Quaternary International 479 (2018) 90e99

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Characteristic grain-size component - A useful process-related parameter for grain-size analysis of lacustrine clastics?

* ** Yin Lu a, , 1, Xiaomin Fang a, , Oliver Friedrich b, Chunhui Song c a CAS Center for Excellence in Earth Sciences and Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, , 100101, b Sedimentology and Marine Paleoenvironmental Dynamics Group, Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234-236, 69120, Heidelberg, Germany c School of Earth Sciences & Key Laboratory of Western China's Mineral Resources of Gansu Province, Lanzhou University, Lanzhou, 730000, China article info abstract

Article history: Lacustrine sediments are important archives for paleoclimate reconstructions. The application of grain- Received 16 November 2016 size analysis as palaeoclimatic proxy in lacustrine clastics is valuable but also difficult because the typical Received in revised form polymodal grain-size distribution in these clastics. To better understand the grain-size distribution of 10 July 2017 lacustrine clastics and to promote the application of grain size in paleoenvironmental interpretation, this Accepted 21 July 2017 study investigates lacustrine clastics from northern and southern China. The grain-size distribution of Available online 22 September 2017 these sediments was decomposed by log-normal distribution function fitting method. Based on the results, and drawing upon the concept of paleomagnetic demagnetization and “Characteristic Remnant Keywords: ” fi Lacustrine sediments Magnetization from paleomagnetism, a conceptual system has been established and de ned for grain- Grain-size distribution size distribution analysis. The system is composed of four components: (I) Characteristic Grain Size Grain-size curve fitting Component (ChGSC), (II) Affiliated Grain Size Component, (III) Meaningful Grain Size Component, and Paleoenvironment (IV) Combination Feature of Grain Size Components (CFGSCs). Based on the proposed system, ChGSC and CFGSCs were used to detect the grain-size distribution of clastics from the different zones inves- tigated. Our results show the number, modal size, and percentage of ChGSC(s) in grain-size distributions are sensitive to changes in the lacustrine environment. The ChGSC(s) mirrors the dominant depositional process and hydrodynamic conditions. The modal size of ChGSC(s) is more sensitive to hydrological conditions than the widely used mean grain-size approach. Thus, the ChGSC(s) provide a useful process- related parameter for paleoenvironmental reconstructions. To test this promising application, we applied this approach to a deep drill core from the Qaidam Basin in the northeastern Tibetan Plateau. © 2017 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction et al., 2015). These conditions in a continental setting, allow for high-resolution palaeoclimatic studies that are crucial to better are typically hypersensitive to climatic changes understand the pattern and dynamics of the global climate system (Verschuren, 2009; Yu and Shen, 2010; Wolff et al., 2011; Herb et al., (An et al., 2011; Brauer et al., 2007; Kashiwaya et al., 2001; Litt et al., 2013; Liu et al., 2017) and therefore provide an ideal archive of 2014; Torfstein et al., 2015; Elbert et al., 2015; Tian et al., 2017). continental climate change because of the preservation of long (a Furthermore, clastic sediment sequences in large lakes have not hundred thousand years to million-year time scale), uninterrupted been investigated sufficiently to examine their climatic and sedi- sedimentary records (Kashiwaya et al., 2001; Wang et al., 2012; Lu mentary significances. As climate change is typically the main driver of the hydrological conditions within a lake, the nature and arrangement of clastic sedimentary facies in lacustrine sediments, both of which are * Corresponding author. closely related to the hydrological conditions, can be used to ** Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Lu), reconstruct (palaeo)climatic changes. Grain size is a particularly [email protected] (X. Fang). valuable indicator of the hydrodynamic evolution of lakes because 1 Present address: Sedimentology and Marine Paleoenvironmental Dynamics it corresponds to the hydraulic energy that is needed for clast Group, Institute of Earth Sciences, Heidelberg University, Germany. http://dx.doi.org/10.1016/j.quaint.2017.07.027 1040-6182/© 2017 Elsevier Ltd and INQUA. All rights reserved. Y. Lu et al. / Quaternary International 479 (2018) 90e99 91 transport, sorting, and deposition. Grain-size analysis provides receives high-suspension dust input (An et al., 2012)(Fig. 1A, C). paleoenvironmental information at the high temporal resolution Angulinao Lake in northern China is controlled by the East Asian that is needed to reconstruct and understand the dynamics of winter monsoon (during the winter half-year) and the Westerlies climate change. However, the typical polymodal grain-size distri- and is therefore characterized by significant high-suspension (Sun bution in lacustrine sediments has hindered the wide application of et al., 2008a,b) and low-suspension (Prins et al., 2007; Sun et al., grain-size analysis as a standard tool. This polymodal grain-size 2008a,b) dust input. distribution comes from different transporting media (e.g., floods, aeolian input, lake currents, and waves) and the re-sorting of 2.2. Grain-size measurement and component decomposition sediment. Within a lake, however, different depositional zones are characterized by distinctive combination of grain size components, Grain-size distribution was determined using a Malvern Mas- as was shown for (Inner Mongolia) (Xiao et al., 2012). tersizer 2000 laser particle sizer after the organic matter and car- This shows that decomposition polymodal grain-size distribution is bonates were removed by H2O2 and HCl, respectively. The fractions a potential tool for paleoenvironmental reconstructions based on <4 mm(84) and >63 mm(44) were regarded as clay and sand, lacustrine sediments. respectively. In between, fractions 4e8 mm(74), 8e16 mm(64) and Recently, two different approaches have been carried out to 16e63 mm indicate very fine silt, fine silt and medium to coarse silt, obtain reliable results from grain-size distribution analyses of respectively. The fractions 63e125 mm(34), 125e500 mm(14) and lacustrine sediments: (1) end-member modeling analysis (EMMA) >500 mm indicate very fine sand, fine to medium sand and coarse (Dietze et al., 2012, 2013, 2014; Ijmker et al., 2012; Liu et al., 2016; sand, respectively. Parris et al., 2009; Yu et al., 2016), and (2) the log-normal distri- Polymodal sediments are typically formed by various combi- bution function fitting method (Xiao et al., 2009, 2012, 2013, 2015; nations of unimodal components (Ashley, 1978; Inman, 1949; Gammon et al., 2017). The log-normal distribution function fitting Tanner, 1964; Visher, 1969) of which the grain-size distribution method is based on single sample fitting and subsequent decom- generally follows a log-normal distribution (Krumbein, 1938; Passe, position. In contrast, the EMMA method requires an eigenspace 1997). Using the log-normal distribution function, the grain-size decomposition with different scaling procedures that extract distribution can therefore be described with sufficient accuracy genetically meaningful end-member grain-size distributions and (Ashley,1978; Passe, 1997; Qin et al., 2005). Based on these findings, their percentages in each sample (Dietze et al., 2013). the log-normal distribution function fitting method described by The log-normal distribution function fitting method has been Qin et al. (2005) was used to quantitatively fit and partition the successfully applied to obtain paleohydrological information of grain-size components within individual distributions of the ancient lakes (Xiao et al., 2009, 2012, 2013, 2015; Gammon et al., sampled lake sediments. 2017). Based on grain-size component fitting and decomposition, The log-normal distribution function fitting method assumes percentage of each individual component was acquired. Subse- that a polymodal grain-size distribution is composed of several quently, the sequence of percentage variation on the individual unimodal log-normal distributions (Qin et al., 2005). The prototype component was compared with other proxies. However, in our formula of the log-normal function is as follows: Miocene-Pleistocene deep drill core studies, we expect to get the 2 3 variation sequence of component(s) that reflects the dominant Z∞ ! Xn 6 C ðX a Þ2 7 sedimentary process, not simply to do statistics of each individual FðXÞ¼ 4 piffiffiffiffiffiffi exp i dX5 s 2p 2s2 component. Therefore, an effective conceptual analysis system is i¼1 i ∞ i needed to extract the most essential elements from the large number of decomposed components. In this study, we establish where X ¼ lnðdÞ, d is the grain size in mm, n is the number of modes, such a system for grain-size distribution analysis using lacustrine c is the content of the ith mode, s is the variance of the ith mode, sediments from southwestern, western, and northern China as i i and a is the mean value of the ith mode's logarithm grain size, a ¼ representative sedimentary archives. We further propose the i i lnðd Þ (Xiao et al., 2009, 2012). The fitting residual is calculated as Characteristic Grain Size Component (ChGSC) as a useful process- i follows: related parameter for paleoenvironmental research and apply this approach to a deep drilling core (SG-1b) from the Qaidam Basin in Xm À À Á À ÁÁ 1 2 the northeastern Tibetan Plateau to test its applicability. dF ¼ F X G X m j j j¼1 2. Material and methods where m is the number of grain-size intervals and GðXÞ is the 2.1. Lacustrine sediment sampling measured grain-size distribution of a sample (Xiao et al., 2009, 2012). The fitting process of each sample is accomplished until a For this study, surface sediments in the deep/central, the minimum fitting residual is yielded. Then, the modal size (median shallow/transitional and the lakeshore zones of , Dian size) and the relative percentage of each component are given. The Lake, Yangzonghai Lake and Yilong Lake in southwestern China technical aspects of this procedure are described in detail by Qin (Fig. 1A and B), the in western China (Fig. 1C) and et al. (2005) and Xiao et al. (2012). Angulinao Lake in northern China (Fig. 1D) were sampled for grain- size distribution measurement and decomposition. Sedimentary 3. Results sequences in these lakes both are dominated by clastics with low levels of organic materials. The location of these lakes and detailed 3.1. Grain-size component decomposition of sediments from the information about the sample collection are given in Fig. 1 and deep/central lake zone Table 1. Lugu Lake, , Yangzonghai Lake and Yilong Lake in southwestern China (-Guizhou Plateau) are mainly The grain-size distributions of sediments from the deep/central controlled by the Indian monsoon and show no significant aeolian lake zone are composed of three to four unimodal components, input (Fig. 1A and B). Qinghai Lake in western China (Tibetan designated C1,C2,C3 and C4, from the finest to the coarsest modes, Plateau) is impacted by the Westerlies throughout the year and respectively (Fig. 2). All analyzed grain-size distributions are 92 Y. Lu et al. / Quaternary International 479 (2018) 90e99

Fig. 1. Location of the study area. (A) Map showing location of the sampled lakes in southwestern, western and northern China and the main atmospheric circulation systems. CLP: Chinese Loess Plateau; TP: Tibetan Plateau. (B) Magnification from rectangle B in Fig. 1A showing the location of four sampled lakes in southwestern China. The Indian monsoon is the main atmospheric circulation system that controls the area. (C) Magnification from rectangle C in Fig. 1A showing the location of Qinghai Lake in western China. The area is impacted by the Westerlies throughout the year. (D) Magnification from rectangle D in Fig. 1A showing the location of Angulinao Lake in northern China. During the winter half year, the East Asian winter monsoon is the controlling climate system in this area. Hulun Lake, Daihai Lake and Dali Lake are the places referred in this study. Detailed sampling-related information of the studied lakes are listed in Table 1. characterized by a unimodal component (red dashed curves in and 13.50%e46.17% (average value: 28.29%), respectively. The Fig. 2) that not only has a non-overlapping area with other adjacent combined total percentage of the two components varies between > 1 > components 2 but also has a percentage value ~10%. The 73.13% and 96.86% (average value: 82.85%). modal size of these components occurs in the very fine silt size e m m range (4.4 7.5 m, the average value is 5.9 m), and observed 3.3. Grain-size component decomposition of sediments from the percentages vary between 75.92% and 90.52% (average value: lakeshore 81.96%) (Fig. 2). The grain-size distributions of lakeshore sediments are 3.2. Grain-size component decomposition of sediments from the composed of four unimodal components, C1,C2,C3 and C4 (Fig. 4). shallow/transitional lake zone Like the grain-size distributions of sediments from the shallow/ transitional lake zones, the grain-size distributions of the sedi- The grain-size distributions of sediments from the shallow/ ments from lakeshores also have two unimodal components (red transitional lake zone are composed of three to five unimodal dashed curves in Fig. 4) that have a non-overlapping area with > 1 > components, C1,C2,C3, C4 and C5 (Fig. 3). In each sample, two other adjacent components of 2 and a percentage of ~10%. unimodal components (red dashed curves in Fig. 3) occur that have The modal sizes of these components occur in (1) the very fine to > 1 fi e m m fi a non-overlapping area with other adjacent components 2 and ne silt size range (5.3 10.8 m, average value: 7.7 m) for the ner a percentage of > ~10%. The modal size of these components occurs component and (2) the very fine to medium sand range in (1) the clay to fine silt size range (2.3e12.4 mm, average value: (68.3e301.3 mm, average value: 189.3 mm) for the coarser compo- 5.2 mm) for the finer component and (2) the fine silt to very fine nent. The percentages of the fine component vary between 16.78% sand size range (11.9e77.4 mm, average value: 50.7 mm) for the and 28.98% (average value: 21.63%), while percentages of the coarser component. Percentages for the finer and coarser compo- coarser component vary between 58.83% and 77.01% (average nents varies between 34.77% and 69.06% (average value: 54.58%) value: 68.45%). The combined total percentage of the two Y. Lu et al. / Quaternary International 479 (2018) 90e99 93

Table 1 Locations and sampling-related information of the studied lakes.

Type Lake Hydrological information Sample Sample location Water depth Plots in Remark name (m) figure

Modern deep/central lake sediments Lugu Lake Lake area: 48.5 km2, MWD: 93.5 m, LgL-1 27.691320 N, 38.4 2A Sag pond AWD: 40.3 m 100.804959 E Yilong Lake Lake area: 38.0 km2, MWD: 6.2 m, YlL-1 23.67744 N, 2.0 2B AWD: 2.4 m 102.5748 E Qinghai Lake Lake area: 4340.0 km2, MWD: 27.0 m, QhL-1, 2 36.811306 N, 2C, D Top 2 cm of core AWD: 17.9 m 100.137083 E QH-1A Angulinao Lake area: 31.7 km2, MWD: 30.0 m, AglnL-1 41.316667 N, 2E, Lake AWD: 19.5 m 114.35 E Ancient deep lake sediments Qinghai Lake QhL-3 36.661861 N, 2F Erlangjian deep 100.389639 E drilling Modern shallow lake (transition Dian Lake Lake area: 297.9 km2, MWD: 5.9 m, DL-1 24.768611 N, 5.4 3B zone) sediments AWD: 2.9 m 102.664167 E Lugu Lake LgL-2 27.695944 N, 35.1 3C Sag pond 100.809250 E Yangzonghai Lake area: 31.7 km2, MWD: 30.0 m, YzL-1 24.90764 N, 22.1 3A Sag pond Lake AWD: 19.5 m 103.01031 E Angulinao AglnL-2, 41.316667 N, 3D, E, F Lake 3, 4 114.35 E Modern lakeshore sediments Qinghai Lake QhL-4, 36.811306 N, 4A, B, C, D Erlangjian deep 5,6, 7, 100.137083 E drilling Angulinao AglnL-5, 6 41.316667 N, 4E, F Lake 114.35 E

Note: MWD: max water depth, AWD: average water depth. Hydrological information about related lakes cited from Wang and Dou (1998).

Fig. 2. Grain-size component fitting and decomposition of sediments from the deep/central lake zone of various lakes. In Fig. 2AeF, the black-coloured curves represent the measured grain-size distribution, the green-coloured curves represent the fitted grain-size distribution (above the black-coloured curves), and the dashed curves with different colours represent the decomposed grain-size components. Modal size and percentage of each component and fitting residual of each sample are given. CM, component modal size. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) components varies between 86.69% and 94.92% (average value: (Qin et al., 2005), the notion of paleomagnetic demagnetization and 90.08%). the concept of “Characteristic Remnant Magnetization” (Tauxe, 1998) from paleomagnetism have been used in this study to establish a conceptual system for grain-size distribution analysis of 3.4. Establishing a conceptual system for grain-size distribution lacustrine clastics. The reasoning behind this argument is that both analyses using the log-normal distribution function fitting method concepts share the same aim, which is to extract the component(s) that carries the main information. The conceptual analysis system Based on the log-normal distribution function fitting method 94 Y. Lu et al. / Quaternary International 479 (2018) 90e99

Fig. 3. Grain-size component fitting and decomposition of sediments from the shallow/transitional lake zone of various lakes. In Fig. 3AeF, the black-coloured curves represent the measured grain-size distribution, the green-coloured curves represent the fitted grain-size distribution (above the black-coloured curves), and the dashed curves with different colours represent the decomposed grain-size components. Modal size and percentage of each component and fitting residual of each sample are given. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) used in herein is composed of four components: (I) the Charac- accuracy of the percentage of the ChGSC. teristic Grain Size Component (ChGSC), (II) the Affiliated Grain Size Component (AfGSC), (III) the Meaningful Grain Size Component 3.4.3. Meaningful Grain Size Component (MeGSC) (MeGSC), and (IV) the Combination Feature of Grain Size Compo- This component has a non-overlapping area with other adjacent nents (CFGSC). Based on the statistical analysis of fitting results 1 1 components between 3 and 2 or a non-overlapping area with from lacustrine clastics (Figs. 2e4), the conceptual analysis system > 1 < other adjacent components 2 but a percentage that is ~10% was defined and interpreted as below. (blue dashed curves in Figs. 2e4). The percentage of individual MeGSC varies between 1% and <20%. The total percentage of 3.4.1. Characteristic Grain Size Component (ChGSC) MeGSC(s) in one grain-size distribution is generally < 20%. Like the This component is characterized by a non-overlapping area with AfGSC, each grain-size distribution of a given sediment can have > 1 > other adjacent components of 2 and a percentage ~10% (red one or more MeGSC(s) (e.g., Figs. 2D, 3F, 6B and 7E), or no MeGSC dashed curves in Figs. 2e4). Each grain-size distribution of a given (e.g., Figs. 2F, 3A, 4E, 6A and 7B). This parameter does not reflect the sediment must have at least one ChGSC. Our grain-size component dominant sedimentary processes but may indicate some sedi- fitting and decomposition results suggest the total percentage of mentary processes that are related to either a special transport ChGSC(s) in one grain-size distribution is generally >70%. Sedi- medium or a change in the dynamic conditions of the respective mentologically, this ChGSC indicates a particular sedimentary transport medium. Its sedimentology meaning still needs to further environment with one dominant sedimentary process. If more than explore. one ChGSC is present, this indicates a sedimentary environment that was dominated by different sedimentary processes. From the 3.4.4. Combination feature of grain size components (CFGSCs) highest to the lowest percentages of the ChGSCs in one sample, The CFGSCs is mainly depicted by the number, modal size and they can be divided into the first ChGSC, the second ChGSC, the percentage of the ChGSC(s) in a grain-size distribution. For third ChGSC, etc. example, only one ChGSC in each grain-size distribution of sedi- ments from the deep/central lake zone (Fig. 2). The combination 3.4.2. Affiliated Grain Size Component (AfGSC) feature makes the grain-size distribution of sediments from the This component has a non-overlapping area with other adjacent deep/central lake zone are different with sediments from the 1 components between 0 and 3 (yellow dashed curves in shallow/transitional lake zone and lakeshore. Grain-size distribu- Figs. 2e4). Our grain-size component fitting and decomposition tions in the latter two types of sediments have more ChGSC(s), results reveal the percentage of individual AfGSC varies between 1% which with different modal size and percentage. CFGSCs indicates a and 15%. The total percentage of AfGSC(s) in one grain-size distri- particular sedimentary environment with dominant sedimentary bution is generally <20%. Each grain-size distribution of a given process (es), and superimposed by some minor processes. sediment can have one or more AfGSC(s) (e.g., Figs. 2F, 3A, 4E, 6CeF We choose the brackets for the components just based on our and 7D). In some case, there is no AfGSC in one grain-size distri- grain-size component fitting and decomposition of ~1100 samples. bution (e.g., Figs. 2D, 3F and 6B). This parameter will improve fitting The value of the “non-overlapping area with other adjacent Y. Lu et al. / Quaternary International 479 (2018) 90e99 95

Fig. 4. Grain-size component fitting and decomposition of lakeshore sediments from various lakes. In Fig. 4AeF, the black-coloured curves represent the measured grain-size distribution, the green-coloured curves represent the fitted grain-size distribution (above the black-coloured curves), and the dashed curves with different colours represent the decomposed grain-size components. Modal size and percentage of each component and fitting residual of each sample are given. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) components” was used to define how one component stands out grain-size distribution shows two ChGSCs. One ChGSC has a modal from the rest. One component that stands out from the rest (i.e., size that is comparable to the ChGSC in sediments from the deep/ having a non-overlapping area with other adjacent components of central lake zone. The other is much coarser and characterized by a > 1 fi fi fi 2), indicates one speci c sedimentary process. The percentage modal size in the ne silt to very ne sand range (Fig. 3). These of one component used to evaluate the relative importance of it. results are consistent with the fact that the shallow/transitional The quantitative definitions of the three components, e.g., ChGSC is lake zone receives much more sediment from transporting media characterized by a non-overlapping area with other adjacent near the lake margin, such as rivers. > 1 > components of 2 and a percentage ~10%, are based on our The grain-size distribution of lakeshore sediments also shows subjective experience. Thus, it is an open system that might need a two ChGSCs. However, the coarser ChGSC has a much higher per- slight adjustment under certain boundary conditions. centage than the finer ChGSC and is generally much coarser than the ChGSCs of sediments from other lake zones (Fig. 4). These re- sults accurately reflect depositional processes at the lakeshore that 4. Discussion are dominated by fluvial transport and lake waves. This strong hydrodynamic regime leads to the deposition of a small amount of 4.1. Sedimentary implications of Characteristic Grain Size fine silt that is combined with a larger amount of sand particles Component (ChGSC) (Schieber et al., 2007). Overall, the similar results of our conceptual analysis system The grain-size distributions of sediments from the deep/central show that the investigated lacustrine deposits have inherent and lake zone are characterized by one ChGSC with an average per- stable grain-size distribution features for a variety of modern lakes fi centage up to ~82% and a modal size in the very ne silt range with different sediment sources and different climatic regimes (i.e., (Fig. 2). This observation accords with the fact that sediments in the the westerlies, Indian monsoon, East Asian monsoon) (Fig. 1A). deep/central lake zone are mainly composed of offshore suspension Moreover, the above mentioned observations suggest that the fi particles (clay to ne silt). Because of the depositional processes in ChGSC(s) of sediments from different lake zones closely mirror the these areas, sediments are deposited under relatively still lake dominant depositional processes and hydrodynamic conditions. water and weak hydrodynamic conditions. Our new grain-size fitting results as well as previous studies of In sediments from the shallow/transitional lake zone, each 96 Y. Lu et al. / Quaternary International 479 (2018) 90e99 lacustrine clastics from Hulun Lake (Xiao et al., 2012), Daihai Lake characterized by one ChGSC and five samples (5.3%) are charac- (Xiao et al., 2013) and Dali Lake (Xiao et al., 2015) in northern China terized by two ChGSCs. Fig. 6 shows representative grain-size dis- indicate that sediments along a transect from the lakeshore to the tributions of the 90 samples. In these samples, observed lake centre are characterized by a decrease in the percentage of percentages of ChGSC vary between ~62.08% and 96.02%, with an their coarser ChGSCs that corresponds to an increase in the per- average value at ~79.46%. Modal size of these ChGSCs vary between centages of their finer ChGSCs. Therefore, the number, modal size, ~5.01 mm and 19.0 mm, with an average value of ~9.1 mm. and percentage of ChGSC(s) in grain-size distributions of lake One may note that the average value in modal size of ChGSC in sediments can be used to obtain paleoenvironmental information. the Core SG-1b (~9 mm) is slightly larger than in the investigated sediments from the deep/central lake zone in the modern lakes fi < m 4.2. Application of Characteristic Grain Size Component (ChGSC) (very ne silt, 8 m). This difference should mainly result from different instruments that used for grain-size measurement. The To test the promising application of ChGSC based on the con- sediments from the deep/central lake zone in the modern lakes are ceptual grain-size distribution analysis system, we applied it to a measured by Malvern Mastersizer (2000) laser particle sizer, while clastics dominated sedimentary sequences in a huge lake, paleo- sediments from Core SG-1b are measured by Microtrac S3500 laser Qaidam Lake. The lake was located in the western Qaidam Basin, particle sizer. Mudstone (late Miocene lacustrine deposits) were northeastern Tibetan Plateau (Fig. 1A), developed during the late collected from one drill core located in the Nanyishan anticline, Oligocene-Quaternary (Yang, 1986; Zhang et al., 1987; Wu and Xue, western Qaidam Basin, northeastern Tibetan Plateau. Twenty par- 1993; Wang et al., 2012). A ~723 m-deep drill core SG-1b (as part of allel samples were pre-treated in the Institute of Tibetan Plateau a joint Sino-German project) was recovered in the paleo-Qaidam Research, Chinese Academy of Sciences. Subsequently, they were Lake with an average sediment recovery rate of ~93% (Zhang measured at the Institute of Tibetan Plateau Research using a et al., 2014; Lu et al., 2015). Detailed paleomagnetic dating of Microtrac S3500 laser particle sizer and in the Key Laboratory of Core SG-1b constrains its age at ~7.3e1.6 Ma (Zhang et al., 2014). Western China's Environmental Systems, Lanzhou University with Previous detailed examination of lithofacies, seismostratigraphy a Malvern Mastersizer 2000 laser particle sizer, respectively in and grain-size records, suggest that the drilling site was in a deep to 2012. The parallel experiments reveal the Microtrac S3500 laser fi semi-deep lake environment during ~7.3e3.6 Ma (~723-245 m), in particle sizer with lower sensitivity to clay and very ne silt par- a shallow lake environment during ~3.6e1.9 Ma (245-35 m), and ticles than the Malvern Mastersizer 2000 laser particle sizer. The finally in a lakeshore-like environment during ~1.9e1.6Ma(35- differences in mean grain-size measured by the two different in- m 0m)(Lu et al., 2015). The millimeter-to centimeter-scale thin struments are varying between 0 and 5 m. Thus, the average value m interbedded marl and limestone layers are mainly deposited in the in percentage (~79.5%) and modal size (~9 m) of ChGSC in the Core core with depth <600 m, gypsum crystals frequently occurrenced SG-1b are identical to the investigated sediments from the deep/ m in the core with depth <250 m (Zhang et al., 2014). Detailed mineral central lake zone in the modern lakes (~82.0% and ~6 m). fi composition study (Fang et al., 2016) reveals the very low content of The ve samples which characterized by two ChGSCs are also halite, gypsum, celestite, calcite and aragonite in the lower part of shown (Fig. 7). Grain-size distribution of these samples are similar the core (with core depth >600 m). Thereby, in this study, we chose with investigated sediments from the shallow/transitional lake fi the lower 100 m of the Core SG-1b (720-620 m) to apply the zone. Among the ve samples, grain-size distribution of four e concept of ChGSC based on the conceptual analysis system. Paleo- samples have one coarse ChGSC (Fig. 7B E). Percentage of the e magnetic dating constrains the age of the investigated core interval coarse ChGSC are in the range of ~8% 10%. Thus, these samples to be between ~7.3e6.8 Ma (Zhang et al., 2014). Lithofacies of the were inferred to be mass movement deposits. fi investigated interval comprise fine siltstones with horizontal These tting and decomposition results of Core SG-1b suggest millimeter-scale laminae (Fig. 5), suggesting that the sediments that the sediments in the investigated core interval are mainly fi were deposited in a deep to semi-deep lake environment (Lu et al., composed of offshore suspension particles (clay to ne silt) and 2015). have been deposited under relatively still lake water and weak Within the investigated core interval, our fitting and decom- hydrodynamic conditions. These quantitative characteristics sug- position experiment reveals fitting residuals that are in the range of gest deposition in a deep to semi-deep lake environment, which is ~2.74e1.18% (average value: ~1.76%), suggesting a very good fitting in agreement with the qualitative observations of the respective process. The grain-size distribution of each sample is composed of lithofacies (Lu et al., 2015). If compared to the mean grain-size, the modal size of the ChGSC three to five unimodal components, designated C1,C2,C3,C4 and C5, from the finest to the coarsest modes, respectively. In the grain-size in the investigated core interval shows a strikingly similar trend distributions of 95 analyzed samples, 90 samples (94.7%) are (Fig. 8). However, the observed variations in the modal size of

Fig. 5. Lithofacies of the investigated section in Core SG-1b. The core interval is characterized by gray to dark-gray fine siltstones with horizontal millimeter-scale laminae (Lu et al., 2015). Y. Lu et al. / Quaternary International 479 (2018) 90e99 97

Fig. 6. Representative grain-size distributions of sediments from the lower part of Core SG-1b. The black-coloured curves represent the measured grain-size distribution, the green- coloured curves represent the fitted grain-size distribution (above the black-coloured curves), and the dashed curves with different colours represent the decomposed grain-size components. Modal size and percentage of each component and fitting residual of each sample are given. These grain-size distributions are both characterized by one ChGSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Grain-size distributions of sediments that inferred to be mass movement deposits in the Core SG-1b. The black-coloured curves represent the measured grain-size dis- tribution, the green-coloured curves represent the fitted grain-size distribution (above the black-coloured curves), and the dashed curves with different colours represent the decomposed grain-size components. Modal size and percentage of each component and fitting residual of each sample are given. These grain-size distributions are both char- acterized by two ChGSCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 98 Y. Lu et al. / Quaternary International 479 (2018) 90e99

Fig. 8. Grain-size component fitting and decomposition of sediments in Core SG-1b. (A) Comparison the modal size of ChGSC(s) with the mean grain-size (from Lu et al., 2015). (B) Percentage of ChGSC in the measured samples. (C) Fitting residual of each sample.

ChGSC show relatively high amplitude variations compared to the Acknowledgements mean grain-size (Fig. 8). This observation suggests that the modal size of ChGSC is more sensitive to fluctuations in the hydrological This study was co-supported by the (973) National Basic conditions than the mean grain-size; this inference is reasonable Research Program of China (2013CB956400), the Strategic Priority because the mean grain-size represents mixed information, while Research Program of the Chinese Academy of Sciences (Grant No. the modal size of ChGSC reflects only information of the dominant XDB03020400) and the Priority Programme 1372 ‘Tibetan Plateau: depositional process. Formation, Climate, Ecosystems (TiP)’ of the German Research Overall, the conceptual grain-size distribution analysis system Foundation (DFG; Grant No. AP34/34-1,2,3). We thank Yougui Song, that is established herein will greatly improve and promote the Hucai Zhang and Zhiqiang Yin for providing some grain-size data. application of the log-normal distribution function fitting method We thank Qiong Li (Lanzhou University) for grain-size measuring. in grain-size distribution analysis of lacustrine clastics by extracting Special thanks to Prof. Jule Xiao for providing the log-normal dis- significant sedimentological processes from a set of sediments tribution function fitting software and Prof. Erwin Appel for sup- deposited in different lake environments. porting Y.L.’s research at the Univ. of Tübingen, Germany, during the year of 2013e2014. We thank Philip L. Gibbard (University of Cambridge), Ge Yu and one anonymous reviewer for their 5. Conclusions constructive comments, which improved the quality of the manu- script substantially. A conceptual system for grain-size distribution analysis of fi lacustrine clastics has been established and de ned based on the References log-normal distribution function fitting method. The system is composed of four components: (I) Characteristic Grain Size An, Z., Clemens, S.C., Shen, J., Qiang, X., Jin, Z., Sun, Y., Prell, W.L., Luo, J., Wang, S., Component (ChGSC), (II) Affiliated Grain Size Component (AfGSC), Xu, H., Cai, Y., Zhou, W., Liu, X., Liu, W., Shi, Z., Yan, L., Xiao, X., Chang, H., Wu, F., Ai, L., Lu, F., 2011. Glacial-interglacial Indian summer monsoon dynamics. Sci- (III) Meaningful Grain Size Component (MeGSC), and (IV) the ence 333 (6043), 719e723. Combination Feature of Grain Size Components (CFGSC). An, Z., Colman, S.M., Zhou, W., Li, X., Brown, E.T., Jull, A.J.T., Cai, Y., Huang, Y., Lu, X., Our data from modern lakes and a drill core show that the Chang, H., Song, Y., Sun, Y., Xu, H., Liu, W., Jin, Z., Liu, X., Cheng, P., Liu, Y., Ai, L., investigated lacustrine clastics from different lake environments Li, X., Liu, X., Yan, L., Shi, Z., Wang, X., Wu, F., Qiang, X., Dong, J., Lu, F., Xu, X., 2012. Interplay between the westerlies and Asian monsoon recorded in lake are characterized by their inherent and stable grain-size distribu- Qinghai sediments since 32 ka. Sci. Rep. 2 (619), 1e7. tion features. Furthermore, the ChGSC(s) of sediments from Ashley, G.M., 1978. Interpretation of polymodal sediments. J. Geol. 86 (4), 411e421. different lake zones are suggested to mirror the dominant deposi- Brauer, A., Allen, J.R., Mingram, J., Dulski, P., Wulf, S., Huntley, B., 2007. Evidence for last interglacial chronology and environmental change from Southern Europe. tional processes and hydrodynamic conditions. Therefore, the Proc. Natl. Acad. Sci. U. S. A. 104 (2), 450e455. modal size of ChGSC is more sensitive to hydrological conditions Dietze, E., Hartmann, K., Diekmann, B., Ijmker, J., Lehmkuhl, F., Opitz, S., Stauch, G., than the widely used mean grain-size approach. The ChGSC(s) Wünnemann, B., Borchers, A., 2012. An end-member algorithm for deciphering modern detrital processes from lake sediments of Lake Donggi Cona, NE Tibetan provide(s) a useful process-related parameter that can be applied in Plateau, China. Sediment. Geol. 243e244, 169e180. paleoenvironmental reconstructions. Dietze, E., Maussion, F., Ahlborn, M., Diekmann, B., Hartmann, K., Henkel, K., Y. Lu et al. / Quaternary International 479 (2018) 90e99 99

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