Sr-Nd isotopic characteristics of the Northeast Sandy Land, China and their implications for tracing sources of regional dust

Catena 184 (2020) 104303

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Sr-Nd isotopic characteristics of the Northeast Sandy Land, China and their implications for tracing sources of regional dust

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Yuanyun Xiea,b, , Lu Liua, Chunguo Kangc, Yunping Chia a

College of Geographic Science, Harbin Normal University, Harbin 150025, China Heilongjiang Province Key Laboratory of Geographical Environment Monitoring and Spatial Information Service in Cold Regions, Harbin Normal University, Harbin 150025, China c Geography Department, Harbin Institute, Harbin 150086, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Sr-Nd isotopic composition Loess Northeast Sandy Land Eolian dust Source tracing

The Sr-Nd isotopic study of the Northeast Sandy Land (NESL), China is of great significance for a thorough understanding of the dust system in this area. The 162 bulk (< 63 μm) and 86 size-fractioned samples of surface eolian sands and river sands in the NESL are very suitable to be added to the elaborate Sr-Nd isotopic composition dataset. The results are as follows. The Nenjiang River drainage system (NR) in the Songnen Sandy Land (SNSL) and the Hulun Buir Sandy Land (HLSL) are characterized by more radiogenic Nd values relative to the other areas within the NESL whereas the Onqin Daga Sandy Land (ODSL), the Horqin Sandy Land (HQSL) and the Songhua River drainage system (SR) exhibit a clear overlap of Sr-Nd isotopic composition. Nd isotopic composition (especially in the ODSL) exhibits significant dependency on time effect, indicating instability of geochemical composition in the dust source areas. Sr isotopic composition is slightly affected by grain size with very small variations of size-separated 87Sr/86Sr values, yet Nd isotopic composition exhibits clear grain sizedependent variability. The substantial proportion of Sr-Nd isotopic values show enrichment in the coarse-grained fraction, which is accounted for by source-rock control. Both the HLSL and the NR to the northwest of Harbin play very little role in contribution to eolian dust deposits in the Northeast Plain, either in the geological past or now, whereas Sr-Nd geochemical composition alone is insufficient to unequivocally distinguish exact provenance of the eolian dust deposits in this region. The integrated consideration, as expected, revealed the derivation of the eolian dust deposits. The eolian loess and the modern dust-storm deposits in Harbin share significantly different source areas: Harbin loess is the product of spring dust-storm weather and has a mixed source with the SR being the main dust contributor, whereas 2011 Harbin dust-storm deposits were derived from the ODSL and beyond which also provided minor amounts of fine dust particles to Harbin loess.

1. Introduction The Gobi (stony desert), deserts and sandy lands widely distributed in the north of China, continuously convey large amounts of dust into the downwind adjoining areas and even into areas further away, and are identified to be a dominant provenance for Asian dust (e.g., Zhang et al., 1997; Sun, 2002; Chen et al., 2007; Rao et al., 2008; Li et al., 2009, 2011; Yang et al., 2009; Shi and Liu, 2011; Che and Li, 2013; Chen and Li, 2013; Nie et al., 2014; Zhang et al., 2016). The geochemical, especially isotopic, investigations into the dust source areas are crucial for Asian dust system study, and also offer the key to a better understanding of the initiation and evolution of deserts, and of the formation mechanism, derivation and migration path of desert materials. More importantly, the isotopic investigations of the dust source

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areas are of great importance to reconstruct the past pattern of atmospheric circulation. The sediments derived from different geologic bodies have significantly different Sr-Nd isotopic composition that are dependent on their origins and ages, and these isotopic ratios show little alternation during the surficial processes of weathering, transportation and deposition (Grousset and Biscaye, 2005). More significantly, the Sr-Nd isotopic composition of sediments are recognized to offer more precise information regarding the composition of source areas, and accordingly, are expected to be more diagnostic in pinpointing source than is element geochemistry (Dou et al., 2012; Israel et al., 2015; Bi et al., 2017; Xie et al., 2018). Therefore, Sr-Nd isotopic ratios have long been widely employed as a powerful tool for deciphering the provenance of eolian dust (e.g., Grousset and Biscaye, 2005; Chen et al., 2007; Rao

Corresponding author. E-mail address: [email protected] (Y. Xie).

https://doi.org/10.1016/j.catena.2019.104303 Received 20 March 2019; Received in revised form 26 September 2019; Accepted 30 September 2019 Available online 15 October 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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were sampled. River sand and eolian sand are well mixed by wind and hydraulic transport and their fine-grained fractions are capable of representing the average composition of a comparatively large area. A total of 162 samples were collected throughout the NESL, 24 from the ODSL, 35 from the HQSL, 13 from the HLSL and 90 from the SNSL. Of the 90 obtained samples in the SNSL, 41 samples were taken from the Nenjiang River drainage system (NR, including Dumeng, Qiqihar, Tailai, Zhenlai and Baicheng) and the rest from the Songhua River drainage system (SR, including Changling, Qian’an, Zhaoyuan, Da’an, Songyuan, Fuyu, Dehui, Yushu). All the samples obtained in the NESL were separated into < 63 μm fraction by dry sieving. Such a selection is principally based on a consideration that only components smaller than 63 μm can be transported in suspension in the air (Moldavg, 1962; Pye, 1987, 1995; Tsoar and Pye, 1987; Rao et al., 2008; An et al., 2012; Vandenberghe, 2013; Nottebaum et al., 2015; Vandenberghe et al., 2018), which could effectively undermine the influence of mineral sorting during eolian transportation and deposition and allow intersample comparison in the same grain-size ranges. In addition, to evaluate the effect of grain size on Sr-Nd isotopic composition, some samples were further split by dry sieving into < 30 μm, < 10 μm, 74–63 μm, 63–30 μm and 30–10 μm fractions. A total of 248 sub-samples were acquired by such a separation: 46 from the ODSL, 63 HQSL, 23 HLSL and 116 SNSL. In order to determine the contribution of the NESL to regional dust, Some Northeast (NE) loess samples combined with the modern duststorm deposits were collected for understanding the relation between eolian dust and the NESL. The 22 collected NE loess samples are from the first loess horizon developed during the last glaciation (equivalent to Malan loess (L1) in the Chinese Loess Plateau) in three sections (see Fig. 1 for position): Xinwopu section (XWP loess, 42°38′N, 119°16′E), Kulungou section (KLG loess, 42°43′N, 121°47′E) and Huangshan section (HS loess, 45°43′N, 126°36′E). These loess deposits are distributed along the margin of the NESL. All loess samples were wet-sieved to obtain < 63 μm fraction. Additionally, two samples from the HS loess and one sample from the KLG loess were split by wet sieving into fractions of 97–63 μm, 63–30 μm, < 10 μm and < 10 μm, respectively. A strong dust-storm event occurred in northeast China on May 11, 2011. The 3 dust-storm deposit (DSD) samples were collected in Harbin at the time of dust storm.

et al., 2008; Li et al., 2009, 2011; Yang et al., 2009; Israel et al., 2015). Clarifying source regions of the dusts demands systematic studies on the Sr-Nd isotopic characteristics of all potential source regions. The pioneering investigations have clearly established the elaborate Sr-Nd isotopic composition of the Gobi and deserts in Northern China (e.g., Chen et al., 2007; Rao et al., 2008; Li et al., 2009, 2011), including the arid lands around the northern boundary of China (NBC), arid lands on the northern margin of the Tibetan Plateau (NMTP) and deserts on the Ordos Plateau (OD), making a significant contribution to Asian dust system study. In the Northeast Sandy Land (NESL), situated at the easternmost edge of the Eurasian desert-loess belt, however, few (for the Onqin Daga Sandy Land and the Horqin Sandy Land and the Hulun Buir Sandy Land) or no (for the Songnen Sandy Land) Sr-Nd isotopic composition can hitherto be obtained, which significantly hinders our understanding of the material mixing pattern linked to the Central Asia Orogen (CAO arid lands, CAO are orogenic belts situated approximately near the NBC, which include the Gobi-Altay Mountain, Tianshan Mountains, and Great Hinggan Mts) and the North China Craton, and of contributions to regional dust. In this regard, extensive sampling was performed in the NESL to determine Sr-Nd isotopic composition of the fine-grained fractions, thereby establishing a detailed Sr-Nd isotopic composition data set of the NESL. In addition, the contribution of the NESL to regional dust is also discussed in this study. 2. Sampling and methods The NESL is composed of the Onqin Daga Sandy Land (ODSL), the Horqin Sandy Land (HQSL), the Hulun Buir Sandy Land (HLSL) and the Songnen Sandy Land (SNSL) (see Fig. 1). The ODSL, with an area of 52000 km2, is situated at the southern side of the Xilin Gol grassland in central Inner Mongolia. The HQSL, the largest sandy land in China, is mainly distributed in the flood plain of the West Liaohe River, covering an area of ca. 62432 km2. The HLSL, with an area of 7435.2 km2, is one of the highest latitude sandy lands in China and the smallest in the NESL, and developed from an alluvial-lacustrine plain. The SNSL, 8355.95 km2, is located in the central and western Songnen Plain and distributed along the flood plain and river terraces of the Nenjiang River and the Songhua River (Qiu, 2008). The river sands and eolian sands which cover most of the NESL,

Fig. 1. (A) Sketch map of East Asia, showing the deserts and sandy lands in northern China believed to be main Asian dust sources; (B) Sketch map of the Northeast Sandy Lands in China, showing location of the study area and sampling section sites. The red arrow represents the prevailing wind direction after Qiu, 2008. 2

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3.2.1. The Onqin Daga Sandy Land The Sr-Nd isotopic composition for bulk samples (< 63 μm) and < 10 μm fractions is both characterized by significantly great variations (see Fig. 2b), ranging from 0.709618 to 0.712293 and from −15.0 to −6.6, from 0.710636 to 0.712339 and from −12.4 to −5.9, respectively. The significant variation indicates the heterogeneity of isotopic composition in the ODSL. The 87Sr/86Sr and εNd(0) values in the ODSL are between the CAO (from 0.710587 to 0.713900 and from −8.7 to −0.9, respectively, Li, et al., 2009) and the the North China Craton (from 0.714824 to 0.719218 and from −17.2 to −11.8, respectively, Li, et al., 2009), which indicates a material mixture from the Great Hinggan Mts to the east and the North China Craton to the south.

All the sub-samples were measured for Sr and Nd isotopic composition. Strontium is enrichment in detrital minerals (such as: feldspars, micas, clays, and carbonates) derived from parent rocks and in secondary carbonates (mainly calcite) formed mostly during pedogenesis (Yang et al., 2005; Wang et al., 2007). Sr contents hosted in secondary carbonates can mask the isotopic characteristics of parent rocks (Újvári et al., 2012). To eliminate the influence of secondary carbonates on Sr isotopic composition and to ensure analysis of only detrital minerals of sediments, the sub-samples were leached in acetic acid (0.5 mol/L) at room temperature for up to 8 h (e.g., Chen et al., 2007; Wang et al., 2007; Újvári et al., 2012). The Sr-Nd isotopic ratios of acid-insoluble residues of all the subsamples were determined by thermal ionization mass spectrometry (TIMS) following the method of Chen et al. (2007). The Sr and Nd isotopic ratios were normalized to correct for mass fraction by using 86 Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The analytical blank was < 1 ng for Sr and < 50 pg for Nd. Reproducibility and accuracy were checked by periodically measuring the Sr standard NBS987 and Nd standard JMC, with a mean 87Sr/86Sr value of 0.710250 ± 7 (2σ external standard deviation, n = 10) and a mean 143 Nd/144Nd value of 0.512109 ± 3 (2σ external standard deviation, n = 7), respectively. Epsilon Nd values (εNd) were calculated using the present-day chondritic uniform reservoir values of 143 Nd/144Nd = 0.512638.

3.2.2. The Horqin Sandy Land The Sr-Nd isotopic data of the total sixty-three sub-samples can document the overall isotopic characteristics of the HQSL well. The 87 Sr/86Sr and εNd(0) data of bulk samples (< 63 μm) and < 10 μm fractions in the HQSL show prominent variations (Fig. 2c), especially in the case of εNd(0) value, varying from 0.709898 to 0.712122 and −12.4 to −5.8, from 0.709436 to 0.712091 and −11.9 to −4.3, respectively, which shows heterogeneous composition. As is the case for the material derivation of the ODSL, the material mixture of both the Great Hinggan Mts to the west and the North China Craton to the south is responsible for the derivation of the HQSL materials, but the Great Hinggan Mts is the main material contributor.

3. Results and discussion

3.2.3. The Hulun Buir Sandy Land The Sr-Nd isotopic data of the 23 sub-samples in this study can roughly reflect the overall isotopic characteristics of the HLSL. The 87 Sr/86Sr and εNd(0) data in the HLSL show slightly smaller variations relative to those of the ODSL and the HQSL (Fig. 2d), ranging from 0.709465 to 0.710361 and −8.9 to −5.0, respectively, which is characteristic of materials supplied by the Great Hinggan Mts. It should be pointed out that Sr-Nd isotopic data (< 75 μm and < 5 μm) reported by Chen et al. (2007) using the same acetic acid treatment as in this study for the HLSL do not fall within our data range, which are significantly higher than those in this study.

3.1. Reliability assessment for the inter-sample difference in Sr-Nd isotopic composition Before discussing the Sr-Nd isotopic data of the studied samples from different regions, it is essential to estimate its analysis error in order to accurately evaluate the reliability of difference between samples (e.g., Wang et al., 2007; Yang, et al., 2009; Rao et al., 2017). The error of 143Nd/144Nd in a single determination is normally within 0.000020, which would cause a variation of εNd(0) within 0.3ε units. Generally, the total experimental error is supposed to comprise multiple determination error, correction error of Standard Reference material, and chemical error of the pretreatment. Based on uncertainty estimates, the total experimental error of 143Nd/144Nd determination is generally within ± 0.000050 (Wang et al., 2007; Yang et al., 2009; Rao et al., 2017), which would introduce a change of εNd(0) within 0.7ε units. Accordingly, we accept that Nd isotopic composition has essentially changed or an isotopic anomaly occurs only when the difference of between-sample εNd(0) values is larger than 1ε units, in accordance with the proposal of Bayon et al. (2015). In other words, Nd isotopic variation between samples is essentially triggered by geological processes rather than experimental error only when it is larger than 1ε units. The experimental error of 87Sr/86Sr determination, as is the case for 143 Nd/144Nd, is generally lower than 0.000050 (Yang, et al., 2009; Rao et al., 2017); this uncertainty value is noticeably lower than the difference between samples, indicating that the samples characterized by the 87Sr/86Sr difference higher than 0.000050 really display significant between-sample variability. That is, the absolute Sr isotopic difference between varying samples bears geological significance only when is greater than 0.000050. There are also different opinions suggesting that Sr isotopic composition are identical if the difference of 87Sr/86Sr ratios between samples is within ± 0.001500 (e.g., Yang, et al., 2009; Rao et al., 2011), which is accepted by this study.

3.2.4. The Songnen Sandy Land The Sr-Nd isotopic data of the total 116 sub-samples perfectly depict the overall isotopic characteristics of the SNSL. According to the isotopic characteristics, the SNSL can be further divided into two sub-regions (Fig. 2e), i.e., the NR sub-region with low 87Sr/86Sr and high εNd(0) and the SR sub-region with high 87Sr/86Sr and low εNd(0). The samples in the NR yield 87Sr/86Sr and εNd(0) values of 0.707394 to 0.710277 and −7.8 to −3.8, respectively, while the samples in the SR are characterized by 87Sr/86Sr and εNd(0) values varying from 0.709198 to 0.713523 and −11.0 to −4.8, respectively. The data of the two subregions are separated into their respective regions with very little overlap. In general, in terms of spatial variations of Nd isotopic composition, the largest variations are in the ODSL, followed by the HQSL, the SNSL and the HLSL, which records well the heterogeneous degree of geochemical composition of the sandy land. 3.3. Temporal variation of Sr-Nd isotopic composition over time in the NESL The temporal variation of Sr-Nd isotopic composition over time reflects the stability of source regions in geochemical composition, which is of importance for deeply understanding the linkage of eolian dust to potential source area. The Sr-Nd isotopic composition of eight profiles in the NESL were investigated to understand their variations over time (Fig. 3). As can clearly be seen from Fig. 3, εNd(0) values in the profiles present clear temporal variations, varying from 1.6ε to 6ε units, which has clearly exceeded the total experimental error of 143Nd/144Nd determination

3.2. Sr-Nd isotopic composition for the NESL The Sr-Nd isotopic composition for the studied samples in the NESL was listed in Supplementary Table1 and graphically drawn in Fig. 2. 3

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Fig. 2. Sr-Nd isotopic characteristics of the Northeast Sandy Land. (a) Plot constructed as showing the spatial variations of Sr-Nd isotopic composition between the studied areas; (b)-(e) Plot graphically showing the outcome of the determined Sr-Nd isotopic composition from the ODSL, HQSL, HLSL and SNSL, respectively.

0.002883. Here, the clear change of geochemical composition of dust source areas with time was referred to as the “time effect” of Nd isotopic composition. It is of great importance for us to fully understand the source identification of the dust system that the Nd isotopic composition of potential dust source areas has shifted significantly through time. Geochemical methods are a powerful tool to trace dust source because the geochemical linkage between dust deposits and the potential source

( ± 0.000050). Further observation revealed that the largest variations for εNd(0) values occurred in the ODSL (6ε units), followed by the HLSL (3.9ε units) and the SNSL (1.6ε to 2.1ε units). The reason for clear temporal variations of εNd(0) values in the ODSL may be due to the variation of relative material contribution of different source areas to the ODSL, as indicated by significant spatial variations of εNd(0) values in the ODSL (8.4ε units). In comparison, however, 87Sr/86Sr values in the profiles show minor temporal variations from 0.000253 to 4

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Fig. 3. Sr-Nd isotopic composition for eight sections in the Northeast Sandy Land, which indicates variation of Sr-Nd isotopic composition through time. Note that Nd isotopic values in the profiles present clear temporal variations, which reflects instability of geochemical composition in dust source areas.

sediments is affected by many factors, not only by parent rocks, but also by grain size and chemical weathering. In general, 87Sr/86Sr values in sediments increase with decreasing grain size because the fine grain component contains more Rb (K)-rich clay minerals (with more positive 87Sr/86Sr values) (Dasch, 1969; Rao et al., 2006, 2017; Feng et al., 2009; Meyer et al., 2011). In some cases, however, the 87Sr/86Sr values of the fine particles are lower than those of the coarse particles (e.g., Derry and France-Lanord, 1996; Li et al., 2015). In the case of our study, grain-size effect on Sr-Nd isotopic composition was well reflected in the 153 obtained size-separated data (Fig. 4 and Supplementary Table1). In summary, the variations of sizeseparated 87Sr/86Sr values are very small, but variation magnitude is distinct in different sandy lands, with greatest variations in the ODSL (0.000055 to 0.001478), then in the HQSL (0.000031 to 0.001115) and the HLSL (0.000340 to 0.000801), and finally in the SNSL (0.000092 to 0.000537). The small 87Sr/86Sr variations in different grain size fractions are less than the between-sample difference threshold of 0.001500 (Yang, et al., 2009; Rao et al., 2011), and also are far less than variation ranges of the dust materials in the CLP (0.003516 to 0.011821, Rao et al., 2006; Chen et al., 2007) and of the bulk samples (< 63 μm) in each individual sandy land (0.002224 to 0.004325). Unlike most of previous studies, the fine-grained fraction (< 10 μm) for the HQSL and the HLSL has a lower 87Sr/86Sr values than has the coarse-grained fraction (< 63 μm or 30–63 μm) (Fig. 4). For the ODSL, however, the distribution of 87Sr/86Sr values in different grain-size fractions shows a competing characteristic, with more radiogenic Sr either in fine particles (< 10 μm) or in coarse particles (63–74 μm). Similarly, the sizeseparated data (< 63 μm, 30–63 μm, 10–30 μm and < 10 μm) of two samples from NR indicates the < 10 μm fraction is enriched in radiogenic Sr, whereas the size-separated data (< 63 μm, < 30 μm and < 10 μm) of ten samples from SR characterize Sr isotopic composition irregularly enriched in different grain-size fraction (see Fig. 4). Although it has been a consensus that the Nd isotopic composition

areas is always deemed to be reliable, based on a consideration of geochemical stability of source areas. However, it has long been a controversial issue that whether the dust source of loess on the Chinese Loess Plateau (CLP) has changed on orbital and/or tectonic timescales (e.g., Sun and Zhu, 2010; Xiao et al., 2012; Che and Li, 2013; Nie et al., 2014; Yan et al., 2014; Zhang et al., 2015; Licht et al., 2016). The variations of loess geochemical composition over time in profiles were generally interpreted as shifting material source. However, in view of a consideration of the “time effect” of Nd isotopic composition, as revealed by our observation, supply of dust materials with shifting geochemical composition in the same source areas will also introduce variations of geochemical composition of dust materials in loess accumulation areas. For this reason, the change of the geochemical composition of dust materials in loess accumulation areas does not necessarily signify a shift in the source area.

3.4. Grain-size effect on Sr-Nd isotopic composition Sr-Nd isotopic composition of sediments is generally dependent on several factors such as chemical weathering, particle sorting and source rocks (Goldstein et al., 1984; Eisenhauer et al., 1999; Tütken et al., 2002; Smith et al., 2003; Rao et al., 2017). In the present study, Sr-Nd isotopic composition of sediments is probably linked to particle sorting in addition to source rocks due to the acid-leaching pre-treatment removing the effect of chemical weathering. Although there are different opinions on the influence of grain size on Sr-Nd isotopic composition (e.g., Derry and France-Lanord, 1996; Smith et al., 2003; Feng et al., 2009; Bayon et al., 2015; Li et al., 2015; Bi et al., 2017), it is generally believed that the Nd isotopic composition of sediments is rarely affected by grain size and essentially inherits the characteristics of its parent rocks (Dasch, 1969; Goldstein et al., 1984; Nakai et al., 1993; Rao et al., 2006, 2017; Chen, et al., 2007; Meyer et al., 2011; Dou et al., 2016). However, Sr isotopic composition of 5

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Fig. 4. The variation of size-separated Sr-Nd isotopic composition. Note that Sr isotopic composition is slightly affected by grain size, and however that Nd isotopic composition exhibits clear grain size-dependent variability. It is of interest to note that The substantial proportion of Sr-Nd isotopic values exhibit enrichment in the coarse-grain fraction.

igneous/metamorphic and volcanic provinces do not show any significant grain-size dependency. The same goes for the study of Bi et al. (2017): Nd isotopic ratios in Core MD06-3040 on the inner shelf of the East China Sea are not correlated with the particle size, whereas εNd(0) values in Core CM97 in the Yangtze River Estuary are positively correlated with grain size, with higher εNd(0) values found in the coarser sediments. Interestingly, based on the study of Feng et al. (2009), the variation pattern of Nd isotopic composition with grain size differ significantly for loess and the 2006 Bejing dust-storm sample: for loess, Nd isotopic ratios are nearly independent of grain size; but for dust-storm

of sediments is fundamentally not affected by particle size, there are different research results. The inconstant relationship between Nd isotopic composition and grain size associated with mineral sorting can been observed in some studies (e.g., Feng et al., 2009; Bayon et al., 2015; Bi et al., 2017). For example, a few large river sediments (i.e., Mississippi, Nile, Volga, Mekong, Don, Fraser and Chao Phraya rivers) are characterized by the significant decoupling of Nd isotopes between clay and silt fractions, with systematical shift towards more radiogenic εNd(0) values in clay fractions (Bayon et al., 2015). In contrast, the Nd isotopic composition of river sediments from mixed/sedimentary, 6

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and thus dust components from different source areas may be separated into different grain-size fractions during dust transport and wind dynamic sorting. Thus, there is some doubt as to how to distinguish whether source rocks or mechanical sorting (and/or chemical weathering) cause the Sr-Nd isotopic decoupling between different size fractions. If it is true for sediments with a single source that more radiogenic Sr is enriched in fine grain-size fraction due to mechanical sorting and/or chemical weathering, and that Nd isotopic composition is basically unaffected by grain size or enriched in fine-grained fraction as interpreted by Bayon et al. (2015), it could be reasonably suggested that, Sr-Nd isotopic composition enriched in coarse-grained fractions for this study, which is clearly not a consequence of dependence of SrNd isotopes on grain size and/or chemical weathering, can be interpreted as being controlled by source rocks.

samples, Nd isotopic ratios depend strongly on grain size but vary irregularly with grain size. In the present study, the dominant proportion of the size-fractioned samples in the NESL exhibit clear grain-size dependent Nd isotopic variability (see Fig. 4), with greatest variations in the ODSL (0.1ε to 9.1ε), then in the HQSL (0.6ε to 6.5ε), then the HLSL (0.7ε to 4.7ε), and finally in the SNSL (0.4ε to 2.7ε). The clear grain size-dependent Nd isotopic variations have exceeded variation ranges of the bulk samples (< 63 μm) in each individual sandy land (8.4ε, 6.6ε and 3.9ε for the ODSL, HQSL and HLSL, respectively), except 4ε of the NR and 6.2ε of the SR. With respect to Nd isotopic distribution in different grain-size fractions, except for very few samples, the vast majority of the samples in the HLSL and the SNSL are represented by lower εNd(0) values in the < 10 μm fraction. However, εNd(0) values in the ODSL show no correlation with grain size. Interestingly, the samples in the east HQSL are characterized by more radiogenic Nd values in the coarser fractions than in the < 10 μm fraction; no correlation was observed between εNd(0) values in the central-west HQSL and grain size. In addition, the size-fractioned variations of Sr-Nd isotopic composition for the HS and KLG loess show slight dependence of Sr-Nd isotopic ratios on grain size (variation range of 0.000707 to 0.000851 and 1.1ε to 2.2ε for Sr and Nd isotopic ratios), with slightly less radiogenic εNd(0) values in the fine grain-size fraction (< 10 μm) and weak tendency towards more radiogenic Sr signatures in the fine grain-size fraction (< 10 μm). As stated above, both sorting-related grain size and chemical weathering may influence Sr isotope ratios of sediments to cause finegrained fractions to tend to be enriched in more radiogenic 87Sr because of high Rb/Sr and 87Sr/86Sr ratios in clay minerals (Feng et al., 2009; Meyer et al., 2011; Rao et al., 2017). Chemical weathering not only decreases mineral grain size, but also causes the Rb/Sr ratios of minerals to increase, and also tends to enrich clay minerals (enriched in more radiogenic Sr) in fine-grained fractions. In addition, there is also evidence indicating that preferential weathering of volcanic (such as basalt) and/or sedimentary rocks relative to other more resistant lithologies during continental weathering possibly leads locally to Nd isotopic decoupling between different size fractions with more radiogenic Nd in the clay fraction (Bayon et al., 2015). Accordingly, in this sense, mechanical separation during sedimentary transport and sorting, and chemical weathering are consistent in influencing Sr and, if any, Nd isotopic composition. The 87Sr/86Sr and εNd values in sediments mainly depend on their Rb/Sr and Sm/Nd ratios (Hu et al., 2012, and references therein), and meanwhile, these element distributions are essentially controlled by the mineral phases in which the elements are bound, as certain elements tend to be held especially in the corresponding mineral phases (Condie et al., 1992; Asiedu et al., 2000; Meinhold et al., 2007; Pe-Piper et al., 2008; Hu et al., 2015; Zaid, 2015; Ahmad et al., 2016). Thus, the sizefractionated variation of Sr-Nd isotopic composition in sediments can be interpreted as the mineral (and hence chemical compositional) control on isotopic composition distribution in mineralogically different grain size fractions (Tütken et al., 2002), such as 87Sr/86Sr values of Cabearing minerals (e.g. calcite, dolomite, and anorthite) which are well known to be lower than for Rb-bearing minerals (e.g. micas, k-feldspar, and clay minerals). The mineral controls on element distribution, to the greatest extent, account for the variability of geochemical composition in sediments during hydraulic sorting (Taylor and McLennan, 1985), as minerals are separated into different grain-size fractions owing to sedimentary processes, such as sorting, resulting in a redistribution of elements in sediments. The size-separated mineral composition for the NESL (unpublished data, see Supplementary Table2) revealed that there are clearly different mineral types, contents and assemblages between different-sized fractions, thereby indicating clear grain-size dependence of mineral composition, as revealed by other studies (e.g., Morton and Hallsworth, 1994; Garzanti et al., 2009; Krippner et al., 2015). Wind dynamic sorting during transport and deposition will cause fine-grained fractions to be separated from coarse-grained fractions,

3.5. Contribution to regional dust The geochemical correlation between loess and the potential source areas reveals that the loess in China is chiefly derived from the adjacent upwind arid lands with a short transport distance (Sun, 2002; Chen et al., 2007; Li et al., 2009, 2011; Chen and Li, 2011; Che and Li, 2013; Chen and Li, 2013), which is consistent with large amounts of silt particles hosted in loess deposits. The near-source characteristics of eolian loess deposits make it reasonable to evaluate contribution of the NESL to regional dust accumulations. The NESL were generally suggested as very minor sources of the Asian eolian dust system, although a few investigations using a regional climate model indicated that the NESL (e.g., the HQSL and the ODSL) played a partial role in regional eolian dust emission and transportation (e.g., Shi and Liu, 2011). Although Sr-Nd isotopic composition has great potential in tracing dust sources, non-source factors (such as sorting and weathering) must first be excluded before identifying dust sources (e.g., Dou et al., 2012; Li et al., 2015; Bi et al., 2017). As stated above, the instability of dust source areas in geochemical composition due to the “time effect” of Nd isotopic composition is very likely to affect dust source identification, but if both vertical and horizontal samples are considered as sourcearea composition, this effect will be negligible due to the expansion of the geochemical composition of source areas. In this study, Sr isotopic ratios are little affected by grain size; although Nd isotopic ratios show clear dependence on grain size, yet the dependence is limited within the range of regional samples. In order to further understand whether the grain size effect would affect source identification, the comparison of Sr-Nd isotopic composition between different grain-size fractions was performed in Fig. 5. It is evident that the domain constructed by Sr-Nd isotopic values of different grain-size fractions in each sandy land highly overlaps, indicating that grain-size effect in this study has little effect on source identification. Comparing Sr-Nd isotopic composition of eolian dust deposits with that of the NESL as the potential provenance and that of granitic and volcanic bedrocks in the Great Hinggan Mts, Less Hinggan Mts and Changbai Mts, reveals following information (see Fig. 6): (1) the Nd isotopic values of the surface sediments in the NESL are clearly lower than that of the bedrocks in NE China, indicating that the sediments in the NESL are not directly from products of weathering and denudation of these bedrocks but products of multicycle of Sedimentary recycling; (2) the OD and NMTP are unlikely to be source areas of the dust deposits in NE China, and meantime, the dust deposits in NE China are of the source areas distinct from the CLP loess; (3) the HLSL and the NR to the northwest of Harbin show little contribution to dust emission throughout the geological past; (4) these eolian dust deposits, including the HS loess, the KLG loess, the XWP loess and the 2011 DSD, are very likely to be of distinct provenance; (5) Sr-Nd geochemical composition alone is insufficient to clearly distinguish exact provenance of these eolian dust deposits due to large overlaps of geochemical composition of the potential dust source areas (including the ODSL, the HQSL and the SR). 7

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Fig. 5. Correlation of Sr-Nd isotopic composition between different grain-size fractions. Note that the domain for Sr-Nd isotopic values of different grain-size fractions highly overlaps.

Fig. 6. Sr-Nd isotopic composition of the sediments in the Northeast Sandy Land, in comparison with bedrocks (volcanics and granites) in NE China (including Great Hinggan Mts, Less Hinggan Mts and Changbai Mts) (A plot), the potential sources of Asian dust and the loess of the Chinese Loess Plateau (CLP) (B plot), and with the eolian dust deposits in the Northeast Plain (C plot). NBC, deserts around the northern border of China; NMTP, arid lands of the northern margin of Tibetan Plateau; OD, Ordos Desert (for locations, see Fig. 1). Note that the data from the Northeast Sandy Land are far away from the fields of the NMTP, OD and CLP loess and close to the NBC. Also note that the NR and the HLSL are of the distinct Sr-Nd isotopic composition from the other NESL, which indicates their poor contribution to the eolian dust deposits in the Northeast Plain throughout the geological past. Also note that these eolian dust deposits share clearly different Sr-Nd isotopic composition, which shows their distinct dust derivations. It is of importance to note that Sr-Nd geochemical composition alone is insufficient to clearly distinguish exact provenance of these eolian dust deposits. The data for the bedrocks in NE China are after Xie and Wang (1988), Wu et al. (2000, 2002), Fan et al. (2001), Sui and Xu (2010) and Zhou et al. (2011). The data for the NBC, NMTP, OD and CLP loess are after Chen et al. (2007), Li et al. (2009, 2011) and Rao et al. (2008). 8

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Despite of dominant activity of EASM, dust accumulation during the Holocene is still ongoing, the accumulation rate of which is dependent on intensity of EASM (Wang et al., 2014), and subsequently forming Holocene-aged loess. The time interval of 3000–1500 a B.P. is a important episode of enhanced dust accumulation into loess deposits because of a sudden retreat of the EASM in the late Holocene (e.g., Huang et al., 2009; Wang et al., 2014), as in the case of 2000-a-aged loess in northeast China (e.g., Mu et al., 2016), 3100-a-to-1500-a loess in north and northwest China (e.g., Huang et al., 2009; Liu et al., 2018), and 1400-a-aged loess in uplands in Illinois in the Midwest United States (e.g., Miao et al., 2018). A more recent dust deposition (e.g., pre-1500-a dust) can be identified in soils because of the eolian dust additions to soils due to the decreased dust deposition rate and enhanced the EASM activity (e.g., McTainsh, 1984; Muhs et al., 2008; Huerta et al., 2015; Munroe et al., 2015). Therefore, the modern eolian dust, a deposited “modern loess” (Yan et al., 2015), is deemed to be the continuation of eolian-dust accumulation in the geological past and to be a modern analogy of loess deposits (Xie et al., 2014; Yan et al., 2015; Xie and Chi, 2016). It remains an open question whether the detrial source of eolian dust on the CLP has shifted on orbital scales (e.g., Xiao et al., 2012; Yan et al., 2014; Zhang et al., 2015; Licht et al., 2016; He et al., 2018; Cheng et al., 2019), and meantime, there is also few example indicating the fact that eolian dust source has changed on the millennial or even shorter time scales (e.g., Huang et al., 2009; Stevens et al., 2013; Muhs et al., 2016), whereas all these cannot influence an undoubtable fact that Holocene is accepted to be the latest interglacial period. Taken together, It is, therefore, reasonable to suggest that the modern eolian dust can be treated as the interglacial deposits, as held by Wang et al. (2014). It is of interest to note that, in this scenario, glacially-deposited loess (such as Harbin HS loess) has diverse dust sources from interglacially-formed dust deposits (such as 2011 Harbin DSD). The progressive eastward expansion of aridification since the Pliocene in the Asia interior (Nie et al., 2014; Li et al., 2018), which has been characterized by the basal ages of eolian dust accumulation in China getting younger eastwards (Li et al., 2018), implies the dynamic changes of dust source areas during the glacial and interglacial periods. An intensified summer monsoon during the interglacial periods would lead to substantial precipitation and dense vegetation cover and the dust source areas would retreat westwards; by contrast, a strengthened winter monsoon during the glacial periods would result in limited precipitation, enlarged areas of exposed river and lake beds, and sparse vegetation cover, and the dust source areas would expand eastwards. Accordingly, in this scenario, dust was mostly near-source accumulation during the glacial periods but was far-source accumulation during the interglacial periods (Stevens et al., 2013; Yan et al., 2014; Ma et al., 2015; Wang et al., 2018). It is understandable that the loess and the modern dust-storm deposits have different material sources. Interestingly, although there is clear disparity of the derivation of main dust particles between the HS loess and the modern dust-storm deposits in Harbin, the transport route of Harbin loess is the same, at least in the Songnen Plain, as that of the modern dust-storm deposits. The transportation and deposition of modern dust in Harbin are mainly carried out in the form of dust storms in spring, whereas the modern dust-storm accumulation, a modern analogue of the loess accumulation, is a continuation of the loess accumulation in the geological past. Accordingly, based on the basic principle of “the present is the key to the past”, we tentatively concluded that the sandy lands in the middle and eastern part of Inner Mongolia fed partial amounts of fine particle dust to Harbin loess.

Large areas of continuous loess accumulation requires: (1) an extensive area of arid lands which are sustainable to provide abundant eolian materials; (2) adequate wind power to transport eolian materials; (3) appropriate geomorphological conditions (such as a fairly flat substrate) for dust accumulation, and (4) a comparatively stable tectonic environment to prevent the deposits from being eroded away. It is of interest to note that although the dominant wind direction, in a few cases, is not always consistent with source areas supplying eolian dust (e.g., Menéndez et al., 2007; Suchodoletz et al., 2009; Huerta et al., 2015), it can provide more accurate source information when combining with the availability of dust particles, the spatial linkage between sandy land and loess, and geochemical data. From the perspective of Sr-Nd geochemical composition, the 2011 DSD, the XWP loess, and to a certain extent, the KLG loess seem to come from the SNSL because their data falls in the SNSL domain but outside or on the edge of the domain of both the ODSL and the HQSL. However, judging from prevailing wind direction and the spatial relation between sandy land and loess (see Fig. 1), the XWP and the KLG loess are unlikely to be fed by the SNSL due to these two loess deposits being located upwind of the SNSL. Therefore, based on integrated consideration of Sr-Nd isotopic composition, prevailing wind direction and the spatial relation between sandy land and loess, we argued that the ODSL and the HQSL contributed substantial amounts of dust particles to the XWP and the KLG loess, respectively. The dust-storm weather in Harbin dominantly occurs in spring when the prevailing wind directions are southwesterly (occupying 85.7%), northwesterly (11.1%) and easterly (only 3.2%), with the annual dominant wind direction being southwesterly. No dust source area is present in the east Harbin areas; therefore, theoretically, the source areas of Harbin eolian dust deposits can be the HLSL and the NR in the northwest direction, and the short-range SR and the distal (including the ODSL, the HQSL and beyond) in the southwest. The dusts deflated from the HLSL are difficult to reach Harbin due to the obstruction of the Great Hinggan Mts and the lack of strong wind passage and the Sr-Nd isotopic data in this study also do not support the contribution of the northwest sandy lands (i.e., the HLSL and the NR) to Harbin eolian dust deposits. However, the sandy lands to the southwest of Harbin (including the SR, the ODSL and the HQSL) feature a wide and flat landform, sparse vegetation, bare sandy land and strong wind, which provides an opportunity to offer abundant materials for the Harbin eolian dust deposits. This is also the reason why the Harbin dust-storm weather is dominantly caused by the southwesterly winds. Therefore, based on integrated consideration of near-source property of loess deposits, Sr-Nd isotopic composition, landform and prevailing wind directions, we concluded that the SR in the SNSL is main dust contributor of Harbin HS loess, and that the 2011 Harbin DSD is very likely to originate from the ODSL and beyond. The viewpoint is also further supported by the meteorological records in this region. The modern dust-storm weather frequently occurring in the SNSL has been demonstrated to be dominantly caused by the southwesterly winds in spring and to originate from central Mongolia and mid-eastern Inner Mongolia (Xie et al., 2014). Holocene is regarded to be complex (e.g., varied and unstable) in climate change than is commonly recognized (e.g., Bond et al., 1997; Jackson et al., 2005; Wen et al., 2010), such as three climatic stages (the early Holocene of 11000–8000 a B.P. with gradually intensified the East Asian summer monsoon (EASM), the mid-Holocene of 8000–3100 a B.P. with a generally strong EASM, and the late Holocene after 3100 a B.P. with a sudden retreat of the EASM retreat, Huang et al., 2009; Wang et al., 2014) and the abrupt cooling events (ca 1.4, 3.0, 4.3, 5.6, 8.0 cal kyr B.P., Xue et al., 2008), whereas it is essentially the present interglacial period following the last glacial period (Jackson et al., 2005; Wen et al., 2010; Chen et al., 2016). Accordingly, the variation during this epoch of postglacial warmth is still non-drastic and modest relative to the change on orbital scale (glacial-interglacial time scale) (Xue et al., 2008).

4. Summary The surface eolian sands and river sands in the NESL were systematically sampled to construct the elaborate Sr-Nd isotopic composition dataset for the Asian eolian dust system study. The Sr-Nd isotopic 9

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composition of the 162 bulk samples (< 63 μm) and the 86 size-separated components revealed the following conclusions.

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(1) The NR in the SNSL and to a certain extent the HLSL exhibit distinct Sr-Nd isotopic values from the other NESL. However, there are clear overlaps of Sr-Nd isotopic composition between the ODSL, the HQSL, and the SR in the SNSL. (2) There is significant time effect (i.e., temporal variations) of Nd isotopic composition in the NESL especially in the ODSL, indicating instability of dust source areas in geochemical composition. (3) Although Sr isotopic composition is slightly affected by grain size, the variations of size-separated 87Sr/86Sr values are very small, while Nd isotopic composition exhibits clear grain size-dependent variability. A substantial proportion of Sr-Nd isotopic values show enrichment in the coarse-grained fraction, which is supposed to be controlled by source rocks. (4) The correlation of Sr-Nd isotopic composition of dust deposits in the Northeast Plain with that of the NESL shows that both the HLSL and the NR to the northwest of Harbin play little role in the formation of dust deposits in the Northeast Plain throughout the whole period from the geological past to now, and that Sr-Nd geochemical composition alone fails to clearly detect exact provenance of the eolian dust deposits in the NESL. (5) The integrated approach, not surprisingly, including near-source property of loess deposits, Sr-Nd isotopic composition, and physical geographical elements (such as the spatial relation between sandy land and loess, landform, prevailing wind directions and meteorological records), unraveled source areas of the eolian dust deposits. (6) The ODSL and the HQSL provided substantial amounts of dust particles to the XWP and the KLG loess, respectively, and Harbin loess, which possesses different dust sources from the modern duststorm deposits, is the product of spring dust-storm weather and has a mixed source with the SR in the SNSL as main dust contributor, whereas the 2011 Harbin DSD is very likely to originate from the ODSL and even further areas which also fed minor amounts of fine particle dust to Harbin loess. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Grant: 41871013, 41471070 and 41601200). We thank Mr. Geoffrey Pearce for detailed language polishing. The authors would like to express our appreciation to Mrs. Mu Liu for their help in Sr-Nd isotopic composition analysis. Thanks also go to Hong Zhang, Yuxi Xie, Peng Wu, Zhenyu Wei, Jiaxin Wang and Lei Sun for their assistance in field sampling. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2019.104303. References Ahmad, I., Mondal, M.E.A., Satyanarayanan, M., 2016. Geochemistry of archean metasedimentary rocks of the aravalli craton, NW India: Implications for provenance, paleoweathering and supercontinent reconstruction. J. Asian Earth Sci. 126, 58–73. An, F.Y., Ma, H.Z., Wei, H.C., Lai, Z.P., 2012. Distinguishing aeolian signature from

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