Geochemical Journal, Vol. 43, pp. 123 to 131, 2009

*Corresponding author (e-mail: fengjl@itpcas.ac.cn)

Copyright © 2009 by The Geochemical Society of Japan.

NOTE

123

Grain size effect on Sr and Nd isotopic compositions in eolian dust:Implications for tracing dust provenance and Nd model age

JIN-LIANG FENG,1* LI-PING ZHU,1 XIAO-LIN ZHEN1 and ZHAO-GUO HU1,2

1Institute of Tibetan Plateau Research, Chinese Academy of Sciences,Shuang Qing Rd. 18, P.O. Box 2871, Beijing 100085, China

2Graduate University of the Chinese Academy of Sciences, Yuquanlu Rd. 19, Beijing 100049, China

(Received April 14, 2008; Accepted October 8, 2008)

Strontium (Sr) and neodymium (Nd) isotopic compositions enable identification of dust sources and reconstruction ofatmospheric dispersal pathways. The Sr and Nd isotopic compositions in eolian dust change systematically with grain sizein ways not yet fully understood. This study demonstrates the grain size effect on the Sr and Nd isotopic compositions inloess and 2006 dust fall, based on analyses of seven separated grain size fractions. The analytical results indicate that Srisotopic ratios strongly depend on the grain size fractions in samples from all types of eolian dust. In contrast, the Ndisotopic ratios exhibit little variation in loess, although they vary significantly with grain size in samples from a 2006 dustfall. Furthermore, Nd model ages tend to increase with increasing grain size in samples from all types of eolian dust.Comparatively, Sr isotopic compositions exhibit high sensitively to wind sorting, while Nd isotopic compositions showgreater sensitively to dust origin. The principal cause for the different patterns of Sr and Nd isotopic composition variabil-ity with grain size appears related to the different geochemical behaviors between rubidium (Rb) and Sr, and the similargeochemical behaviors between samarium (Sm) and Nd. The Nd isotope data indicate that the various grain size fractionsin loess have similar origins for each sample. In contrast, various provenance components may separate into differentgrain size fractions for the studied 2006 dust fall. The Sr and Nd isotope compositions further confirm that the 2006 dustfall and Pleistocene loess in Beijing have different sources. The loess deposits found in Beijing and those found on theChinese Loess Plateau also derive from different sources. Variations between Sr and Nd isotopic compositions and Ndmodel ages with grain size need to be considered when directly comparing analyses of eolian dust of different grain size.

Keywords: eolian dust, Sr and Nd isotope, Nd model age, grain size effect, isotopic tracing

ing grain size, indicating more radiogenic strontium infine particles than in coarse ones (Biscaye et al., 1997;Asahara, 1999; Asahara et al., 1999; Zhou and Ding,2004). Correspondingly, some researchers have reportedfinding Nd isotopic ratios (143Nd/144Nd) in eolian depos-its to vary little or to be independent of particle size (Nakaiet al., 1993). On the other hand, Yokoo et al. (2004) re-ported that Nd isotopic ratios in loess tend to be high whenthe grain size is small.

Variations in the Sr and Nd isotopic compositions ofeolian deposits in different grain size fractions reportedtogether in a number of studies suggest more complicatedrelationships for Sr and Nd isotopic ratios.

Kanayama et al. (2005) reported Sr and Nd isotopedata for the total suspended particles and finer fraction(<5 µm) in the total suspended particles, and showed theSr isotopic composition of aerosols increased noticeablyin the <5 µm fraction, even though εNd(0) values showedno significant change. Similarly, Svensson et al. (2000),Smith et al. (2003) and Rao et al. (2006) observed that

INTRODUCTION

Investigators have used Sr and Nd isotopes for identi-fying dust sources and reconstructing atmospheric dis-persal pathways (Grousset and Biscaye, 2005). Winds mixand sort dust grains during transport. Hence, understand-ing the grain size dependence of isotopic tracers providesgeochemical constraints on the origin of the dust (Basileet al., 1997; Bory et al., 2002). Investigations have re-ported variations in the Sr and Nd isotopic compositionsin different grain size fractions of eolian deposits, but theresults have not been consistent nor well documented(Basile et al., 1997; Smith et al., 2003; Grousset andBiscaye, 2005; Kanayama et al., 2005).

A number of researchers showed that Sr isotopic ra-tios (87Sr/86Sr) of eolian deposits increase with decreas-

124 J.-L. Feng et al.

the 87Sr/86Sr ratios tend to increase in finer fractions, butsaw no significant sorting effects on the εNd(0) values forthe dust deposits.

Grousset et al. (1992) presented Sr and Nd isotopedata for a series of fine fraction (<5 µm) and bulk samplepairs in aerosols and loess collected from Australia, Af-rica and South America. These showed that fine fractions(<5 µm) and bulk samples have significantly different Srand Nd isotope compositions, while the 87Sr/86Sr ratiostend to increase in fine fractions and the εNd(0) have noclear trends. Delmonte et al. (2004) also reported similarresults.

This study intends to further investigate grain sizedependence of isotopic characteristics in eolian dustgrains. As a first step, we analyzed Sr and Nd isotopiccompositions of silicate minerals in seven size fractionsand whole silicate minerals (WS) from two loess depos-its and one 2006 dust fall. In doing so, we evaluate thedependence of Sr and Nd isotopic tracers on grain size.This paper also discusses possible geochemical causes ofthe Rb–Sr and Sm–Nd distributions and fractionation.Furthermore, we discuss the implications of the grain sizeeffect for tracing dust provenance and for Nd model ages.

MATERIALS AND METHODS

MaterialsIn this study, we chose three samples to investigate

the effect of grain size on Sr and Nd isotopic ratios. Onelate Pleistocene loess sample was collected from Lantianin the Chinese Loess Plateau (Sample No. LT-25), andanother was collected from Zhaitang, west of Beijing(Sample No. ZT-2). We collected one contemporary dustsample (Sample No. WJ-1) from a heavy dust fall inBeijing, on April 16–17, 2006. Feng et al. (2008) reportedthe grain size distributions and other principal propertiesof these samples.

Analytical methodsDuring dust transport and after dust deposition, dust

particles undergo various degrees of dissolution and/orweathering. Unstable minerals (carbonate, halite, gypsum,etc.) decompose, and secondary Fe–Mn oxides and hy-droxides form, along with carbonate neoformation in theterrestrial dust deposits (e.g., loess, paleosols, and gla-cier cover). In the oceans, seawater partly dissolves un-stable minerals from the deposited dust (Nakai et al.,1993; Weber et al., 1998), while other minerals (e.g., opal,silicate, etc.) are incorporated into the marine dust sedi-ment. Therefore, the stable silicate minerals provide moreappropriate research material than bulk dust samples. Wetreated our samples with 20% hydrochloric acid (HCl)solution at 97°C in a Teflon vessel for 45 minutes (asdescribed by Yokoo et al., 2004). Yokoo et al. (2004) re-

ported that the HCl-residues had 87Sr/86Sr and 143Nd/144Ndvalues similar to those for the acetic acid (HOAc)residues. Therefore, the 87Sr/86Sr and 143Nd/144Nd valuesfor samples in this work do not appear affected signifi-cantly by pretreatment. The silicate residues were washedfour times with super-pure water (18.2 MΩ). The silicateminerals were separated into seven gain size fractions (<2µm, 2–5 µm, 5–10 µm, 10–20 µm, 20–32 µm, 32–50 µm,and >50 µm) based on Stokes’ Law with the sequentialextraction approach. The Rb, Sr, Sm and Nd concentra-tions, Sr and Nd isotopic compositions were determinedfor whole silicate mineral residues (WS) and for the sevengrain size fractions by isotope dilution method using aFinnigan MAT 262 thermal ionization mass spectrometer,installed at the Institute of Geology and Geophysics, Chi-nese Academy of Sciences (CAS), following the analyti-cal procedures described by Chen et al. (2000, 2002).

The measured 87Sr/86Sr and 143Nd/144Nd ratios werecorrected for mass fractionation by normalizing to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively.Measured AMES Standard 143Nd/144Nd and NIST987Standard 87Sr/86Sr yielded 0.512144 ± 0.000012 (2σ, N =30) and 0.710252 ± 0.000014 (2σ, N = 19), respectively.Total procedural blanks indicated approximately 100 pgfor Sr and 50 pg for Nd.

RESULTS AND DISCUSSION

Rb–Sr systemThe concentrations and isotopic ratios of Rb and Sr in

different grain size fractions from the sampled loess andthe 2006 dust fall vary significantly with particle size(Table 1 and Fig. 1). The analytical results show that Rbconcentrations decrease with increasing grain size (Fig.1A). In contrast, Sr concentrations increase with increas-ing grain size (Fig. 1B). Hence, the Rb/Sr ratios decreasewith increasing grain size (Fig. 1C). The 87Rb/86Sr ratiosvary widely from 1.1 to 5.5, depending on grain size (Fig.1D). The 87Sr/86Sr ratios strongly depend on grain sizeand clearly increase with decreasing grain size. Thismeans that more radiogenic Sr exists in the fine particles(Fig. 1E). Sr model ages (TSr) in loess and particles fromthe sampled 2006 dust fall increase notably with increas-ing grain size (Fig. 2F). 87Rb/86Sr and 87Sr/86Sr can pro-vide pseudo isochrons (Fig. 1G). Furthermore, the varia-tions of the 87Sr/86Sr isotopic ratios with grain size aresimilar to those of the 87Rb/86Sr isotopic ratios and theRb/Sr ratios (Figs. 1G and 1H).

Sm–Nd systemFor our samples from the Pleistocene loess and the

2006 dust fall, 147Sm/144Nd and 143Nd/144Nd isotopic ra-tios show disparate patterns with respect to grain size(Table 1 and Fig. 2). With increasing grain size, Sm and

Grain size effect on Sr and Nd isotopic compositions in eolian dust 125

Nd concentrations in loess and particles from the 2006dust fall show increasing trends (Figs. 2A and 2B). TheSm and Nd concentrations in various dust particles cor-relate positively. The Sm/Nd ratios, 147Sm/144Nd isotopicratios, fSm/Nd and εNd(0) values, however, show differentpatterns of change with grain size for loess and the 2006dust. For loess, the Sm/Nd ratios, 147Sm/144Nd isotopicratios, and fSm/Nd values are consistent and tend to increasewith increasing grain size (Figs. 2C, 2D and 2E). TheεNd(0) values in loess, however, change less than 1ε unitwith each grain size fraction (Fig. 2F). From the 2 µm to

32 µm size particles in loess, the εNd(0) values remainalmost constant (Fig. 2F). In contrast, the Sm/Nd ratios,147Sm/144Nd isotopic ratios, fSm/Nd and εNd(0) values fromthe 2006 dust fall do not exhibit consistent variations withincreasing grain size (Figs. 2C, 2D, 2E and 2F). The εNd(0)values fall into two groups in the different fractions fromthe 2006 dust fall, with a sharp transition between thetwo groups (Fig. 2F). The εNd(0) values in the less than10 µm fractions have high values. Meanwhile, the >10µm fractions have low εNd(0) values in the 2006 dust fallsample (Fig. 2F). In contrast, Nd model ages (TNd) in loess

Sample Rb Sr 87Rb/86Sr 87Sr/86Sr TSr(a) Sm Nd 147Sm/144Nd 143Nd/144Nd fSm/Nd(b) εNd(0)(c) TNd

(d)

(ppm) (ppm) (±2σ × 10−6) (Ga) (ppm) (ppm) (±2σ × 10−6) (Ga)

LT-25<2 µm 175.5 104.1 4.884 0.726454 (12) 0.353 2.658 15.87 0.1012 0.512017 (13) −0.485 −12.11 1.53

2−5 µm 142.7 122.7 3.371 0.722994 (12) 0.440 3.434 19.67 0.1056 0.512045 (13) −0.463 −11.56 1.55

5−10 µm 128.9 126.5 2.953 0.721792 (13) 0.474 3.434 19.36 0.1073 0.512050 (14) −0.455 −11.48 1.57

10−20 µm 102.2 146.4 2.023 0.718858 (13) 0.590 3.856 21.24 0.1097 0.512047 (14) −0.442 −11.52 1.61

20−32 µm 82.5 157.3 1.518 0.717300 (15) 0.715 4.023 21.60 0.1126 0.512042 (13) −0.428 −11.62 1.67

32−50 µm 77.3 161.9 1.384 0.716996 (13) 0.770 4.150 22.18 0.1131 0.512068 (13) −0.425 −11.12 1.64

>50 µm 76.0 163.3 1.347 0.717056 (14) 0.794 5.048 26.63 0.1146 0.512078 (14) −0.417 −10.93 1.65

WS(e) 106.9 139.0 2.228 0.719498 (12) 0.556 3.687 20.40 0.1093 0.512077 (11) −0.444 −10.95 1.56

ZT-2<2 µm 169.3 88.7 5.531 0.726694 (15) 0.315 2.669 15.87 0.1017 0.512075 (13) −0.483 −10.98 1.46

2−5 µm 143.4 129.8 3.199 0.721244 (12) 0.425 3.218 18.46 0.1054 0.512104 (12) −0.464 −10.41 1.47

5−10 µm 124.3 136.3 2.641 0.719903 (15) 0.479 3.561 19.97 0.1078 0.512100 (10) −0.452 −10.49 1.51

10−20 µm 100.0 155.1 1.868 0.717688 (13) 0.595 3.754 20.48 0.1108 0.512107 (13) −0.437 −10.37 1.54

20−32 µm 85.2 179.8 1.372 0.715501 (11) 0.700 3.765 20.21 0.1127 0.512096 (13) −0.427 −10.58 1.59

32−50 µm 77.6 187.4 1.199 0.714954 (14) 0.769 3.458 18.58 0.1125 0.512098 (13) −0.428 −10.53 1.58

>50 µm 74.6 193.9 1.113 0.714612 (14) 0.808 3.801 20.82 0.1104 0.512057 (13) −0.439 −11.34 1.61

WS(e) 103.9 154.4 1.949 0.717225 (13) 0.554 3.725 20.75 0.1085 0.512069 (14) −0.448 −11.09 1.56

WJ-1<2 µm 148.4 90.7 4.742 0.724194 (11) 0.330 2.919 17.01 0.1038 0.512025 (12) −0.473 −11.96 1.56

2−5 µm 120.9 139.8 2.505 0.718489 (12) 0.466 3.104 18.58 0.1010 0.512000 (11) −0.486 −12.45 1.55

5−10 µm 110.1 147.2 2.167 0.718100 (14) 0.526 3.236 19.07 0.1026 0.512001 (11) −0.479 −12.43 1.57

10−20 µm 96.0 162.8 1.707 0.716837 (12) 0.617 4.004 24.33 0.0995 0.511933 (12) −0.494 −13.75 1.62

20−32 µm 84.2 192.6 1.266 0.715383 (15) 0.752 4.678 29.34 0.0964 0.511883 (13) −0.510 −14.73 1.64

32−50 µm 77.7 195.5 1.151 0.715095 (12) 0.811 4.670 27.94 0.1011 0.511923 (12) −0.486 −13.95 1.66

>50 µm 75.1 197.7 1.100 0.715185 (15) 0.855 5.988 35.20 0.1028 0.511931 (12) −0.477 −13.78 1.67

WS(e) 95.2 165.0 1.671 0.716611 (14) 0.621 4.470 27.74 0.0974 0.511923 (13) −0.505 −13.95 1.60

Table 1. Rb, Sr, Sm, and Nd concentrations and isotopic data by grain size fraction in the loesses and the 2006 Beijing area dustfall

(a)The TSr is the Rb–Sr crustal residence age calculated relative to depleted mantle (DM) reservoir. TSr ages calculated using an analogousequation to TNd with (87Sr/86Sr)DM = 0.702, (87Rb/86Sr)DM = 0.02, λRb = 1.42 × 10–11 yr–1 (Nakai et al., 1993).(b)The fSm/Nd notation describes deviation of 147Sm/144Nd from a chondritic uniform reservoir (CHUR). fSm/Nd = [(147Sm/144Nd)Sample/(

147Sm/144Nd)CHUR

– 1], where (147Sm/144Nd)CHUR = 0.1967.(c)The εNd(0) notation describes deviation of 143Nd/144Nd (in part per 104) from a CHUR reservoir. εNd(0) = [(143Nd/144Nd)Sample/(

143Nd/144Nd)CHUR

– 1] × 104, where (143Nd/144Nd)CHUR = 0.512638 (Jacobsen and Wasserburg, 1980).(d)The TNd is the Sm–Nd crustal residence age calculated relative to depleted mantle (DM) reservoir. TNd = (1/λSm)ln[((143Nd/144Nd)Sample –(143Nd/144Nd)DM)/((147Sm/144Nd)Sample – (147Sm/144Nd)DM) + 1], where (143Nd/144Nd)DM = 0.51315, (147Sm/144Nd)DM = 0.2137, λSm = 6.54 × 10–12

yr–1 (Jahn et al., 2001).(e)WS, Whole silicate minerals sample.

126 J.-L. Feng et al.

Fig. 1. Variations of (A) Rb concentrations, (B) Sr concentrations, (C) Rb/Sr ratios, (D) 87Rb/86Sr isotopic ratios, (E) 87Sr/86Srisotopic ratios and (F) TSr values, with grain size, and (G) relationships between 87Rb/86Sr and 87Sr/86Sr, and (H) Rb/Sr and 87Sr/86Sr from the loesses and 2006 Beijing area dust fall. LT-25, Chinese Loess Plateau sample; ZT-2, Zhaitang loess sample, west ofBeijing; WJ-1, 2006 dust fall sample from Beijing. WS, whole silicate mineral sample.

Grain size effect on Sr and Nd isotopic compositions in eolian dust 127

Fig. 2. Variations of (A) Sm concentrations, (B) Nd concentrations, (C) Sm/Nd ratios, (D) 147Sm/144Nd isotopic ratios, (E) fSm/Nd

values, (F) εNd(0) values and (G) TNd values with grain size, and (H) relationship between 87Sr/86Sr and εNd(0) from the loessesand 2006 dust fall. LT-25, Chinese Loess Plateau sample; ZT-2, Zhaitang loess sample, west of Beijing; WJ-1, 2006 dust fallsample from Beijing. WS, whole silicate mineral sample.

128 J.-L. Feng et al.

and particles from the 2006 dust fall show increasingtrends with increasing grain size (Figs. 2G). Comparedto Sr model ages (TSr), Nd model ages (TNd) are older forthe same grain size fraction for each eolian deposit (Ta-ble 1; Figs. 1F and 2G). Furthermore, the εNd(0) valuesin the loess vary little with increasing 87Sr/86Sr isotopicratio, whereas the εNd(0) values in the 2006 dust samplesvary significantly with increasing 87Sr/86Sr ratio (Fig. 2H).

Reasons for grain size effectDuring weathering processes, large chunks of rock

break into smaller ones or decompose through chemicalreactions with water. In general, the intensity of weather-ing varies with particle size. Some of our observationsrelated to particle size include: (1) The finer particlesexhibit evidence of extended weathering processes andintensive weathering. (2) The fine fractions contain moreof the minerals resistant to weathering than the coarsefractions. (3) The secondary minerals, especially claysform mainly within the <2 µm fraction.

The mineral characteristics and geological age prin-cipally control the Sr and Nd isotopic compositions ofsilicate minerals. Samples of loess and contemporary dustcontain of a mixture of surface sediment and soils fromdistinctly different source areas with different Rb–Sr andSm–Nd system characteristics. Therefore, the mineralcharacteristics and geological age of minerals of differ-ent grain sizes affect the pattern of 87Sr/86Sr and 143Nd/144Nd ratios in the different grain size fractions.

Rb and Sr have significantly different physical andchemical properties. This difference in properties leadsto Rb and Sr being incorporated into different silicateminerals. These differences also result in the Rb and Srbehaving differently during weathering processes. Rb andSr are mobile and can be lost during long periods of weath-ering (Dasch, 1969; Marques et al., 2004). But, Rb-bearing minerals (micas and K-feldspar) tend to be moreresistant to chemical weathering than the Sr-bearing min-erals (plagioclase and calcite) (Bottino and Fullagar, 1968;Dasch, 1969; Faure, 1986). Therefore, Sr is more readilylost from minerals than Rb during weathering processes.On the other hand, Brass (1975) and Faure (1986) reportedthat chemical weathering of rocks releases Sr into solu-tion, where the 87Sr/86Sr ratio is less than that of the origi-nal minerals or rocks. As a result, chemical weatheringnot only decreases mineral grain size, but also causes theRb/Sr ratios of minerals to increase, and tends to enrichweathered minerals in radiogenic 87Sr (Faure, 1986).

Our data show that the Sr isotope ratios increaseslightly with decreasing grain size for coarser particles(Table 1 and Fig. 1E). This suggests that the incipientweathering does not strongly change Sr isotopic compo-sitions for coarser particles (Giannetti and Masi, 1989;Price et al., 1991; Ma and Liu, 2001). With more advanced

weathering, plagioclase, which is enriched in non-radiogenic Sr, begins to break down (Price et al., 1991).Consequently, weathering results in Sr isotopic ratios in-creasing significantly as grain sizes decrease further. TheRb/Sr, 87Rb/86Sr and 87Sr/86Sr ratios increase sharply inthe <2 µm fractions in loess and the 2006 dust (Table 1;Figs. 1C, 1D and 1E). This most likely results from theirhigher content of clay and weathered micas (biotite andmuscovite). Many studies (Dasch, 1969; Faure, 1986;Kanayama et al., 2005) corroborate that high 87Sr/86Srratios, along with relatively high Rb and low Sr concen-trations are generally inherited by clays from their parentminerals. Because of significantly chemical weathering,the Rb/Sr isotope system does not follow the closed sys-tem behavior for eolian dust, and the pseudo isochronsdo not represent any age of geological significance. More-over, Goldstein (1988) suggested that Sr is recycled fromthe continents to the mantle preferentially to Nd. He alsopointed out that the Rb/Sr ratio of continents might beincreased by subduction of ocean-floor carbonates. Bothprocesses would lead to TSr < TNd for eolian deposits.

Sm and Nd have similar geochemical properties. Theselead to Sm and Nd behaving similarly during weathering(McCulloch and Wasserburg, 1978; Goldstein, 1988).

The Pleistocene loess studied here, is typically a sedi-mentary amalgamation from many dust storm events, andtends to be quite homogeneous in composition (Jahn etal., 2001). As a result, the Nd isotopic compositions donot vary significantly with grain size, especially for theintermediate fractions (Table 1 and Fig. 2F). Accordingly,our data further confirm previous Kanayama’s, Rao’s andothers results regarding Sr and Nd isotope variations withvarious grain size (Kanayama et al., 2005; Rao et al.,2006; see above introduction).

In contrast, the 2006 dust sample examined in thiswork came from a single dust storm event. This sample isnot nearly as homogeneous, due to wind sorting by grainsize. The less than 10 µm grain size fractions could haveremained airborne for a longtime and been transported agreat distance by atmospheric currents (Pye, 1987). There-fore, the less than 10 µm size fractions very likely resultfrom a mixing of large dust storm source areas that havebeen very well mixed. The coarse grains (>10 µm grainsize fractions) from the dust fall, on the other hand, havenot been mixed nearly as well. Finally, the geographicaldifferences in source areas can cause the Nd isotopic ra-tios to fall into two groups (Table 1 and Fig. 2F).

Tracing dust provenanceThe relationships between the three samples (the Chi-

nese Loess Plateau loess, the Beijing area loess, and the2006 Beijing dust fall), and their potential source areasare still debatable. Liu et al. (1981) and Zhang (1984)suggested that contemporary dust falls and Pleistocene

Grain size effect on Sr and Nd isotopic compositions in eolian dust 129

loess in Beijing derived from the same source area as theloess on the Chinese Loess Plateau, and that they all de-rived from the north-western part of the He-xi Corridor(Gansu province) and western Inner Mongolia. Xiong etal. (2002) suggested that the Pleistocene loess in theBeijing area came mainly from the “Hunshandake SandyLand” area (north to northwest of Beijing) based on grainsize variation. In contrast, Yang et al. (2007) put forwardthat the Pleistocene loess found in the Beijing area is oflocal origin, based on rare earth element analysis; andthat majority of contemporary dust falls in Beijing comefrom distant regions such as the Badain Jaran Desert inwestern China. Moreover, Nakano et al. (2004) suggestedthat contemporary dust falls in Beijing originated mainlyfrom adjacent northwestern to western areas, rather thanfrom the remote Taklamakan desert, the southwesternGobi desert, or the Chinese Loess Plateau. Based on ouranalyses of geochemical properties, quartz content, andoxygen isotope compositions of quartz etc. from (1) the2006 Beijing dust fall (WJ-1 and other samples), (2)Pleistocene loess samples (ZT-2 and other samples) fromthe Beijing area, and samples (LT-25 and other samples)from the Chinese Loess Plateau, Feng et al. (2008) sug-gested that the dust fall in 2006 and Pleistocene loess inBeijing have different sources. The loess deposits foundin Beijing and in the Loess Plateau also appear to differin provenance.

The Sr and Nd isotope data with various grain sizereported here show significant differences among (1) theloess on the Chinese Loess Plateau (LT-25), (2) the Beijingarea loess (ZT-2) and (3) the 2006 Beijing dust fall (WJ-1; Table 1, Figs. 1 and 2). These significant differencesindicate that the potential source areas of LT-25, ZT-2and WJ-1 differ from one another. Nakano et al. (2004)and Feng et al. (2008) also favored this conclusion.

The Sr and Nd isotope data for loess samples of LT-25 and ZT-2 reported here show that the 143Nd/144Nd ra-tios vary little with grain size, though the 87Sr/86Sr ratiosvary significantly with grain size. As described above,the increase of 87Sr/86Sr ratios with decreasing grain sizeresults principally from a grain size effect rather than asignificant separation of source areas among the grain sizefractions. Accordingly, the Nd isotopic compositionswithin each loess sample vary little with grain size, whichsuggests similar origins for the various grain size frac-tions.

The Nd isotopic compositions in the 2006 dust fallsample differ systematically with grain size, indicatingthat components from different source areas may sepa-rate into different grain size fractions during dust stormformation and wind sorting processes. Additionally, oxy-gen isotopic compositions within quartz from this dustfall vary with grain size and correlate with the variationsin Nd isotopic composition, which supports the above idea

of grain size separation by source area (Feng et al., 2008).In general, different patterns of Sr and Nd isotopic

composition with grain size need to be considered whenmaking direct comparisons between analyses of eoliandeposits of different grain sizes.

Implication for Nd model ageEolian deposits can represent the average composi-

tion of the continental crust on a large scale (Taylor etal., 1983; Goldstein et al., 1984; Goldstein, 1988). TheNd model age of eolian deposit generally reflects the aver-age crustal residence ages of the rocks in the source ter-rain (Taylor et al., 1983; Goldstein, 1988). Accordingly,the Nd model ages have been used to understand the evo-lution of crust (Taylor et al., 1983; Goldstein et al., 1984;Goldstein, 1988), and to trace the provenance of eoliandeposits (Taylor et al., 1983; Nakai et al., 1993; Liu etal., 1994; Gallet et al., 1996, 1998; Jahn et al., 2001).

The Nd model ages associated with various grain sizesreported here show a tendency to increase with grain sizein samples from all types of eolian dust (Table 1; Fig.2G). Model ages for a single dust deposit, based on dif-ferent size fractions, differ by as much as 150 Ma. Theeolian samples studied in this work mainly consist of siltsize particles (Feng et al., 2008). For other eolian depos-its, the more the samples vary in grain size, the greaterlikelihood that their Nd model ages will vary within de-posits from very similar provenance. Moreover, this grainsize effect on Nd model age could partially explain whythe Nd model ages of dust deposits in the Pacific Oceanappear younger than those of Chinese loess samples(Nakai et al., 1993). We thus suggest that grain size ef-fect need to be considered when using Nd model ages toconstrain the origins of eolian deposits.

CONCLUSION

Based on our measurements and the discussion above,we find that the variation patterns of Sr and Nd isotopiccompositions with grain size differ significantly for loessand our sample from the 2006 Beijing dust fall. For ho-mogeneous loess, Sr isotopic ratios depend strongly ongrain size, viz. 87Sr/86Sr ratios increase with decreasinggrain size (with more radiogenic Sr found in the fine par-ticles), while Nd isotopic ratios are almost independentof the grain size. For the less homogeneous 2008 Beijingdust fall, the variations of Sr isotopic ratios with grainsize resemble those of loess, but the Nd isotopic ratiosdiffer significantly with grain size. Furthermore, the Ndmodel ages tend to increase with increasing grain size insamples from all types of eolian dust.

The Sr isotopic compositions display high sensitivityto wind sorting, whereas Nd isotopic compositions showgreater sensitivity to the origin of the dust. The principal

130 J.-L. Feng et al.

cause for the different patterns of Sr and Nd isotopic com-position variability with grain size appears related to thedissimilar geochemical behaviors between Rb and Sr, andthe similar geochemical behaviors between Sm and Nd.

For late Pleistocene loess samples collected fromLantian in the Chinese Loess Plateau and from Zhaitang,west of Beijing, the Nd isotopic compositions remainsimilar for different grain sizes, which suggests similarorigins for the various grain size fractions for each sam-ple. In contrast, the various grain sizes in the 2006 Beijingarea dust fall show systematic differences in Nd isotopiccomposition, indicating that components from differentsources may separate into different grain size fractionsduring dust storm formation and wind sorting processes.

The Sr and Nd isotopic compositions further confirmthat the dust from the 2006 dust fall event and Pleistoceneloess in Beijing come from different sources. As suggestedin our previous study, the loess deposits found nearBeijing and those from the Chinese Loess Plateau alsoderive from different source areas.

Because of the impact of grain size on Sr and Nd iso-topic composition and Nd model ages, grain size need tobe considered when comparing eolian deposits with dif-ferent particle sizes.

Acknowledgments—We thank Prof. Chen F., Dr. Chu Z. Y.and Dr. Li C. F. for isotope analysis. Dr. Yang S. L. was thankedfor providing sample LT-25. We thank Prof. William Isherwoodfor his suggestions and linguistic help. We also thank Dr.Asahara Y., Dr. Yokoo Y. and Dr. Amakawa H. for their thought-ful comments. This work was supported by NSFC (Grant No.40771028).

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