Magnesium isotopic variations in loess: Origins and implications

Earth and Planetary Science Letters 374 (2013) 60–70

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Magnesium isotopic variations in loess: Origins and implications Kang-Jun Huang a,b,n, Fang-Zhen Teng b,c,d,n, Amira Elsenouy c, Wang-Ye Li e, Zheng-Yu Bao a a

State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China Isotope Laboratory, Department of Geosciences, University of Arkansas, Fayetteville, AR 72701, USA Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA d Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA e CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China b c

art ic l e i nf o

a b s t r a c t

Article history: Received 15 January 2013 Received in revised form 30 April 2013 Accepted 4 May 2013 Editor: B. Marty Available online 2 June 2013

Loess deposits serve as important continental archives for studying Quaternary climatic variations and for estimating the average chemical composition of the upper continental crust. Here, we report highprecision Mg isotopic data for 19 loess samples from China, Argentina and Europe, which were previously used to estimate the composition of the upper continental crust. The results show that global loess samples have heterogeneous Mg isotopic compositions, with δ26Mg (per mil deviation of the 26Mg/24Mg ratio from the DSM3 standard) values ranging from −1.64‰ to +0.25‰ and a weighted average of −0.89‰, which is lighter than both crust and mantle silicates. MgO content and δ26Mg of loess positively correlate with CaO/Al2O3 ratio, suggesting a two-component mixing between carbonates and secondary silicate minerals. The large variation in Mg isotopic composition of loess results from a combination of factors, including source heterogeneity, eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit. Our results suggest that Mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change, but may not represent the average Mg isotopic composition of the upper continental crust due to mixing, sorting of isotopically distinct components and isotope fractionation during loess deposit formation. & 2013 Elsevier B.V. All rights reserved.

Keywords: Mg isotopes loess upper continental crustal composition paleoclimatic change

1. Introduction Knowledge on the Mg isotopic composition of the upper continental crust is crucial for understanding interactions between the crust and mantle and constraining global Mg cycles. The Mg isotopic composition of the upper continental crust estimated from granites, loess, shales and upper crustal composites is highly heterogeneous, with δ26Mg varying from −0.52‰ to +0.92‰ (Shen et al., 2009; Li et al., 2010; Liu et al., 2010), and on average heavier than the mantle, which has a relatively homogeneous Mg isotopic composition (i.e., δ26Mg ¼−0.257 0.07‰, Teng et al., 2007, 2010a; Handler et al., 2009; Yang et al., 2009; Bourdon et al., 2010; Chakrabarti and Jacobsen, 2010; Liu et al., 2011; Huang et al., 2011; Pogge von Strandmann et al., 2011; Xiao et al., 2013). The highly variable Mg isotopic composition of the upper continental crust is mainly attributed to the lithological complexity of the continental crust and Mg isotope fractionation during low-temperature water–

n Corresponding authors at: State Key Laboratory of Geological Processes and Mineral Resources & Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China. E-mail addresses: [email protected] (K.-J. Huang), [email protected] (F.-Z. Teng).

0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.05.010

rock interactions (Tipper et al., 2006, 2008; Pogge von Strandmann et al., 2008, 2012; Li et al., 2010; Teng et al., 2010b; Wimpenny et al., 2010, 2011a; Huang et al., 2012; Opfergelt et al., 2012). These make accurate estimates of the average Mg isotopic composition of the upper continental crust challenging. Loess is a good proxy for constraining the average composition of the upper continental crust due to the fact that loess is formed by particles from a large area of exposed crust and undergoes limited chemical weathering during its formation (e.g., Goldschmidt, 1922; Taylor et al., 1983; Gallet et al., 1998; Barth et al., 2000; Peucker-Ehrenbrink and Jahn, 2001; Hattori et al., 2003; Teng et al., 2004; Hu and Gao, 2008; Park et al., 2012). Furthermore, loess–paleosol sequence preserves a terrestrial record of environmental changes during the Quaternary (Heller and Liu, 1984; Liu and Ding, 1998). A number of proxy indicators, including magnetic susceptibility, grain-size distribution, mineral composition, and geochemical signature have been employed to characterize paleoclimatic changes during the Quaternary (Heller and Liu, 1984; Bronger and Heinkele, 1990; An et al., 1991; Ding et al., 1994; Liu and Ding, 1998; Chen et al., 1999; Guo et al., 2000; Wang et al., 2007; Liang et al., 2009). Large isotope fractionation during low-temperature water–rock interactions and limited fractionation during igneous differentiation (Teng et al., 2007, 2010a;

K.-J. Huang et al. / Earth and Planetary Science Letters 374 (2013) 60–70

61

Al2O3+Fe2O3

Si ltsiz ed fe lds pa

PAAS

rs an

Cl

ay

m

s

ine

ica

ra

m

ls

d

UCC

China Europe Argen na New Zealand North America

MgO+CaO

Fig. 1. Map showing the world distribution of loess deposits and sampling localities (modified after Pésci, 1990). Loess samples from Argentina, Europe, and China measured in this study are marked as black solid triangles, quadrangles and circles, respectively. Open squares and circles stand for the previously studied loess samples from the USA and New Zealand (Li et al., 2010).

Liu et al., 2010) make Mg isotopes a potentially powerful tracer of chemical weathering, which exerts a fundamental role in regulating the global carbon cycle, and in turn controls the Earth's climate (Berner et al., 1983). Hence, studies of Mg isotopic composition of loess may potentially shed light on the average Mg isotopic composition of the upper continental crust and paleoclimatic change (Wimpenny et al., 2011b). To date, limited Mg isotopic data of loess have been reported and they display large ( 41‰) variations (Young and Galy, 2004; Li et al., 2010; Immenhauser et al., 2010; Wimpenny et al., 2011b). The detailed mechanisms that control Mg isotopic variations in loess are still unclear, and systematic investigations of the major loess deposits of different continents are needed. Here, we analyzed Mg isotopic compositions of well-characterized loess samples from the major loess deposits in the world (Europe, China and Argentina, Fig. 1) (Taylor et al., 1983; Gallet et al., 1998; Jahn et al., 2001; Peucker-Ehrenbrink and Jahn, 2001; Teng et al., 2004) to (1) determine the general factors controlling Mg isotopic composition of loess, (2) discuss possible application of Mg isotopic geochemistry of loess as a tracer of paleoclimatic change, and (3) investigate whether Mg isotopic composition of loess can be used to evaluate the average isotopic composition of the upper continental crust. Our results demonstrate that the Mg isotopic composition of loess has potential as a tracer for reconstructing paleoclimatic change, but is not suitable for estimating the average Mg isotopic composition of the upper crust due to isotope fractionation during eolian sorting, chemical weathering and dilution by carbonates.

2. Samples Loess is an eolian deposit of silt-sized clastic particles, which is formed essentially by mechanical erosion of large geographical areas in arid and semiarid climate (Bryan, 1945; Smalley, 1966; Liu, 1985; Pécsi, 1990; Pye, 1995; Smalley et al., 2011). Owing to the difference in the surficial geology and the effectiveness of sediment mixing processes in the individual source region, mineral compositions of loess vary greatly within different regions (Pye, 1995). In general, the dominant minerals in loess are siltsized quartz, feldspar, carbonate minerals, clay minerals and heavy minerals (Liu, 1985; Pécsi, 1990; Eden et al., 1994; Gallet et al., 1998). The ternary diagram, i.e. (Na2O+K2O)–(MgO+CaO)–(Al2O3+Fe2O3), has been used to reflect the relative abundance of silt-sized feldspars and micas, carbonates and clay minerals in loess (Muhs

Carbonates

Na2O+K2O

Fig. 2. The ternary diagram showing the relative abundances of carbonates (MgO +CaO), clay minerals (Al2O3+Fe2O3) and silt-sized feldspars and micas (Na2O+K2O) in loess samples in this study (bigger color icons) and previous studies (smaller gray icons) as well as post-Archean Australian average shale (PAAS, Taylor and McLennan, 1985; McLennan, 2001) and the upper continental crust (UCC, Rudnick and Gao, 2003). The Chinese loess data are from Taylor et al. (1983), Gallet et al. (1996), Ding et al. (2001) and Jahn et al. (2001). The Argentinean loess data are from Gallet et al. (1998). The European loess data are from Gallet et al. (1998). The New Zealand loess data are from Graham et al. (2001) and Li et al. (2010). The USA loess data are from Muhs et al. (2008) and Li et al., (2010). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

et al., 2008). Relative to silt-sized feldspars and micas, carbonates and clay minerals are main components of global loess, with loess plotting between clay minerals (Al2O3+Fe2O3) and carbonates (MgO+CaO) on a trajectory pointing away from carbonates toward clay minerals (Fig. 2). Nineteen loess samples from 8 localities on three continents were investigated here (Fig. 1). The chemical compositions of these loess samples cover the compositional ranges of global loess (Fig. 2), suggesting that our samples have good representativeness for the global loess. Among these samples, 10 were collected from Europe covering most of the western part of the continent, including Belgium, France, Germany, Hungary, the United Kingdom and Spitsbergen (Svalbard), seven were from the Chinese Loess Plateau, and two were from Pampean, Argentina. Detailed descriptions on these samples have been presented in previous studies (Taylor et al., 1983; Gallet et al., 1998; Jahn et al., 2001). Thus, only a brief description is given below. 2.1. European loess Ten samples from Europe that were previously used to estimate the composition of the upper continental crust (Taylor et al., 1983; Gallet et al., 1998) were studied here. Four samples were collected from France: samples NS4 and NS6 (Nantois section, Brittany), sample PR (Port-Racine section in Normandy), and sample HOT (Gallet et al., 1998). Two samples (K1 and K2) were from the Kaiserstuhl section, Rhine Valley, Germany (Taylor et al., 1983). One peculiar loess sample (SCIL) was collected from Scilly Islands, UK, and dated to be 18 ka (Gallet et al., 1998). Sample K14 was collected from the Kesselt section in Belgium and sample H was from Hungary. One Holocene loess sample (LO94) was collected from the Adventdalen valley on Spitsbergen Island in Svalbard, Norway, where loess was generally produced through the combined erosive powers of salt and ice smashing in addition to abrasion to the underlying bedrocks by glacial erosion (Bryant, 1982; Gallet et al., 1998). The European loess samples are dominated by quartz, calcite, clay minerals, muscovite, chlorite–vermiculite, and micas (Swineford and Frye, 1955). The two samples from Kaiserstuhl, Rhine Valley, Germany, have high carbonate content, but other

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eastern part of the Loess Plateau. JX-1 and JX-8 are from the Holocene paleosol (S0) and Pleistocene paleosol (S1-2), respectively, and JX-10 is a loess sample from the bottom of the section (L2). Holocene paleosol sample XN-1 and loess sample XN-10 are from the top (S0) and the bottom (L2) of Xining section situated in the western part of the Chinese Loess Plateau, respectively. A single loess sample, XF-6, was collected from the middle of the Xifeng section (L1-5) located in the central part of the Loess Plateau. Loess from the Chinese Loess Plateau is mainly composed of quartz, plagioclase, K-feldspar, amphibole, mica, pyroxene, calcite, and clay minerals, with small amounts of heavy minerals and dolomite (Liu, 1985; Eden et al., 1994; Zheng et al., 1994; Yokoo et al., 2004). The clay minerals are dominated by illite, vermiculite, chlorite, and kaolinite, with subordinate amounts of smectite and interlayer minerals (Bronger and Heinkele, 1990; Eden et al., 1994; Zheng et al., 1994; Jahn et al., 2001; Yokoo et al., 2004). Chinese loess samples studied here have limited variation in CIA value from 58 to 65 (Table 1), suggesting that they were from provenances of moderate weathering (Gallet et al., 1998). All major elements, except CaO, in the loess samples from these three sections (Xining, Xifeng and Jixian) display nearly the same variation pattern when normalized to their underlying paleosols (Jahn et al., 2001). Xining and Xifeng sections show weak loss or

loess samples contain only minor amounts of carbonate. Although the origin of the carbonates in these loess samples is still debated, a primary origin in the Rhine Valley samples is suggested on the basis of the large amounts of limestone available in the Alps (Taylor et al., 1983). The chemical index of alteration, which is defined as CIA ¼molar [Al2O3/(Al2O3+CaOn+Na2O+K2O)]  100, where CaOn is the amount of CaO in the silicate fraction (Nesbitt and Young, 1982), is widely used to determine the degree of weathering of loess. The European loess samples have low CIA values ranging from 55 to 65 (cf. fresh igneous rocks typically have CIA ¼50 75, Nesbitt and Young, 1982), suggesting that they were derived from moderately weathered areas (Gallet et al., 1998). 2.2. Chinese loess One Chinese loess sample (CH) and other six loess samples collected from three separate sections (Jixian, Xifeng and Xining) on the Chinese Loess Plateau have been analyzed here (Jahn et al., 2001; Peucker-Ehrenbrink and Jahn, 2001). These three sections represent three different climatic conditions, ranging from cold and dry in the west (Xining) to more humid and warm in the east (Jixian) across the Chinese Loess Plateau (Jahn et al., 2001). Three samples (JX-1; JX-8; JX-10) were collected from the Jixian section in the

Table 1 Magnesium isotopic compositions of standards and loess samples from China, Argentina and Europe. Sample no.

Location

SiO2 (wt%)

Al2O3 (wt%)

MgO (wt%)

CaO (wt%)

TiO2 (wt%)

CIA (%)

a

87

Sr/86Sr

143

Nd/144Nd

Standards KH-olivine Duplicate e Duplicate Seawater Duplicate China XN-1 XN-10 XF-6 JX-1 JX-8 JX-10 CH Argentina 24–26 Replicate f 52–54 Europe LO94 SCIL HOT PR Duplicate NS4 NS6 K14 H K1 K2

δ26Mg (‰)

b

2SD

c

δ25Mg (‰)

2SD

Δ25Mg′

−0.34 −0.31 −0.28 −0.92 −0.92

0.06 0.08 0.08 0.05 0.07

−0.17 −0.18 −0.09 −0.46 −0.48

0.05 0.06 0.06 0.06 0.03

0.01 −0.02 0.06 0.02 0.00

Qinghai Qinghai Gansu Shanxi Shanxi Shanxi

62.36 65.07 64.62 71.25 70.12 63.83

12.82 12.31 13.50 14.55 14.74 11.56

2.60 2.73 2.63 1.79 2.16 1.93

12.22 10.30 8.85 1.52 2.12 13.98

0.66 0.64 0.72 0.79 0.85 0.61

61 58 62 64 65 60

0.714457 0.714457 0.715376 0.718385 0.717661 0.714365

0.512109 0.512139 0.512147 0.512112 0.512016 0.512093

−0.87 −0.93 −0.75 −0.41 −0.44 −0.79 −0.67

0.06 0.06 0.06 0.06 0.06 0.06 0.07

−0.47 −0.48 −0.40 −0.20 −0.15 −0.41 −0.36

0.05 0.05 0.05 0.09 0.09 0.05 0.03

−0.02 0.00 −0.01 0.01 0.08 0.00 −0.01

Mercedes

62.39

15.01

1.35

10.94

0.72

59

0.706334

0.512558

Mercedes

69.5

13.59

1.56

4.84

1.09

65

0.709443

0.512311

−0.44 −0.37 −0.29

0.06 0.06 0.06

−0.22 −0.17 −0.15

0.05 0.05 0.05

0.01 0.02 0.00

Spitsbergen UK France France

78.73 83.94 75.59 77.72

10.85 8.50 8.34 6.23

1.04 0.59 1.42 1.11

0.56 0.46 7.83 9.71

0.61 0.57 0.7 0.44

60 65 59 55

0.716768 0.725683 0.714324 0.712963

0.511966 0.512039 0.512087 0.51208

France France Belgium Hungary Germany Germany

71.56 75.07

10.59 10.65

1.21 1.29

7.05 3.93

0.93 0.97

60 60

0.714849 0.715267 0.730248

0.512188 0.512084 0.511992

59.9 59.1

7.78 7.98

3.45 4.37

23.11 22.90

0.32 0.29

65 65

0.70956 0.71001

0.511323 0.511406

0.25 −0.10 −1.09 −1.40 −1.33 −0.33 −0.63 0.02 −1.56 −1.64 −1.63

0.06 0.06 0.05 0.07 0.05 0.07 0.07 0.07 0.07 0.07 0.07

0.13 −0.07 −0.53 −0.71 −0.70 −0.18 −0.37 −0.02 −0.79 −0.87 −0.86

0.05 0.05 0.06 0.03 0.06 0.03 0.03 0.03 0.03 0.03 0.03

0.00 −0.02 0.04 0.02 −0.01 −0.01 −0.04 −0.03 0.02 −0.02 −0.01

d

Major elemental and Sr–Nd isotopic data of Chinese loess are from Jahn et al. (2001), and Argentinean and European loess are from Taylor et al. (1983) and Gallet et al. (1998). The two loess samples (H and CH) and loess sample K14 are from Diane McDaniel and E. Juvigné, respectively, whose major element contents are not available. a CIA refers to the chemical index of alteration, CIA ¼ molar [Al2O3/(Al2O3+CaOn+Na2O+K2O)]  100, where CaOn is the amount of CaO in slicates (Nesbitt and Young, 1982). b x δ Mg ¼ [(xMg/24Mg)sample/(xMg/24Mg)DSM3−1]  1000, where x ¼25 or 26 and DSM3 is a Mg solution made from a piece of pure Mg metal (Galy et al., 2003). c 2SD ¼2 times the standard deviation of the population of n (n4 20) repeat measurements of the standards during an analytical session. d Δ25Mg′ is defined as δ25Mg′  0.521  δ26Mg′ following Young and Galy (2004), where δxMg'=1000  ln[(δxMg +1000)/1000] with x ¼ 25 or 26 and is reported largely as a quality control on the data, with values that should be close to zero. e Duplicate¼ repeat instrumental measurement of the same purified Mg solution. f Replicate¼ repeat column chemistry and instrumental measurement of different aliquots of the same stock solution.

K.-J. Huang et al. / Earth and Planetary Science Letters 374 (2013) 60–70

gain of CaO, and the Jixian section shows a significant CaO loss especially in the JX-1 and JX-8 samples (Jahn et al., 2001). No REE fractionations between loess and paleosols are observed in these three sections, suggesting no change in eolian dust source composition between the periods of loess deposition and soil formation (Jahn et al., 2001). All loess samples from these three sections display a restricted range of 143Nd/144Nd ratios (0.5120–0.5121) and typical upper crustal 87Sr/86Sr ratios (0.714–0.718), indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust. The 87Sr/86Sr variations of loess samples can partly be attributed to two-component mixing between a less radiogenic carbonate component and a more radiogenic silicate one (Jahn et al., 2001).

63

The long-term precision based on replicate runs of natural and synthetic multi-element standards is o 70.07‰ (2SD) (Teng et al., 2010a). The internal precision of the measured 26Mg/24Mg ratio based on at least 4 repeat runs of the same sample solution during a single analytical session is o 70.1‰ (Teng et al., 2010a). At least one standard was analyzed with each group of samples during the course of an analytical session to assess accuracy and reproducibility. Multiple analysis of in-house Kilbourne Hole olivine standard (KH-olivine) yielded δ26Mg values ranging from −0.34‰ to −0.28‰ (Table 1), which is consistent with published values (Li et al., 2010; Teng et al., 2010a). Results for seawater standard analyzed during the course of this study (Table 1) also agree with previously published data (Foster et al., 2010; Ling et al., 2011, and references therein).

2.3. Argentinean loess Two Pampean loess samples (24–26, 52–54) were collected at different depths of a loess drilling hole, located near the city of Mercedes, about 87 km west of Buenos Aires, Argentina (Gallet et al., 1998). The sample 24–26 was collected at a depth from 24 to 26 m and the sample 52–54 was collected at a depth from 52 to 54 m. The genesis and evolution of the Pampean loess have been related to the last glacial events in Patagonia. The origin of Pampean loess is still debated, with some studies suggesting a single Andean source (e.g., Sayago, 1995), whereas other data highlight the possibility of multiple source areas, including the central and northern part of Argentina (e.g., Morrás, 1999), and the floodplains of nearby Colorado and Negro rivers (Zárate and Blasi, 1993). The Pampean loess has a coarser grain size and a different mineral composition compared to the loess from Europe and China, with the presence of considerable amounts of volcanic glass and plagioclases, and a scarcity of quartz and carbonate (Zárate and Blasi, 1993).

3. Analytical methods Magnesium isotope analysis of loess samples was performed at the Isotope Laboratory of the University of Arkansas, Fayetteville, following the established procedures (e.g., Yang et al., 2009; Li et al., 2010; Teng et al., 2010a); as such, only a brief description is provided below. 1–25 mg of sample powder was dissolved in a mixture of Optima-grade HF, HNO3 and HCl in Savillex screw-top beakers in order to obtain 50 μg Mg for high-precision isotopic analysis. Separation of Mg was achieved by cation exchange chromatography using Bio-Rad AG50W-X8 resin (200–400 mesh) in 1 N HNO3 following the methods reported in previous studies (Teng et al., 2007; Yang et al., 2009; Li et al., 2010). Two standards (seawater and olivine) were processed with samples for each batch of column chemistry. The column chemistry was carefully calibrated to ensure 100% Mg yield (Teng et al., 2007, 2010a). The same column procedure was processed twice for all samples in order to obtain a pure Mg solution with matrix element/Mg o0.05% (Teng et al., 2007, 2010a; Yang et al., 2009; Li et al., 2010). To correct for instrumental mass bias, Mg isotopic compositions were analyzed by the standard-sample bracketing method using a Nu plasma Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). The three Mg isotopes (24, 25 and 26) were measured simultaneously in separate Faraday cups (H5, Ax and L4) in low-resolution mode. The background Mg signals for 24Mg (o10−4 V) were negligible relative to the sample signals (2–4 V). Magnesium isotope data are reported in δ-notation relative to the standard DSM3 made from pure Mg metal (Galy et al., 2003), defined as δxMg¼ [(δxMg/24Mg)sample/(δxMg/24Mg)DSM3–1]  1000, where x is either 25 or 26.

4. Results Magnesium isotopic compositions are reported in Table 1 for reference materials (KH-olivine and seawater) and loess samples, along with published major elemental and Sr-Nd isotopic data (Taylor et al., 1983; Gallet et al., 1998; Jahn et al., 2001). Overall, δ26Mg values of loess from different parts of the world range from −1.64‰ to +0.25‰ (Table 1), with a weighted average/median value of −0.89‰/−0.67‰. When plotted in a δ25Mg versus δ26Mg diagram, all samples analyzed in this study fall along the terrestrial equilibrium mass-dependent fractionation curve with a slope of 0.52 (Young and Galy, 2004; Li et al., 2010; Teng et al., 2010a). European samples have the most fractionated Mg isotopic composition among all loess samples studied here (Table 1). Loess samples from arctic Spitsbergen (LO94) and Scilly Island (SCIL) have relatively heavier Mg isotopic composition (δ26Mg¼+0.25‰ and −0.1‰) and extremely low MgO and CaO contents. By contrast, two loess samples from Germany (K1 and K2) have the highest CaO and MgO contents and the lowest δ26Mg values (−1.64‰ and −1.63‰). Compared to European loess, δ26Mg values of samples from the Chinese Loess Plateau display a relatively narrower range, varying from −0.93‰ to −0.41‰ (Table 1), and display an overall increasing trend from the northwest to the southeast of Chinese Loess Plateau. Two samples from the Xining section in northwestern Chinese Loess Plateau have relatively lighter Mg isotopic composition (δ26Mg ¼−0.93‰ and −0.87‰), and three samples from the Jixian section in southeastern Chinese Loess Plateau have higher δ26Mg values, ranging from −0.41‰ to −0.79‰ (Table 1). The loess sample from Xifeng section located in the central Chinese Loess Plateau has an intermediate δ26Mg value of −0.75‰. Two Pampean loess samples from Argentina have similar Mg isotopic compositions with δ26Mg ¼−0.40‰ for sample 24-26 and −0.29‰ for sample 52-56 (Table 1).

5. Discussion In this section, we combine our Mg isotope data with those from the New Zealand and USA previously studied by Li et al. (2010) to first explore the general controls on Mg isotopic composition of global loess, and then evaluate the processes and factors potentially influencing the distribution of Mg isotope in loess. Finally, we use these data to investigate the potential applications of Mg isotope systematics as proxies of paleoclimatic change and estimate of the average Mg isotopic composition of the upper continental crust. 5.1. Mineralogical control of Mg isotopic composition of loess As the dominant minerals in loess (Fig. 2), carbonate minerals and clay minerals are the main hosts of Mg and control the

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Fig. 3. (A) The MgO and (B) δ26Mg as a function of CaO/Al2O3 ratio in loess. Error bars in this and subsequent plots represent 2SD. The solid lines represent binary mixing lines for these data. Loess samples from Spitsbergen (LO94, the red line) and Scilly Island (SCIL, the blue line) are assumed to be the aluminosilicate end members based on their highest Mg isotopic compositions and lowest CaO/Al2O3 ratios, whereas the loess sample from Kaiserstuhl (K2) with the lowest δ26Mg value and highest CaO/Al2O3 ratio is assumed to represent the carbonate end member. Mass-balance equations are: MgOloess ¼ f  MgOAl þ ð1−f Þ  MgOCa

ð1Þ 

CaO=Al2 O3loess ¼ ½ f  CaOAl þ ð1−f Þ  CaOCa = f  Al2 O3Al þ ð1−f Þ  Al2 O3Ca 

ð2Þ

δ26 Mgloess ¼ ½f MgOAl δ26 MgAl þ ð1−f ÞMgOCa δ26 MgCa =½f MgOAl þ ð1−f ÞMgOCa 

ð3Þ

where f is the proportion of aluminosilicate in loess and the subscripts Al and Ca represent aluminosilicate and carbonate end member. Magnesium isotopic data of loess in New Zealand and the USA are from Li et al. (2010) (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

5.2. Processes affecting Mg isotopic composition of loess Heterogeneous source materials, eolian sorting and different degrees of chemical weathering during formation of loess deposit are considered as the potential factors controlling the variations in mineralogical and chemical composition of loess (Smalley, 1966; Liu, 1985; Pécsi, 1990; Nesbitt et al., 1996; Smalley et al., 2011). Below, we discuss how these processes can potentially affect Mg isotopic composition of loess.

20

BSE

Carbonate rock 10

Frequency

variation of MgO content in loess. The positive correlation between MgO and CaO/Al2O3 ratio of loess sample investigated here (Fig. 3A) suggests that MgO content of loess is linked to the abundance of carbonate (CaO-rich) and clay (Al2O3-rich) minerals. Similarly, δ26Mg of loess samples is negatively correlated with the CaO/Al2O3 ratio (Fig. 3B), suggesting that carbonate and clay minerals play an important role in controlling the Mg isotopic composition of loess samples. Silicate and carbonate rocks have distinct Mg isotopic signatures (Fig. 4). Silicate rocks and minerals from the continental crust have δ26Mg values ranging from −0.75‰ to 0.44‰, whereas the δ26Mg values of carbonate rocks tend to be lower, between −4.84‰ and −0.47‰ (Fig. 4). The positive correlation between MgO and CaO/Al2O3 ratio, and negative correlation between δ26Mg and CaO/Al2O3 ratio in loess can thus be well modeled by a binary mixing between clay-rich loess samples LO94/SCIL (high δ26Mg, low MgO content and low CaO/Al2O3 ratio) and a carbonate-rich loess sample K2 (low δ26Mg, high MgO content and high CaO/ Al2O3 ratio) (Fig. 3). Although this two-end member mixing is oversimplified since both clays and carbonates can have variable MgO contents, CaO/Al2O3 ratios and δ26Mg values, nonetheless, this calculation suggests that Mg isotopic signatures of loess are most likely controlled by their variable proportion of carbonate to clay minerals. In the subsequent section, we explore the potential factors affecting distribution of carbonates and clays during formation of loess deposits.

50 40 30 20 10 25 20 15 10 5 15

Granite and granitc mineral

Soil and clay mineral

Shale

10 5 8 6 4 2 0

Loess

-5

-4

-3

-2

-1

0

1

δ 26 Mg (‰) Fig. 4. Histograms of Mg isotopic compositions of different continental crustal rocks. Carbonate rock data are from references (Galy et al., 2002; Buhl et al., 2007; Brenot et al., 2008; Bolou-Bi et al., 2009; Jacobson et al., 2010; Pokrovsky et al., 2011; Riechelmann et al., 2012); shale data are from Teng et al. (2007) and Li et al. (2010); granite and granitic mineral data are from Shen et al. (2009), Li et al. (2010), Liu et al. (2010), Ryu et al. (2011) and Tipper et al. (2012); and soil and clay mineral data are from references (Brenot et al., 2008; Pogge von Strandmann et al., 2008, 2012; Teng et al., 2010b; Tipper et al., 2006, 2010, 2012; Huang et al., 2012; Opfergelt et al., 2012). Loess data from this study are marked as diagonal lines, and the other loess data are from Li et al. (2010). The vertical dashed line represents the average δ26Mg value of the bulk silicate earth (BSE) (δ26Mg¼ −0.2570.07‰; Teng et al., 2010a).

5.2.1. Source heterogeneity The provenance of loess from around the world is diverse. For example, Chinese loess samples derive mainly from non-glacial

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alluvium in the Gobi desert (Liu, 1985; Liu et al., 1994; Sun, 2002), whereas loess samples from Argentina have multiple source areas, including extra-Andean regions of Patagonia, the Parana River basin, and the Pampean Hills (Zárate and Blasi, 1993; Sayago, 1995; Smith et al., 2003; Iriondo and Kröhling, 2007). The origin of the Western European loess material is still controversial, but it has been proposed to come from local sources of paleo-estuaries bordering the English Channel and/or shallow marine shelves that emerged during glacial times (Lebret and Lautridou, 1991; Gallet et al., 1998). Holocene loess (LO94) from Spitsbergen was probably produced by glacial grinding of bedrocks in Svalbard, which is characterized by large exposures of Jurassic black shales (Bryant, 1982; Peucker-Ehrenbrink and Jahn, 2001). The loess samples from Banks Peninsula, New Zealand, are mainly from Mesozoic greywacke in the Southern Alps and were produced by mountain glacial erosion, and the source materials of loess in the Midwestern USA are probably dominated by river outwash from the Rocky Mountains (Taylor et al., 1983; Teng et al., 2004). Such diverse source regions of individual loess deposits in turn produce loess with different chemical, isotopic and mineralogical compositions. For example, Sr and Nd isotopic compositions of global loess vary significantly and mainly reflect source signatures (Taylor et al., 1983; Yokoo et al., 2004; Chen et al., 2007; Pett-Ridge et al., 2009; Újvári et al., 2012). The source heterogeneity likely plays an important role in controlling the Mg isotopic composition of global loess. Similar to 87Sr/86Sr and 143Nd/144Nd ratios, δ26Mg values of global loess vary significantly (−1.64‰ to +0.25‰) and display significant geographical variability, but the individual provinces show less isotopic variability, with the exception of the loess samples from China and France (Fig. 5). As shown in Fig. 4, Mg isotopic compositions of carbonate and silicate rocks are distinct. Thus, the co-variations between Sr, Nd and Mg isotopic ratios in the loess from different parts of the world (Fig. 5) may reflect characteristics of their source regions. For examples, among all loess samples analyzed here, those loess samples from Argentina exhibit mantle-like Mg isotopic signatures (−0.37‰ and −0.29‰), the least radiogenic Sr values (0.7063 and 0.7094) and the most radiogenic Nd values (0.51256–0.51231), reflecting a significant contribution of a young volcanic source region (Zárate and Blasi, 1993; Smith et al., 2003). By contrast, loess samples from the USA have the least radiogenic Nd, relatively more radiogenic Sr and heavier δ26Mg values (−0.32‰ to −0.01‰), implying a greater contribution of differentiated crustal materials. The relatively homogeneous Mg, Sr and Nd isotopic compositions of loess in the New Zealand appear to indicate that their source materials were well mixed through sedimentary recycling.

65

Loess samples from Argentina, USA and New Zealand experienced limited post-alteration during their formation and have restricted Mg isotopic variations. By contrast, loess samples from China and Europe, especially France, are characterized by relatively constant 87Sr/86Sr and 143Nd/144Nd ratios but highly variable δ26Mg values (Fig. 5). The similar 87Sr/86Sr and 143Nd/144Nd ratios of these loess samples indicate similar source materials. This is in agreement with the fact that Chinese loess has its ultimate source in High Asia (Gallet et al., 1998). The large Mg isotopic variations of these loess samples, therefore, may be attributed to additional processes during formation of loess deposits, as discussed below. 5.2.2. Eolian transport Considerable eolian sorting of loess materials occurs during transport from the source area to the final depositional area (Liu, 1985; Pécsi, 1990; Pye, 1995). This process not only leads to a change in grain size of loess, but also induces variations in the mineral and chemical composition of loess. For example, loess in the Chinese Loess Plateau displays a decreasing grain-size gradient from northwest (sandy loess zone) to southeast (clayey loess zone), reflecting sorting of eolian particles during transport from the Gobi Desert to the Chinese Loess Plateau (Liu et al., 1993; Eden et al., 1994; Liu and Ding, 1998; Yokoo et al., 2004). This grain-size gradient is also reflected in chemical gradients. For example, the concentrations of Si, Na and Mg decrease from north to south of the Chinese Loess Plateau, and increasing trends are observed for the concentrations of Al, Fe, K and Mn (Eden et al., 1994). In addition, Wang et al. (2007) noted that the fine grain-size fractions (o2 μm) of loess from the Chinese Loess Plateau have higher 87 Sr/86Sr ratios than those of coarser grain-size fractions. Similar phenomena are also observed in other loess deposits such as the Argentinean loess region (e.g., Smith et al., 2003). δ26Mg values of Chinese loess samples show an overall increasing trend from the northwest (Qinghai) to the southeast (Shanxi) of the Chinese Loess Plateau (Table 1). This is coincident with the decreasing granulometric gradient from northwest (sandy loess zone) to southeast (clayey loess zone) (Liu et al., 1993; Eden et al., 1994; Liu and Ding, 1998; Yokoo et al., 2004), suggesting that the mineral sorting during eolian transport is likely to be an additional process affecting the Mg isotopic compositions of Chinese and French loess. This speculation is supported by the negative correlation between SiO2/TiO2 molar ratios and δ26Mg values of loess samples from the same loess deposit, particularly those from China and France (Fig. 6). The molar SiO2/TiO2 ratio is generally used as a proxy for grain size of eolian particles, which ultimately reflects winter monsoon intensity (Liu et al., 1995; Jahn et al., 2001; Peng and Guo, 2001). Because SiO2 and TiO2 are among the

Fig. 5. (A) 87Sr/86Sr and (B) 143Nd/144Nd ratios as a function of δ26Mg value for loess samples. Sr and Nd isotopic data source: Chinese loess from Jahn et al. (2001), Argentinean and European loess from Taylor et al. (1983) and Gallet et al. (1998), New Zealand and the USA from Taylor et al. (1983). Magnesium isotopic data of loess in New Zealand and the USA are from Li et al. (2010).

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0.4

0.4 China

0.0

France Argen na

δ26Mg (‰)

δ26Mg (‰)

0.0

USA

-0.4

JX-8

JX-1

New Zealand

XF-6 JX-10 XN-1 XN-10

-0.8

-0.4 -0.8

China France USA New Zealand Argenna

-1.2 -1.6

-1.2

53

55

57

59

61

63

65

67

CIA (%) -1.6 50

100

150

200

250

SiO2/TiO2 (molar rao) 26

Fig. 6. δ Mg as a function of the SiO2/TiO2 molar ratio of loess. Magnesium isotopic data of loess in New Zealand and the USA are from Li et al. (2010). The solid line represents a linear regression fitted through the Chinese loess data.

most stable components, they are relatively resistant to chemical weathering. Moreover, quartz as a dominant detrital mineral in loess tends to accumulate in the coarse fraction of loess (Liu, 1985; Pécsi, 1990; Eden et al., 1994; Peng and Guo, 2001). The coarse fraction consequently has relatively higher SiO2 content, whereas the TiO2 loess displays little variations between different grainsize fractions (Yang et al., 2006). As a result, eolian sorting would lead to a decrease in the SiO2/TiO2 ratio with increasing transport distances from the source area. The negative correlation between SiO2/TiO2 ratios and δ26Mg values for loess samples from China and France therefore reflects an eolian sorting progress. As grain sizes of eolian particles decrease, Mg isotopic composition of loess becomes heavier. This also reflects the fact that clay minerals, which generally have heavier Mg isotopic signatures (Fig. 4), tend to be enriched in the fine fraction of loess, whereas detrital carbonates with lighter Mg isotopic composition are more dominant in the coarse loess fraction (Liu, 1985; Chen et al., 2007).

5.2.3. Chemical weathering Before being transported by winds to the final depositional area, loess materials have undergone various changes in their source provenance such as chemical weathering (Gallet et al., 1998; Ding et al., 2001; Peucker-Ehrenbrink and Jahn, 2001). After deposition, loess material undergoes additional chemical weathering during interglacial periods when paleosols form, but experiences negligible chemical alteration during dry and cold glacial periods (Liu and Ding, 1998; Ding et al., 2001). The chemical index of alteration (CIA) has been widely used to estimate the weathering intensity of loess (Liu et al., 1995; Gallet et al., 1996; Jahn et al., 2001; Yang et al., 2004; Újvári et al., 2008). Because the CIA value reflects the entire alteration processes that detrital particles have experienced (McLennan et al., 1993), it could be an index of the relative contribution of altered source rocks to loess. The CIA values of loess from the same loess deposit (especially China, France and USA) positively correlate with their δ26Mg values (Fig. 7), indicating Mg isotope fractionation during chemical weathering, with preferential enrichment of heavier Mg isotopes in loess that has undergone more intense weathering (i.e., higher CIA value). This is in agreement with the general expectation that continental weathering fractionates Mg isotopes, with light Mg isotopes enriched in the dissolved phase and heavy Mg isotopes concentrated in the weathered residue (e.g., Tipper et al., 2006, 2008, 2010, 2012; Brenot et al., 2008; Pogge von Strandmann et al., 2008, 2012; Li et al., 2010; Teng et al., 2010b;

Fig. 7. The CIA as a function of the δ26Mg of loess. CIA refers to the chemical index of alteration, CIA ¼molar [Al2O3/(Al2O3+CaO*+Na2O+K2O)]  100 where CaO* is the CaO content in the silicate fraction of the sample (Nesbitt and Young, 1982). Magnesium isotopic data of loess in New Zealand and the USA are from Li et al. (2010).

Wimpenny et al., 2010; Huang et al., 2012; Opfergelt et al., 2012). Secondary clay minerals are ultimately produced at the expense of the upper continental crustal materials during chemical weathering (Nesbitt and Young, 1989; McLennan, 1993), and generally have heavier Mg isotopic composition than carbonates (Fig. 4). Thus, those loess samples with higher CIA values and heavier Mg isotopic composition may have undergone more intense weathering and contain a higher abundance of clay minerals. The combination of Mg isotope fractionation during eolian sorting with Mg isotope fractionation during chemical weathering can account for the narrow ranges of 87Sr/86Sr and 143Nd/144Nd ratios but large variations of δ26Mg values in the loess from China and France (Fig. 5). In summary, in addition to source heterogeneity, both the processes of eolian sorting and chemical weathering during formation of loess deposit have potentials to affect the distribution of carbonate and clay minerals in loess, which in turn controls the Mg isotopic signatures of loess. Eolian transport preferentially delivers fine-grained fractions (enriched in clay minerals) from source regions to final loess deposits, whereas chemical weathering transforms primary minerals into secondary minerals dominated by clay minerals (Fig. 8). Since clay minerals have heavier Mg isotopic composition than carbonates (Fig. 4), both eolian sorting and chemical weathering tend to drive the Mg isotopic composition of loess toward heavy values. Although it is difficult to tease out the exact contribution of chemical weathering from eolian sorting for samples studied here, the directions of these two processes on fractionating Mg isotopes are consistent. With the exception of loess samples from New Zealand and some loess samples from the USA and Europe, most of the loess samples studied here have significantly lighter Mg isotopic compositions compared to crustal igneous rocks (Li et al., 2010). This may reflect dilution by carbonates from source regions with light Mg isotopic signatures (Fig. 4). Thus, source heterogeneity plays the most important role in controlling Mg isotopic signatures of the loess, enhanced by eolian sorting and chemical weathering. 5.3. Implications for paleoclimatic change The thick loess–paleosol sequence in the Chinese Loess Plateau, which records a history of alternation between winter and summer monsoon dominance, is regarded as one of the best archives of environmental changes during the Quaternary (Heller and Liu, 1984; Liu, 1985; Liu and Ding, 1998). Consequently, a number of proxy indicators involving loess–paleosol sequences have been used to characterize paleoclimatic change. Among these proxy indicators, magnetic susceptibility and grain-size

K.-J. Huang et al. / Earth and Planetary Science Letters 374 (2013) 60–70

67

Physcial weathering Chemicalweathering

(2)Eolian sorng: preferenal transport of fine-grained clays enriched in heavy Mg isotopes

(3)Chemical weathering (pedogenesis): fraconate Mg isotopes with preferenal enrichment of heavy Mgisotopes

(1)Source heterogeneity:

inweathered residual Silicate rock δ26Mg= -1.22‰to 0.44‰

Wind

Carbonate rock δ26Mg= -4.84‰to -0.47‰ Outwash /Desert basin

Loess deposit

Fig. 8. Schematic diagram of the controls on Mg isotopic compositions of loess. The Mg isotopic composition of the loess reflects the relative proportions of carbonates to clays in loess. Three factors: (1) source heterogeneity, (2) eolian sorting during transport and (3) chemical weathering, play important roles in controlling the distribution of carbonates and clays in loess.

distribution are closely related to past changes of chemical weathering (pedogenesis) and wind intensity respectively, and have been widely used to reconstruct the summer and winter monsoon in East Asia (e.g., An et al., 1991; Liu and Ding, 1998; Jiang and Ding, 2010). Geochemical indicator proxies such as CIA, elemental ratios of Rb/Sr and SiO2/Al2O3, and isotopic ratios of 86Sr/87Sr have been shown to be more sensitive to paleoclimatic changes than magnetic susceptibility and grain size and, therefore, are widely used to reconstruct paleoclimatic changes (Gallet et al., 1998; Chen et al., 1999; Yang et al., 2000; Ding et al., 2001; Guo et al., 2004; Wang et al., 2007; Liang et al., 2009). Nonetheless, each of these proxies needs to be verified by other independent indices. Thus, it is important to develop additional proxy indices. A recent study of Mg isotopic records in a loess–paleosol sequence from the Chinese Loess Plateau has shown that the large oscillations in δ26Mg values closely match the changes in other weathering tracers such as magnetic susceptibility, grain size and elemental ratios of Rb/Sr and Ca/Ti (Wimpenny et al., 2011b). These findings indicate that Mg isotopes also have the potential to characterize changes in paleoclimatic conditions. Our study of loess samples from world-wide loess deposits reveals that Mg isotopic composition of loess is not only controlled by source rocks but also influenced by the processes of eolian sorting and chemical weathering, which are sensitive to the paleoclimatic change. The significant correlations between Mg isotopic ratios and indexes of weathering (e.g., CIA) imply that loess has been subjected to different degrees of chemical weathering, which imprinted on their distinct Mg isotopic compositions. In addition, our results indicate that Mg isotopic ratios of the loess, especially those from the Chinese Loess Plateau, correlate with grain-size changes (e.g., SiO2/TiO2), which depend on wind intensity during eolian transport. All these indicate that Mg isotopic systematics in loess can potentially be a useful tool to enhance our understanding of paleoclimatic changes. Nonetheless, more studies of Mg isotopes, especially on well-characterized loess–paleosol sequences, are needed before using Mg isotopes in loess as a new geochemical proxy for paleoclimatic changes. 5.4. Implications for the average Mg content and isotopic composition of the upper continental crust A number of studies, starting with the pioneering work of Taylor et al. in the 1980s, have used the chemical composition of loess to derive the average upper continental crust composition (Taylor et al., 1983; Gallet et al., 1998; Barth et al., 2000; Peucker-Ehrenbrink and

Jahn, 2001; Hattori et al., 2003; Teng et al., 2004; Hu and Gao, 2008; Újvári et al., 2008; Li et al., 2010; Park et al., 2012). The results from these studies have found that most major and trace elemental concentrations in loess fall within the range of previous estimates of the upper continental crust (Taylor et al., 1983; Gallet et al., 1998; Peucker-Ehrenbrink and Jahn, 2001), suggesting that the chemical composition of loess could reflect that of the average upper continental crust. In addition, Taylor et al. (1983) suggested that even fluid-mobile elements may not be fractionated in loess, as reflected by the fact that trace element abundances in loess are similar to those of the upper crust, and do not correlate with elemental solubility and mobility. For instance, as a fluid-mobile element, Li is known to be fractionated during weathering (Kısakürek et al., 2004; Rudnick et al., 2004), but loess samples from around the world are found to have similar Li concentrations and isotopic compositions (Teng et al., 2004). This suggests that loess could also provide constraints on the average upper crustal composition for soluble elements. MgO contents of loess vary considerably from 0.59 to 4.37 wt%, depending on MgO contents of source rocks and relative proportion of silicates to carbonates in loess. To eliminate these effects, we examined the positive correlation between MgO and insoluble elements considered to be not significantly fractionated during weathering and sedimentary processes, such as La, Th, Y and Nb (McLennan, 2001; Rudnick and Gao, 2003). The MgO content of global loess from this and previous studies moderately correlates with these insoluble elements (Fig. 9). Using these correlations and the average upper continental crust concentrations of these elements (Rudnick and Gao, 2003), we estimate the MgO content of the upper continental crust from each correlation (Fig. 9), ranging from 1.48 wt% to 2.39 wt%. The average of these results is about 1.92 wt%, which is obviously lower than the more recent estimates of the MgO content of the upper continental crust (2.48 wt%, Rudnick and Gao, 2003). Furthermore, as noted above, δ26Mg values of the global loess studied here show a greater range of variation than representative upper continental crust samples (Fig. 5), suggesting that the Mg isotopic composition of loess may not represent the original Mg isotopic composition of the upper continental crust. There are several lines of evidence to support such a statement. First, Mg is fluid-mobile and Mg isotopes are significantly fractionated during chemical weathering (Tipper et al., 2006, 2008; Pogge von Strandmann et al., 2008, 2012; Li et al., 2010; Teng et al., 2010b; Wimpenny et al., 2010; 2011a; Huang et al., 2012; Opfergelt et al., 2012). Thus, loess will not record crustal Mg elemental and isotopic abundances. Second, as

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Fig. 9. MgO content versus concentration of insoluble elements La, Y, Nb and Th in loess. The gray circles refer to the loess data from references (Taylor et al., 1983; Gallet et al., 1996, 1998; Graham et al., 2001; Jahn et al., 2001; Muhs et al., 2008; Újvári et al., 2008; Liang et al., 2009). The solid lines represent linear regressions fitted through the data. The dashed lines represent the elemental concentrations in the upper continental crust from Rudnick and Gao (2003) and the estimated MgO values based on the linear correlations. See text for details.

demonstrated above, mineral sorting during eolian transport appears to modify Mg isotopic signatures of loess. Finally, carbonates have significantly lighter Mg isotopic compositions than silicates, resulting in the relatively lighter Mg isotopic composition of loess. The isotopically light carbonates will not significantly change the Mg content of loess, but clearly shift their Mg isotopic compositions toward lighter value. However, carbonates only account for a small proportion of the upper continental crust, whereas silicates dominate the mineralogy of the upper continental crust (Gao et al., 1998). Estimates of the average Mg isotopic composition using loess alone hence will lead to light δ26Mg value. Nonetheless, Mg isotopic compositions of loess might place the maximum limit on the average δ26Mg value of the carbonate component in the upper crust.

6. Conclusions Based on high-precision Mg isotopic analyses of global loess samples, we conclude: (1) Loess from different parts of the world has a heterogeneous Mg isotopic composition with δ26Mg varying from −1.64‰ to +0.25‰ and a weighted average δ26Mg ¼−0.89‰, which is lighter than the mantle and upper continental crust. (2) δ26Mg of loess from different parts of the world can be modeled by binary mixing between clay-rich and carbonaterich end members, which reflects the mineralogical control of Mg isotopic composition of loess. (3) Source heterogeneity, eolian sorting during transport of loess and chemical weathering during formation of loess deposit affect the distribution of carbonates and clays in loess, and account for the highly heterogeneous Mg isotopic composition of loess.

26 (4) Significant correlations between δ Mg of loess from the same loss deposit and climatic indices such as CIA and SiO2/TiO2 ratio indicate that Mg isotopic composition of loess may provide insights into paleoclimatic changes. (5) Due to isotope fractionation during eolian sorting, chemical weathering and dilution by carbonate, loess sample is not suitable for estimating the average Mg isotopic composition of the upper continental crust but can be used to constrain the maximum δ26Mg of the average crustal carbonate.

Acknowledgments We are grateful to Bernhard Peucker-Ehrenbrink, Bor-ming Jahn and Roberta Rudnick for providing loess samples and Bernhard Peucker-Ehrenbrink for comments on an earlier version of the manuscript. We thank Shan Ke, Wei Yang and Sheng-Ao Liu for help in the lab, and Johnnie Chamberlin and Shui-Jiong Wang for stimulating discussions. The manuscript was significantly improved by the constructive reviews from two anonymous reviewers. The efficient editorial handling of Bernard Marty is greatly appreciated. This work was financially supported by the National Science Foundation (EAR-0838227, EAR-1056713 and EAR-1340160). K.J.H. is partially supported by the China Scholarship Council.

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