Variation and paleoclimatic significance of organic carbon isotopes of Ili loess in arid Central Asia

Organic Geochemistry 63 (2013) 56–63

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Variation and paleoclimatic significance of organic carbon isotopes of Ili loess in arid Central Asia Zhiguo Rao a,⇑, Yuanbin Xu a, Dunsheng Xia a, Luhua Xie b, Fahu Chen a a MOE Key Laboratory of Western China Environmental Systems, Collaborative Innovation Centre for Arid Environments and Climate Change, Lanzhou University, Lanzhou 73000, China b Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 8 January 2013 Received in revised form 23 July 2013 Accepted 2 August 2013 Available online 9 August 2013

a b s t r a c t As an indicator for terrestrial paleovegetation, the stable isotopic composition of total organic matter (d13Corg) in loess sediments has been widely used for paleoclimatic reconstruction in western Europe, the Great Plains of North America and the Chinese Loess Plateau (CLP). However, little is known about the variation and paleoclimatic significance of the loess d13Corg in arid Central Asia (ACA). We report d13Corg data from an Axike (AXK) loess/paleosol profile from the eastern Ili Basin, eastern Central Asia. Along the profile, the d13Corg values were more negative in the paleosol layers observed in the field and were confirmed by environmental magnetic proxies and a higher concentration of total organic carbon (TOC), consistent with results for western Europe and the northwestern CLP. Our results demonstrate that the loess d13Corg in this region documents mainly the response of d13C of locally predominant C3 plants to paleoclimatic variation, especially paleoprecipitation. Our results also suggest that the loess d13Corg values in the area have the potential for quantitative paleoprecipitation reconstruction on the basis of detailed d13Corg results from modern plants and surface soils in the future. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction According to their photosynthetic pathways, terrestrial higher plants are mostly C3 and C4 plants. The carbon isotopic composition (d13C) of modern C3 plants ranges from 20‰ to 34‰, with the most frequent values around 27‰, and they have competitive growth advantages under environmental conditions of humid, cold and high atmospheric CO2 concentration. On the other hand, the d13C values of modern C4 plants range from 9‰ to 19‰, with the most frequent values ca. 13‰. These plants have relatively competitive growth advantages under arid and hot environmental conditions and low atmospheric CO2 concentration (Deines, 1980; O’Leary, 1981, 1988; Farquhar et al., 1989; Sage et al., 1999). Therefore, d13Corg values obtained from geological sediments that derive from terrestrial higher plants can theoretically be used to reconstruct past variation in C3/C4 relative abundance and related paleoenvironmental and paleoclimatic conditions. The approach is especially effective for loess, due to its eolian nature. As a terrestrial eolian deposit, the organic matter (OM) in loess is derived mainly from the local vegetation cover, with a negligible contribution from the OM of aquatic plants or lower organisms, as they are rarely found in the loess region. For loess sediment, the uncertainty in d13Corg as a paleo-C3/C4 indicator ⇑ Corresponding author. Fax: +86 931 8912330. E-mail address: [email protected] (Z. Rao). 0146-6380/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.orggeochem.2013.08.007

comes mainly from 3 aspects: (i) variation in d13Corg during the burial and decomposition of plant remains (Melillo et al., 1989; Wang et al., 2008), (ii) degradation of the original loess OM by the microbial community feeding on root exudate and root remains (Gocke et al., 2011, 2013) and (iii) smoothing of the d13Corg signal by the post sedimentary input of younger roots (Gocke et al., 2010, 2011; Huguet et al., 2012). Studies have demonstrated that the decomposition of plant remains occurs mainly in the early stage of deposition, with d13Corg becoming more positive (Melillo et al., 1989), normally by ca. 1–3‰ (Wang et al., 2008). At a specific location, d13Corg variation during the burial and decomposition of plant remains can be treated as a ‘‘systematically positive excursion’’. Results from loess profiles with a high accumulation rate have clearly shown a series of rapid d13Corg variations (locations in Fig. 1) with relatively significantly magnitude, such as Nußloch and Achenheim profiles in western Europe (Hatté et al., 1998, 1999, 2001), Yuanbao profile in western Chinese Loess Plateau (CLP; Rao et al., 2005; Chen et al., 2006). These results indicate that the influence of the penetration of younger roots is not significant within a certain time resolution. In the CLP, compound-specific carbon isotopic studies of long chain n-alkanes derived from terrestrial higher plants have been conducted on three loess profiles, including Xunyi and Luochuan profiles (Zhang et al., 2003) and a Xifeng profile (Liu et al., 2005a); the results showed an overall similar changing pattern for the d13Corg values from the same profile (such as Xunyi and Xifeng profile, Zhang et al., 2003; Liu et al.,

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Fig. 1. Map showing distribution of study sites for paleo-C3/C4 variation. Upper, (a), study sites in mid- and high-latitudes of Eurasia, including: a – Achenheim and Nußloch profiles in western Europe; b – Surduk profile in Northern Serbia; c – Chashmanigar site in southern Tajikistan; d – Lake Baikal; e – Beringia in northeastmost Asia; f – AXK profile in arid Central Asia (this study). Lower, (b), study sites in the Chinese Loess Plateau, including: 1 – Yuanbao; 2 – Jingyuan; 3 – Huanxian; 4 – Xifeng; 5 – Lingtai; 6 – Baoji; 7 – Xunyi; 8 – Luochuan; 9 – Jiaodao; 10 – Lantian; 11 – Weinan; 12 – Jixian; 13 – Yanshi (modified from Rao et al., 2006).

2005a,b) and other adjacent profiles (Gu et al., 2003; Liu et al., 2005b; locations in Fig. 1). These results partially demonstrated that the activity of the microbial community did not affect the loess d13Corg values significantly, because long chain n-alkanes are mainly from leaf wax, and are quite stable in younger sediments. This suggests that loess d13Corg is still a feasible and reliable indicator for past C3/C4 variation. During the past few decades, loess d13Corg has been widely used in the mid-latitudes of the northern Hemisphere as a paleovegetational and paleoclimatic indicator, including (Fig. 1) western Europe (e.g. Hatté et al., 1998, 1999, 2001), the Great Plains of North America and adjacent regions (e.g. Kelly et al., 1991; Nordt et al., 1994, 2002; Fredlund and Tieszen, 1997; Tieszen et al., 1997; Johnson and Willey, 2000; Panno et al., 2004), and the CLP (e.g. Gu et al., 2003; Zhang et al., 2003; Vidic and Montañez, 2004; Rao et al., 2005; Liu et al., 2005a,b, 2011; Chen et al., 2006). Arid Central Asia (ACA) falls within a similar latitudinal band to western Europe and the CLP (Fig. 1), with a vast area of loess distribution. Recently, the paleoclimatic variation in the ACA has been widely studied (e.g. Chlachula, 2003; Herzschuh, 2006; Cheng et al., 2012; Chen et al., 2008, 2010). Due to the same eolian nature as the loess in the CLP, the loess in the ACA has also been increasingly studied recently (e.g., Ye, 2000, 2001a,b; Yang and Ding, 2006; Song and Shi, 2010; Feng et al., 2011; E et al., 2012; Jia et al., 2012a,b; Song et al., 2012). However, the variation in, and paleoclimatic significance of, the loess d13Corg in the area has rarely been studied, with only a 1.77 Myr loess d13Corg record from the Chashmanigar site in southern Tajikistan (Fig. 1) reported (Yang and Ding, 2006). Therefore, the loess d13Corg in the area requires further study to help make its paleoclimatic significance clearer. We report d13Corg data from a typical loess profile in the Ili Basin in the eastern ACA. With a clear loess/paleosol alternation confirmed by environmental magnetic studies (Jia et al., 2012a), the profile is particularly appropriate for a d13Corg study.

2. Profile and methods The Ili Basin, in the eastern part of the ACA, is surrounded by the western Tien Shan Mountains. The whole basin, with a mean annual precipitation of 257 to 512 mm and a temperature ranging from 2.6 to 10.4 °C (Song et al., 2012), has an apparently a typical temperate continental climate. Correspondingly, the local vegetation in the basin is mainly temperate steppe. The basin has a trumpet-shaped terrain, as the altitude gradually increases along the west to east slopes. Therefore, eolian dust from the vast Gobi desert in Central Asia and water vapor brought in by the westerly wind from the west are deposited favorably on the topographic uplift. The Ili River cuts through the central basin from east to west, with thick loess widely distributed on its terraces (Song and Shi, 2010). The loess profile, labelled Axike (AXK), at 43°310 4800 N, 83°190 0000 E and 860 m above sea level (m a.s.l.) is from the easternmost part of the Ili Basin (also the headwater of the Ili River) in the steppe zone at the foothills of the Tien Shan Mountains (Axike Village, Zeketai Town, Xinyuan County, Xinjiang Autonomous Region, northwest China; Fig. 1). The profile is several km from the Zeketai (ZKT) profile, which has been studied for > 10 yr (Ye, 2000, 2001a,b; Feng et al., 2011; E et al., 2012). The whole profile, with a depth of 17.8 m, can be divided into three main pedostratigraphic units. The top ca. 1.5 m (Unit 1) is an organic- and carbonate-enriched soil layer remarkable for its relatively reddish colour. The second unit (Unit 2) is a structureless yellowish loess layer, with no observable marks of biological activity, and spans a depth from ca. 1.5 to ca. 9 m. The third (Unit 3) is a paleosol pedocomplex spanning a depth from ca. 9 to 17.8 m and is made up of three poorly developed paleosol layers with two intercalated yellowish loess layers. The dating results for the loess at the site using different methods, including optically stimulated luminescence (OSL) and thermoluminescence (TL) and radiocarbon, are not completely

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consistent at present (Ye, 2001b; Feng et al., 2011; E et al., 2012; the results for OSL and soil snail dating are shown in Fig. 2). We speculate that loess at the study site accumulated since the last glacial, although the accurate age of the loess needs further study. However, the alternation of the loess/paleosol layers was clearly observed in the field and was confirmed recently from environmental magnetic studies (Jia et al., 2012a). The AXK profile was therefore considered suitable for our study of the d13Corg variation in loess and paleosol layers and its paleoclimatic significance in the area. In total, 296 loess samples were collected at 6 cm intervals from the profile. To confirm the d13Corg measurements for the top part of the profile, 95 samples were taken from a nearby parallel section of the top 4.05 m (AXKB) at 4 cm intervals, with the topmost 0.25 m not sampled (Fig. 3), because field observation indicated that the topmost 0.25 m of AXKB was much more loose and organic-enriched, seeming having been pushed to cover the sampling site as a result of agriculture activity. All 391 loess samples were oven-dried at low temperature (< 40 °C), ground and sieved through 80 lm mesh after removal of modern rootlets that occasionally existed in the top parts of the study profiles with depth limited to several decimetres (Fig. 3) and small carbonate nodules that occasionally existed in the samples from the lower part of the study profile (Fig. 3). Subsamples of ca. 0.5 g were digested for 6 h with < 10% HCl at room temperature and then heated in a water bath for up to 2 h at 80 °C to completely remove carbonate. They were again oven-dried at 80 °C after being washed at least 6 to pH > 6 with distilled water, ground again, and then weighed and sealed in Sn boats for on-line d13Corg measurement using an Elementar IsoPrime 100 isotope ratio mass spectrometry (IRMS) instrument interfaced with an Elementar PYRO cube instrument for elemental analysis (EA). Two standards (IAEA-CH3 and IAEA-CH7) were measured multiple times to ensure the accuracy of the measurements. Likewise, 31 samples were measured in duplicate or, occasionally, in triplicate. All the results are reported in per mil units (‰) relative to Vienna Peedee belemnite (VPDB). Statistical analysis of the d13C data from the standards IAEA-CH3 (n = 20) and IAEA-CH3 (n = 11) indicated a total error of <± 0.2‰. TOC concentration was measured via EA, and the pretreatment of the TOC subsample identical to that for d13Corg measurement. Therefore, TOC concentration is reported relative to sample wt. after carbonate removal, not relative to original bulk sample. All the samples were pretreated at the Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University, and instrumental analysis was performed at the Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

Fig. 2. Soil snail and OSL dating results of Zeketai loess profile close to AXK and AXKB profiles (modified from Feng et al., 2011).

3. Results and discussion The TOC concentration and loess d13Corg data from the top ca. 4 m of the AXK and AXKB profiles are shown in Fig. 4. There was an apparent negative correlation between TOC concentration and corresponding loess d13Corg data in both profiles, i.e. the loess d13Corg became more positive with decreasing TOC concentration. The d13Corg value of the topmost sample was 26.1‰ for AXK and 23.4‰ for AXKB, clearly indicating that the OM is derived predominantly from C3 plants. The more positive value for AXKB is consistent with the lower TOC concentration (1.43% for AXK and 0.64% for AXKB). TOC decreased quickly with depth, with the d13Corg values increasing quickly. Both datasets were much more stable below 1 m, with low TOC concentration and more positive d13Corg values. As a whole, the d13Corg values for AXKB were slightly more negative than AXK between 1 and 4 m, especially 3–4 m (Fig. 4). This might be caused by the slight differences in loess accumulation rate and carbon isotopic composition of terrestrial plants at specific locations on different topography. Studies have demonstrated that the decomposition of the OM after deposition occurs mainly during the early stage, with increasing d13Corg values (Melillo et al., 1989). Therefore, the variation in TOC and d13Corg values in the top 1 m of both profiles may mainly reflect the large OM contribution from modern plants and their gradual decomposition, not the climatic imprint alone or an increasing C4 signal. Whatever the reason, our data for the topmost part of the two profiles clearly demonstrate that overlying modern vegetation is dominated by C3 plants with a negligible C4 signal, consistent with an investigation of the distribution of modern C4 plants in China (Yin and Li, 1997). An investigation of 10 modern surface soils sampled from the eastern Ili Basin indicated that magnetic susceptibility was not a reliable indicator of the pedogenetic intensity in the region (Jia et al., 2012b); however, the frequency-dependent magnetic susceptibility (MSfd) is an ideal indicator of loess pedogenesis intensity in the Ili loess (Jia et al., 2012a). The TOC and loess d13Corg data for the lower 4–17.8 m of AXK are shown in Fig. 5. Recent MSfd data (Jia et al., 2012a) are also plotted for comparison. The higher MSfd values in the lower part of AXK, i.e., the pedocomplex of Unit 3, clearly indicate three stages of stronger pedogenic intensity (Fig. 5), consistent with the field observation of three paleosol layers with two intercalated yellowish loess layers. It is reasonable that the paleosol layer was formed under relatively warmer and wetter climatic conditions. If this is the case, the warmer and wetter climate should normally favor OM decomposition and adversely affect the conservation of soil OM due to enhanced activity of the microbial community under such climatic conditions. A higher TOC concentration in the paleosol layer (Fig. 5) should indicate a much larger TOC input from local vegetation under warmer and wetter climatic conditions. Therefore, TOC is an approximate indicator of warmer and wetter climatic conditions, as well as an indicator of loess pedogenic intensity in the area. Together, both the TOC and MSfd data confirm the field observations of the pedostructure of the lower part of the AXK profile (Fig. 5). The loess d13Corg data for the lower part range from 18.6‰ to 22‰, with an average value of 20.4‰. The loess d13Corg data down the profile also show three clear stages with peak negative values, broadly corresponding to the three paleosol layers with higher TOC and MSfd values (Fig. 5). Therefore, the d13Corg data show in general a ‘‘negative correlation model’’ with the paleosol layers with stronger pedogenic intensity at our study site. In western Europe, the negative loess d13Corg data (with most values more negative than 24‰) from two sites (Achenheim and Nußloch profiles, Fig. 1), with more negative values in the paleosol layers formed during the Holocene and the last

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Fig. 3. Fieldwork photos of sampling site AXKB. (a) overview of whole study profile that formed by construction; (b) lower part of the profile; (c) feature of a small carbonate nodule in lower part loess; (d) top part of profile AXKB; (e) feature of the modern rootlet; (f) feature of the top part of profile AXKB after sampling; note the topmost 0.25 m was not sampled.

Fig. 4. Variation in the loess d13Corg (d) and TOC concentration (N) in the top part of the AXK profile (a) and the parallel AXKB profile (b) plotted vs. depth.

interglacial along a temporal sequence (Fig. 6), have been thought to record the response of the d13C signal of predominantly terrestrial C3 plants to climate change (Hatté et al., 1998, 1999). Because the results demonstrate that the d13C values of modern C3 plants are mainly a response to the variation in local precipitation (Stewart et al., 1995), attempts have been made to use loess d13Corg data from western Europe for paleoprecipitation reconstruction (Hatté et al., 2001). In the CLP, during the past decades, compound-specific carbon isotopic studies of long chain n-alkanes derived from terrestrial higher plants or/and d13Corg studies have been conducted on more than 10 loess profiles accumulated since the last glacial or interglacial (e.g. Gu et al., 2003; Zhang et al., 2003; Vidic and Montañez, 2004; Rao et al., 2005; Liu et al., 2005a,b, 2011; Chen et al.,

2006; Fig. 1). The data indicate that the majority of the CLP was dominated by C3 plants during both glacial and interglacial periods. In most of the profiles, the loess d13Corg data along a temporal sequence were more positive in paleosol layers formed under warmer and wetter climatic conditions and were more negative in loess layers accumulated under arid and colder climate conditions (Rao et al., 2006). According to the general C3/C4 trend in the CLP along a spatial gradient, with the climate gradually becoming arid and colder, the relative abundance of C4 plants should decrease northwestwardly (Rao et al., 2006). Data from a profile from Yuanbao (Fig. 1) in the western CLP demonstrated that the local vegetation was dominated greatly by C3 plants during the last glacial and the contribution from C4 plants was negligible (Rao et al., 2005; Chen et al., 2006). Recently reported loess d13Corg data for a profile from Jingyuan (Fig. 1) in the northwesternmost part of the CLP exhibit more negative values in paleosol layers formed during the Holocene and the last glacial interstadial (Fig. 7), indicating that the local vegetation has been dominated greatly by C3 plants since the last glacial, with the contribution from C4 plants being negligible (Liu et al., 2011). In the southwest of the Ili Basin, a 1.77 Myr loess d13Corg record from the Chashmanigar site in southern Tajikistan (Fig. 1), indicated a clear C3 predominance, with most values more negative than 24‰ (Yang and Ding, 2006). In Chashmanigar loess, the negative d13Corg peaks normally existed in paleosol layers. Especially since the last interglacial, the most positive d13Corg peaks seem to exist in the loess layer formed during the last glacial maximum (Fig. 7). In the northeast of the Ili Basin, compound-specific carbon isotopic studies of long chain n-alkanes from terrestrial higher plants in the lacustrine sediments from Lake Baikal (Fig. 1), spanning the last 20 kyr, clearly indicated that C3 plants exclusively predominated the Baikal watershed since the last glacial (Brincat et al., 2000). In the northwesternmost Asia, carbon isotopic data from remains of megaherbivores that were buried in permafrost of Beringia (Fig. 1) also indicated that the study area was also exclusively dominated by C3 plants during the last glacial (Bombin and Muehlenbachs, 1985). Considering the western Europe, Tajikistan, Lake Baikal and the western CLP data (Fig. 1), our data should also record a d13C signal derived predominantly from C3 plants. They do show a similar ‘‘negative correlation’’ with the paleosol layers (Fig. 5), consistent

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Fig. 5. Loess d13Corg (a), TOC concentration (b) and MSfd (c) variation vs. depth for the lower part of the AXK profile; note the ‘‘negative correlation’’ of loess d13Corg with paleosol layers indicated by horizontal gray bars.

Fig. 6. Loess d13Corg records from Achenheim (a) and Nußloch (b) profiles in western Europe since the last interglacial (modified from Hatté et al., 1998); note the ‘‘negative correlation’’ of loess d13Corg with paleosol layers indicated by horizontal gray bars.

with the ‘‘model’’ for western Europe (Achenheim and Nußloch profiles; Fig. 6) and the northwestern CLP (Jingyuan profile; Fig. 7) and southern Tajikistan (Chashmanigar site; Fig. 7). The loess d13Corg data from the AXK profile may therefore also record the d13C signal derived predominantly from C3 plants. That the loess d13Corg data from the AXK profile record the variation in C3/C4 relative abundance is unlikely. Numerous records from the mid-latitudes of the northern Hemisphere show an overall trend of C4 relative abundance change, with a higher relative abundance during interglacial periods with warmer and wetter climatic conditions (Rao et al., 2012), i.e. more positive sedimentary d13Corg data from paleosol layers in the central loess area of the CLP (e.g. Gu et al., 2003; Zhang et al., 2003; Vidic and Montañez,

2004; Liu et al., 2005a,b) and the Great Plains of North America and adjacent regions (e.g. Kelly et al., 1991; Nordt et al., 1994, 2002; Fredlund and Tieszen, 1997; Tieszen et al., 1997; Johnson and Willey, 2000; Panno et al., 2004). If the loess d13Corg data for the AXK profile did record the variation in C3/C4 relative abundance, then a lower C4 relative abundance in the paleosol layers indicated by more negative d13Corg values would be inconsistent with the overall change characteristic of the mid-latitudes of the northern Hemisphere. In addition, a higher C4 relative abundance during glacial periods, with relative cold and arid climatic conditions, only occurs at low-latitude tropical areas (e.g. Street-Perrott et al., 1997; Huang et al., 2001). Ultimately, the AXK loess d13Corg data do not indicate the variation in C3/C4 relative abundance but

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Fig. 7. Loess d13Corg records from Chashmanigar site in southern Tajikistan since the last interglacial (a, modified from Yang and Ding, 2006) and from the Jingyuan profile in the northwesternmost Chinese Loess Plateau since the last glacial (b, modified from Liu et al., 2011); note the ‘‘negative correlation’’ of loess d13Corg with paleosol layers that indicated by horizontal gray bars.

the response of predominantly C3 plants to climate change, the contribution from C4 plants to the loess of the area being negligible. It is well known that C4 plants have a relatively competitive growth advantage under arid and hot environmental conditions and low atmospheric CO2 concentration (Deines, 1980; O’Leary, 1981, 1988; Farquhar et al., 1989; Sage et al., 1999). Investigation of the distribution of modern plant species indicates that, in the region below 46°S, almost no C4 plants have been found. Correspondingly, only few C4 species distribute in limited small areas with relatively warm environment in the region above 60°N (Sage et al., 1999). The results are consistent with paleovegetation studiess from Lake Baikal (Brincat et al., 2000) and Beringia (Bombin and Muehlenbachs, 1985). In the area of the Tibet Plateau, with the altitude ranging from 2210–5010 m a.s.l., only 8 modern C4 species have been found (Wang et al., 2004). All the evidence for the distribution of modern C4 plant species indicates that, under a certain ‘‘threshold’’ temperature, C4 plants would be absent or extremely rare in modern vegetation. However, the determination of the ‘‘threshold’’ temperature value for the growth of C4 plants is not easy, because of the co-variation of temperature with other climatic factors, such as precipitation. In a comparative study of the variation in modern C3/C4 relative abundance in three regions around the North Pacific, it was found that when the mean annual temperature (MAT) was < 12 °C, the contribution of C4 plants to local biomass was negligible no matter how much precipitation changed (Rao et al., 2010). If so, the MAT at our study site (the MAT at Xinyuan city, close to the AXK profile, is ca. 8.1 °C, Jia et al., 2012a) in the ACA is apparently < 12 °C; likewise the MAT at Achenheim and Nußloch sites in western Europe (the MAT at Karlsruhe/Germany, close the Nußloch profile, is ca. 10.7 °C; the MAT at Strasbourg/France, close to the Achenheim profile, is ca. 9.8 °C, Rao et al., 2007) and Chashmanigar site in southern Tajikistan (the MAT at this site is ca. 11 °C, Yang and Ding, 2006). Obviously, it is reasonable that the local vegetation at these sites is dominated by C3 plants and the contribution from C4 plants is negligible, as mentioned above. Studies indicate that the d13C composition of modern C3 plants responds mainly to the variation in local precipitation (Stewart et al., 1995). In Asia, the d13C values of modern C3 plants from the loess area of northern China (Wang et al., 2003), the CLP (Liu et al., 2005c) and the eastern CLP (Zheng and

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Shangguan, 2007) all demonstrate a negative correlation between the d13C of modern C3 plants and local mean annual precipitation (MAP), i.e. the d13C values of C3 plants decrease with increasing precipitation. Recently, d13C data for 334 woody plant species (all woody plants are C3 species) at 105 sites on a global scale were systematically summarised, and a strong negative correlation was revealed between MAP and the d13C data for the plants (Diefendorf et al., 2010). A similar viewpoint comes from another recent review, also for a global scale (Kohn, 2010). Studies of modern surface soils from the CLP and central-east Asia (Lee et al., 2005; Feng et al., 2008) (areas covered predominantly by modern C3 plants) also showed a negative correlation between surface soil d13Corg data and the corresponding local amount of precipitation, consistent with studies of modern C3 plants. Based on the results from modern C3 plants and surface soils, we believe the loess d13Corg data from the AXK profile reflect mainly the response of the d13C of C3 plants to the variation in precipitation, i.e. the more negative loess d13Corg values in the paleosol layers were caused by the more negative d13C values of C3 plants resulting from a larger amount of precipitation. Although the d13C signal from predominantly C3 plants was recorded, the loess d13Corg values from the AXK profile (Fig. 5) are much more positive than those recorded from the loess in western Europe (Fig. 6) and southern Tajikistan (Fig. 7). The difference might be caused mainly by the oceanic climate in Europe being much wetter than the continental climate of the Ili Basin (the MAP at Karlsruhe/Germany, close to the Nußloch profile, is ca. 822 mm, Rao et al., 2007; the MAP at Xinyuan city, close to the AXK profile, is ca. 480 mm, Jia et al., 2012a). For southern Tajikistan, to the south of AXK, there is also a much higher amount of precipitation than for AXK site (the MAP at the Chashmanigar site is ca. 842 mm, Yang and Ding, 2006). Such evidence supports our assumption that the loess d13Corg in our study area is controlled mainly by local precipitation. Irrespectively, more detailed studies of modern C3 plants and surface soils in the ACA are required to confirm that loess d13Corg data from the area have the potential to be used for paleoprecipitation reconstruction. The differences amongst AXK d13Corg, TOC concentration and MSfd, including the peak positive d13Corg values in the two intercalated loess layers in the pedocomplex in the lower part of the profile, do not correspond completely with the lowest TOC concentration and MSfd values (Fig. 5), and there was no apparent change trend in MSfd data between 4–10 m of the AXK profile; however the d13Corg has an overall positive change trend (Fig. 5), that may be attributed mainly to the slight differences in the climate-driven mechanism and climatic sensitivity. For example, the loess d13Corg in the study area responds mainly to the amount of local precipitation. However, the MSfd, as an indicator of the intensity of pedogenesis (Jia et al., 2012a,b), may be controlled by not only precipitation, but also temperature.

4. Conclusions In the top part of the AXK profile and the parallel AXKB profile in the eastern Ili Basin, in the eastern ACA, the loess d13Corg values increase rapidly down the profile with decreasing TOC concentration. The loess d13Corg values from the top part may reflect mainly the OM contribution from modern plants and their gradual decomposition rather than a climatic imprint from an increasing C4 signal. However, the values for the topmost samples clearly demonstrate an input from predominantly C3 plants. The loess d13Corg data from the lower part of the AXK profile show a ‘‘negative correlation model’’ with paleosol layers. In the paleosol layers observed in the field and confirmed via higher frequencydependent magnetic susceptibility and TOC concentration values, the corresponding loess d13Corg values are generally more negative,

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which is consistent with loess d13Corg results from western Europe, southern Tajikistan and the northwesternmost CLP. Together, all the data from the above sites indicate that sedimentary loess d13Corg data reflect the response of the d13C of locally predominant C3 plants to climate change. Considering the results for modern C3 plants and surface soils, we conclude that the loess d13Corg data for the ACA have a link with precipitation variation via the d13C signals of C3 plants. Paleoclimatic studies of the ACA based on loess may benefit from our results. Acknowledgements The work was supported financially by the National Natural Science Foundation of China (Grant Nos. 41171091, 40901055 and 41130102), the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2012-k49), the Program for New Century Excellent Talents in Universities (Grant No. NCET-10-0468) and the National Innovative Research Team Project (Grant No. 41021091). Three anonymous reviewers provided detailed comments that helped improve the manuscript, and they are gratefully acknowledged. Associate Editor—B. van Dongen References Bombin, M., Muehlenbachs, K., 1985. 13C/12C ratios of Pleistocene mummified remains from Beringia. Quaternary Research 23, 123–129. Brincat, D., Yamada, K., Ishiwatari, R., Uemura, H., Naraoka, H., 2000. Molecularisotopic stratigraphy of long-chain n-alkanes in Lake Baikal Holocene and glacial age sediments. Organic Geochemistry 31, 287–294. Chen, F.H., Rao, Z.G., Zhang, J.W., Jin, M., Ma, J.Y., 2006. Variations of organic carbon isotopic composition and its environmental significance during the last glacial period on western Chinese Loess Plateau. Chinese Science Bulletin 51, 1593– 1602. Chen, F.H., Yu, Z.C., Yang, M.L., Ito, M., Wang, S.M., Madsen, D.B., Huang, X.Z., Zhao, Y., Sata, T., Birks, H.J.B., Bomer, I., Chen, J.H., An, C.B., Wünnemannn, B., 2008. Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history. Quaternary Science Reviews 27, 351–364. Chen, F.H., Chen, J.H., Holmes, J., Boomer, I., Austin, P., Gates, J.B., Wang, N.L., Brooks, S., Zhang, J.W., 2010. Moisture changes over the last millennium in arid central Asia: a review, synthesis and comparison with monsoon region. Quaternary Science Reviews 29, 1055–1068. Cheng, H., Zhang, P.Z., Spöt, C., Edwards, R.L., Cai, Y.J., Zhang, D.Z., Wang, W.C., Tan, M., An, Z.S., 2012. The climatic cyclicity in semiarid-arid central Asia over the past 500,000 years. Geophysical Research Letters 39, L01705. Chlachula, J., 2003. The Siberian loess record and its significance for reconstruction of Pleistocene climate change in north-central Asia. Quaternary Science Reviews 22, 1879–1906. Deines, P., 1980. The isotopic composition if reduced organic carbon. In: Fritz, P., Fontes, J.C. (Eds.), Handbook of Environmental Isotope Geochemistry, The Terrestrial Environment, vol. 1. Elsevier, Dordrecht, pp. 339–345. Diefendorf, A.F., Mueller, K.E., Wing, S.L., Koch, P.L., Freeman, K.H., 2010. Global patterns in leaf 13C discrimination and implications for studies of past and future climate. Proceedings of the National Academy of Sciences of the USA 107, 5738–5743. E, C.Y., Lai, Z.P., Sun, Y.J., Hou, G.L., Yu, L.P., Wu, C.Y., 2012. A luminescence dating study of loess deposits from the Yili River basin in western China. Quaternary Geochronology 10, 50–55. Farquhar, G.D., Ehleringer, J.R., Hubick, K.T., 1989. Carbon isotope discrimination and photosynthesis. Annual Review Plant Physiology and Plant Molecular Biology 40, 503–537. Feng, Z.D., Wang, L.X., Ji, Y.H., Guo, L.L., Li, X.Q., Dworkin, S.I., 2008. Climatic dependency of soil organic carbon isotopic composition along the S–N Transect from 34°N to 52°N in central-east Asia. Palaeogeography Palaeoclimatology Palaeoecology 257, 335–343. Feng, Z.D., Ran, M., Yang, Q.L., Zhai, X.W., Wang, W., Zhang, X.S., Huang, C.Q., 2011. Stratigraphies and chronologies of late Quaternary loess-paleosol sequences in the core area of the central Asian arid zone. Quaternary International 240, 156– 166. Fredlund, G.G., Tieszen, L.L., 1997. Phytolith and carbon isotope evidence for late Quaternary vegetation and climate change in the Southern Black Hills, South Dakota. Quaternary Research 47, 206–217. Gocke, M., Kuzyakov, Y., Wiesenberg, G.L.B., 2010. Rhizoliths in loess – evidence for post-sedimentary incorporation of root-derived organic matter in terrestrial sediments as assessed from molecular proxies. Organic Geochemistry 41, 1198– 1206.

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