Magnetic, granulometric and geochemical characterizations of loess sections in the eastern Arid Central Asia: Implication for paleoenvironmental interpretations

Journal Pre-proof Magnetic, granulometric and geochemical characterizations of loess sections in the eastern Arid Central Asia: Implication for paleoenvironmental interpretations Guanhua Li, Dunsheng Xia, Hao Lu, Youjun Wang, Jia Jia, Xianbin Liu, Xiaoqiang Yang PII:

S1040-6182(20)30004-5

DOI:

https://doi.org/10.1016/j.quaint.2020.01.003

Reference:

JQI 8108

To appear in:

Quaternary International

Received Date: 10 April 2019 Revised Date:

15 December 2019

Accepted Date: 6 January 2020

Please cite this article as: Li, G., Xia, D., Lu, H., Wang, Y., Jia, J., Liu, X., Yang, X., Magnetic, granulometric and geochemical characterizations of loess sections in the eastern Arid Central Asia: Implication for paleoenvironmental interpretations, Quaternary International, https://doi.org/10.1016/ j.quaint.2020.01.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Magnetic, granulometric and geochemical characterizations of loess sections in the eastern Arid Central Asia: Implication for paleoenvironmental interpretations Guanhua Lia,b, Dunsheng Xiab*, Hao Lub, Youjun Wangb, Jia Jiab, Xianbin Liub , Xiaoqiang Yanga a

Southern Laboratory of Ocean Science and Engineering (Guangdong, Zhuhai), Department of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai 519000, Guangdong, China. b

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MOE Key laboratory of western China’s Environmental Systems, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, Gansu, China.

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Corresponding author: Dunsheng Xia (E-mail address: [email protected])

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Abstract

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Loess deposits in the Arid Central Asia (ACA) contain sensitive messages concerning the evolution of the Westerlies and the interior aridification of Asia. In this study, we conducted a combined study of geophysical and geochemical characterization on loess sections in the eastern ACA, to discuss possible paleoclimate development since the late Pleistocene Era. Results show that the ACA loess chiefly exhibited similar geophysical and geochemical compositions with loess deposits in other regions, like the Chinese Loess Plateau. There are various accumulation rates that would have been responsible for the depositional difference within the ACA loess profiles. In addition, there are three magnetic responses to the bulk grains and geochemical proxies in this study. At the bottom segment of the section, magnetic enhancement was associated with coarse grain sizes and water leaching. Magnetic enhancement responded to stronger pedogenesis and moderate paleo-wind intensity in the uppermost parts. Moreover, there is a transitional segment in the middle part of the section, characterized by inordinately detrital inputs and weak pedogenic contributions. Thus, it may be better to evaluate the magnetic climatology of the ACA loess through different depositional stages. Furthermore, this study generally reveals that the paleoclimate pattern in the ACA was probably characterized by relatively low moisture during warm intervals, and vice versa, in the middle and bottom parts of the profiles. But relatively enhanced moisture was indicated during the Holocene period. Further studies are still necessary to obtain reliable proxies and chronology for a detailed paleoclimate interpretation of the ACA loess.

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Keywords: Loess; Paleoclimate; Rock magnetism; Grain size; major element; The Arid Central Asia

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1. Introduction

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Loess has been regarded as one of the most significant research archives for the Quaternary paleoclimate around the world, as synchronous variation of climatic records among loess, marine sediments and polar ice cores was discovered in the past few decades (e.g. Heller and Liu, 1984; Bloemendal et al., 1995; Porter and An, 1995; Guo et al., 2002; Maher et al., 2003; Balsam et al., 2005; Deng et al., 2005; Kravchinsky et al., 2008; Sun et al., 2010, 2015; Újvári et al., 2016; Zeeden, et al., 2018). There is an enormous loess belt across the Eurasian continent through the mid-latitudes of the Northern Hemisphere (e.g. Pécsi, 1990; Haase et al., 2007; Smalley et al., 2011; Djordjije et al., 2014; Zeeden et al., 2018; Costantini et al., 2018) and thick loess provides pivotal terrestrial sediments for paleoclimate studies. On the Chinese Loess Plateau (CLP), paleoclimate studies of loess have attracted attention from many scholars (e.g. Liu et al., 2007, 2015). Multidisciplinary approaches including rock magnetic, geochemical, and granulometric analyses were applied to assess paleoclimate variations on the CLP and gave rise to fruitful achievements regarding loess-paleoclimate studies (e.g. Heller and Evans, 1995; Maher and Thompson, 1999; Hao and Guo, 2001; Guo et al., 2002; Deng et al., 2005; Hao et al., 2008; Sun et al., 2011; Liu et al., 2016; Muhs, 2018). Loess deposits are also widely distributed in the ACA from the Caspian Sea to the western margin of the Altai Mountains, where the modern climate is largely dominated by the Westerlies (Ye, 2001; Smalley et al, 2011). The ACA loess not only offers the potential to record a sensitive message of the Asian interior acidification and the evolution of a global dust source (Duce et al., 1980; Fang et al., 2002; Machalett et al., 2008; Zan et al., 2010, 2013; Song et al., 2018a), but it also has been regarded as an alternative archive to decipher the paleoclimate teleconnection between the monsoonal CLP and the Northern Atlantic (e.g. Forster and Heller, 1994; Ye, 2001; Youn et al., 2014; Li et al., 2016b; Li et al., 2018).

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Previous studies of loess-paleosol sequences in the ACA have revealed the overall trend of local aridification since the middle Quaternary (e.g. Fang et al., 2002; Zan et al., 2010). However, the detailed pattern of paleoclimate evolution during glacial and interglacial cycles since the late Pleistocene still necessitates clarification. The complexity of local geomorphology has been encoded in the ACA loess, bringing about curious variations of traditional proxies that have been widely used in the CLP (e.g. Ye, 2001). Magnetic susceptibility (MS), for instance, has been regarded as a primary proxy for paleomonsoon strength in the CLP as high values of the MS were mostly found in the paleosol horizons (e.g. Banerjee et al., 1993; Hao and Guo, 2001; Liu et al., 2007; Hao et al., 2008; An et al., 2014; Maher, 2016). The magnetic characteristics of loess have also been widely revealed in the ACA, especially in the eastern ACA (e.g. Forster and Heller, 1994; Ye, 2001; Chlachula, 2003; Zhu et al., 2003; Matasova et al., 2010; Liu et al., 2008, 2013; Li et al., 2015a, 2015b; Zan et al., 2010, 2013, 2015; Jia et al., 2012, 2018; Song et al., 2018b). However, no specific correlation between the magnetic susceptibility and pedogenic intensity was observed in certain loess sections,

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which could be interpreted solely by neither the pedogenic model from the CLP (e.g. Zhou et al., 1990; Chen et al., 2005; Liu et al., 2007, 2015), nor the ‘wind vigor’ model (e.g. Begét et al., 1990; Chlachula, 2003; Zhu et al., 2003) from the high latitudes. Generally, pedogenic and diagenetic processes were dominated by local conditions, and would usually embody in the geochemical composition and grain size variations (e.g. Gallet et al., 1998; Wang et al., 2015; Sun et al., 2004, 2008; Muhs, 2018), which potentially affected the magnetic properties. A recent study has proposed that there are very small influences from organic material and soluble components on the magnetic concentration, while the carbonates have a relatively higher impact on the magnetic susceptibility of loess in the ACA, when compared to in the CLP (Song et al., 2018b). Nevertheless, the possible influences on magnetic enhancement in the ACA have not been well evaluated by multiple means.

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In order to better assess the magnetic enhancement and the paleoenvironmental pattern in this region, it is quite necessary to obtain a comprehensive understanding of the magnetic, geochemical and granulometric records. On the other hand, there has been a great deal of loess-paleoenvironment research with a combination of these three approaches in the CLP and Europe during the last two decades (e.g. Maher et al., 2009; Galović et al., 2011; Wang et al., 2015; Zhang et al., 2007, 2012; Guan et al., 2016). However, similar efforts have been quite insufficient in the ACA loess so far. In this paper, two loess profiles from Tacheng (TC), in the eastern ACA, were adopted for a combined analysis of rock magnetic, geochemical and grain size parameters. An intersectional comparison was made between two profiles in Tacheng and other loess profiles in other parts of the ACA, in order to explain the variability of the combined proxies with respect to environmental circumstances. This study aims at the further assessment of magnetic enhancement and contribution to a better understanding of the paleoenvironmental pattern derived from the three sets of parameters, which would be beneficial for paleoclimate study in the ACA since late Pleistocene.

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2. Materials and Methods

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The study area (Fig.1a) lies in the eastern part of the ACA, where vast deserts distributed in the main basins are surrounded by high mountains of the northern branches of the Himalayan-Tibetan Plateau. This area has also been regarded as a key area for the evaluation of climatic interactions between the North Atlantic and monsoonal Asia climate regimes (e.g. Porter and An, 1995; Ye, 2001). In the eastern part of the ACA, loess is generally distributed on the river terraces and provides ideal materials to monitor the evolution of the Westerlies and the aridification of the ACA during the period since the middle Quaternary (e.g. Fang et al., 2002; Lu et al., 2009; Zan et al., 2010; Li et al., 2016b). The Tacheng loess belt is distributed on the southern piedmont of the Tarbahatai Mountains in northwestern Xinjiang. The modern climate is characterized by a mean annual temperature of about 7 °C and a mean annual precipitation of 150 mm to 250 mm, respectively. Two loess profiles (46°53′N， 83°14′E) underlain by a c.50 cm thick river gravel formation consist of the late Quaternary loess–palaeosol sequences with a depth of 8m and 13m, respectively. The

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stratigraphy for the TCA profile has been demonstrated in a previous study (Li et al., 2015a). The other one hereinafter referred to as the TCB profile could match well with the TCA profile, although their thicknesses are quite different. The TCB profile is about 1km far away from the TCA profile and has been explored on the third river terrace of the Abdullah river. As shown in Fig.1b, three depositional horizons could be generally identified by field observation in the TCB profile, which matches well with the TCA profile. The topsoil horizon ranging from 0 to 0.6m was developed by soil formation processes during the Holocene period. A loess horizon (0.6-10.3m) with a light-yellow color is situated in the middle part of the section. There are two weakly developed paleosols at the depths of 3m and 6.5m, respectively, in the loess horizon. The paleosols were formed at the bottom of the profile with an intercalated sandy silt layer. The paleosols present a dark-brown color, and contain carbonate pseudomycelium and have a dense texture. All samples were obtained at a 2cm interval and ground into fine powders for further measurements after drying naturally in the laboratory.

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Fig. 1. The map showing the study area (a) and comparison of two loess profiles in Tacheng (b). The base maps are modified from Google Earth image and Geomap APP (Ryan et al., 2009), respectively. (b) Magnetic susceptibilty and median grain sizes (Md) of the TCA profile are modified from Li et al., 2015a.

The temperature dependent susceptibility (κ−T curve) of representative samples in the TCB profile was measured between -192 and 700 ⁰C using an AGICO KLY-3 Kappabridge. The determination of magnetic susceptibility including mass specific susceptibility (χ) and the percentage frequent-dependent susceptibility (χfd%) was similar with the methods in Li et al., (2015a). Anhysteretic remanence magnetization (ARM) was gained in a peak alternating field (AF) of 200 mT with a bias field of 100 µT with a DTECH D2000 Demagnetizer and then measured using a Molspin spinner magnetometer. The ARM susceptibility (χARM) is derived from normalization of the ARM by the bias DC field. A series of isothermal remanent magnetization (IRMs) were produced using a MMPM10 pulse magnetizer. The IRM in a field of 1.5 T is defined as saturation isothermal remanence (SIRM). The SIRM was then stepwise demagnetized from 0 to 1T in an alternating field and from room temperature to 700⁰C in a MMTD80 thermal demagnetizer, respectively. All IRMs were measured with a

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Molspin spinner magnetometer. According to the IRM measurements, three related parameters were calculated as Soft=(SIRM-IRM-20mT)/2, hard (HIRM=(SIRM ＋ IRM-300mT)/2) and S-ratio=-IRM-300mT/SIRM. Hysteresis parameters and backfield remanences were determined under maximum field of 0.8T using a Lake Shore PMC MicroMag 2900 Series AGM. The magnetic measurements were performed in Lanzhou University and Tübingen University.

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Additionally, the grain-sizes of all samples were measured using a Malvern Mastersizer 2000 laser diffraction particle size analyzer, after pretreatment with H2O2 and HCl for reducing the effects of organic matter and carbonates. Furthermore, ultrasonic dispersion with certain 10% (NaPO3)6 were applied before the measurement. Component partitions of grain size distribution were analyzed using a logarithmic normal distribution fitting (Xiao et al., 2014, 2015). Samples at 10cm interval were selected from the TCA profile and ground thoroughly matching 200 mesh sieves. Then, 4g of powder was made into a rounded measuring wafer surrounded by boric acid powder. Major elements were detected with the Netherlands Philips Magix PW2403 X-Ray spectrometer. Based on the preliminary measurements of major elements, the chemical index of alteration (CIA) (Nesbitt and Young, 1982; Mclennan, 1993) was calculated as Al2O3/(Al2O3+CaO+Na2O+K2O), in which the CaO indicates the content of CaO in the silicates in moles. The depletion coefficient of alkali (DCA) during weathering represents the degree of leaching differences of alkalis, which was derived from (CaO+Na2O)/Al2O3 (e.g. Yu and Wen, 1991). The eluvial coefficient (EC) was calculated as (Al2O3+Fe2O3)/(CaO+Na2O+MgO) (e.g. Yu and Wen, 1991). The high degree of relative moisture is characterized by low values of the depletion coefficient of alkali and high values of the eluvial coefficient. The percent variation △% of a given element relative to the Al2O3 in different paleosol layers was derived from the formula as △%=(Xp/Al2O3Paleosol)/((Xl/Al2O3Loess)-1)*100%, in which Xp,Xl indicate the content of a given element in the paleosol and loess horizons, respectively (e.g. Chen et al., 2001). Due to the relatively stable activity in loess, Al2O3 was adopted as a reference element. The Al2O3Paleosol and Al2O3Loess represent the content of Al2O3 in the paleosol and loess layers, respectively. △<0 denotes depletion of a given element relative to the Al2O3, and vice versa. All measurements above were conducted in Lanzhou University.

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3. Results

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3.1. Rock magnetic properties

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The temperature-dependent magnetic susceptibility (κ-T) is a useful tool for estimating subtle variations in magnetic assemblages as well as magnetic grain sizes during heating/cooling cycles (e.g. Mullender et al., 1993; Banerjee et al., 1993; Dunlop and Özdemir, 1997; Evan and Heller, 2003; Deng et al., 2004; Liu et al., 2005; Just and Kontny, 2012; Evdokia and Enzo, 2018). As demonstrated in Fig. 2a-d, low-temperature curves indicate sudden changes around 120K, revealing the Verwey transition of magnetite (e.g. Verwey, 1939; Muxworthy et al., 2003). Above the

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Verwey transition, a maximum-type shape denotes the isotropic point of magnetocrystalline anisotropy of magnetite (Syono, 1965). This finding is supported by the well-known decrease in susceptibility near to the Curie-Point of 580°C in heating curves as an indicator for magnetite. Two prominent peaks are located around 250 and 500 ⁰C in the heating curves along with an increasing trend during cooling processes, possibly reflecting the unblocking of magnetic grain size (e.g. Liu et al., 2005). There is a gradual decline of magnetic susceptibility in the heating curves between 300 and 450 ⁰C, especially in the topsoil (Fig. 2a) and paleosol (Fig. 2d) samples, indicating the presence of maghemite (e.g. Oches & Banerjee., 1996; Deng et al., 2004; Liu et al., 2005). Relatively abundant organic matter in the topsoil would be responsible for newly formed magnetic minerals, causing an obviously enhanced κ after cooling steps (e.g. Liu et al., 2005). Fig.2e-h show hysteresis loops of typical samples, which provide similar information of magnetic assemblages with the TCA profile (Li et al., 2015a).

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Fig. 2. Plots showing the temperature dependent magnetic susceptibility curves (a-d) and hysteresis loops (e-h) of representative samples in the TCB section. (a-d) The red and blue lines indicate magnetic susceptibility variations during the heating and cooling runs, respectively. The low temperature κ-T curves are marked by green lines. (e-h) Hysteresis loops derived from original measurements are presented by the grey lines, while the black lines represent the loops after slope corrections.

The incremental acquisitions of IRM and stepwise backfield curves are shown in Fig.3a. All typical samples put up a primary rise before 280 mT and nodical points from c.35 to 65mT between the backfield curve and the abscissa. Remanence of demagnetized SIRM by alternating filed and thermal demagnetization, respectively, is demonstrated in Fig.3b-c. The ratio of remanence versus field manifests fluctuation before 30 mT by AF demagnetization, indicating major loss of remanence. The median demagnetization field of the topsoil is about 28mT, while the loess and paleosol samples have a median demagnetization field between 25 and 40mT, reflecting the

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main carrier of low coercivity minerals (Evans and Heller, 2003). There are no more than 18% of the SIRMs left after demagnetization with peak AF of 100mT, which were completely demagnetized by a peak AF of 300mT, denoting the presence of several incomplete antiferromagnetic minerals with a relatively high coercivity. Such high-coercivity minerals present again in the thermal demagnetization steps, where a prominent remanence depletion at c.120 ⁰C and a continuous decrease after 600 ⁰C would indicate the possible appearance of goethite and hematite, respectively. As shown in Fig. 3d, the representative samples plot in the pseudo single domain (PSD) and multidomain (MD) area of the Day plot (Day et al., 1977; Dunlop, 2002). Similar domain states of magnetic grain sizes are further clarified by the Dearing plot (Dearing et al., 1997) in Fig. 3e, in which the ratio of χfd% and χARM/SIRM is applied for the determination of the magnetic grain sizes. Moreover, paleosol samples are specified to contain finer grain sizes as shown by the Day, Dearing and King plots (King et al., 1982).

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Fig. 3. The IRM curves (a-c) and magnetic grain size analyses (d-f) in the TCB profile. (a) The IRM acquisition and back field curves of representative samples. (b-c) The remanence plots of demagnetized SIRM by AF and thermal demagnetization, respectively. (d) The Day plot (Day et al., 1977) of representative samples. Ratios between saturated remanent magnetization (Mrs) versus saturation magnetization (Ms) and remanent coercivity (Hcr) versus coercivity (Hc) derived from hysteresis measurement have been applied in the Day Plot. (e) The Dearing plot (Dearing et al.,1997) of all samples. (f) The King plot (King et al., 1982) showing binary analyses of χARM versus χ. The SSD, PSD and MD are the abbreviations of the stable single domain, pseudo single domain and multi-domain, with respect to the magnetic grain size.

Concentration-related magnetic parameters are illustrated in Fig. 4, showing that both TCA (Li et al., 2015a) and TCB profiles share similar variations. The χ values of

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the TCB profile vary between 32.6 and 85.2×10−8 m3 kg−1, averaging around 48.3×10−8 m3 kg−1. Similar to that in the TCA section, the χ is basically enhanced in the loess and topsoil layers of the TCB profile with mean values of 49.8 and 62.9×10−8 m3 kg−1, respectively. Variations in the two profiles are comparable between the χ and SOFT, confirming magnetic control by mainly ferrimagnetic minerals. The HIRM in the TCB profile presents little difference between loess and paleosol horizons in the lower parts, although high values of HIRM are observed in the topsoil samples. Similarly, the χARM/SIRM depletion in the loess horizon of the TCB profile is not as prominent as in the TCA record. The χARM/SIRM ratio of the TCB profile ranges from 0.10 to 0.33 ×10-3mA-1 with a mean value of 0.14×10-3mA-1, similar to the mean value in the TCA profile. In the loess horizon of the TCB profile, the mean value of χARM/SIRM is 0.13×10-3mA-1, lower than that of 0.17×10-3mA-1 in the paleosol layer. Compared with the TCA profile, the χfd% values in the TCB profile relatively increased in the paleosol at the bottom. Nevertheless, the χfd% values in the two sections are confined within 4%. It is noteworthy that there are large oscillations of the main magnetic parameters in the middle segment ranging from 1.5 to 3 m in the TCA profile without a recurrence in the TCB profile. In the TCB profile, the corresponding segment is characterized by a relatively low magnetic concentration with three gentle fluctuations.

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Fig. 4. Comparison of concentration-related magnetic parameters between the TCB and the TCA profiles. Yellow bars indicate the comparable oscillation of magnetic parameters, while grey bars denote the variations in the paleosols.

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3.2. Granulometric characteristics

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Grain sizes provide useful information about the dust source, transportation and post depositional alteration in loess (e.g. Liu, 1985). Grain size has been widely applied as an effective tool for evaluation of the Asian paleomonsoon variation (e.g. Ding et al., 1992, 2005; Nugteren et al., 2004; Sun et al., 2004, 2008; Oldfield et al., 2009; Sun et al., 2010, 2013; Yang et al., 2014; Guan et al., 2016). According to the prevalent

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grading standard (e.g. Blott and Pye, 2001), grain sizes in the two sections are estimated by three fractions, clay (<2 µm), fine silt (2-63 µm) and sand (>63 µm). Fig.5a-b presents variations of these fractions with depth in the two profiles. There is an obvious dominance of the silt fraction, taking up more than 45% of total grain size fractions. Sand, accounting for up to about 30%, constitutes the second abundant fraction and is enhanced significantly in the loess horizon. With regard to the distribution of grain sizes, frequency distribution curves (Fig. 5c-f) of typical samples generally display bimodal or trimodal patterns that are consistent with other loess deposits (e.g. Sun et al., 2002, 2004, 2008). Previous studies have put forward that two important components, including a short-suspension component and a long-suspension component, are associated with different wind dynamics in typical loess deposits (e.g. Tsoar and Pye, 1987; Pye, 1987; Sun et al., 2002, 2004, 2008; Wang et al., 2003). In order to better decipher the relationship between grain-size components and specific depositional processes, an analysis of the component partition by logarithmic normal distribution fitting (Xiao et al., 2014, 2015) has been conducted in the typical samples as illustrated in Fig. 5c-f. With fitting residuals less than 4%, all typical samples present four unimodal distributions that correspond to four different components, based on the primary range of each unimodal size. The first component with modelled grain sizes less than 1.5 µm varies between 3.58 and 6.66%. The fine component exhibits modelled grain sizes of 8-9 µm in loess samples and 9-14 µm in the paleosols, respectively, which is slightly coarser than that of the CLP (e.g. Sun et al., 2004, 2008). The content of the fine component is confined within 40% in the loess samples, but exceeds 50% in the paleosol samples. The coarse component of loess samples presents modelled grain sizes between 50 and 83 µm, while paleosol samples put up model sizes between 25 and 50 µm.

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Fig. 5. Granulometric properties in the TCA and TCB profiles. (a-b) The distribution of clay, fine silt and sandy silt fractions in two loess profiles. (c-f) Frequency distribution curves of four representative samples and grain size component partitioning with the lognormal distribution function based on frequency distribution fitting. Four components were identified by polymodal distributions and indicated by C1 (component 1) through C4 (component 4) from fine to coarse modes along with corresponding contents.

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3.3. Results of Geochemical measurements

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Due to the comparability of the two sections and fluctuation behavior of magnetic records in the middle-upper part of the TCA profile, this study chose samples at 10cm intervals from the TCA profile to carry out the geochemical measurements. Results show that the TCA profile shares similar composition of major elements with loess deposits in other depositional regions, like the CLP and Siberia (e.g. Chen et al., 2001; Ding et al., 2001; Pullen et al., 2011; Zhang et al., 2012; Wang et al., 2015; Skurzyński et al., 2018), characterized by relatively high concentration of Si, Al and Ca oxides. As shown in Fig.6, the content of SiO2 was essentially enhanced in the loess layer with a mean value of 52.4%, compared with the paleosol segment, where the mean content of SiO2 is 49.7%. A decreasing trend was roughly observed in the variation of Na2O content, although loess samples show a higher Na2O content than paleosol samples. The CaO content varies between 9.9 and 25.6% with a mean of 14.6%. Unlike in the Chinese Loess Plateau (e.g. Chen et al., 2001; Peng et al., 2018), the TCA profile exhibits the enhancement of CaO content in the paleosol layers, causing an approximated reversal trend with SiO2 variation. There is a similar trend among the oxides MgO, K2O, Fe2O3, and Al2O3, showing a general downward increasing pattern. The detailed features delineate some different reservations of the oxides above. Variations of the MgO and Fe2O3 are close to each other in the very bottom segment, while the other two oxides exhibit a reversal trend in the same segment. The content of MgO ranges from 1.7 to 3.1% with a mean content of 2.5%, while the Fe2O3 content varies between 4.6-6.6%, averaging at 5.7%. A peak type in the Fe2O3 curve is similar to that of CaO at c.1.7m in the middle part of the section, which is contrary to the significant depletion of the SiO2 and Na2O contents. Such an oscillation may denote the fortuitously uneven remoulding of the original input elements in the studied section, which possibly accounts for the oscillation of magnetic parameters in the TCA as indicated in Fig.4 during this interval. The variation of K2O and Al2O3, ranging from 3.2 to 4.2% and from 11.9 to 14.2%, respectively. For K2O and Al2O3, a gradual decrease in the loess horizon and obvious fluctuation in the paleosol layers probably indicate post depositional alteration of the eolian matrix in paleosols.

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The binary plot (Fig.6b) between the elements and average of upper continental crust (UCC) (Taylor and Mclennan, 1985, 2008) delineates similar differentiation between loess and paleosol samples. But it is apparent to detect the distinction in the absolute deviation of different elements with respect to the UCC composition. Basically, there is a similar composition of oxides SiO2, Al2O3, Fe2O3, K2O and MgO with respect to the UCC mean, while a large difference is observed in the Na2O and

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CaO content. Furthermore, the element enrichment in paleosols relative to basic loess has been estimated by △% in Fig.6c. The enrichment of Fe2O3, CaO, K2O and MgO is found in the buried paleosols, while depletion of Fe2O3, CaO, MgO and Na2O is significant in the topsoil samples, indicating a pedogenic difference between the upper and lower paleosols. Fig.6d shows the A (Al2O3)–CN (CaO & Na2O)–K (K2O) diagram (e.g. Nesbitt and Young, 1982, 1984), in which samples are distributed in a line parallel to the CN axis towards the terrigenous shale point. The A-CN-K diagram shows that the weathering intensity is quite low in the TCA loess, which is characterized by removal of silicate Ca and Na elements from sodium amphibole in the parent materials. Such low weathering property is also indicated in the binary plot of the CIA versus Na (Na2O)/ K (K2O) ratio (Fig.6e). Compared with the records from the CLP (e.g. Liu, 1985; Chen et al., 2001) and the Yili basin (e.g. Ye, 2001; Li et al., 2016b), samples in this study are basically distributed in the low weathering domain with a weathering index of no more than 60.

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Fig. 6. Characteristics and analyses of major elements in the TCA profile. (a) Plots showing variations of representative elements in terms of the oxides content. The grey bars highlight the element variations in paleosols. (b) The binary plot of mean values for major element contents in the TCA profile with respect to the upper continental crust (UCC) means. (c) The variation of a given element content relative to the Al2O3 content in different paleosol layers (d) The A–CN–K trigonal diagram of representative oxides in the TCA profile. (e) Comparison of the Na/K and CIA ratios between the TCA profile and other records in the (Luochuan) LC (red color) and YL (Yili) (blue color) loess profiles. The different areas in the diagram indicate weathering degrees in terms of low, high and strong intensities.

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4. Discussions

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4.1. Estimation of depositional differences by the magnetic record in the ACA loess

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Previous studies have revealed large discrepancies on depositional properties, like the thickness of the ACA loess, although the loess profiles were deemed to be

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contemporaneous deposits (e.g. e.g. Feng et al., 2011; E et al., 2012; Li et al., 2015a, 2015b; Zan et al., 2013, 2015; Jia et al., 2012, 2018; Song et al., 2018a). Due to the chronological controversy, the depositional differences have largely complicated paleoenvironment correlations among the loess profiles (e.g. Ye, 2001; Li et al., 2016c). In order to better clarify the depositional differences, this study further compares the loess profiles from the northern piedmont of the Tianshan Mountains (ZL section, Wei et al., 2013), the Yili basin (TLD section, Shi, 2005) and the Tacheng basin (Li et al., 2015a), in terms of mass-specific susceptibility (χ). Although chronological data indicate that the main body of these profiles were formed since the last glacial period, the total thicknesses of the four sections are quite different (Fig.7a). Additionally, there is a significant distinction regarding the stratum depth in the loess horizons (Fig.7b), indicating different deposition patterns among these loess profiles. The depositional differences of loess may result from three main reasons, including dust accumulation rates, hiatuses by post depositional erosion, and various local circumstances (e.g. Roberts and Winklhofer, 2004; Lu et al., 2006; Zhu et al., 2007; Deng, 2008). Especially, depositional hiatus has been regarded as a very important factor for different accumulation rates in loess (e.g. Lu et al., 2006; Zhu et al., 2007; Li et al., 2016a). In this study, the χ values in the four profiles share comparable variations in general (Fig.7a). Particularly, the fundamentally consistent variation of magnetic susceptibility in the bottom paleosol layers suggests that these loess sections record similar paleoclimate variations. In addition, there was no apparent evidence accounting for the depositional hiatus during the field observation. Thus, the depositional hiatus could generally be excluded for the depositional distinction through a long-term perspective.

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Fig. 7. Analyses of depositional differences of the loess profiles in the ACA. (a) Comparison of magnetic records and stratigraphic properties of the ZL (Wei et al., 2013), TLD (Shi, 2005; Liu et al., 2012) and two TC profiles in this study. Grey bars indicate paleosol horizons and

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chronological data are from OSL measurements. (b) Diagrams showing the assessment of depositional units at 10cm intervals.

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Previous studies have pointed out that the ACA loess was largely affected by the complex landforms and source region nearby (e.g. Ye, 2001). Due to approaching the dust source, variations of the dust source and the underlying surface during deposition may possibly exert a great impact upon the dust accumulation at different locations (e.g. Ye, 2001; Song et al., 2014; Li et al., 2016c). Such an impact would have given rise to differences of grain size distribution in different loess profiles. As shown in Fig.8, there are subtle differences of key granulometric components between the TCA and TCB profiles. The slightly higher content of coarse components (grain size fraction above 50 µm) in the TCA profile and relatively higher content of fine components (grain size fraction between 1 and 25 µm) in the TCB profile potentially indicate differences upon dust accumulation. Therefore, the depositional differences among the ACA loess profiles could possibly be attributed to various accumulation rates of local eolian dusts, with respect to specific locations. In addition, these profiles could be regarded as contemporaneous deposits, which recorded similar paleoenvironmental variations in general. Nevertheless, in consideration of these detailed differences of magnetic records among these profiles, caution should be taken to the unique and complex property of an individual section, in order to retrieve a reliable paleoclimate record.

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Fig. 8. Comparison of grain size groups in terms of mean contents between the TCA and TCB profiles, based on the differentiation of the grain size distribution.

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4.2. Paleoenvironmental implication for rock magnetic parameters.

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Reinforced studies on the CLP confirm a generally pedogenic enhancement of magnetic concentration in paleosols associated with fine/ultrafine magnetic grain size assemblages (e.g. Heller and Liu, 1984; Kukla et al., 1988; Bloemendal et al., 1995; Porter and An, 1995; Maher and Thompson, 1999; Nie et al., 2008; Sun et al., 2010; Liu et al., 2015). Such pedogenic enhancement has also been revealed in loess profiles in Eastern Europe and other parts of Central Asia (e.g. Vasiljević et al., 2014; Basarin et al., 2014). However, due to an absence of abundant ultrafine magnetic constituents in most of the ACA loess/paleosol sequences, the process of magnetic enhancement forced by pedogenesis remains unclear. Magnetic enhancement was widely attributed to variations of wind vigor, partly pedogenic contributions, carbonate leaching and gleization caused by waterlogging (e.g. Evans and Heller, 2003; Liu et al., 2008; 2013). Recent efforts have also underlined a complicated linkage between magnetic characteristics and stratigraphic properties of the ACA loess deposits (e.g. Liu et al., 2012, Li et al., 2015a,b; Zan et al., 2013, 2015; Jia et al., 2012, 2018; Song et al., 2014 2018a; Li et al., 2018). This complexity of magnetic records could be further visualized by two aspects, both involving magnetic mineral distribution and magnetic responses to stratigraphic variations. In this study, different correlations of magnetic parameters may denote the distinction of magnetic constitutes and magnetic grain size distribution within different stratigraphic layers (Fig. 9a). Detailed analyses combined with magnetic properties, bulk grain sizes and geochemical parameters are conducted to further clarify the main factors that controlled the magnetic variations within the different layers (Fig.9c-f). The positive correlation between χ and the sandy silt fraction in paleosol P3-4 and loess L2 probably reveals magnetic contribution from the inputs of coarse grain size fractions. Similarly, the positive correlation χARM/SIRM and clay fraction in the paleosol P3-4 may suggest that the fine grain size fraction contained relatively high content of fine magnetic minerals. There are weakly negative relationships between χ and sandy silt fraction at the loess layer L1 and the paleosol P1-2, where relatively depleted Md values were observed, possibly suggesting an attenuated influence of wind vigor on magnetic enhancement. Similar to the CLP record (e.g. Liu et al., 2007), negative correlations between χ and the sandy silt fraction along with positive correlations between χARM/SIRM and the clay fraction are presented in the topsoil (P1), suggesting a magnetic variation associated with enhanced pedogenesis. Moreover, a positive correlation between the EC index and χ in paleosols and loess L3 may denote a possible influence of leaching on magnetic records in the bottom paleosols and interbedded sandy silt layer. In addition, a positive relationship between the CIA index and χARM/SIRM at the bottom indicates a pedogenic contribution on fine magnetic particles. In the upper loess layer L1, faint negative correlations between magnetic proxies and geochemical indexes may indicate the composite interference from detrital inputs and uneven leaching. Thus, this may possibly be responsible for the oscillation zone in the TCA profile rather than depositional alteration by tectonic or great paleoclimatic change. Based on the analyses above, there are three magnetic responses to the bulk grain sizes and geochemical proxies. First, the bottom layers were characterized by the magnetic enhancement related to coarse grain sizes and water leaching. Second, a

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concomitant relationship between magnetic enhancement and intensified pedogenesis was found in the upper soils. The third pattern was illustrated in the transitional segment in the middle part of the profile, showing inordinately detrital inputs and a weak pedogenic contribution.

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Fig. 9. Binary analyses of magnetic and nonmagnetic parameters in the TCA profile. (a-b) Bivariate plots indicating the relationship among χ, χARM/SIRM and SIRM. (c-d) Bivariate plots showing the correlation between magnetic parameters and grain size proxies. (e-f) Bivariate analysis between magnetic parameters and geochemical parameters. Red circles indicate samples from the loess L1 and paleosol P1 in the upper part of the profile, while red lines denote the linear fitting curves. The blue circles and lines represent the data distribution and linear fitting curves of samples from L2 and P2 in the middle segment of the profile. Black circles show the data distribution of samples from L3, P3 and P4 at the bottom of the profile, and black lines indicate the linear fitting curves of correspondent samples.

In the CLP, the sandy silt fraction always reflected the strength of the winter monsoon that caused accumulation of coarse fractions and the background of magnetic minerals in loess horizons (e.g. Maher and Thompson, 1999; Maher et al., 2003; Deng et al., 2004; Liu et al., 2007; Sun et al., 2010). Inversely, pedogenesis during interglacial periods would give rise to an increase of the ultrafine clay fraction along with fine magnetic grain size components that enhanced the content of magnetic minerals. In this study, the low χfd% values indicate a limited contribution of ultra-fine superparamagnetic particles associated with high-level pedogenesis (e.g. Liu et al., 2007). Generally, magnetic enhancement at the lower segments of the TCA profile is possibly controlled by coarse grained minerals. This coarse fraction was mainly transported by a land-based wind system during short distance movements (Pye, 1987), and was possibly associated with the strengthened Westerlies and affiliated surface winds in the studied area. On the other hand, paleosols in the CLP usually underwent relatively strong leaching that caused the accumulation of calcareous nodules in the

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loess horizons (e.g. Liu, 1985; Yang et al., 2005). However, the paleosols in the TCA profile showed a limited leaching intensity as indicated by the EC index and the observed assembly of calcium carbonate mycelium. Nevertheless, impacts from possible gleization in this section could not be completely excluded due to the magnetic mineralogical differences between topsoil, loess and paleosol samples. In the Siberian loess sections, low values of magnetic susceptibility had been ascribed to gleization in paleosols, where relatively high coercive forces were determined (e.g. Chlachula et al., 2003; Zhu et al., 2003; Liu et al., 2013). As shown in Fig. 2, the relatively high Hc and Hcr with high maghemite content in paleosols would possibly indicate certain gleization in the paleosol (Bidegain et al., 2009). In the topsoil, increased magnetic minerals with a relatively depleted sandy silt content may indicate that the intensified pedogenesis similar to the CLP record lead to magnetic enhancement in this segment. This hypothesis was further supported by positive correlations between the χARM/SIRM and CIA index in the topsoil and the visibly dark-brown color in this horizon. The unit ranging from 0.8 to 2.6m, including the L1 and P2 horizons, may possibly serve as a transitional deposition zone. As no specific correlation between magnetic and other proxies was observed, this transitional deposition zone seems to be dominated by inordinately detrital inputs and a weakly pedogenic contribution. Basically, the fluctuations of the magnetic enhancement in this zone hint to possible abrupt changes of paleo-wind intensity (wind vigor) and relatively irregular leaching (Ye, 2001).

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Based on the aforementioned analysis, it could be deduced that both wind vigor and pedogenesis exert pivotal influence upon the magnetic enhancement. Therefore, paleoclimatic interpretations of solely magnetic concentration are somewhat weak in their reliability in most parts of the ACA loess. At the bottom of the profile, magnetic properties were controlled mainly by dust input related to paleo-wind intensity. In the upper segment of the profile, magnetic variations were dominated by pedogenesis, while simultaneously only relatively moderate paleowind intensities prevailed. Very likely, the middle intermediate zone reveals an influence on the magnetic enhancement by both wind-vigor or pedogenesis. Nevertheless, these findings are only applicable to the loess sections in the eastern part of the ACA, and are hardly transferred to other loess regions in the world. This study suggests that local deposition conditions from comprehensive analysis should be taken into account for magnetic interpretation in loess. With regard to the ACA loess, it may be better to evaluate paleoclimate changes in terms of specific deposition horizons, where dominant factors of magnetic variations have fully been assessed.

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4.3. Paleoenvironmental variation pattern revealed by the multiple parameters.

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Paleoenvironment variations during the late Quaternary are not well discerned in the ACA, due to the aridity background and geomorphologic complicacy. The modern climate in the ACA is mainly controlled by the northern branch of the Westerlies and the southward Siberian High, leading to different transportation processes of evapotranspiration (Ye, 2001; Shi and Sun, 2008; Li et al., 2016c). A remarkable synthesis of paleoclimate records with a reliable chronology revealed an

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unsynchronized pattern of the Holocene paleoclimate in the ACA, compared with that in monsoonal Asia (e.g. Chen et al., 2008). With regard to the glacial/interglacial cycles, some studies emphasized a pattern similar to monsoonal Asia, with warm temperature corresponding to abundant precipitation (Ye, 2001). However, some scholars have proposed that increased moisture may occur during cold periods in the ACA, in consideration the arid circumstances (e.g. Li, 1990) In this study, a similar variation of the χ, DCA and EC indexes generally indicates a relatively high humidity in the loess horizon (Fig.10). On the other hand, the relative depletion of the EC index and increase of the DCA index may basically reveal a relatively low humidity in the buried paleosols, although high CIA values are presented. Such a finding was not a singular trait confined to the studied profile, but was also revealed in other sections in this area (e.g. Ye, 2001; Song et al., 2014). These findings may support the hypothesis that the ACA paleoclimate during the late Quaternary was dominated by relatively low humidity during warm periods and high humidity during cold periods (Li, 1990; Ye, 2001). Meanwhile, comparable variations among combined parameters are observed in the paleosols, which are characterized by decreased χ, Md, and EC simultaneously with relatively enhanced DCA, CIA, 2-10 µm grain size and χARM/SIRM parameters. This behavior possibly reflects a relatively high degree of the post-alteration in the buried paleosol layers, in spite of low humidity. Except for the coarse grain size fraction, synchronous variations of these parameters in the topsoil are observed, suggesting an increased contribution of pedogenesis to magnetic enhancement (Fig.10). In addition, the paleowind intensity, dust source expansion and snowline variations might affect local paleoclimate records in this area (Ye, 2001). During glacial periods, the strengthened wind intensity and the applicable dust source nearby would give rise to the input of coarse sediment of the ACA loess, which contains abundant magnetic minerals. Due to the blockage of water vapor by the Tibetan Plateau, water vapor feeding the rainfall in the ACA was mostly transported by the mid-latitude Westerlies from the North Atlantic Ocean and inland lakes (e.g. Böhner and Lehmkuhl, 2005). Theoretically, precipitable water vapor density in the northern Tianshan area is slightly lower than that in the Tarim Basin, although the former possesses higher relative moisture (e.g. Shi and Sun, 2008). In consideration of the arid background, relative moisture might largely depend on the degree of water vapor evaporation. During cold periods, glacier expansion in the Northern Hemisphere along with increased snow cover on the local mountains would reinforce the frigidness of the loess area, causing restricted evaporation. Although low temperature inhibited the weathering process, loess layers that were formed during glacial periods show relatively higher humidity. On the contrary, the warm temperature under moderate wind vigor would have an intensified weathering degree in the buried paleosols, leading to aggregation of fine bulk grained materials along with fine magnetic minerals. The weak pedogenesis had a limited response to effective moisture in the ACA loess, possibly because of the dramatic evaporation under arid background during

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interglacial periods. This hypothesis is supported by the low values of the EC index and the field observation showing calcium carbonate pseudomycelium in the paleosols, which was assumed as possible precipitation of secondary lime. In the topsoil, the covariant pattern of χARM/SIRM, χ, fine silt faction, CIA, DCA and EC might suggest a coherent variation of water and heat conditions, denoting a high weathering degree corresponding to high effective moisture. This pattern is completely different from the buried paleosols, but is coherent to the Holocene paleoclimate records in this area (e.g. Chen et al., 2016; Jia et al., 2018). The generally warm-humid pattern in the topsoil has also received approval from climatological data, which indicates a warm and humid environment conducive to soil development along with moderate wind vigor in recent years (Shi et al., 2003). In summary, the ACA loess may contain different paleoclimatic patterns between topsoil (formed during the Holocene) and the other parts of the section (possibly formed since the last interglacial period). The loess and buried paleosols might exhibit a paleoclimatic pattern of relatively high moisture during cold intervals and vice versa, while the topsoil reflects similar paleoclimate variation with other Holocene records in the ACA. The periodical variation of the Westerlies and local temperature alteration driven by the northern Hemisphere climatic system are likely the dominant reason for the paleoclimatic variation in the ACA.

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Fig. 10. Variances of combined geophysical and geochemical parameters in the TCA and TCB profiles. Grey bars indicate the paleosol horizon, while dashed lines highlight the comparison between the TCA and TCB profiles.

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5. Conclusions

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Based on geophysical and geochemical properties, magnetic enhancement and paleoenvironment pattern were preliminarily evaluated in the ACA loess. The main conclusions of this study are drawn as follows: (1) Magnetic method could serve as an alternative tool for stratigraphic comparison in the ACA loess. Comparison of magnetic records in this study revealed generally similar pattern of paleoenvironmental variations in eastern ACA since last interglacial. Depositional differences among these loess profiles possibly resulted from various accumulation rates of local eolian dusts, rather than significant hiatus.

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More attempts are still needed to the mechanism accounting for various accumulation rates in different loess profiles in such close sites. (2) This study further suggests that both wind vigor and pedogenesis exert pivotal influence on the magnetic records in the ACA loess, but the dominant factor varies a lot within different deposition horizons. At the bottom part of the studied section, magnetic enhancement was generally related to coarse grain size variations and water leaching. Magnetic enhancement associated with enhanced pedogenesis was observed in the topsoil. The middle part of the section exhibits a seemingly transitional pattern of magnetic variations in between the topsoil and the bottom parts. (3) Paleoclimate variations in the ACA loess were possibly characterized by relatively low humidity during interglacial periods and high humidity during glacial periods. However, this study may indicate a relatively high humidity with moderate wind vigor during the Holocene in the topsoil. In addition, this study suggests that paleoclimate assessment in the ACA loess should be conducted in consideration of the specific deposition horizons, where the dominant factor of magnetic variations has fully been understood. Moreover, further research is still necessary so as to obtain detailed interpretations of late Pleistocene paleoenvironmental evolution, along with more reliable chronology and more robust proxies in the ACA loess.

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Acknowledgments

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The authors would like to acknowledge Prof. Erwin Appel for experimental support of this study and precious suggestions. Special thanks go also to the editor Prof. Thijs van Kolfschoten and the reviewers for their insightful comments and constructive suggestions. The authors also thank Mr. Garrett Thompson from the School of International Studies, Sun Yat-sen University for the language review. This study was supported by the National Natural Science Foundation of China (41704069, 41877444 and 32110-41030366) and Guangdong Province Introduced Innovative R&D Team of Geological Processes and Natural Disasters around the South China Sea (2016ZT06N331). The authors declared that there is no conflict of interest and the colored illustrations are in the online version.

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Declarations of interest:

The authors declared that there is no conflict of interest.

Guanhua Li (First author)