Paleoenvironmental significance of the magnetic fabrics in Chinese loess-paleosols since the last interglacial (

Paleoenvironmental signi¢cance of the magnetic fabrics inChinese loess-paleosols since the last interglacial (6 130 ka)

Rixiang Zhu a;�, Qingsong Liu b, Michael J. Jackson b

a Paleomagnetism Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR Chinab Institute for Rock Magnetism and Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA

Received 7 July 2003; received in revised form 5 January 2004; accepted 29 January 2004

Abstract

We compare new anisotropy of magnetic susceptibility (AMS) data with previously-determined paleoclimaticallysensitive parameters for a loess profile at Lingtai, central Loess Plateau, China, to unravel factors that control loessAMS. Magnetic lineations are clustered along NE^SW and NW^SE orientations during cold and warm periods,respectively, which probably corresponded to paleowind directions of both winter and summer monsoons.Anomalously steep magnetic foliations coincidentally occur at the paleoclimatic stage boundaries, indicatingsediment disturbance. Lineation strength appears to be sensitive to millennial-scale rapid paleoclimatic fluctuations.Generally, the winter monsoon produces a lower degree of lineation than the summer monsoon. Furthermore, bystudying lineation subsets for the cold and warm periods, we find that the cold-interval lineation strengths co-varywith grain size over the last V70 ka. During the last interglacial (about 74^128 ka), the lineation and declination ofthe susceptibility maximum (Dec-Kmax) are sensitive to rapid fluctuations in winter and summer monsoons. Long-termtrends of magnetic foliation and anisotropy degree replicate grain size variations, with superimposed short-term peaksrelated to post-depositional compaction, as suggested by density. The AMS shape parameter (T) indicates oblatefabrics. These new findings will not only improve our understandings of loess AMS, but also expand the possibilitiesof paleoclimatic studies in the Chinese loess.; 2004 Elsevier B.V. All rights reserved.

Keywords: AMS; Chinese loess; Lingtai; paleowind strength; paleowind direction; monsoon

1. Introduction

The anisotropy of the low-¢eld magnetic sus-ceptibility (AMS) is a rapid and precise tool fordetermining the fabric of rocks and sediments byquantitatively measuring the average preferred

mineral orientation [1^4]. During the past severaldecades, AMS has been widely used to estimatemagma £ow directions [5] and principal tectonicstrains [6]. For such studies, detailed knowledgeof the sources of magnetic anisotropy is usuallynot critically important. However, applications ofAMS to eolian sediments (e.g. Chinese and Alas-kan loess) are rare [7]. The main reason is that theAMS of eolian sediments is controlled by a com-plex set of processes including both depositionalfactors (wind strength, types of detrital eolian

0012-821X / 04 / $ ^ see front matter ; 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(04)00103-7

* Corresponding author. Tel. : +86-10-62007912;Fax: +86-10-62010846.E-mail address: [email protected] (R. Zhu).

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Earth and Planetary Science Letters 221 (2004) 55^69

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minerals), and post-depositional alteration (e.g.pedogenesis). The AMS of eolian sediments rep-resents the sum of contributions from detrital andauthigenic minerals and is a convolution of pri-mary and secondary fabrics.

The Chinese loess (Fig. 1) is a climatically sen-sitive wind-blown deposit, a¡ected by both sum-mer and winter monsoons in a region of half amillion square kilometers [8]. Generally, eolianinputs were carried by the winter monsoon fromnorthern source areas and were deposited on theLoess Plateau during cold/dry periods. Eolian in-puts dramatically decreased and soils developedduring warm/humid periods.

The earliest AMS study on the Chinese loess byHeller et al. [9] was an attempt to evaluate theuniformity of loess deposition rate through timeat Luochuan. Paleowind directions were later es-timated by AMS lineation directions [10^12]. Therelationship between wind direction and orienta-tion of Kmax (where Kmax and Kmin are, respec-tively, the maximum and minimum principalaxes of the AMS ellipsoid) was further examinedby wind-tunnel simulation experiments [13]. Theseexperiments showed a strong correlation, with de-viations less than 20‡. Liu et al. [10] found thatredeposited loess has a higher degree of aniso-

tropy (1.0329PrKmax/Kmin 9 1.064) than wind-blown deposits (1.0029P9 1.032). Recently, Zhuet al. [14] and Pan et al. [15] conducted parallelAMS and paleomagnetic studies to identify thepossible e¡ects of deformation or disturbance onthe ¢delity of natural remanent magnetization(NRM) records. They found that AMS parame-ters are fairly stable and uniform since the lastinterglacial period, and thus that anomalousNRM directions can be attributed to ¢eld behav-ior rather than to disturbance of the sediments.AMS has also been measured for Alaskan loess[7], where the AMS faithfully records eolian trans-port directions, which coincide with major paleo-climatic changes over the Alaskan region, e.g. therecorded wind direction shifted from a NW^SE toN^S direction from glacial to interglacial periods,respectively.

Despite these previous attempts, there is still adearth of magnetic fabric studies for the Chineseloess/paleosols, especially in relation to their pa-leoclimatic signi¢cance. We systematically in-vestigate the AMS parameters and their correla-tions with widely-accepted paleoclimatic param-eters (e.g. susceptibility, grain size, etc.) of loesssamples from Lingtai (Fig. 1), Gansu Prov-ince, China, since the end (V130 ka) of Marine

Fig. 1. Schematic map showing the location of the Chinese loess plateau (left panel) and the sampling site (right panel). In theright panel, the gray region marks the distribution of loess deposits, and the dotted region shows the distribution of deserts. Theopen arrows represent the northwesterly winter monsoon winds; those with lighter outlines are the inferred directions of the win-ter monsoons a¡ected by the Liupan Mountains [17]. The solid gray arrows indicate the southeasterly summer monsoon winds.

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R. Zhu et al. / Earth and Planetary Science Letters 221 (2004) 55^6956

Oxygen Isotope Stage 6 (MIS6) (Loess unit L2).Speci¢cally, we aim to provide a basis for inter-preting loess AMS and its paleoclimatic signi¢-cance.

2. Site description and sampling

The Lingtai section (107‡30P33QE, 35‡00P75QN)is located in the central part of the Chinese LoessPlateau (Fig. 1), and consists of 175 m of Pleisto-cene loess-paleosol sediments in the upper partand 130 m of Pliocene red-clay sediments in thelower part [16,17]. The thickness of the Holocenesoil S0 is about 1.5 m. The loess unit L1, fromdepths of 1.5^7.0 m, consists of light yellow-graysandy silt. In the underlying 3 m, paleosol unit S1(MIS5) is characterized by relatively higher sus-ceptibility.

Samples were collected along a well-exposedgully wall after removing a half-meter thicknessof exterior sediments, to eliminate weathered ma-terial and disturbance due to vegetation. Largeblock samples, with approximately 10-cm sidesand 30-cm height, were cut and orientated insitu using a magnetic compass. Cubic specimens(8 cm3) were prepared from these blocks in thelaboratory.

3. Experimental methods

To determine the magnetic mineralogy andgrain size distribution, rock magnetic measure-ments were made, including temperature-depen-dent susceptibility, hysteresis loops, and fre-quency-dependent susceptibility (Mfd and Mfd%,as de¢ned below).

Stepwise temperature-dependent susceptibilitycurves were measured in an argon atmosphereusing a KLY-3 Kappabridge equipped with aCS-3 high-temperature furnace (Agico Ltd.,Brno). The peak experimental temperatures were200, 300, 400, 500, 600, and 700‡C. Each runstarted with V300 mg of fresh material. Magnetichysteresis loops were measured for small chipsamples on a MicroMag 2900 Alternating Gra-dient Magnetometer (Princeton Measurements

Corp.). A Day-plot [18] (Mrs/Ms vs. Bcr/Bc, whereMs and Mrs are saturation magnetization and sat-uration isothermal remanent magnetization; Bcr

and Bc are, respectively, remanence coercivityand coercive force) was then constructed after re-moving the e¡ects of high-¢eld susceptibility car-ried by paramagnetic minerals.

AMS of all samples was measured using aKLY-3s Kappabridge (Agico Ltd., Brno) withan automated sample handling system. Each sam-ple was rotated through three orthogonal planes.The susceptibility ellipsoid was calculated by aleast-squares method. The large number of mea-surements permits precise determination of AMSellipsoids even though the degree of AMS is fairlylow.

Frequency-dependent susceptibility was ob-tained by measuring susceptibilities at two fre-quencies (470 and 4700 Hz) with a BartingtonInstruments MS2 magnetic susceptibility meter.Mfd and Mfd% were calculated as M400Hz�M4000Hz,and 100*(M400Hz�M4000Hz)/M400Hz, respectively.

4. Results

4.1. Temperature-dependent susceptibility

Temperature-dependent susceptibilities (M-T)for characteristic loess (6.02 m) and paleosol(8.98 m) samples are shown in Fig. 2. M-T curvesare sensitive to mineralogical changes during heat-ing [19^22]. A loess sample after heating to 200‡Chas a room temperature susceptibility V8%above its initial value (Fig. 2a). Between 165and 290‡C (Fig. 2b), the susceptibility is almosttemperature-independent. Above 290‡C (Fig. 2c),susceptibility begins to decrease with increas-ing temperatures. Signi¢cant neoformation ofstrongly magnetic minerals occurs above V450‡C(Fig. 2d), most of which occurs above 600‡C (Fig.2e,f).

Paleosol samples share most of the M-T featuresof loess samples. Susceptibility increases up to250‡C (Fig. 2g,h) and decreases while heatingabove that temperature (Fig. 2h). The 400‡C runof the paleosol and loess samples shows similarbehavior (Fig. 2c,i). The 500‡C and 600‡C runs

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R. Zhu et al. / Earth and Planetary Science Letters 221 (2004) 55^69 57

(Fig. 2j,k) do not show as much production offerrimagnetic minerals as the same runs for theloess (Fig. 2d,e). New ferrimagnetic minerals areproduced only above 600‡C. For the coolingcurve after the 700‡C thermal treatment, both

loess and paleosol samples show a dominantpeak at around 400‡C.

The susceptibility decrease between 300‡C and450‡C has been widely interpreted as the mineralinversion from strongly magnetic maghemite to

Fig. 2. Temperature-dependent susceptibility curves (normalized by the initial value) showing heating to di¡erent maximum tem-peratures for a representative loess and paleosol sample.

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weakly magnetic hematite [20^25]. It is, however,noted that the ¢nal susceptibility values at roomtemperature (ME) after heating to 500‡C are higherthan or equal to those of the fresh samples (Fig.2d,j for loess and paleosol, respectively). If pedo-genic maghemite has inverted to hematite above300‡C during the heating experiments, the resul-tant loss of ME must be compensated by formationof new ferrimagnetic material even below 500‡C.

4.2. Day-plot

Hysteresis results are illustrated in the Day plot[18] in Fig. 3. The single domain/pseudo-single-domain (SD/PSD) and PSD/multidomain (PSD/MD) boundaries are based on Dunlop [26,27].Clearly, both loess and paleosol data are locatedin the PSD grain size region (Fig. 3a). The tworatios (Mrs/Ms and Bcr/Bc) are positively corre-lated (Fig. 3b). This phenomenon has been attrib-uted to low-temperature oxidation (LTO) [28].Generally, LTO results in formation of partiallyoxidized magnetite, with an inner magnetite corecovered by a maghemitized rim, occurring mainlyat the crystal surface or in ¢ssures, which causedsigni¢cant increases in Mrs and Bcr [29,30]. Thus,the (Mrs/Ms and Bcr/Bc) ratios will increase simul-taneously. Alternatively, Dunlop [27] interpretedthe right-upward departure of previously-pub-lished Chinese loess data from the synthetic trendestablished by Day et al. [18] as the consequenceof a mixture of SD+10-nm superparamagnetic(SP) grains. However, Liu et al. [28] pointed outthat this interpretation is inconsistent with thecommon observation that SD+SP particles areless important for loess compared to paleosol

samples. Therefore, we attribute the hysteresis ra-tio trend to a greater proportion of partially-oxi-dized magnetite in the loesses.

LTO e¡ects are more evident in coarser-grainedmagnetic assemblages because the ¢ner-grainedmagnetic particles eventually become fully oxi-dized. Thus, the positive correlation revealed be-tween Mrs/Ms and Bcr/Bc suggests that the mag-netic carriers are dominated by coarser-grainedparticles.

4.3. AMS and paleoclimatic proxies

AMS can be described in terms of an ellipsoidwith three orthogonal principal axes, designatedthe maximum, intermediate, and minimum sus-ceptibility (Kmax, Kint, and Kmin) axes, respectively.These quantities can be combined in various waysto describe the shape of the ellipsoid and the fea-tures of the corresponding magnetic fabrics [31].The major AMS parameters examined in thisstudy are the:

lineation ðLÞ ¼ Kmax=K int; ð1Þ

foliation ðFÞ ¼ K int=Kmin; ð2Þ

degree of AMS ðPÞ ¼ Kmax=Kmin; ð3Þ

and

shape parameter of AMS ðTÞ ¼

ð2R23R13R3Þ=ðR13R3Þ; ð4Þ

where R1,R2 and R3 are ln(Kmax), ln(Kint) andln(Kmin), respectively.

The other parameters are the orientation ofKmax, speci¢ed by inclination (Inc-Kmax) and de-clination (Dec-Kmax), as well as the inclination ofKmin (Inc-Kmin). Parameters F12, F23 are used toevaluate the statistical signi¢cance of the lineationand the foliation, and generally, values largerthan 4 indicate statistically signi¢cant anisotro-pies. E12 indicates the angular uncertainty in thedirection of Kmax within the magnetic foliationplane. Because the measured foliation is generallyalmost horizontal, E12 represents the 95% con¢-dence angle for the azimuth of Kmax.

Almost all of these samples have statistically

Fig. 3. Day plots [18] for loess and paleosol samples. Thedashed line in (b) shows the positive correlation betweenMrs/Ms and Bcr/Bc, which we attribute to the e¡ects of low-temperature oxidation.

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signi¢cant magnetic lineations, with F12s 4 andE126 20‡ (Fig. 4a). F12 has a weak positive cor-relation with susceptibility (Fig. 4b), probably dueprimarily to the increasing importance of randommeasurement errors for samples with lower bulksusceptibility. Even extremely weak lineations(L6 1.001) are generally determined with a highstatistical signi¢cance (Fig. 4e) and low azimuthaluncertainty (Fig. 4d). All samples have well-re-solved magnetic foliations (F23s 10, Fig. 4c).

Our AMS results are compared with previ-ously-published paleoclimatically sensitive param-eters in Fig. 5. Susceptibility, grain size and drybulk density are used to construct the pedostratig-raphy, with the main boundaries, MIS6/5, MIS5/4, MIS4/3, MIS3/2, and MIS2/1 located at 10.0,7.0, 5.3, 3.0, and 1.5 m, respectively. Sub-unitswithin MIS5 are further distinguished by drybulk density results (Fig. 5c). Generally, paleosolshave high susceptibility, ¢ne grain size and high

density. It appears that density is better able toresolve ¢ne-scale details than susceptibility orgrain size. This is mainly because susceptibilityis sensitive to pedogenic alteration. Neoformationof a small amount of maghemite/magnetite par-ticles can signi¢cantly enhance susceptibility. Inaddition, pedogenesis always proceeds downwardinto the underlying material, which obscures theoriginal paleoclimatic boundaries [32]. However,dry bulk density is determined by the non-mag-netic matrix, which seems to be sensitive to eolianinputs and post-depositional compaction, but lesssensitive to pedogenic alteration.

To reduce experimental errors, measurementswith F126 4 and E12s 20‡ were rejected, andthe curves were smoothed by ¢ve-point movingaverages, which still enables resolution of millen-nial-scale variations. The lineation ‘polarities’have been chosen so that Dec-Kmax is in the in-terval 0^180‡. The AMS foliation (Fig. 5d) andmedian grain size (Fig. 5b) have similar varia-tions, with dominant peaks between 10 m and10.5 m, and a broad minimum within paleosolS1 (7^10 m). The two records are inconsistentabove 4.5 m, where the median grain size gradu-ally increases from V8 Wm to V14 Wm, whileshort-term £uctuations in foliation seemingly cor-relate with variations in density.

Stratigraphic variations of Dec-Kmax and Inc-Kmax are shown in Fig. 5e,f, respectively. A fea-ture of the Inc-Kmax record is the sharp increase at5.5^5.3 m. Most of the samples have Inc-Kmin s 70‡ (Fig. 5g). However, three Inc-Kmin

anomalies are observed at V5.2, 7.0, and 8.2 m,respectively. These three anomalies approximatelycoincide with the paleoclimatic boundaries MIS4/3, MIS5a/4, and MIS5b/5a, respectively.

Frequency-dependent susceptibilities (Fig. 5h,i)are used to estimate the contributions of pedogen-ic SP grains [33]. Variations in the absolute valueof Mfd replicate most of the features of the bulksusceptibility record. However, the relative valueMfd% shows that even for intermediate paleosolsamples (3.0^5.3 m), Mfd% reaches maximum val-ues of V12%. The peaks of Mfd% correspond towarm periods with relatively strong pedogenesis,while the Mfd% minimum values appear to markrelatively colder periods.

Fig. 4. Statistical signi¢cance and angular uncertainties forthe AMS (see text for details). The gray box in (a) marksthe samples with F126 4 and E12s 20‡.

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The shape parameter (T) of the AMS ellipsoidsis generally located in the oblate region (Fig. 6a).However, T systematically departs from oblateshapes toward the neutral line (T= 0) or even to-ward the prolate region (T6 0) for the matureand intermediate paleosols. There is also an ap-parent tendency toward decreasing T in the upperhalf of the section (Fig. 6a). This tendency is alsoseen in generally decreasing P and F (Fig. 6b,c).

Fig. 6. Correlation between (a) T, (b) P, and (c) F. The thicklines indicate linear trends between 1.5 and 6 m. The hori-zontal thick and thin dashed lines in (a) show the upper lim-its and the neutral lines (T= 0), respectively. Vertical dashedlines correlate peaks in T, P and F. The light gray bar in (c)shows the background value (F6 1.003) below which theshape of the AMS ellipsoid is shifted from the oblate regionto the neutral or even prolate region.

6

Fig. 5. Depth plots of AMS parameters and paleoclimaticallysensitive parameters: (a) low-¢eld magnetic susceptibility; (b)median grain size [17]; (c) density; (d) foliation F ; (e) Dec-Kmax ; (f) Inc-Kmax ; (g) Inc-Kmin ; (h) absolute frequency-de-pendent susceptibility Mfd ; and (i) relative frequency-depen-dent susceptibility Mfd (%). Vertical dashed lines represent themajor paleoclimatic boundaries. Gray-bars mark the sub-units S1S1^S1S3.

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Variations in L (1.001 V 0.001) are much smallerthan those in F (1.012 V 0.012) (Fig. 7), which iswhy the F and P curves (Fig. 6) are nearly iden-tical.

5. Discussion

In this section, we link the AMS parameterssystematically to other well-determined paleocli-matic proxies in the Chinese loess/paleosol se-quence. Our working hypothesis is that if anAMS parameter exhibits temporal evolution pat-terns similar to one or more paleoclimatically sen-sitive parameters, then the AMS parameter wasprobably controlled by the same paleoclimati-cally-driven mechanisms. First, we will provide ageneral framework for understanding the loessAMS signal. Second, we will propose a depthâge transformation model. Finally, we will discussthe paleoclimatic signi¢cance of the major AMSparameters.

5.1. Interpretive framework for the loess AMS

The primary loess fabrics are acquired duringits accumulation, and are determined by the com-position, size, shape and concentration of eoliandetrital grains as well as by the paleowind

strength. The primary fabrics are further a¡ectedby post-depositional factors, e.g. mainly by com-paction and by neoformation of ferrimagneticparticles through pedogenesis. All minerals (in-cluding dia-, para-, and ferri-magnetic grains)contribute to AMS. Among them, the majorityof paramagnetic silicates (micas, pyroxenes, am-phiboles, clays) have crystallographically-con-trolled grain-scale magnetic susceptibility ellip-soids that roughly coincide with grain shape[34,35]. Grain-scale anisotropy is typically shape-controlled for ferrimagnetic particles. However,‘inverse’ AMS can be important in ¢ne-grainedferrimagnetic assemblages containing SD grains[36]. Therefore, knowledge of the grain size distri-bution of the magnetic particles is essential forevaluating the origin of AMS.

The mineralogical di¡erences [37^39] and grainsize di¡erences [33,38,40] between loess and paleo-sol have been extensively studied. It is well ac-cepted that magnetite, maghemite, and hematiteare the dominant magnetic minerals in the Chi-nese loess/paleosol sequences, and that the sizedistribution of strongly magnetic grains rangesfrom below 30 nm (SP) to above 30 Wm (MD).Fortunately, the dominant AMS signal of theloess appears to be carried by one major fraction,i.e. ferrimagnetic minerals. Paramagnetic mineralsand antiferromagnetic minerals (hematite andgoethite) make only minor contributions becauseof their weak magnetic susceptibilities.

It is well determined that the eolian ferrimag-netic minerals are coarser-grained particles, pro-ducing primarily oblate AMS ellipsoids for loessunits, which correspond to cold periods. In con-trast, during warm periods, pedogenesis producedsecondary ¢ne-grained particles through decom-position of paramagnetic and silicate minerals.Statistically, these ¢ne-grained particles will inher-it the sedimentary fabrics (but with an inverseAMS) of these paramagnetic and silicate mineralsbecause they are usually formed within or adhereto the cleavage planes of laminated paramagneticiron-bearing clays, micas, etc. [3]. Thus, the mea-sured AMS is the sum of the primary oblateellipsoids and pedogenically-produced secondaryinverse prolate or oblate ellipsoids that aredependent on the grain size distribution and spa-

Fig. 7. Plot of L against F. The dashed line corresponds tothe triaxial shape. Clearly, the shape of the AMS ellipsoid ismainly controlled by F. When F is larger than V1.003, theAMS ellipsoid is oblate.

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tial distribution of these newly-formed magneticparticles.

5.2. Age model and depthâge transformation

The pedostratigraphy of the studied section wasdetermined by a combined approach including¢eld color, magnetic susceptibility, grain size,and density, and was further constrained by the14C and thermoluminence (TL) dating [15]. Themain paleoclimatic boundaries of MIS6/MIS5(128 ka), MIS5/MIS4 (74 ka), and MIS2/MIS1(10 ka) are at 10, 7, and 1.5 m, respectively.Sub-units within S1 (S1S1-S1S3) are further rec-ognized using dry bulk density data (Fig. 5c). Asample taken from a depth of 3.5 m gave a 14Cage of 26.5 ka [15]. Two samples taken from 4.8and 5.52 m gave TL ages of 38.1 and 47.5 ka,respectively. These age control points are shownin Fig. 8. Clearly, the sedimentation rate sharplyincreased from 5.6 to 12.5 cm/kyr across theMIS4/3 boundary (V5.2^5.3 m). Two lineartrends can be ¢tted to the data below and above5.25 m, respectively. R2 for each linear trend is0.99. The lines in Fig. 8 de¢ne an age^depthtransformation that is used in Figs. 9 and 10.

AMS and paleoclimatically sensitive parametersare shown vs. age in Fig. 9 along with the inde-pendently-dated GISP2 oxygen isotope records.Although there are inevitable chronological un-certainties in making such comparisons, there ap-pear to be some signi¢cant similarities betweenthe GISP2 and loess records. In particular, theorientation and strength of the magnetic lineationin the loess (Fig. 9b,c) exhibit strong and system-atic variations over time scales similar to those of

Fig. 8. Age model for the studied pro¢le from Lingtai. Thethick and thin dashed lines are the ¢tted trend above and be-low 5 m. The slopes for these two lines are 17.97 kyr/m(above 5.2 m), and 7.97 kyr/m (below 5 m), which corre-spond to sedimentation rates of 5.6 and 12.5 cm/kyr, respec-tively. The sedimentation rate increased suddenly at around40 ka.

Fig. 9. Age plots of (a) the GISP2 oxygen isotope record,AMS parameters (b) Dec-Kmax, and (c) lineation. Grain size[17] and absolute Mfd are shown in (d) and (e). Inc-Kmax isplotted in (f). The solid and open stars in (c) represent thelineation values during the peak cold and warm periods, re-spectively. Thick dashed lines (GISP2 age) and thin dashedlines (loess age) are used to further correlate these two re-cords. Slight shifts are due to age uncertainties of these twodi¡erent records, but within age errors (about several ka).

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the dominant isotope variations in the GISP2 rec-ord (Fig. 9a). Relatively small shifts in the Lingtaichronology would allow major features in theAMS record to be correlated to correspondingfeatures in the GISP2 record. Keeping in minduncertainties in the correlation, it is interestingto explore in more detail the correspondence ofAMS and climate cycles. Minimum values of Dec-Kmax (Fig. 9b) correspond to cold periods inGISP2 (Fig. 9a). In Fig. 9c, we separately markchanges in lineation of cold and warm periods.Clearly, to ¢rst order, lineation variations duringcold periods can be described as a ‘V’ shape witha minimum between 34 and 40 ka. In contrast,variations in lineation during warm periods seemto have an inverse relationship with those during

cold periods. Inc-Kmax also has a strong positiverelationship with pedogenesis (Fig. 9e,f).

Fig. 10 is used to scrutinize the AMS behaviorwithin paleosol unit S1. The cold events namedC19^C26 (Fig. 10a) [41] have counterparts withinthe chronological uncertainties on the L curve(Fig. 10b). We observe that during cold periods,dominant directions of Dec-Kmax are oriented to-ward the NE (Fig. 10c). However, there is an in-verse relationship between the ¢rst-order trend ofL and grain size (Fig. 10d).

5.3. Inc-Kmin, Inc-Kmax, and Dec-Kmax

The three major Inc-Kmin anomalies are locatednear paleoclimatic transition boundaries (Fig. 5g).This coincidence suggests that paleoclimaticchanges strongly a¡ected the transportation anddeposition of eolian material, which resulted inhigher disturbance of the loess deposits.

Unlike Inc-Kmin, Inc-Kmax (Fig. 5f) shows a‘step-like’ pattern at around 5.2^5.3 m, which di-rectly relates to the sharp change in sedimentationrate at this depth (Fig. 8). The sedimentation ratesuddenly increased from 5.6 cm/kyr below 5.2^5.3 mto 12.5 cm/kyr, which indicates a sharp changein the overall paleoclimatic pattern in this pro¢le(at around 40^45 ka), which corresponds to theMIS4/MIS3 paleoclimatic boundary. Experimentsinvolving sediments deposited in water under lab-oratory conditions show that under weak or mod-erate currents, the long-axes of prolate grains pro-duce a lineation parallel to the direction of £ow[42,43]. The imbrication is generally less than 10‡.The magnetic foliation is still essentially restrictedto the bedding plane. However, under strongercurrents, the magnetic fabric is more imbricated,tilting by 5^20‡ away from the bedding plane.

Loess, of course, is a wind-blown deposit. Ifeolian deposition is analogous to deposition under£owing water, we may deduce that the large in-crease in Inc-Kmax at around 40^45 ka (5.2^5.3 m)is an imbrication caused by increased wind inten-sity.

The loess deposits are controlled by both thewinter and summer monsoons. Pedogenesis alsosharply increases above 5.2^5.3 m (Fig. 5h,i).We therefore conclude that the corresponding in-

Fig. 10. Age plots of (a) cold events, (b) L, (c) Dec-Kmax

and (d) grain size between 60 and 140 ka. Symbols C19^C26and the corresponding arrows in (a) are the cold eventsdocumented in the MD95-2036 sea surface temperature rec-ord over the subpolar Atlantic region by Lehman et al. [41].The gray arrow shows an alternative interpretation of C23.The solid and open stars in (b) represent the lineation valuesduring the peak cold and warm periods, respectively. Thedashed line in (a) is the third-order polynomial trend for thewhole curve.

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crease in Inc-Kmax re£ects an increase of thesummer monsoon intensity. To ¢rst order, Inc-Kmax gradually decreased since V40 ka. This pat-tern is also recorded by variations in lineationduring warm periods (Fig. 9c, open stars).

Generally, the initial fabrics of loess are mainlydetermined by gravitational settling and windstrength. When the eolian inputs are relativelyuniform through time, we expect that statisticallyhigher lineation will correspond to higher windstrength. However, deposition of loess is con-trolled by the coexisting winter and summer mon-soons. The winter monsoon brings eolian inputsfrom northern sources to the Loess Plateau. Incontrast, the summer monsoon does not bringmaterial for deposition; it reworks the surfacematerial deposited during cold periods, and de-stroys the primary lineation produced by wintermonsoons by realigning the long-axes of magneticminerals (generally SE^NW). Thus, the declina-tion of Kmax could partly record the directionsof both winter and summer monsoon winds. Bycomparing Dec-Kmax and the GISP2 oxygen iso-tope record, we observe that Dec-Kmax systemati-cally shifts from SE directions during warm peri-ods to NE directions during cold periods. Thisindicates that the dominant wind directions forwinter and summer monsoons in this region areNE^SW and SE^NW, respectively (Fig. 11).

A possible alternative explanation is suggestedby the results of £owing water deposition experi-ments, which show that under some £ow condi-tions, elongated particles roll around their longaxes, producing a horizontal magnetic lineationperpendicular to the £ow axis. The near-perpen-dicular groupings shown in Fig. 11 could there-fore conceivably be due to signi¢cant changes indepositional £ow regime rather than the changesin wind direction. The lack of signi¢cant di¡er-ence in the imbrications of the two data sets, how-ever, seems inconsistent with this interpretation,and we therefore favor the wind-direction hypoth-esis.

We note that the observed NE^SW direction ofthe winter monsoon winds based on AMS is notconsistent with the view that the winter monsoonsprevail along the N^S or NW^SE direction. Thismay imply that the direction of the winter mon-

soon is locally a¡ected by the surrounding moun-tains (Fig. 1). However, this hypothesis needs fur-ther testing by studying the spatial distribution ofDec-Kmax along some well-designed loess tran-sects. If the wind direction model in Fig. 1 isreasonable, then we would expect some systematicshifts in the wind direction along the Jingbian^Xifeng^Lingtai transect (Fig. 1).

Our interpretation above is based on the hy-pothesis that AMS lineation orientations accu-rately re£ect the eolian-controlled preferred orien-tation of elongate grains. It is also common thatlineation is produced by the superposition of twofabrics. Thus, lineation does not necessarily rep-resent either fabric, but rather it might parallelthe intersection of two competing foliations [6].Nevertheless, we suggest that the rapid £uctua-tions in Inc-Kmax re£ect the interaction of thecompeting processes of the summer and wintermonsoons in this region.

5.4. F, L, T and P

AMS foliation is controlled by the grain sizedistribution of ferrimagnetic minerals and bypost-depositional compaction. Gravitational forcesalign the long-axes of the coarser-grained particleswithin the horizontal plane more e⁄ciently thanfor the ¢ner-grained particles. The grain sizes ofeolian inputs (including the magnetic minerals)are in turn determined by the intensity of the

Fig. 11. Results of AMS principal directions for samples cor-responding to the peak of (a) cold and (b) warm periods rec-ognized in Figs. 9 and 10 since the last interglacial (V130ka). Squares and circles represent Kmax and Kmin, respec-tively. Cold and warm periods refer to the millennial scalepaleoclimatic £uctuations.

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winter monsoon. Thus, the intensity of wintermonsoon, grain sizes of eolian inputs, and folia-tion are somewhat inter-related. To ¢rst-order,the higher intensity of the winter monsoon willcarry coarser-grained eolian material, thereby re-sulting in a larger foliation during deposition.Post-depositional compaction also contributes tothe observed foliation by rotating the long-axes ofmagnetic minerals into the bedding plane. Thus, ahigher degree of compaction will increase folia-tion as well as the density of samples. Long-term variations in foliation are controlled by thegrain size distribution (Fig. 5b^d). However, thesuperimposed short-term peaks are more relatedto changes in sediment density, which is a proxyfor compaction.

The GISP2 oxygen isotope record (Fig. 9a)contains numerous abrupt paleoclimatic oscilla-tions during the last interglacial, with millennialscale warming intervals, referred as to Dans-gaardÔeschger (DÔ) events. The e¡ects of theseDÔ events and Heinrich events are not con¢nedto the North-Atlantic region, but have global im-portance, e.g. in the Norwegian Sea [44], Asia[45], and Europe [46]. Generally, winter andsummer monsoons prevailed over the Loess Pla-teau during cold and warm periods, respectively.Fluctuations of the wind-direction proxy Dec-Kmax re£ect the interactions of these two windsystems. Dec-Kmax minima (Fig. 9b) appear tocoincide with cold events (low N

18O values inFig. 9a). Thus, we can separately investigate thesubset variations of both Dec-Kmax and lineationduring warm and cold periods by connecting themajor peaks and troughs for each.

The cold subset curve (solid stars in Fig. 9c)exhibits a ‘V’ shape, which almost replicates the¢rst-order trend of grain size. This suggests thatthe cold subset curve of lineation re£ects thestrength of the winter monsoon. However, thereis a more complex relationship between the line-ation of the warm subset curve and the summermonsoon intensity. The ¢rst e¡ect of the summermonsoon is to disturb the winter monsoon linea-tion. Therefore, we should expect a relativelystrong lineation when the summer monsoon isweak, due to preservation of the winter lineation.As the summer monsoon gets stronger, partial

disruption and replacement of the winter lineationshould result in gradually diminishing L. Finally,when the summer monsoon is strong, it shouldimpose the dominant lineation, and only thenshould we expect a positive correlation. There-fore, variations in cold and warm subset curvesof lineations during the glacial and interglacialperiods should faithfully re£ect the £uctuationsin winter and summer monsoon intensities, re-spectively.

Between 12 and 34 ka, the summer and wintermonsoon intensity do not closely correlate, butgenerally parallel patterns are observed between40 and 70 ka. Between 70 and 130 ka (Fig. 10),Dec-Kmax minima also appear to coincide withcold events revealed from the subpolar Atlanticregion [41]. The subset curves of lineation (Fig.10a) show that both winter and summer mon-soons are strong around 100^120 ka. The coldsubset lineation variations con£ict with the grainsize variations. Liu et al. [28] proposed that grainsize is not consistently a reliable winter monsoonproxy especially during interglacial periods be-cause pedogenesis can signi¢cantly decrease thesize of clay minerals. However, for samples youn-ger than 70 ka, pedogenesis was relatively weak.For that interval, grain size variations can be usedto gauge the winter monsoon variations, as dem-onstrated by the similar ‘V’ shapes of the grainsize curve and the cold-subset lineation curve.

A further signi¢cant observation is that thewinter monsoon, which dominates during coldintervals, produces lower lineations than thesummer monsoon. This is probably because thesummer monsoon produces a secondary lineationby reworking the loess deposited during cold pe-riods, whereas the winter monsoon directly con-trols the lineation through deposition.

5.5. Uncertainties in this study and future work

This study provides a new interpretation ofAMS results from Chinese loess/paleosol depositsin terms of paleoclimatic £uctuations. Millennialscale variations in various AMS parameters andtheir similarities to £uctuations in paleoclimati-cally sensitive parameters (density, grain size, sus-ceptibility, frequency-dependent susceptibility, etc.)

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suggest that the AMS results encode rapid paleo-climatic changes. However, we acknowledge theuncertainty in our correlation with the GISP2 rec-ord, due to limitations in the chronological con-trols for Lingtai. These uncertainties easily allowthe proposed correlations, but they do not allowde¢nitive establishment of contemporaneity orleads/lags between the signals.

Our age model mainly relies on the pedostrati-graphic boundaries that represent the MIS6/5,MIS5/4, MIS4/3, and MIS3/2 boundaries. How-ever, Liu et al. [32] argued that pedogenesis pro-gresses downward into the underlying glacialdeposits, thus obscuring the original climaticboundaries. Therefore, the observed susceptibilityor the pedostratigraphic boundaries may notmark the authentic climatic boundaries. Withthese limitations in developing accurate age mod-els, the tentative correlation in Fig. 9 is somewhatspeculative, but it is reasonable, because for ex-ample, the SE^NW magnetic lineation direction,which we associate with warm periods, corre-sponds with the prevailing wind direction of themodern summer monsoon. In contrast, the NE^SW magnetic lineation can be most reasonablyassigned to the Asian winter monsoon corre-sponding to cold periods. If our correlations arecorrect, the magnetic lineations provide the mostrobust indications of the possible recording ofDÔ cycles at Lingtai. The direction of Kmax

may therefore provide a means to distinguishthe Asian summer and winter monsoons. How-ever, further AMS studies, especially from thewestern Loess Plateau, are necessary to furthercon¢rm these conclusions.

6. Conclusions

By linking AMS parameters with previously-de-termined paleoclimatic records, we can infer thepaleoclimatic signi¢cance of each AMS parame-ter. Our main conclusions are:1. The primary AMS displays a horizontal mag-

netic foliation and a well-de¢ned magnetic lin-eation. We conclude that Dec-Kmax recordscompeting processes associated with summerand winter monsoons. However, further stud-

ies on the origin of the lineation, especially thespatial pattern of Dec-Kmax over a broad re-gion, are necessary to faithfully link Dec-Kmax

to paleowind directions. Unlike the Alaskanloess, the eolian inputs of the Chinese loessare carried only by the winter monsoon. Thesummer monsoon reworks the loess depositsduring cold periods, yielding a generally higherlineation than the winter monsoon. Apparent-ly, the winter and summer monsoon are dom-inated at this site by NE and SE winds, respec-tively.

2. The AMS foliation F and degree parameter Pexhibit almost identical features. Both param-eters show long-term trends that are related tograin size variations, with superimposed short-term peaks caused by compaction.

3. Bulk density is recommended as a useful paleo-climatically sensitive parameter for the Chineseloess. It seems that density is more pedogene-sis-independent than magnetic parameters be-cause the former is determined by the ‘non-magnetic’ matrix. The sub-units within S1 aremost clearly identi¢ed by bulk density varia-tions.

In summary, the magnetic lineation and Dec-Kmax are sensitive proxies for rapid £uctuations inwinter and summer monsoons. Major cold andwarm events since 130 ka have been correlatedwith those in the GISP2 oxygen isotope record,even though the sedimentation rate for the Ling-tai loess pro¢le is not particularly high. Furtherstudy of the spatial features of AMS by combin-ing several pro¢les is necessary to improve ourunderstanding of the loess AMS, and especiallyits paleoclimatic signi¢cance.

Acknowledgements

We are very grateful to Dr. Z.L. Ding for pro-viding us with grain size data sets. We thank An-drew Roberts, Jan Bloemendal and France La-groix for their insightful comments and helpfulsuggestions for improvement of this manuscript.This work was supported by NSFC Grant40221402 and US NSF Grants EAR 0003421and EAR/IF 9818704.[VC]

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