High resolution OSL dating of aeolian activity at Qinghai Lake, Northeast Tibetan Plateau

Catena 183 (2019) 104180

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High resolution OSL dating of aeolian activity at Qinghai Lake, Northeast Tibetan Plateau

T

⁎

E. ChongYia,b, , Zhang Jinga, Chen ZongYana, Sun YongJuana,b, Zhao YaJuana, Li Pinga, Sun ManPinga, Shi YunKuna a

Qinghai Provincial Key Laboratory of Physical Geography and Environmental Processes, School of Geographical Science, Qinghai Normal University, Xining 810008, China b Provincial Key Laboratory of Geology and Environment of Salt Lakes, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Aeolian activity High resolution OSL dating Qinghai Lake Northeast Tibetan Plateau Late Glacial

Qinghai Lake is the largest lake on the Tibetan Plateau (TP), located between the extremely arid Qaidam Basin to the west and the severely desertified Gonghe Basin to the south. Extensive aeolian sediment at Qinghai Lake is ideal material to reconstruct regional aeolian activity, and to better understand the relationship between desertification and climatic change in the lake basin. Aeolian sand accumulation is usually accompanied by erosion, hence, depositional hiatuses and disconformities must be identified for reliable reconstruction of regional aeolian activity. To date, the low density of age sampling around Qinghai Lake has hindered identification of hiatuses. In this study we report the first high density OSL dating; 41 OSL ages were obtained from two aeolian sand sections, Dongwei (DW) and Niaodao (ND). Sand accumulation trends recorded in the high-density OSL sampling sections were consistent with previously published probability density function (PDF) ages for the northeast TP. The middle Holocene (~7 to ~4 ka) was characterized by very low accumulation rates, with rapid sedimentation in the Late Glacial to Early Holocene (~14 to ~7 ka) and the Late Holocene (after ~4 ka). A ~3 ka hiatus in accumulation between ~7 to ~4 ka was identified in the DW section, but the ND section showed successive accumulation since the Late Glacial (~14 ka), on sub-orbital and millennial scales. Our environmental reconstructions are consistent with previously published aeolian data and paleoshorelines records. The combined evidence shows: strong aeolian activity since ~14 ka to ~9 ka; initiation of pedogenesis at ~9 to 7 ka; intensified soil development between ~7 ka and ~4 ka (with the most intense pedogenesis and least aeolian activity between ~6 to ~4 ka); relatively weak paleosol formation from ~4 to ~2 ka; and renewed aeolian activity after ~2 ka.

1. Introduction As the highest, youngest and largest plateau on Earth, the Tibetan Plateau (TP) is very sensitive to global climate change and human activity. Frequent strong wind, sparse vegetation and abundant sand sources mean that the TP has been subject to aeolian activity both in the geologic past and through to the present day (Dong et al., 2017). Qinghai Lake is the largest on the TP, located between the extremely arid Qaidam Basin in the west and the severely desertified Gonghe Basin in the south. The large closed-basin body of water acts as a natural ecological barrier in preventing desert expansion eastward and northward. Global warming and enhanced human activity in recent years has exacerbated aeolian activity in the region (Zhang et al., 2003)

and highlighted the necessity of reconstructing the history of aeolian activity and its climatic drivers. The paleoenvironmental record contained in the extensive aeolian deposits at Qinghai Lake provide a reference for predicting future desertification trends, and a source of paleoclimatic information that is independent of the lake sediment record (Lu et al., 2015). Normally, lake sediments can provide high resolution and continuous climatic records. However, it is often hampered by the presence of a lake reservoir effect (LRE, also known as ‘dead carbon’ or ‘old carbon’ effect) on radiocarbon dating for sediment cores from TP lakes (Hou et al., 2012; Mischke et al., 2013). LRE from Qinghai Lake core sediments suffers from the problem of age uncertainty, and varies temporally and spatially (Chen et al., 2016; E et al., 2018a; Hou et al., 2012; Jull et al., 2014; Mischke et al., 2013; Yu

⁎ Corresponding author at: Qinghai Provincial Key Laboratory of Physical Geography and Environmental Processes, School of Geographical Science, Qinghai Normal University, Xining 810008, China. E-mail addresses: [email protected], [email protected] (E. ChongYi).

https://doi.org/10.1016/j.catena.2019.104180 Received 17 October 2018; Received in revised form 11 July 2019; Accepted 21 July 2019 Available online 08 August 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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between 165–240 cm and 310–350 cm, and a weakly developed soil between 100 and 130 cm. In the top half of the section, the aeolian sand is fine grained, while below 350 cm it is mainly of medium grain size. The section is underlain by poorly sorted, angular gravels of unknown depth. OSL samples were collected at 30 cm intervals on aeolian sand stratum and 15 cm intervals on sandy paleosols, giving 26 OSL samples in total. Grass ashes were excavated at a depth of 260 cm (sample NDash) and these were radiocarbon dated to provide an independent age against which the reliability of the OSL ages could be assessed. Sample ND-ash sent to Beta Analytics Inc. for accelerator mass spectrometry radiocarbon dating, The calibration of 14C dates was performed using the program ‘CALIB REV 7.0.2’ (Stuiver and Reimer, 1993) and the INTCAL 13 and MARINE 13 calibration curve (Reimer et al., 2013). The DW section (36°18′7″N, 101° 2′27″E, 3316 m asl) lies at the margin of modern active dunes on the southeastern edge of the Qinghai Lake basin. The bulk of the 300 cm section comprises aeolian sand, with a weakly developed paleosol at 75–90 cm and a dark gray paleosol at 105–135 cm (Fig. 3). OSL samples were collected at 30 cm intervals below 135 cm and at 15 cm intervals in the upper 135 cm of the section, 15 OSL samples in total. In both the ND and DW sections, bulk samples were collected continuously at 5 cm intervals for measurements of GS and MS.

et al., 2007). In addition, conflicting proxies in lake sediments indicate that there is still some controversy over Holocene climatic changes at Qinghai Lake (Chen et al., 2016; E et al., 2018a; Hou et al., 2012; Jull et al., 2014; Mischke et al., 2013; Yu et al., 2007). Aeolian sediments have some significant advantages for recontructing paleoclimatic records: (1) the environmental significance of routine proxy indicators in aeolian sediment such as grain size (GS), magnetic susceptibility (MS) and total organic content is direct and clear; (2) OSL dating of aeolian sediment is robust because of abundant dating materials (mainly quartz or feldspars) that are well-bleached prior to deposition. The excellent bleachability of quartz from different sediments at Qinghai Lake is detailed in Long et al. (2019). However, aeolian sediments also have some shortcomings, namely their low time resolution and discontinuous deposition. Therefore, it is essential to determine the continuity of aeolian sedimentation on different time scales before making paleoenvironmental inferences. Previous studies in the Qinghai Lake area used probability density functions (PDF) based on the mean and the standard deviation of each OSL age (Chen et al., 2016; Liu et al., 2012; Lu et al., 2011; Stauch, 2015). Individual PDFs are summed to provide a cumulative PDF, and peaks in the PDF are used to indicate periods of maximum deposition. However, age clusters in PDF plots may be an artifact of variations in the density of sampling from aeolian sections (Singhvi et al., 2001). With few ages to constrain sequences, it has been impossible to distinguish whether a dune profile represents sporadic and intense deposition, or continuous low-intensity deposition (Telfer and Thomas, 2007). Numerical modeling also suggests that the resolution of sampling may affect the capacity to distinguish the impact of major externally-forced events, such as regional climate transitions, from stochastic disturbances affecting sediment accumulation and preservation (Telfer et al., 2010). OSL dating using high-resolution sampling may resolve these problems, and has been successfully applied to aeolian sediments (Buylaert et al., 2008; E et al., 2018b; Kang et al., 2013, 2015; Lai et al., 2007; Li et al., 2016; Long et al., 2017; Lu et al., 2006; Stevens et al., 2006, 2007; Sun et al., 2010; Wang et al., 2015). However, in the Qinghai Lake area, high density OSL sampling sections are scarce, which limits understanding of regional environmental changes. In this study, high density OSL dating was employed for the first time at Qinghai Lake to obtain a high resolution chronological framework for two aeolian sand sections, Dongwei (DW) and Niaodao (ND). The analysis verified the depositional continuity on sub-orbital and millennial scales in the two sections, and the broad trend of aeolian activity since the Late Glacial was reconstructed based on GS and MS proxies.

3. Laboratory analysis MS and GS analysis and OSL dating were undertaken at the Qinghai Provincial Key Laboratory of Physical Geography and Environmental Processes, Qinghai Normal University. MS was measured using a Bartington MS2B sensor after samples were air dried below 40 °C. GS was measured using a Malvern Mastersizer 2000 laser particle size analyzer after pre-treatment to remove organic matter and secondary carbonates, followed by dispersion with sodium metaphosphate (Lu et al., 2011). OSL sample processing and preparation was carried out in dark room. Two to three centimeters was removed from each end of the tube samples and reserved for environmental dose rate determination. Mineral grains of 90–125 μm were retrieved by wet-sieving. Sieved fractions were treated with HCl (10%) and H2O2 (10%) to remove carbonates and organic material, and concentrated HF (i.e., 40%) for 60 min to remove any remaining feldspar contamination and the outer alpha-irradiated layer. Extracted quartz was checked for purity using the IR depletion ratio test (Duller, 2003). Equivalent doses (De) were determined by the single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000; Wintle and Murray, 2006). All measurements were carried out on a standard Risø TL/OSL DA-20 reader (Bøtter-Jensen et al., 2010). For De calculations, the initial 0 to 0.32 s of the OSL signal minus an early background signal of the following 0.32 to 0.64 s was selected (Ballarini et al., 2007; Cunningham and Wallinga, 2010). Typically, 12–24 aliquots were measured for each sample and the weighted mean De (with one standard error uncertainty) calculated. De rejection criteria were restricted by two test measurements (e.g., 0.9 < R5/R1 < 1.1, and R4/ N < 5%).

2. Aeolian sediment in the study area and sample collection Qinghai Lake (altitude 3194 m asl) is the largest inland brackish lake in China at the present day, with a water surface area of 4260 km2, water volume of 7.16 × 109 m3 and a catchment area of ~29,660 km2 (Wang et al., 2014a). Data from Gangcha weather station (1975–2011), located 10 km to the north of Qinghai Lake, shows an average annual temperature of −0.6 °C and total precipitation of 370 mm. Wind speed peaks at 18 m s −1 in spring and the prevailing wind direction is west and northwest. Aeolian sediments are widely distributed around Qinghai Lake (Fig. 1). Previous studies of aeolian sediment have mainly been concentrated in the east and south of the lake (Liu et al., 2012; Lu et al., 2011, 2015). Given that aeolian sand around the desert boundary is more sensitive to climate change and human activity than that in the inner desert, this study focused on sediments in the west (ND section) and south east (DW section) of Qinghai Lake. The two sections were systematically sampled in 2012 and 2014. The ND section (37°2′10″N, 99°44′33″E, 3215 m asl) is located on the second terrace of Buha River and has an outcrop thickness of 600 cm (Fig. 2). The upper 30 cm of the section comprises the modern soil with plant roots. Most of the section comprises aeolian sand, with two relatively dark, consolidated, erosion-resistant sandy paleosols

4. Quartz luminescence characteristics and dose rate Fig. 4A shows typical dose response and OSL decay curves for aeolian sand sample ND-265. The natural OSL signal from ND-265 was bleached to the measurement background within 2 s (Fig. 4A), indicating a fast component dominated signal. Natural preheat plateau and dose recovery preheat plateau tests were carried out on ND-265 in order to selecting an appropriate preheating regime (Fig. 4B). The preheat temperature was increased from 160 to 300 °C in 20 °C increments, and the cut-heat temperature prior to test dose OSL measurements was 40 °C (i.e., 160–260 °C) below the preheat temperature. Fig. 4B shows a De plateau from 160 to 300 °C for sample ND-265. In the 2

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Fig. 1. Location of the Qinghai Lake study area. The inset map shows location of the lake in the northeast of the Tibetan Plateau. Red diamonds show the location of the two sections investigated in this study (ND and DW). Pink circles show the location of previously published aeolian sand sections (Lu et al., 2011; Liu et al., 2012; Lu et al., 2015). Red double circle stands for Gangcha meteorological station (GC). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in DW, with median grain size of 29–300 μm in ND and 38–247 μm in DW. According to Lu et al. (2011, 2015), aeolian sand and loess can be differentiated from paleosols on the basis of various proxy indicators. Aeolian material has relatively coarse mean particle size, low organic matter content, low MS, low Rb/Sr and low chemical index of alteration (CIA), while paleosols have higher organic carbon content, finer particle size, much higher MS and at least slightly higher Rb/Sr and CIA. The environmental significance of MS and GS in aeolian sediment is direct: MS can be used as a proxy indicator of the rate of pedogenesis (higher MS = greater soil development); and median GS indicates the intensity of aeolian activity (coarser median GS = more intense activity/stronger winds). Results of the OSL dating are summarized in Table 1. For the ND section, the basal age of aeolian sand that mantles the underlying gravels was determined as 16.5 ± 1.7 ka, which equates to the beginning of last deglaciation period, just after the Last Glacial Maximum. This result is consistent with the first luminescence age from the Qinghai Lake of 15.1 ± 1.6 ka (Porter et al., 2001), which was from an icewedge fill marking the end of wedge activity and the start of aeolian sedimentation. A Late Glacial age for the commencement of aeolian sedimentation has been identified in many other locations in the northeast TP, including the Gonghe Basin (Stauch et al., 2018), the Qaidam Basin (Yu and Lai, 2014), the Anyemaqen Shan range (Lehmkuhl et al., 2014), the central and western Qilian Shan area (Zhang et al., 2015) and the catchment of Lake Donggi Cona (Stauch et al., 2012). The radiocarbon age of the ND-ash sample at 260 cm depth was determined as 2950 ± 30 BP, 3001–3207 cal. BP. The calibrated radiocarbon age is in good agreement with the OSL age of the adjacent sample ND-265 (3.14 ± 20 ka), indicating the good reliability of the OSL ages. The OSL ages are generally consistent with stratigraphic position, except for several age inversions in both sections

dose recovery preheat plateau tests, the dose recovery ratios for the sample are within 10% of unity for the 160 to 300 °C preheat temperature intervals (Fig. 4B). Both the natural and dose recovery preheat plateaus indicated that the preheat and cut-heat temperature ranges of 160–300 °C and 160–280 °C were suitable for De measurement, and a routine preheat temperature of 260 °C and cut-heat temperature of 220 °C were employed for De measurements. Fig. 5 shows measured dose plotted against given dose for 16 randomly selected samples. All ratios lie within 10% of the slope of unity, indicating that the given dose is well recovered for these samples. Concentrations of U, Th and K were measured by neutron activation analysis in the Chinese Atomic Energy Institute in Beijing. All measurements were converted to beta and gamma dose rates using the conversion factors of Guérin et al. (2012). The cosmic-ray dose rate was calculated for each sample as a function of depth, altitude and geomagnetic latitude (Prescott and Hutton, 1994). Adjustment of the dry dose rate for aeolian sands and sandy paleosols was based on a longterm water content of 5% and 7%, respectively, estimated using previous estimates of water content and that of modern soil (QLA) (E et al., 2015; Liu et al., 2012). Radionuclide concentrations, water content and dose rates are summarized in Table 1. 5. Results Variations in the proxy indices measured in the laboratory are consistent with the field pedostratigraphical observations. In both the ND and DW sections, aeolian sand is characterized by a relatively coarse median GS and low MS, and the sandy paleosols have finer GS and higher MS (Figs. 2 and 3). MS varied from 7.97 to 48.91 × 10−8 m3 kg−1 (average 23.61 × 10−8 m3 kg−1) in ND and from 5.17 to 38.30 × 10−8 m3 kg−1 (average 16.91 × 10−8 m3 kg−1) 3

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Fig. 2. Stratigraphy and dating of the ND section, Qinghai Lake. From the left, the diagram shows: OSL ages, lithography, median grain size (Mz), low frequency magnetic susceptibility (LFMS) and clay/silt/sand percent.

the DW section, dated by the high resolution OSL sampling (15 cm intervals) to 7–4 ka (Fig. 6A). The internal stratigraphy and abrupt change in proxy indicators imply that the hiatus was due to post-depositional erosion. In the ND section, none of the luminescence dates fall into the period between 5.2 ka and 4.1 ka, even when one standard deviation errors are taken into account (Fig. 6A). There are no stratigraphic or proxy indicators to suggest any significant post-depositional erosion occurred. However, the gap in dates coincides with a peak in MS and a low in GS, implying low rates of aeolian activity. The lowest sand accumulation rate and strongest pedogenesis occurred between 5.2 ka and 4.1 ka. Thus, we conclude that the gap in OSL dates is due to the low accumulation rate, and that the ND section is overall successive on suborbital and millennial scales. Yang et al. (2018) reported similar low accumulation rates accompanied by paleosol formation between during the period 5–3 ka, based on OSL sampling (40 cm intervals) of a megadune in the eastern Qinghai Lake basin. The continuity of deposition in the ND section suggests it makes a suitable basis for reconstructing the broad trends of aeolian activity since the Late Glacial at Qinghai Lake. In order to construct a continuous paleoenvironment record, age-depth modeling was undertaken using the Bacon program to interpolate an

during the early Holocene, probably because of frequent dune reactivating and fixing under an unstable climate condition. Similar age inversions during the early Holocene were also observed at the BHPA section at Qinghai Lake (Lu et al., 2011). Both the ND and DW sections show similar trends of sand accumulation during the Holocene, with three distinct stages: rapid sedimentation in the early Holocene, very low sedimentation during the middle Holocene and rapid sedimentation in the Late Holocene. The ND section also records another period of rapid accumulation in the Late Glacial.

6. Discussion 6.1. Verification of accumulation continuity Verification of the continuity of sequences on different time scales is a vital prerequisite for reconstructing environmental changes. Hiatuses represent a lack of net accumulation due to a reduction in sediment deposition and/or an increase in the reworking or erosion of previously deposited units (Leighton et al., 2013). As this study focuses on suborbital and millennial scales, hiatuses smaller than 1 ka were not taken into account. A distinct accumulation hiatus of ~3 ka was identified in 4

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Fig. 3. Stratigraphy and dating of the DW section, Qinghai Lake. From the left, the diagram shows: OSL ages, lithography, median grain size (Mz), low frequency magnetic susceptibility (LFMS) and clay/silt/sand percent.

picked up in the Qinghai Lake PDF plots (Fig. 6B). This demonstrated the importance of employing a sufficiently high resolution OSL sampling strategy in order to accurately reflect variations in aeolian deposition in a region. Telfer and Thomas (2007) also emphasize that careful and systematic choice of sampling density, combined with high density sampling can minimize the potential impact of artifact from variations in sampling density in aeolian sections.

age for each of the 5 cm interval proxy data sample (Blaauw, 2010; Blaauw and Christen, 2013). The 95% confidence intervals in the agedepth model were estimated from Monte Carlo age-depth simulations. The age-depth model is shown in Fig. 6A. In order to produce a synthesis of aeolian activity at Qinghai Lake since the Late Glacial, aeolian sand OSL ages from previous studies in the region were collated and their PDFs plotted in Fig. 6B. The data comprise 32 ages from 11 sections at Qinghai Lake from Liu et al. (2012) and Lu et al. (2011, 2015) and 93 ages from 39 sections in the northeast TP, including Qinghai Lake, Gonghe Basin and Qaidam Basin, from Chen et al. (2016). Comparison of age clusters from the PDF plots with dates of sand accumulation in ND and DW (Fig. 6A and B) shows that both our high density OSL sampling and the relatively low density mutiple-sections at Qinghai Lake and the northeast TP follow similar trends in aeolian activity, with two major peaks in aeolian activity in the Late Glacial (~15–12 ka) and early Holocene (~10.5–7.0 ka). The PDF plots in Fig. 6B show a very limited number of OSL ages in the middle and Late Holocene, however, this period is well covered in the ND section which clearly records a rapid sand accumulation event at ~4 ka. Chen et al. (2016) reported a period of strong aeolian activity between ~1.5–0 ka in the northeast TP, and this is also indicated by both the OSL ages and proxy indicators for ND and DW, but it is not

6.2. Aeolian activity and climatic history at Qinghai Lake Fig. 7 plots MS, GS and Bacon age-depth modeling from the present study with a range of proxy indicators from previous studies as a basis for interpreting aeolian activity and climatic history since the Late Glacial. The proxy indicators comprise total pollen concentration, redness of lake sediments eroded from nearby red beds or loess deposits and are transported by fluvial means into the lake (Ji et al., 2005), and salinity from Sr/Ca data in ostracods, which are regarded as reliable indicators of moisture in the northeast TP, and paleoshorelines, which provide independent and direct geomorphic evidence of lake level (Chen et al., 2016). The reconstructed aeolian activity and climatic history can be divided into four stages, designed I to IV. Stage I: Late Glacial to Early Holocene (14–9 ka) (595–395 cm in 5

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due to meltwater from glaciers and permafrost (Liu et al., 2015). In the early Holocene, the lake level was about 20 m shallower than today, indicating much lower effective moisture (Yu, 2005). Stage II: Early to Middle Holocene transition (9–7 ka) (395–370 cm in ND). The lithostratigraphic unit in the ND section shifted from aeolian sand into sandy paleosols during this stage. From ~9 to ~7 ka, sand content rapidly decreased from 94 to 38% and median GS from 256 to 79 μm, while MS increased from ~11 to 39 × 10−8 m3 kg−1 (Fig. 7A and B). The DW section shows similar trends. The changes indicate a marked weakening of aeolian activity and local sand input in the early Holocene. Similar rapid transitions are observed in the salinity (decreasing) and redness (increasing) proxies (Fig. 7E and F). Lu et al. (2015) also reported weakened aeolian activity in the latter part of the early Holocene, due to a gradually warmer and wetter, but variable climate. The westerlies index reconstructed from the Qinghai Lake sediments also indicate a stronger westerlies during the early Holocene than the middle to late Holocene (An et al., 2012). Stage III: Holocene Climatic Optimum (7–4 ka) (370–340 cm in ND). From ~7 to ~4 ka, the sand accumulation rate was at its lowest, GS at its finest, mainly comprising silt, and MS stabilized around 40 × 10−8 m3 kg−1 (Fig. 7A, B and C), indicating a shift to stable warm and wet conditions. The paleoshoreline and salinity proxies are also consistent with a warm and wet climate. Fig. 7D shows the highest Holocene lake level of 9.1 m above present was attained at ~5.1 ka (Liu et al., 2015), and salinity fluctuated and decreased from ~7 ka to its lowest level at ~5.5 ka (Fig. 7E). In a multi-proxy study of Qinghai Lake, Yu (2005) suggested that the highest Holocene lake level of about 10 m above the present day occurred from ~5.5 ka to ~4 ka. Stage IV: Late Holocene (4–0 ka) (295–0 cm in ND). The lithostratigraphic unit in ND is composed of paleosol, weakly developed paleosol and aeolian sand. Frequent fluctuations in MS and GS indicate unstable climate conditions. The stage can be divided into two phases of climate fluctuation (4–2 ka) and enhanced aeolian activity (after 2 ka). Changes in a number of indicators are evident around 4 ka – a sharp decline in MS and coarsening of median GS (Fig. 7A and B), clustering of OSL ages (Fig. 6) and high aeolian sand accumulation rate (Fig. 7C) – all of which suggest a strengthening of aeolian activity and drier and colder climate conditions. A change in climate at this time is supported by other proxies. A shift to lower lake sediment redness around 4 ka (Fig. 7F) is consistent with a drier climate (Ji et al., 2005), and palaeoshoreline deposits, fluvial sediments and aeolian sands near the modern lake shore suggest that the level of Qinghai Lake fell by > 8 m after 3.7 ± 0.4 ka as a result of an increase in aridity (Liu et al., 2011, 2015). The timing of the shift to a drier climate at Qinghai Lake corresponds with the 4.2 ka abrupt global climate change event that was characterized by aridification and a predominant temperature drop (Wang, 2005). However, the impact of this cold and dry event appears to have been relatively limited at Qinghai Lake, with a gradual increase in MS and gradual decrease in median GS from 4 ka to 2 ka (Fig. 7A and B). Paleosol development continued in the ND and DW sections between 4 and 2 ka, but the sand accumulation rate was much higher than in the Middle Holocene. Previous studies have recorded similar paleosol development at other sections in the Qinghai Lake area at this time (Lu et al., 2011, 2015). After ~2 ka, MS decreased and median GS increased continuously, the sand accumulation rate increased to its highest level and pedogenic intensity was much weaker, indicating a further strengthening of aeolian activity and colder and drier climate conditions. Paleosol development ceased after ~1 ka as aeolian activity continued to increase, with median GS approaching the early Holocene level. The upturn in aeolian activity is supported by other studies and proxies. Increased probability of aeolian sand OSL ages was used by Chen et al. (2016) to infer renewed aeolian activity after ~1.5 ka BP in the northeast TP. Paleoshoreline evidence (Fig. 7D) suggests lake levels at Qinghai Lake declined from ~4 to ~3 ka, increased after ~3 ka, reached a Late Holocene high at ~2 ka and then declined continuously to the present level (Liu et al., 2015).

Fig. 4. OSL characteristics for coarse-grained quartz sample ND-265. (A) Growth curve and corresponding decay curve (inset). (B) Preheat plateau tests for equivalent dose De and dose recovery for temperatures between 160 and 300 °C at 20 °C intervals. Solid black circles represent the equivalent dose De and the black line is the average De; open blue triangles represent dose recovery ratio, dashed lines represent the range and error bars represent one standard error of 3 aliquots measured at each temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ND). The lithostratigraphic unit in the ND section is characterized by aeolian sand, and there is a peak in median GS, indicating that the strongest aeolian activity occurred during this period (Fig. 7A and B). Median GS fluctuates from 128 to 300 μm, indicating strong, variable and frequent wind conditions. MS is at its lowest in this period, varying between 8 and 19 × 10−8 m3 kg−1, indicating reduced pedogenesis. Both the low MS and high median GS indicate dry climate conditions from the Late Glacial to Early Holocene; this is consistent with previous studies of aeolian deposits, indicating low effective moisture, sparse vegetation and strong aeolian activity (Chen et al., 2016; Lu et al., 2011, 2015). Low redness and high salinity (Fig. 7E and F) strongly indicate very low river recharge and low lake level. Yu and Kelts (2002) found that Qinghai Lake was very shallow before 11,600 14C yrs BP, with carbonate production and organic productivity much lower than in the Holocene, suggesting a colder and drier climate. Shoreline evidence of Qinghai Lake highstands in the Late Glacial (Fig. 7D) may be 6

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Overall, the reconstructed Holocene environmental record from the ND and DW sections is consistent with previously published aeolian and paleoshoreline records in the Qinghai Lake area, demonstrating consistency by the different types of OSL-dated terrestrial evidence. However, less consistency was found when these records are compared to proxy indices from the lacustrine sediments, which represent the aquatic environment with salinity (Fig. 7E), redness (Fig. 7F), and especially total pollen count (Fig. 7G) showing very different trends at the millennial scale. The inconsistencies may be due to different LRE ages that complicate interpretation of radiocarbon ages from lake sediments (Chen et al., 2016), and different temporal resolution between aeolian and lacustrine sediments. Furthermore, the number of high resolution OSL-dated sections is still limited, therefore suggesting an urgent needs for more aeolian sections with high resolution OSL dating.

6.3. Comparison with other records in the northeast TP In order to obtain a broad trend of aeolian activity and climatic change in the northeast TP, the ND records and aeolian records at Qinghai Lake were compared with other regions in the northeast TP, especially the Gonghe basin, where a large number of OSL dates have been produced. The previously published 43 dates from aeolian sand and 24 from the sandy paleosol in the Gonghe Basin were summarized after Stauch et al. (2018). Similarly, dates from the Qinghai Lake area, 43 from aeolian sand and 33 from sandy paleosol, were also included.

Fig. 5. Given doses versus measured doses for 16 randomly selected samples. The solid black line is the slope of unity, where the ratio is 100%, and the dashed lines denote 10% above and below the line. Error bars represent one standard error.

Table 1 Characteristics of OSL dating samples from the ND and DW sections at Qinghai Lake. The number of aliquots used for determination of the equivalent dose De is given as a subscript in parentheses. Sample-No

Depth (cm)

Water content (%)

Grain size (mm)

K (%)

ND-40 ND-70 ND-100 ND-130 ND-160 ND-170 ND-180 ND-190 ND-220 ND-235 ND-250 ND-265 ND-280 ND-295 ND-315 ND-330 ND-345 ND-375 ND-395 ND-415 ND-445 ND-475 ND-505 ND-535 ND-565 ND-595 DW-15 DW-30 DW-45 DW-60 DW-75 DW-90 DW-105 DW-120 DW-135 DW-150 DW-175 DW-205 DW-235 DW-265 DW-295

40 70 100 130 160 170 180 190 220 235 250 265 280 295 315 330 345 375 395 415 445 475 505 535 565 595 15 30 45 60 75 90 105 120 135 150 175 205 235 265 295

5 5 5 7 7 5 7 7 5 5 7 5 7 7 5 7 7 7 7 5 5 7 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125 90–125

1.47 1.57 1.43 1.79 1.57 1.86 1.67 1.80 1.78 1.65 1.64 1.58 1.53 1.66 1.68 1.66 1.67 1.47 1.44 1.41 1.41 1.42 1.48 1.50 1.46 1.56 1.96 1.92 1.96 1.90 2.02 1.89 1.87 1.99 1.80 1.83 1.75 1.86 1.79 1.89 1.88

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

U (ppm) 0.05 0.05 0.05 0.06 0.05 0.06 0.06 0.06 0.06 0.05 0.05 0.05 0.05 0.05 0.06 0.05 0.06 0.05 0.05 0.05 0.05 0.05 0.10 0.11 0.10 0.11 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04

1.57 1.75 1.49 1.91 2.32 2.42 2.16 2.27 2.56 2.21 2.20 2.04 1.89 2.14 2.45 2.40 2.58 1.48 1.45 1.42 1.38 1.48 1.13 1.29 1.15 1.30 1.89 1.91 1.64 1.46 1.47 1.56 1.54 1.63 1.60 1.43 1.26 0.99 0.92 0.88 1.10

7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.07 0.08 0.07 0.08 0.09 0.10 0.09 0.09 0.10 0.09 0.09 0.09 0.08 0.09 0.10 0.10 0.10 0.07 0.07 0.07 0.07 0.07 0.06 0.06 0.06 0.07 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.3

Th (ppm)

Dose rate (Gy/ka)

De (Gy)

Age (ka)

8.02 ± 0.25 8.86 ± 0.27 7.92 ± 0.25 10.5 ± 0.29 10.4 ± 0.29 13.6 ± 0.37 11.6 ± 0.32 12.2 ± 0.34 13.2 ± 0.36 10.8 ± 0.30 11.1 ± 0.31 10.8 ± 0.30 9.24 ± 0.28 10.6 ± 0.30 11.3 ± 0.32 10.9 ± 0.30 11.0 ± 0.31 6.88 ± 0.23 6.88 ± 0.23 6.87 ± 0.23 5.78 ± 0.20 5.85 ± 0.20 5.35 ± 0.16 5.82 ± 0.17 5.23 ± 0.16 5.57 ± 0.17 10.44 ± 0.7 11.40 ± 0.7 8.72 ± 0.6 8.13 ± 0.6 8.62 ± 0.6 7.93 ± 0.6 7.66 ± 0.5 9.04 ± 0.6 8.26 ± 0.6 7.35 ± 0.5 5.68 ± 0.5 5.66 ± 0.5 5.61 ± 0.5 5.18 ± 0.5 5.59 ± 0.5

2.63 2.73 2.51 3.04 2.90 3.47 3.03 3.21 3.37 3.01 2.96 2.90 2.66 2.91 3.10 2.98 3.02 2.34 2.30 2.31 2.22 2.20 2.19 2.27 2.16 2.30 3.37 3.36 3.14 2.99 3.12 2.96 2.91 3.13 2.88 2.81 2.58 2.61 2.52 2.57 2.62

1.38 ± 0.25(13) 1.74 ± 0.29(12) 2.91 ± 0.70(12) 4.34 ± 0.36(17) 3.26 ± 0.32(16) 5.16 ± 0.37(13) 6.78 ± 0.79(16) 7.01 ± 1.20(12) 7.66 ± 0.43(18) 10.71 ± 0.50(13) 11.31 ± 0.36(15) 9.12 ± 0.37(17) 8.46 ± 1.00(18) 11.08 ± 0.67(15) 17.33 ± 0.66(18) 21.63 ± 1.21(18) 26.38 ± 0.58(14) 21.64 ± 0.80(17) 18.84 ± 1.26(17) 23.73 ± 1.39(17) 20.02 ± 1.19(16) 29.30 ± 2.69(15) 26.46 ± 1.47(18) 30.22 ± 1.76(17) 29.62 ± 2.17(17) 37.93 ± 3.25(12) 1.41 ± 0.25(24) 2.03 ± 0.11(17) 3.02 ± 0.16(23) 5.64 ± 0.58(18) 4.78 ± 0.12(23) 5.14 ± 0.49(18) 8.48 ± 0.44(18) 11.11 ± 1.19(18) 25.50 ± 1.62(17) 21.24 ± 2.28(16) 21.72 ± 1.12(12) 20.02 ± 2.16(12) 24.00 ± 2.63(12) 24.09 ± 2.27(12) 25.34 ± 3.24(12)

0.52 ± 0.10 0.64 ± 0.11 1.16 ± 0.28 1.43 ± 0.14 1.13 ± 0.12 1.49 ± 0.13 2.24 ± 0.28 2.19 ± 0.39 2.27 ± 0.17 3.55 ± 0.24 3.83 ± 0.22 3.14 ± 0.20 3.18 ± 0.41 3.80 ± 0.30 5.59 ± 0.35 7.27 ± 0.54 8.73 ± 0.47 9.25 ± 0.57 8.19 ± 0.68 10.28 ± 0.79 9.03 ± 0.70 13.31 ± 1.39 12.08 ± 0.97 13.32 ± 1.10 13.74 ± 1.29 16.51 ± 1.72 0.42 ± 0.08 0.61 ± 0.04 0.96 ± 0.07 1.89 ± 0.21 1.53 ± 0.08 1.74 ± 0.19 2.92 ± 0.21 3.55 ± 0.42 8.84 ± 0.71 7.57 ± 0.89 8.43 ± 0.60 7.66 ± 0.91 9.51 ± 1.14 9.36 ± 0.99 9.68 ± 1.33

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.11 0.12 0.11 0.13 0.13 0.16 0.13 0.14 0.15 0.13 0.13 0.13 0.12 0.13 0.14 0.13 0.14 0.10 0.10 0.10 0.10 0.10 0.12 0.12 0.12 0.13 0.15 0.15 0.14 0.13 0.14 0.13 0.13 0.14 0.13 0.13 0.12 0.12 0.11 0.12 0.12

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Fig. 6. Aeolian sand and paleosol OSL ages from the Qinghai Lake area, the Gonghe Basin and the northeast TP. (A) Aeolian sand and paleosol OSL agedepth relationships for the DW and ND study sections. Error bars represent one standard error. Median ages (yellow line) and 95% probability age range (gray shading) for the ND are based on a Bacon age-depth model. (B) Probability density function (PDF) plot of previously published OSL ages from the Qinghai Lake area and the northeast TP, based on data from Chen et al. (2016), Liu et al. (2012) and Lu et al. (2011, 2015). (C) Circles: aeolian sand OSL ages; (D) Triangles: sandy paleosol OSL ages. OSL data in the Gonghe Basin are from Qiang et al. (2013, 2016), Liu et al. (2013), Stauch et al. (2018); OSL data in the Qinghai Lake area are from Lu et al. (2011), Liu et al. (2012) and Lu et al. (2015). Green shading represents intervals of strong aeolian sand accumulation and the light blue shading represents renewed aeolian sand activity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

accumulation of aeolian sand from 7.5 to 2.5 ka in the Gonghe Basin (Stauch et al., 2018). Markedly reduced aeolian sand accumulation from 6.4 to 3.6 ka was observed in the eastern Qaidam Basin (Yu and Lai, 2014). With the reduction of the aeolian sand accumulation, sandy paleosols developed. Clustered OSL dates from sandy paleosols concentrated during 6 to 2.5 ka in the Gonghe Basin and the Qinghai Lake area (Fig. 6D). In ND section, high values of MS indicate a strong pedogenesis process during 7 to 4 ka, but with high fluctuation during 4 to 2 ka. In the eastern Gonghe Basin, warm and wet conditions based on several proxies from the peat deposit were recorded during 7.1to 3.8 ka (Liu et al., 2014). The Lake Donggi Cona pollen record reveals a wet period between 7 and 4.5 ka (Wang et al., 2014b). Two mid-Holocene paleosols that date to about 8.5 ka to 7 ka and ~5.5 ka to 4 ka are recorded at SHD section in the Anyemaqen Shan, ~300 km to the south of the Qinghai Lake, reflecting more humid climate during the mid-

All aeolian records at different basins in the northeast TP show a generally consistent trend. In the Qinghai Lake area, both ND section and clustered OSL dates indicate an enhanced sand accumulation started at least before 14 ka, and lasted until 7 ka (Fig. 6C). Similarly, enhanced accumulation of aeolian sand started at 12 ka, and lasted until 7.5 ka in the Gonghe Basin (Fig. 6C, Stauch et al., 2018). In the Qaidam Basin, enhanced aeolian accumulation took place from 13.1 ka to 6.4 ka (Yu and Lai, 2014; Stauch, 2015). The timing of the highest rates of sand accumulation in the Late Glacial and Early Holocene is consistent with other records in the regions (Stauch, 2015). Sporadic sandy paleosols developed during this period (Fig. 6D), which may be attributed to either occasionally wet and variable climatic conditions or localized topographic changes. Less aeolian sand accumulation was recorded during 7.5to 2.5 ka in the Qinghai Lake area (Fig. 6C). There was only an occasional 8

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Fig. 7. Comparison of environmental proxy indicators for the ND and DW sections at Qinghai Lake (QHL) with previously published paleoenvironmental records. Interpolated ages for proxy data in the DW section were obtained using the BACON program with upper (0–130 cm) and lower (135–295 cm) segmentations. (A) Low frequency magnetic susceptibility (LFMS) at the ND and DW sections; (B) Median grain size (Mz) at the ND and DW sections; (C) Deposition rates at the ND and DW sections; (D) Reconstructed lake level based on paleoshorelines (Liu et al., 2015); (E) Salinity changes (Zhang et al., 1989); (F) Redness from core QH-2000 (Ji et al., 2005); (G) Total pollen percent from core QH-2000 (Shen et al., 2005).

(Stauch et al., 2018). In addition, Chen et al. (2016) reported a period of strong aeolian activity during ~1.5 to 0 ka in the northeast TP. Thus, the heterogenous reactivation time of aeolian activity in the northeast TP may be related to the stronger human influence during late Holocene. Archeological evidence in the northeast TP indicates that permanent settlement above 3000 m in elevation was established after 3.6 ka, with the introduction of an agro-pastoral economy facilitating year-round habitation at higher altitudes (Chen et al., 2015). Intensive grazing could lead to destruction of the vegetation cover, thereby enhancing aeolian entrainment (Schlütz and Lehmkuhl, 2009). Although the total area of cultivation on the TP was limited, the potential destruction was likely greater.

Holocene (Lehmkuhl et al., 2014). In summary, a broadly consistent humid climate condition with intensified soil development occurred during ~7 to ~4 ka in the northeast TP. The time of resumed aeolian activity differs at different basins in the northeast TP during late Holocene. In the Qinghai Lake area, the number of OSL dates is very limited during late Holocene compared to the Gonghe Basin (Fig. 6C). But in ND section, the sand accumulation rate increased to its highest level, and pedogenic intensity indicated by MS was much weaker after ~2 ka. According to Stauch (2015), aeolian accumulation of sand resumed at around at 1.9 ka at the Qinghai Lake, 3.6 ka in the Qaidam Basin and 3.2 ka at the Donggi. At the eastern side of the Gonghe basin, a general increase in aeolian accumulation started from around 2.5 ka, including the reactivation of aeolian fine sand 9

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7. Conclusions

measurements. Radiat. Meas. 37, 161–165. E, C.Y., Lai, Z.P., Hou, G.L., Cao, G.C., Sun, Y.J., Wang, Y.X., Jiang, Y.Y., 2015. Age determination for a Neolithic site in northeastern Qinghai-Tibetan Plateau using a combined luminescence and radiocarbon dating. Quat. Geochronol. 30 (Part B), 411–415. E, C.Y., Sun, Y.J., Liu, X.J., Hou, G.L., Lv, S.C., Sun, M.P., 2018a. A comparative study of radiocarbon dating on terrestrial organisms and fish from Qinghai Lake in the northeastern Tibetan Plateau, China. The Holocene 28. https://doi.org/10.1177/ 0959683618788671. E, C.Y., Sohbati, R., Murray, A.S., Buylaert, J.P., Liu, X.J., Yang, L., Yuan, J., Yan, W.T., 2018b. Hebei loess section in the Anyemaqen Mountains, northeast Tibetan Plateau: a high-resolution luminesecence chronology. Boreas 47, 1170–1183. https://doi.org/ 10.1111/bor.12321. Guérin, G., Mercier, N., Nathan, R., Adamiec, G., Lefrais, Y., 2012. 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In this study, high density OSL dating of aeolian sand from two sections (DW and ND) around the Qinghai Lake was employed for the first time to obtain high resolution chronological frameworks. Both sections showed a similar trend of sand accumulation: rapid sedimentation during ~10.5–7.5 ka and after ~4 ka, with a very low accumulation rate during the middle Holocene (7–4 ka). The overall sand accumulation trend is consistent with previously published ages based on low resolution OSL sampling at different sections. Careful and systematic choice of sampling density, combined with high density sampling, can minimize the potential impact of artifact due to variations in sampling density and reflect genuine variations in regional aeolian deposition. A 3 ka accumulation hiatus from ~7 to ~4 ka was identified in the DW section, but the ND section was found to be overall successive since the Late Glacial (~14 ka) at millennial scale. The reconstructed environmental record for the ND section is consistent with previous aeolian sand records and paleoshoreline evidence. The strongest sand accumulation occurred during~14 to ~9 ka, pedogenesis was initiated from ~9 to ~7 ka, soil development intensified between ~7 ka and ~4 ka (with the most intense pedogenesis and least aeolian activity from 6 to 4.5 ka), paleosol formation was relatively weak at ~4 to ~2 ka and aeolian activity renewed after ~2 ka. This broad trend of aeolian activity and climatic change in the northeast TP is consistent before 4 ka, and composed of a strong aeolian activity period during late Glacial to ~7 ka and an intensified pedogenesis during ~7 to ~4 ka. In contrast, during late Holocene, the reactivated time of aeolian sand is heterogenous in different basins. Acknowledgements This study was supported by National Natural Science Foundation of China (NSFC) grant (grant number 41761042), NSF grant of Qinghai Province (grant number 2017-ZJ-901), Chinese Academy of Sciences (CAS) "Light of West China" Program. We thank Yuan Jie, Li Fan and Yang Long for his help in the field. We would like to thank Drs. Barbara Rumsby and Miao Xiaodong for correcting the English and constructive suggestions. Special thanks to anonymous reviewers and editors whose constructive suggestions and detailed comments helped to clarify and improve the paper. References An, Z., Colman, S.M., Zhou, W., Li, X., Brown, E.T., Jull, A.T., Cai, Y., Huang, Y., Lu, X., Chang, H., 2012. Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka. Sci. Rep. 2, 1–7. Ballarini, M., Wallinga, J., Wintle, A.G., Bos, A.J.J., 2007. A modified SAR protocol for optical dating of individual grains from young quartz samples. Radiat. Meas. 42, 360–369. Blaauw, M., 2010. Methods and code for ‘classical’ age-modelling of radiocarbon sequences. Quat. Geochronol. 5, 512–518. Blaauw, M., Christen, J.A., 2013. Bacon Manual V2. 2. (11 pp.). Bøtter-Jensen, L., Thomsen, K.J., Jain, M., 2010. Review of optically stimulated luminescence (OSL) instrumental developments for retrospective dosimetry. Radiat. Meas. 45, 253–257. Buylaert, J.P., Murray, A.S., Vandenberghe, D., Vriend, M., Corte, F.D., Haute, P.V.D., 2008. Optical dating of Chinese loess using sand-sized quartz: establishing a time frame for Late Pleistoceneclimate changes in the western part of the Chinese Loess Plateau. Quat. Geochronol. 3, 99–113. Chen, F.H., Dong, G.H., Zhang, D.J., Liu, X.Y., Jia, X., An, C.B., Ma, M.M., Xie, Y.W., Barton, L., Ren, X.Y., Zhao, Z.J., Wu, X.H., Jones, M.K., 2015. Agriculture facilitated permanent human occupation of the Tibetan Plateau after 3600 BP. Science 347, 248–250. Chen, F.H., Wu, D., Chen, J.H., Zhou, A.F., Yu, J.Q., Shen, J., Wang, S.M., Huang, X.Z., 2016. Holocene moisture and East Asian summer monsoon evolution in the northeastern Tibetan Plateau recorded by Lake Qinghai and its environs: a review of conflicting proxies. Quat. Sci. Rev. 154, 111–129. Cunningham, A.C., Wallinga, J., 2010. Selection of integration time intervals for quartz OSL decay curves. Quat. Geochronol. 5, 657–666. Dong, Z.B., Hu, G.Y., Qian, G.Q., Lu, J.F., Zhang, Z.C., Luo, W.Y., Lyu, P., 2017. Highaltitude aeolian research on the Tibetan Plateau. Rev. Geophys. 55, 864–901. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence

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