Re-arrangement of atmospheric circulation at about 2.6Ma over northern China: evidence from grain size records of loess-palaeosol and red clay sequences

*Corresponding author. Tel.: #86-10-6200-8111; fax: #86-10-6491-9140.

E-mail address: [email protected] (Z.L. Ding).

Quaternary Science Reviews 19 (2000) 547}558

Re-arrangement of atmospheric circulationat about 2.6 Ma over northern China:

evidence from grain size recordsof loess-palaeosol and red clay sequences

Z.L. Ding!,",*, N.W. Rutter#, J.M. Sun!, S.L. Yang!, T.S. Liu!

!Institute of Geology, Chinese Academy of Sciences, P.O.Box 9825, Beijing 100029, People's Republic of China"State Key Laboratory of Loess and Quaternary Geology, Xian 710054, People's Republic of China#Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada T6G 2E3

Abstract

Recent studies have shown that the red clay sequence underlying the Quaternary loess of the Chinese Loess Plateau is wind-blownin origin. Continuous atmospheric dust deposition in the past 7.0 Ma has been documented. To address the wind system thattransported the Tertiary red clay, two north}south transects were studied in the Chinese Loess Plateau. One of the transects wasdesigned to study spatial changes in grain size of the last glacial}interglacial loess records, and the other to observe particle changes ofthe Tertiary red clay underlying the Quaternary loess. The loess transect consists of nine sections, and the red clay transect of foursections. Analyses of closely spaced samples show that there is a strong southward decrease in grain size of both loess and palaeosolhorizons of the Late Pleistocene, which is consistent with the idea that the aeolian materials of the Quaternary in the Loess Plateauare transported by the northerly winter monsoonal winds. Grain size distribution of the red clay sequences, however, does not showsuch a change. From north to south along the red clay transect, the particle size distribution is almost identical in the four sections,suggesting that the winter monsoonal winds might have played a less important role in transporting the red clay material. It issuggested that the red clay may have been transported by the westerlies from the dust-source regions of northwestern China onto theLoess Plateau. A remarkable re-arrangement of atmospheric patterns at about 2.6 Ma, therefore, has been recorded by the redclay-loess shift. It is speculated that this re-arrangement of atmospheric patterns may have been caused by the onset of glaciation inthe Northern Hemisphere. ( 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

It has been convincingly demonstrated that the Quat-ernary loess-palaeosol sequence of the Chinese LoessPlateau is a valuable archive of regional climate history(Liu, 1985; Kukla et al., 1988; Kukla and An, 1989; Rutteret al., 1991; An et al., 1991a; Ding et al., 1993; Liu andDing, 1993). In the middle and southern parts of theLoess Plateau, loess sections have a thickness rangingfrom about 130 to 180 m, and span an interval from2.4}2.6 Ma to the Holocene (Heller and Liu, 1982, 1984;Liu, 1985; Rutter et al., 1990; Ding et al., 1992). Over 30

intercalated palaeosols have been recognized in the loesssequences (Kukla and An, 1989; Ding et al., 1993). Astandard interpretation for the formation of the loess-soilsequence is that during glacial periods of the Pleistocene,high atmospheric dust in#ux was deposited ontothe Loess Plateau under a climate dominated by wintermonsoonal winds, whereas the dust in#ux was greatlyreduced during interglacial periods, and soils weredeveloped under an intensi"ed summer monsoonalclimate (Liu, 1985; Li et al., 1988; Kukla and An, 1989;An et al., 1991a, b; Rutter and Ding, 1993). The alterna-tion of loess and soil units can thus be regarded asthe product of the oscillatory monsoon system overEast Asia.

At many sites of the Loess Plateau, reddish clay-siltsized sediments of varying thickness are seen to underliethe oldest loess unit L33 (Ding et al., 1992). As the

0277-3791/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 1 7 - 7

Fig. 1. Schematic map showing the zonation of the Last Glacial loessgrains in the Loess Plateau: zone I } sandy loess, zone II } loess and zoneIII } clayey loess. The decrease in loess grains from northwest to southeastis consistent with the northwesterly winter monsoon winds over EastAsia, as indicated by the arrow. The deserts (dotted) and mountains(black) around and within the Loess Plateau are indicated (modi"edfrom Liu, 1985). The north}south transect of last glacial-interglacialloess sequences is from Yulin to Weinan (see text). The studied red claysections are located in Jingbian, Jiaxian, Lingtai and Baoji.

sediments commonly contain Hipparion fossils, they wereoriginally called Hipparion red earth (de Chardin andYoung, 1931). In the 1980s, Liu and his co-workers (Liu,1985) named the reddish earth beneath the loess the redclay formation. Palaeomagnetic measurements at di!er-ent sites on the Loess Plateau showed that the contactbetween the red clay and the overlying loess is around theMatuyama/Gauss magnetic reversal (Heller and Liu,1982; Liu et al., 1988a; Kukla and An, 1989; Rutter et al.,1990; Zheng et al., 1991; Ding et al., 1992).

The most striking di!erence between the red clayand the overlying loess lies in the fact that the palaeosolsin the loess deposits are separated by loess beds ofvarying thickness, whereas few loess horizons can beseen in the red clay. The loess horizons within the Quat-ernary loess deposits are yellowish and massive in struc-ture, implying a cold and dry climate over the LoessPlateau during their accumulation. The repeated occur-rence of non- or weakly-weathered loess horizons andwell-developed palaeosols re#ects cold}warm anddry}wet climatic #uctuations of large amplitude inthe past 2.4}2.6 Ma. The entire red clay formationunderlying the loess, however, appears to be stronglypedogenically developed, thus suggesting a relativelycontinuous warm}wet climate. The relatively abruptshift from red clay accumulation to loess deposition,which happened at about the M/G magnetic reversal,is therefore regarded as a major regional climatic eventin the late Cenozoic of northern China (Ding et al.,1992).

Previous studies, based on "eld and micromorphologi-cal observations and magnetic susceptibility anisotropymeasurements (Liu et al., 1988b; Evans et al., 1991; Moand Derbyshire, 1991; Ding et al., 1997), have suggestedthat the red clay formation underlying the Quaternaryloess is also wind-blown in origin. More recently, thishypothesis has been supported by sedimentological andgeochemical studies of di!erent red clay sections in theLoess Plateau (Ding et al., 1998a, b; Sun et al., 1998).These studies show that the red clay has sedimentologicaland geochemical characteristics that are almost the sameas those observed in the overlying loess. Palaeomagneticmeasurements on the thickest red clay sequences present-ly known in the Loess Plateau show that the red clayformation has a basal age of at least 7.0 Ma (Ding et al.,1998b; Sun et al., 1998). Therefore, the loess-soil sequenceof the last 2.6 Ma can be regarded as the continuation ofatmospheric dust deposition beginning in the lateMiocene. This raises the question: Was the red claymaterial transported by the East Asia winter monsoonalwinds in the manner of the overlying loess or by someother wind system.

To answer this question, we recently studied twonorth}south transects in the Loess Plateau. One of thetransects consisted of nine loess sections, and the otherwas composed of four red-clay sequences.

2. North}south transect of the Last Glacial}Interglacialloess

The loess transect starts from Yulin in the transitionalzone between the Mu Us desert and the Loess Plateau,and ends at Weinan in the southernmost part of theLoess Plateau. Nine loess sections, located at Yulin,Jiaxian, Wupu, Yanchang, Yichuan, Huanglong, Baishui,Pucheng and Weinan (Fig. 1), were observed and sam-pled. The straight-line distance from Yulin to Weinan isabout 420 km. At present, the mean annual temperatureat Yulin is about 8.13C and the annual precipitationabout 450 mm, while at Weinan the annual temperatureand precipitation values are 13.63C and 620 mm, respec-tively. From Yulin to Weinan, both the mean annualtemperature and rainfall increase gradually. Above thebase of the Last Interglacial soil S

1, samples were taken

in the nine sections at 2 or 5 cm intervals for soils and at5 or 10 cm intervals for loess beds. Grain size was ana-lyzed for all the samples, using a PRO-700 SK LaserMicron Sizer. Ultrasonic pre-treatment with addition of20%(NaPO

3)6

solution was used to disperse the samplesfor particle determination. Analysis of 20 replicatesshows a precision of 3.5% for this procedure. This par-ticle analysis machine can read 17 size classes witha measurable range from 0.2 to 500 lm.

Changes in the median grain size of the sections areshown in Fig. 2. According to both "eld observations andthe grain size records, the standard stratigraphic units ofthe Last Interglacial soil S

1, the Last Glacial loess L

1and

548 Z.L. Ding et al. / Quaternary Science Reviews 19 (2000) 547}558

Fig. 2. Median grain size records of the nine sections along the loess transect. The portion from the base of the Last Interglacial soil S1to the top of the

Holocene soil S0

is shown. In most of the sections, the Last Glacial loess L1

can be subdivided into "ve parts, designated, respectively, as L1v1

, L1}2

,L1}3

, L1}4

and L1}5

. In the Jiaxian section, only the part above L1}2

is exposed. From Yulin south to Weinan, the median size of both loess and soilhorizons shows a clear decrease (for localities, see Fig. 1). The stratigraphic positions where the data are selected to study spatial changes of di!erentgrain size parameters (shown in Fig. 4) are marked in each section of the transect.

the Holocene soil S0

(Liu, 1985) are readily recognized.The Holocene soil in the Yulin section is covered bymodern desert deposits. Within the Last Glacial loess L

1,

there are three aeolian sand horizons (L1v1

, L1}3

andL1}5

) composed of medium-sized sand particles, and twothick loess horizons (L

1}2and L

1}4). TL dating results

(Sun et al., 1995) show that L1v1

was deposited in theOxygen Isotope Stage (OIS) 2, L

1}5in OIS 4, and

L1}2}L

1}4in OIS 3. The presence of sand horizons within

L1

suggests the Mu Us desert advanced into the Yulinarea during cold times. This suggestion is reinforced bythe wide distribution of palaeo-sand-dunes within theMu Us desert.

Complete loess-soil deposits above S1

at Jiaxian (Fig. 1)have not yet been found. In this loess section, the Holo-cene soil S

0is dark in colour and has a relatively high-

organic matter content. There is a sand horizon about1.5-m thick below S

0. The grain size of this horizon is

much "ner than that of L1v1

at Yulin (Fig. 2). A loesshorizon about 2-m thick, designated as L

1}2, is exposed

below this sand horizon.South of Jiaxian, the remaining seven sections are

composed of loess and soil units, and no sand horizon is

found within L1. The Last Glacial loess deposit L

1can

also be subdivided into "ve parts (Fig. 2). This means thatsynchronous dust deposition at di!erent time intervalsover the Loess Plateau is documented by di!erent loesshorizons, thus allowing observation of spatial changes insedimentological characteristics of the loess-soil units.

3. Grain size changes in the loess transect

Previous studies (Liu, 1966) have shown that both thegrain size and thickness of the Last Glacial loess depositsdecrease from northwest to southeast over the LoessPlateau, parallel to the dominant direction of the wintermonsoonal winds (Fig. 1). The grain size records ob-tained in this study (Fig. 2) clearly show an overallsouthward decrease in particle size for both loess and soilhorizons. This implies the persistence in the southwardsorting of aeolian particles during subaerial transport inboth glacial and interglacial periods.

To demonstrate temporal grain size changes in detail,variations of the di!erent grain size fractions and themedian grain size are plotted against depth for the

Z.L. Ding et al. / Quaternary Science Reviews 19 (2000) 547}558 549

Fig. 3. Changes in median grain size and the percentage of di!erent grain size fractions in the Yichuan section. The stratigraphic units are labeled. Seethe similarity of changes in di!erent grain size parameters.

Yichuan section (Fig. 3). It is evident that "ne particlefractions ((20 lm) are relatively concentrated in soils,whereas the content of the coarse grain fractions('30 lm) is much higher in the loess units. Most of thecurves indicate that variations in the di!erent size frac-tions have a similar pattern, except for the 20}30 lmfraction which shows much less variation. Another im-portant feature of the grain size distribution at Yichuan isthat the sand particle content ('63 lm%) is close tozero in the soils and in the middle part of L

1, whereas it is

about 10% in the typical loess horizons of L1v1

and L1}5

.A similar pattern of grain size changes is also observed inother sections along the loess transect.

During marine OIS 2 and 4 (L1v1

and L1}5

in the loessstratigraphy), the southern border of the Mu Us desertwas located in the Yulin area, as indicated by the palaeo-sand horizons of L

1v1and L

1}5in the Yulin section (Fig. 2).

During the Last Interglacial (S1) and the interstadials

(L1}2

and L1}4

), the desert border retreated to the northand west (Sun et al., 1998). This observation implies thatloess grain size changes are controlled to a signi"cantdegree by changes in the location of the desert margins innorthern and northwestern China. Nevertheless, spatialchanges in grain size distribution over a speci"c intervalcan still be employed to indicate the direction of thedominant winds transporting the aeolian particles. Fig. 4shows changes in four grain size parameters with dis-tance from Yulin for four di!erent loess-soil units (L

1v1,

L1}4

, L1}5

and S1). The parameters include the sand

particle percentage ('63 lm%, top left in Fig. 4), the"ne particle fraction ((10 lm%, top right in Fig. 4), thecoarse particle fraction ('30 lm%, bottom left in Fig. 4)and the median grain size (bottom right in Fig. 4).L1v1

and L1}5

are typical loess horizons, yellowish incolour and massive in structure. L

1}4is a weakly de-

veloped palaeosol in most of the sections along the tran-sect, and S

1is a typical soil. Three samples were selected

from each of the four horizons in each of the sections.Selection was based on the grain size records shown inFig. 2. The loess samples were selected from the largestmedian grain size parts of L

1v1and L

1}5(marked in Fig.

2), with the soil samples being taken from the smallestmedian grain size parts of L

1}4and S

1(marked in Fig. 2).

Each of the data in Fig. 4 was averaged from the threeselected samples.

The sand particle percentage of L1v1

and L1}5

showsan abrupt decrease over a distance of about 100 km fromYulin. It then decreases more gradually, reaching almostto near zero at Weinan. The overall sand particle contentis low in the horizons of L

1}4and S

1, and as expected, it

shows a more gentle southward decrease (top left in Fig. 4).The pattern of north}south change in the median grainsize is similar to that in the sand particle percentage.Although the median grain size of S

1is small in all of the

sites, a gradual southward decrease in this parameter isstill evident (bottom right in Fig. 4). The median grain


Fig. 4. Changes of grain size parameters with distance from Yulin in di!erent horizons. Four grain size parameters ('63, (10, '30 lm and Md)are plotted. L

1v1and L

1}5are two loess horizons, and S

1and L

1}4are two soils.

size of S1

at Yulin is about 15 lm, whereas it is about6 lm at Weinan.

North}south changes in the parameters of both(10 lm% and '30 lm% are remarkable. Concentra-tions of the particles smaller than 10 lm are much higherin S

1than in the loess horizons, whereas the parameter of

'30 lm% is consistently lower in S1

than in L1v1

,L1}5

and L1}4

. From Yulin to Weinan over a distance ofabout 420 km, the "ne-sized particle fraction((10 lm%) increases from 40% to about 70% in S

1,

while it increases from 7% to about 52% in L1v1

. Thesouthward decrease of the coarse-sized fraction('30 lm%) in S

1is from 26% at Yulin to about 7% at

Weinan. The north}south decrease of this parameter inboth L

1v1and L

1}5is from about 90% at Yulin down to

about 25% at Weinan.All the evidence shown above indicates a strong south-

ward sorting of the aeolian particles during their sub-aerial transport. It leaves no doubt that northerly windspersisted in transporting atmospheric dust from the de-serts to the Loess Plateau throughout di!erent intervalsof the last glacial}interglacial period. As suggested byseveral authors (An et al., 1991b; Ding et al., 1992; Xiao et

al., 1995; Porter and An, 1995), this northerly wind is thewinter monsoon, currently the dominant wind from Sep-tember to May over northern China. It also implies thatthe southerly summer monsoonal #ow is an insigni"canttransporter of "ne dust even in interglacial periods. Spa-tial changes in grain size of aeolian deposits can thereforebe used to reconstruct the direction of the transportingwind system. In the remainder of this paper, spatialchanges in grain size of the upper red clay sequences willbe presented and the question of the dominant winddirection involved in the transporting of the red clayparticles addressed.

4. Loess}red-clay contact

Fig. 5 shows the magnetic susceptibility records of theBaoji and Lingtai (Fig. 1) sections from the top of S

24in

the Wucheng Loess to the red clay horizon where theGauss/Gilbert magnetic reversal is found. In both theBaoji and Lingtai sections, the loess-soil units ofthe Wucheng Loess are readily recognizable in the "eld,and they are also clearly expressed in the magnetic


Fig. 5. Magnetic susceptibility records of the Baoji and Lingtai redclay}loess sections. The lower part of the Pleistocene loess and theupper part of the Pliocene red clay are shown. The upper and lowerboundaries of the Olduvai Subchron are de"ned respectively in L

25(between S

24and S

25) and the lower part of S

26. The Matuyama/Gauss

palaeomagnetic boundary is located within L33

in both sections (Rutteret al., 1990; Ding et al., 1998b). The contact between the red clay and theloess is placed at the base of L

33.

susceptibility records. Between S24

and S26

in the loesssections, there are three well-developed palaeosols(S

24}S

26). Within each of the intervening loess horizon

(L25

and L26

), there is one relatively weakly-developedsoil. S

26is a thick soil complex, and it is used as a strati-

graphic marker in "eld work (Rutter et al., 1991; Dinget al., 1992). The upper and lower boundaries of theOlduvai magnetic polarity subchron are distributed, re-spectively, within L

25and in the lower part of S

26(Rutter

et al., 1990; Ding et al., 1998b).L27

and L32

are two other stratigraphic markers in theloess sequence. The thickness of L

27and L

32in the both

sections is about 6}8 m. Two or three weakly-developedsoils are recognizable within both L

27and L

32. Between

L27

and L32

, there are "ve closely spaced palaeosol units(S

27, S

28, S

29, S

30and S

31). In the classic loess sections

(e.g. Luochuan; Fig. 1), L27

and L32

correspond, respec-tively, to W

L}2and W

L}3. The base of the loess deposits

was de"ned at the base of WL}3

(Liu, 1985). However,subsequent stratigraphic observations have shown that,in the southern part of the Loess Plateau, another loess-soil couplet (S

32}L

33) is recognizable below L

32(Ding

et al., 1992). Accordingly, the base of loess sequence isnow "xed at the base of L

33. Micromorphological obser-

vations of the loess-soil sequence at Luochuan (Brongerand Heinkele, 1989) also indicate the presence of other

loess horizons below WL}3

. However, the loess horizoncorresponding to L

33at Baoji and Lingtai is mixed with

the underlying red clay materials in the Luochuan andXifeng sections.

Previous palaeomagnetic measurements (Heller andLiu, 1982; Liu, 1985; Liu et al., 1988a) showed that theMatuyama/Gauss magnetic boundary is located about3 m below the base of W

L}3, (i.e. in the red clay), whereas

this boundary is in the upper part of L33

(the oldest loessunit) at Baoji and Lingtai (Fig. 5). This apparent con#ictis the result of di!erent views on the precise location ofthe base of the loess. The unit L

33is a coarse-grained and

relatively thick loess horizon, and it is readily identi"edin the "eld in the southern part of the Loess Plateau.Therefore, the loess-red clay contact may be reasonablyplaced at the base of L

33with an age of about 2.6 Ma.

Obviously, this boundary between the loess and the redclay is consistent in age with the onset of glaciation in theNorthern Hemisphere (Shackleton et al., 1984, 1990;Raymo et al., 1989).

5. Grain size records of red clay sequences

The hypothesis that the Tertiary red clay deposits arewind-blown in origin has been supported recently bygrain size and rare earth element (REE) geochemicalstudies. Ding et al. (1998a) measured the REE contents of28 red clay samples at Jiaxian (Fig. 1) and 6 loess-soilsamples at Weinan (Fig. 1). The chondrite-normalizedREE patterns of the red clay and loess-soil samples arealmost identical. They are all characterized by LREEenrichments with a relatively #at HREE, and a slightlynegative Eu anomaly. The samples for grain size analysisat Jiaxian were taken at 3.3 cm intervals. Results ob-tained show that the pattern of grain size distribution ofthe entire red clay is almost the same as that of the loessdeposits (Ding et al., 1998a). The veri"cation of aeoliansedimentation of the red clay thus provides the basis onwhich the transporting wind system of the sediments maybe determined. In this study, four red clay sections, situ-ated at Jingbian, Jiaxian, Lingtai and Baoji (Fig. 1), weresampled at 3.3 or 5 cm intervals, and the grain sizedistribution and magnetic susceptibility of the sampleswere analyzed.

Fig. 6 shows the magnetic susceptibility records of theoldest loess horizon L

33and the upper part of the four

red clay sequences. L33

is a relatively thick loess horizonwith low susceptibility values in all of the four sections.The Matuyama/Gauss magnetic reversal is located with-in this horizon. The red clay sequences shown in Fig. 6were all deposited in the Gauss magnetic polarity chron.In the Jingbian, Jiaxian and Lingtai sections,the red clay of the Gauss Chron is about 25 m thick,whereas it is only about 17 m thick at Baoji. In thePleistocene loess-soil records, magnetic susceptibility


Fig. 6. Correlation of magnetic susceptibility records between the Jingbian, Jiaxian, Lingtai and Baoji sections over the Gauss magnetic polarityChron.

shows consistently higher values in palaeosols than inloess horizons. However, this phenomenon is not evidentin the red clay. It has been observed that some well-developed soils in the Lingtai red clay sequence showrelatively low-magnetic susceptibility (Ding et al., 1998b),implying that magnetic susceptibility is not as useful inthe red clay as in the overlying loess for stratigraphiccorrelation and palaeoclimatic reconstruction. The re-duced magnetic susceptibility values in the well ped-ogenically developed parts of the red clay may be a resultof destruction of magnetic minerals under seasonal wat-erlogging and reducing conditions, as suggested by abun-dant dark Fe}Mn "lms on the soil structural surfaces(Ding et al., 1998b). Nevertheless, the four susceptibilitycurves are broadly comparable (Fig. 6). At the same time,the grain size distributions show a more consistentchange in all four red clay sections, the details of whichare presented below.

5.1. Jingbian section

The Jingbian section (ca. 252-m thick) is located in thetransitional zone between the Loess Plateau and the MuUs desert (Fig. 1). The present mean annual rainfall inthis area is about 400 mm. One of the speci"c features ofthis section is that there are several palaeo-sand horizonswithin the loess deposits. These occurred mostly abovethe loess unit L

15formed at about 1.2 Ma. They can be

interpreted as the re#ection of an advance of the Mu Usdesert during dry periods. A red clay sequence about30-m thick is exposed immediately below the oldest loessunit L

33at Jingbian.

Fig. 7 shows the grain size record from the base of thered clay to the upper part of the loess horizon L

33at

Jingbian. Changes in the median grain size (Md) and thepercentage of clay ((2 lm), very "ne silt (2}5 lm), "nesilt (5}16 lm), medium silt (16}30 lm), coarse silt(30}63 lm) and sand ('63 lm) are plotted againstdepth pro"le. It can be seen that grain size is very "nethroughout the red clay, except for that part between255 m and 259 m that is a loess-like horizon. The mediangrain size of the red clay samples mainly centres around4}8 lm, which is similar to that of the palaeosols in theloess-palaeosol sequences in the southernmost part of theLoess Plateau. Changes in all the grain size parametersshow a similar pattern along the depth pro"le, althoughthere is an inverse correlation between "ne and coarsefractions. Another outstanding characteristic of the redclay grain size distribution is that sand particle sizes('63 lm%) are almost lacking. Moreover, the percent-age of coarse silt (30}63 lm) is also very low, beingmostly no more than 10% (Fig. 7).

5.2. Jiaxian section

Jiaxian is situated about 110 km southeast of Jingbian(Fig. 1). Here, the red clay sequence is about 60-m thickbelow L

33. Palaeomagnetic measurements (Ding et al.,

1998a) have shown that the red clay at this site hasa basal age of about 5.2 Ma. The grain size results of thatpart of the section from S

32to the Gauss/Gilbert

palaeomagnetic boundary are shown in Fig. 8. The grainsize characteristics recognized in the Jingbian section(Fig. 7) are again evident. As seen in Fig. 8, the clay


Fig. 7. Grain size parameter changes with depth at Jingbian. Seven di!erent grain size parameters are plotted. The oldest loess unit L33

and the upperred clay formation are shown. The Gauss/Gilbert palaeomagnetic reversal is found at about the bottom of the red clay sequence at Jingbian. Note thesimilarity of changes of di!erent grain size fractions.

Fig. 8. Changes in seven grain size parameters over the portion from S32

to the depth of 30 m at Jiaxian. The Matuyama/Gauss magnetic boundary isde"ned in L

33, and the Gauss/Gilbert magnetic reversal is found at about the depth of 28 m at Jiaxian (Ding et al., 1998a).


Fig. 9. Changes in seven grain size parameters of the oldest loess unit L33

and the upper part of the red clay in the Lingtai section.

particle content ((2 lm%) in the upper part of thesequence varies between 25% and 35%. Material of sandsize is also negligible, and the coarse silt content is mostlybelow 10%. In sum, most of the upper part of the red claysequence at Jiaxian consists of particles of less than30 lm in diameter.

5.3. Lingtai section

The Lingtai section is located in the middle part of theLoess Plateau (Fig. 1). In the Lingtai section, the thick-ness of the Quaternary loess}soil sequence is about175 m, while the Tertiary red clay sequence is about130-m thick. Palaeomagnetic study suggests a basal agefor the red clay at Lingtai of about 7.0 Ma (Ding et al.,1998b), which makes it the thickest and oldest red claydeposit presently known in the Loess Plateau. Fig. 9shows the grain size records of the Lingtai section fromthe middle of L

33to a depth of 210 m, at which the

Gauss/Gilbert palaeomagnetic polarity reversal wasmeasured. Again, the characteristics of grain size distri-bution at Lingtai are similar to those observed at Jin-gbian and Jiaxian.

5.4. Baoji section

The loess-red-clay sequence at Baoji (ca. 190-m thick)is located in the southernmost part of the Loess Plateau

(Fig. 1). Measurements of palaeomagnetic samples(Evans et al., 1991) suggest a basal age of about 4.3 Mafor the sequence. However, the lower part of the red claydeposit may be lacustrine, while a wind-blown origin hasbeen suggested for the upper part (Evans et al., 1991). Thegrain size of the upper red clay sequence at Baoji is very"ne, and the same grain size changes are observed atBaoji as at the other sites discussed above (Fig. 10).

Taking all the grain size results mentioned above, thesimilarity in the grain size distributions of the four redclay sequences may be summarized with reference to twoparticular aspects. First, most of the particles are "nerthan 30 lm, particles larger than this accounting for nomore than 10%. In the relatively "ne-sized fractions, theclay percentage ((2 lm%) lies mainly in the range of25}35%, the very "ne silt percentage (2}5 lm%) of20}30%, the "ne silt percentage (5}16 lm%) of 25}30%,and the medium silt percentage (16}30 lm%) of 10}20%.Second, changes in the median grain size (Md) in theupper part of all four red clay sequences are minor, themedian grain size varying only over the short range from4 to 8 lm.

6. Discussion and conclusions

The straight-line distance is over 400 km from theloess-desert transitional zone to the southernmost part of


Fig. 10. Changes in seven grain size parameters of the oldest loess unit L33

and the upper part of the red clay in the Baoji section.

the Loess Plateau. However, the grain size characteristicsof the four red clay sequences examined above show onlyslight variation from north to south. This is in sharpcontrast with the overlying loess}soil sequence. The grainsize of the loess deposits of the last glacial}interglacialperiod clearly shows a remarkable southward decrease atdi!erent intervals. A southward decrease of grain size inthe lowermost loess unit (L

33) is also observed (Figs.

7}10). These results simply suggest that the dominantdirection of the wind system that transported aeolianparticles across this region was quite di!erent before andafter 2.6 Ma BP.

Two atmospheric systems exert an important degree ofcontrol on the present climate of northern China inwinter half year. These are the northerly winter monsoonand the westerlies. Winter monsoonal winds are steeredby the Siberian high pressure system, and are consideredresponsible for the transport of aeolian particles fromdeserts to the Loess Plateau both at present and in thePleistocene (An et al., 1991b; Ding et al., 1992). Therelatively rapid southward decrease in the grain size ofboth loess units and soil horizons (Fig. 4) strongly sup-ports this idea. The westerlies may have also playeda role in dust transport, particularly during the inter-glacial periods of the Quaternary when the deserts re-treated to the northwest (Sun et al., 1998). However, thenorth}south sorting of loess grain size is stronger than isthe west}east sorting, as indicated by spatial analyses ofparticle size over the Loess Plateau (Fig. 1; Liu, 1966,1985). Therefore, it may be concluded that the winter

monsoonal #ow was the most important wind for thetransporting of aeolian particles onto the Loess Plateauduring the past 2.6 Ma.

What wind system transported the red clay material?The answer may depend partly on the source regions ofthe aeolian deposits before 2.6 Ma. Geological evidencehas shown that the desertization of northwestern Chinamay have occurred long before 2.6 Ma BP (Ding et al.,1992), as the rising Tibetan Plateau blocked penetrationof the moisture-laden air#ow from the Indian Ocean. Itfollows from this that the vast inland basins of north-western China acted as dust source regions before2.6 Ma, as demonstrated by the Neogene aeolian recordsderived from the North Paci"c (Rea, 1994; Rea et al.,1998). A preliminary calculation of atmospheric dustin#uxes for the Lingtai and Jiaxian loess-red-clay se-quences shows that the dust depositional rate during theperiod of 7.0}2.6 Ma was similar to that during theinterglacial periods of the Quaternary (Ding et al.,1998a, b). Therefore, a dust source region similar in ex-tent to that during the interglacials of the past 2.6 Ma,may have been present in northern China before thatdate. However, the consequent southward decrease in thegrain size of the Pleistocene palaeosols in the LoessPlateau is not observed in the Pliocene red clay sections.This strongly suggests that the northerly winter mon-soonal winds were less important as a transporting me-dium for the red clay material from the dust sourceregions and onto the Loess Plateau. This raises the possi-bility that it was the westerlies that transported and


deposited the red clay aeolian dusts, although this cannotbe tested until the west}east changes in red clay grainsizes have been investigated.

Field observations of palaeosols within the Quater-nary loess show a clear decline in their development fromsoutheast to northwest over the Loess Plateau (Liu,1985), a characteristic that is consistent with precipita-tion patterns of this region. As the summer monsoonair#ow derives from the low-latitude oceans, summermonsoonal rainfall shows a rapid northwestward de-crease in northern China, thus in#uencing strongly phe-nomena such as soil development. Recently, somepreliminary observations of the pedogenic characteristicsof the red clay sequences in the Loess Plateau have beenundertaken. A clear trend of changes in soil features canalso be seen. More speci"cally, the red clay in the south-ern part of the Loess Plateau contains abundant trans-located clay skins and dark Fe}Mn "lms. The content ofclay skins is generally higher in the red clay than in thepalaeosols within the Pleistocene loess. However, trans-located clay skins in the red clay decrease signi"cantly inthe northern part of the Plateau. In the Jingbian section,only a few clay skins can be seen in the entire sequence.This observation implies that during the accumulation ofthe red clay, the summer monsoon was also predominantand a similar climate gradient to that found in thepalaeosols also operated.

The sedimentological characteristics of the redclay/loess sequences in the Loess Plateau suggest forma-tion or intensi"cation of the winter monsoonal windsover East Asia at about 2.6 Ma, supporting an earlierspeculation (Ding et al., 1992). Marine Oxygen Isotoperecords have demonstrated that ice sheets became greatlyenlarged in the Northern Hemisphere at about 2.6 Ma(Shackleton et al., 1984, 1990; Raymo et al., 1989), beingconsistent in timing with the re-arrangement of the atmo-spheric pattern over East Asia. The winter monsoonalwinds #ow from the surface high-pressure cell over theMongolian}Siberian region, centred at about 1053E,503N. This cold high-pressure cell (the Siberian High) hasa central pressure value over 1040 mb in January,the strongest in the Northern Hemisphere. Meteorologi-cal observations (Tao and Chen, 1957) have shown thatmost of the cold air accumulating in the Siberian regionhas its sources in polar areas. About 56% of the aircomes from the Barents Sea with a northwesterly path-way, while the rest comes from the Kara Sea (25%) to thenorth and the North Atlantic Ocean (15%) to the west. Itis likely that by 2.6 Ma BP, gradual cooling led to in-crease in ice cover of the seas, thereby steering more coldair masses down to the Siberian region and causing theformation or intensi"cation the winter high. Therefore, itis preliminarily concluded that the atmospheric re-arrangement of northern China at about 2.6 Mamight have been caused by the onset of glaciation in theNorthern Hemisphere.

Acknowledgements

We are greatly indebted to the critical comments ofProfessor. E. Derbyshire and W. Ruddiman on an earlierversion of the manuscript. This research is supported bythe NNSF of China (49525203), CAS (KZ951-A1-402)and the Natural Sciences and Engineering ResearchCouncil of Canada.

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Documents

Re-arrangement of atmospheric circulation at about 2.6Ma over northern China: evidence from grain size records of loess-palaeosol and red clay sequences