions at the two the two aeolian sequences and the loess transect: sites studied (Ding et al., 2001a), suggesting that (1) the regional wind intensity, on average, is the two ‘red clay’–loess sequences can be regar- relatively constant between glacial and intergla- ded as the most complete and continuous cial periods; and (2) the production of grains over aeolian sedimentary series for the last 7Æ0– all size ranges in the source region is relatively 7Æ7 my in the Chinese Loess Plateau. Given the constant over the last 7Æ7 my. quality of the data on regional and global During the last glacial maximum, the minimum climate changes provided by the Pleistocene distance from the desert margin to the Yangling loess in this region, it is reasonable to expect section was about 360 km, whereas it was much that a longer and more detailed climatic history more remote during the last interglacial. In gen- for eastern Asia will emerge from further study eral, the skewness, kurtosis and median grain- of the ‘red clay’. size values of the two ‘red clay’ sequences are The spatial changes in the particle size of the lower than those for the soil units S1 and S2-1 loess transect suggest that distance to the source from the loess transect (Figs 5 and 11). In the region is the principal factor inﬂuencing the ternary diagrams (Figs 7 and 13), the two ‘red grain-size characteristics of loess deposits. Thus, clay’ data points are lower down the skewness the particle-size parameters of aeolian dust pro- axis than those for soil units S1 and S2-1 along vide a valuable indicator of changes in distance to the Hongde–Yangling transect. This suggests that source through time. On the basis of comparison the source–sink distance in the case of the of ‘red clay’ and loess, it is inferred that the Tertiary ‘red clay’ was greater than for the late distance to source was greater during the Neogene Pleistocene palaeosols. than in the last and penultimate interglacials. The dust transport distance was greatest for the ‘red clay’ and least for the loess, implying that there CONCLUSIONS has been progressive expansion of the dust source region in north-western China since the late The particle-size characteristics of the ‘red clay’ Miocene. at Lingtai and Jingchuan are similar to the

2004 International Association of Sedimentologists, Sedimentology, 51, 77–93 92 S. L. Yang & Z. L. Ding

ACKNOWLEDGEMENTS Ding, Z.L., Rutter, N.W., Sun, J.M., Yang, S.L. and Liu, T.S. (2000) Re-arrangement of atmospheric circulation at about This study is funded by the Chinese Academy of 2.6 Ma over northern China: evidence from grain size records of loess-palaeosol and red clay sequences. Quatern. Sciences (KZCX2-SW-133), the National Natural Sci. Rev., 19, 547–558. Science Foundation of China (grant 40202016) Ding, Z.L., Yang, S.L., Hou, S.S., Wang, X., Chen, Z. and Liu, and the NKBRSF project (G1999043402). The T.S. (2001a) Magnetostratigraphy and sedimentology of the authors wish to thank Professor E. Derbyshire Jingchuan red clay section and correlation of the Tertiary and Dr C. D. Rokosh for their critical comments eolian red clay sediments of the Chinese Loess Plateau. J. Geophys. Res., 106, 6399–6407. and language improvements on an earlier version Ding, Z.L., Sun, J.M., Yang, S.L. and Liu, T.S. (2001b) Geo- of this paper. Thanks are also extended to S. S. chemistry of the Pliocene red clay formation in the Chinese Hou, X. Wang and Dr S. F. Xiong for field Loess Plateau and implications for its origin, source prov- assistance, and to Drs J. T. Han, Z. Y. Gu and enance and paleoclimate change. Geochim. Cosmochim. Z. T. Guo for valuable discussions. 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 REFERENCES Caffee, M.W. (1997) Weathering histories of Chinese loess deposits based on uranium and thorium series nuclides 10 An, Z.S., Kukla, G., Porter, S.C. and Xiao, J.L. (1991) Late and cosmogenic Be. Geochim. Cosmochim. Acta, 61, Quaternary dust flow on the Chinese Loess Plateau. Catena, 5221–5231. 18, 125–132. Gu, Z.Y., Ding, Z.L., Xiong, S.F. and Liu, T.S. (1999) A seven Cande, S.C. and Kent, D.V. 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