Quaternary Science Reviews 200 (2018) 65e84

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Quaternary Science Reviews

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A 14 ka high-resolution d18O lake record reveals a paradigm shift for the process-based reconstruction of hydroclimate on the northern Tibetan Plateau

* ** Bernd Wünnemann a, b, c, , Dada Yan a, c, , Nils Andersen d, Frank Riedel c, Yongzhan Zhang b, Qianli Sun a, Philipp Hoelzmann c a State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan North Road, Shanghai, 200062, China b School of Geographic and Oceanographic Sciences, Nanjing University, 163 Xianlin Ave., Nanjing, 210023, China c Department of Earth Sciences, Freie Universitat€ Berlin, Malteserstr. 74-100, 12249, Berlin, Germany d Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, Christian Albrechts Universitat,€ Max-Eyth-Str. 11, 24118, Kiel, Germany article info abstract

Article history: The influence of the mid-latitude westerlies (MLW) competing with the Asian summer monsoons (ASM) Received 1 May 2018 over the Tibetan Plateau (TP) remains a matter of discussion on how and to which extent both atmo- Received in revised form spheric systems have been controlling hydro-climate during the Holocene. Depleted oxygen isotopes in 26 September 2018 lake deposits were commonly interpreted in terms of enhanced summer monsoon moisture supply, Accepted 28 September 2018 implying a migration of the ASM deep into the interior of the plateau during Holocene periods. In order Available online 6 October 2018 to test this relationship we used a high resolution oxygen isotope record (mean 20 yr resolution) in combination with carbonates and mineral phases, titanium flux, grain size and ostracod abundances Keywords: Holocene derived from a 6.84 m long sediment core in the endorheic Kuhai Lake basin, north-eastern TP. The fi d18 Paleoclimatology results con rm 1) continuous positive co-variance between enriched Ocarb and total carbonates during China the last 14 ka, indicative of dominant seasonal influence on multi-decadal to centennial scale isotopic Sedimentology signatures in lake water and respective carbonate precipitation, 2) negative co-variance between 18 Lakes allochthonous sediment flux and d Ocarb (and carbonates) attributed to relative increase of flux rates Oxygen isotopes during non-summer seasons, 3) correspondence of lake level variations with carbonate mineral phases Hydroclimate and the occurrence/disappearance of ostracod assemblages, and 4) inverse relationships between iso- Westerlies 18 topic signatures in ASM-dominated and MLW-controlled lake records across the TP. Enriched d Ocarb in Asian summer monsoon Kuhai Lake sediments was primarily a result of high evaporation during the summer seasons, while ASM- related rainfall amount did not play an important role, likely counterbalanced by isotopic signatures from 18 different water sources. Conversely, depleted d Ocarb was mainly attributed to water supply during non- summer seasons of colder temperatures and generally light isotopic signatures from MLW-derived sources. This finding may lead to a paradigm shift in such way that depleted d18O in carbonates is pri- marily not the result from ASM-related rainfall as previously assumed. The reconstructed hydro-climatic history of Kuhai Lake indicates the dominance of westerly-derived climate during the Younger Dryas interval (12.8e11.5 ka) under very shallow pond-like conditions. Despite climate amelioration during the early Holocene (11.5e7.5 ka) hydrological conditions remained unstable with frequent alternations be- tween dominance of summer and winter seasons. During the middle Holocene (7.5e5.5 ka) the lake experienced highest lake levels dominated by summer monsoon-related water supply, assigned to the Holocene hydro-climatic optimum. Frequent high-amplitude fluctuations afterwards (5.1e2.9 ka) refer to cooling/drying events under enhanced MLW influence accompanied by a strong lake level decline. The

* Corresponding author. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan North Road, Shanghai, 200062, China. ** Corresponding author. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan North Road, Shanghai, 200062, China. E-mail addresses: [email protected] (B. Wünnemann), ddyan@sklec. ecnu.edu.cn (D. Yan). https://doi.org/10.1016/j.quascirev.2018.09.040 0277-3791/© 2018 Elsevier Ltd. All rights reserved. 66 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84

late Holocene (2.9 ka- Present) period experienced moderate isotopic variations and fluctuating lake levels in response to variable influence of summer- or winter-related hydro-climatic conditions. 18 This seesaw-like pattern with amplitudes of >10‰ in d Ocarb resembles fluctuations in cave records and variations between air and seawater (Dole effect). High correspondence with cooling events derived from North Atlantic drift ice and meltwater discharge indicate close ties to northern hemispheric climate transmitted by the MLW across the TP. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction Yao et al., 2013; Yang and Yao, 2016; Li and Garzione, 2017) already indicate regional differences under modern climate settings and The Asian monsoon system (winter and summer monsoon) in- also during the Holocene (Rao et al., 2016a). They highlight the teracts with the mid-latitude westerlies over large parts of China. importance of seasonality and type of precipitation that control this Both are considered the most prominent atmospheric systems spatial pattern over the entire TP (Yao et al., 2013; Curio and which control climate in China and adjacent regions of Asia (Wang Scherer, 2016; Li and Garzione, 2017). A closer relationship be- 18 et al., 2001; Hu et al., 2008; Liu et al., 2014, Fig. 1). An increasing tween modern d Op and annual temperature variations apart from number of speleothem d18O records along the trajectories of the summer monsoon-affected sites were reported for the arid/semi- ASM show a close relationship with changes in rainfall intensity arid regions of the TP and adjacent regions (Pang et al., 2011; Yao (precipitation amount) in response to orbital- and millennial-scale et al., 2013, Fig. 1A). variations of the northern hemisphere insolation, ice volume and A comparison of d18O and d13C in lake records from different associated northern high-latitude climate, and sea level changes parts of the world (Horton et al., 2016) shows, that their in- (An et al., 2012; Cheng et al., 2016). terpretations cannot be simply transferred to all lake systems. Several past and recent studies proposed a migration of the However, the authors highlight the importance of hydrological Asian summer monsoon deep into the interior of the plateau during balance effects in lake systems by evaporation-induced enrichment the early and middle Holocene (Gasse et al., 1991, 1996; Yao et al., of isotopes in response to changes in the precipitation/evaporation 1997; Hou et al., 2017; Qiang et al., 2017) followed by a general ratio (P/E), as also discussed in detail by several other authors decline of summer monsoon strength after about 6 ka, in accor- (Talbot, 1990; Fontes et al., 1996; Leng and Marshall, 2004). dance with the northern hemisphere insolation trend (e.g., Wanner Although it is known that isotopic fractionation in lake water is a et al., 2008). Such millennial-scale relationships have been dis- complex interplay between different processes (Leng and Marshall., cussed in light of sedimentary sequences from terrestrial and ma- 2004; Zhang et al., 2011), the timing and sources of precipitation, rine records (An et al., 2012; Nagashima et al., 2013; Chen et al., related temperature and geographical location in addition to P/E 2015). Most of them using d18O and partly also carbonates from ratios are important factors (Zhang et al., 2011 and references lakes among other proxies, were interpreted in terms of enhanced therein) that might be still underestimated in respect to different summer monsoon influence during this time span (e.g., Lister et al., regions on the TP. 1991; Gu et al., 1993; Gasse et al., 1996; Wei and Gasse, 1999; So far, high resolution isotopic records from lake sediments Morrill et al., 2006; An et al., 2012), independent of regional covering the entire Holocene hydro-climatic history of the TP and different hydro-climatic conditions and almost in line with adjacent regions are still very rare and do not discuss potential monsoon records derived from isotopic signals preserved in spe- seasonality effects on isotopic signatures in lake carbonates as a leothems. Hence, depleted d18O signals in cave records as indicators response to changes in the timing and source of precipitation be- for ASM strength were consequently transferred to other sites on side an overall evaporation background process within local to the TP (Liu et al., 2007; Mischke et al., 2008; Zhang et al., 2011; An regional hydro-climatic settings. Hence, our aim was to test and et al., 2012; Li et al., 2017). possibly modify the influence of the interplay between the ASM Some records close to the boundary of summer monsoon impact and MLW on the northern part of the TP by comparing highly (transitional zone) and along the north-eastern monsoon realm of resolved proxy data and involved processes with other records China, mainly controlled by the East Asian summer monsoon from the larger region. (EASM), however, suggest a shift of maximum effective moisture We selected the hydrologically closed Kuhai Lake on the north- supply from the early to the middle Holocene (e.g., An et al., 2000; eastern TP at the boundary of modern summer monsoon influence Chen et al., 2008, 2015; Rao et al., 2016b), indicating a delayed and MLW impact as an ideal location for detecting past climatic response to insolation forcing (Zhang et al., 2017). influence by both atmospheric systems. Recent hydro-climatic in- Many interpretations lack an in-depth discussion of implicit and terpretations based on records from this lake provided different interacting processes which are fundamental preconditions for the results in respect of timing and sediment sources, vegetation his- understanding of hydro-climatic variations over longer time scales, tory and stable oxygen isotopes (Mischke et al., 2010; Wischnewski thus limiting a simple transfer of isotopic signals in cave records to et al., 2011; Li et al., 2017) that encouraged us to reconcile climate other sites of the TP. Some researchers already noticed in- impact by process-based studies. High-resolution stable oxygen consistencies in the interpretation of stable oxygen isotopes in lake isotopes from authigenic carbonates in combination with other records and argued that perhaps not all depleted d18O values can be selected sediment proxies are considered most important tracers fully assigned to summer monsoon rainfall amount (Henderson for the detection of hydro-climatic regimes in the local catchment et al., 2010; Qiang et al., 2017) when considering local to regional and adjacent regions during the past 14 ka. conditions. Observational and modelling results on spatial distribution 2. Study site 18 patterns of oxygen isotopes in modern precipitation (d Op)in China combined with the detection of potential water vapour Lake Kuhai, a closed saline lake (approx. 49 km2 in area, sources and related seasonal rainfall patterns (Dayem et al., 2010; maximum water depth of 22.3 m and ~13.6 psm), located in the B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 67

Fig. 1. Major atmospheric systems, climate domains in China (A), Lake Kuhai site (B) and lake basin characteristics (C). Coloured arrows: AWM ¼ Asian winter monsoon (dotted blue), ISM ¼ Indian summer monsoon (red), EASM ¼ East Asian summer monsoon (orange); major position of westerly jet in summer (blue) and winter (dotted blue); the modern boundary of summer monsoon influence (dotted black lines) derived and modified after Yao et al. (2013) and Chen et al. (2015). Numbers indicate selected locations within the monsoon domains (red dots), transitional zone (yellow dots) and westerly domain (blue dots). Squares mark locations mentioned in the text and in Fig. 9:A¼ Kuhai Lake; B¼Hala Lake; C ¼ Manas Lake; D ¼ Sumxi Co; E ¼ Bangong Co; F¼ Seling Co; G ¼ Koucha Lake and H ¼ Dongge Cave. Numbered sites are listed in Table S1 (suppl. information.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 68 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 north-eastern part of the TP at 4132 m above sea level (Fig. 1) and composition in core KH17 was similar to the other cores, except the surrounded by a non-glaciated catchment of 712 km2 was selected lack of an algae layer at 10e12 cm sediment depth, we applied the for this study. The lake water is stratified during the spring and results from KH15 (nearest to KH17 site, Fig. 1B) for calculating the summer season with a thermocline at 7e9 m water depth. Surface depth for zero BP (1950 AD) and inherent reservoir errors (RE) water reaches temperatures of up to 15 C during summers while down-core. Radionuclide dating from cores KH14 and KH17 was water temperatures below the thermocline decreased to not possible because the obtained material was extremely soft 2.5e0.8 C. Bottom waters remained oxygenized. Several rivers (highest water content of up to 80% in the upper 10 cm) resulting in enter the lake of which the majority is episodically active in sum- too less material for multiple sediment analyses (KH14) or was just mer (Fig. 1). The lake is frozen between December and March. missing (KH17). Permafrost is widely distributed in the entire catchment, thus Thirty-two radiocarbon AMS ages for cores KH14/17 from plant limiting groundwater discharge. The lake is surrounded by several remains and various organic fractions (bulk, alkali-soluble and paleo-shorelines, of which the highest one was found ca. 8e10 m insoluble fractions) on selected 1-cm sediment slices down-core above the present lake level (Mischke et al., 2010). A former outflow were provided by Beta Analytics, U.S. Results are listed in Table S2 was neither reported nor detected during our intensive morpho- (supplement). All ages revealed a RE calculated on the base of logical studies. 210Pb/137Cs results and linear regression between dated samples, Only 57% of the modern mean annual precipitation (Maduo resulting in a mean RE of 2495 years for core KH17. A regression up station, 120 km further south: ~332 mm, period: 1976e2010, http:// to the core top as commonly used in other age-depth models was www.cma.gov.cn.) falls during the major monsoon period between avoided because it would imply that the upper sediments had the July and mid of September, while 43% is related to off-monsoon same sediment density (compaction coefficient) as down core, precipitation mainly as snowfall in early spring and late autumn. which was not the case. Hence, linear regression to zero cm depth Annual mean evaporation of about 1000 mm exceeds the precipi- would result in a reduction of the RE by about 130e140 years, not tation by about three times. Mean annual air temperature is 3.3 C consistent with all radionuclide time series for the upper 10e15 cm with a maximum of ~14 C in July. Low temperatures, close to or of the cores and also not supported by sediment composition. In below zero C, commonly occur between September and April in order to consider the extreme low compactions in all upper cores this region. Vegetation in this high altitude region consists of sparse with a mean resolution of 7 yr cm 1 based on 137Cs data (for the grassland used as pasture by local residents. down-core part: mean 20 yr cm 1), the 210Pb/137Cs age (1950 AD) at 10 cm sediment depth and the respective RE seem reasonable. 3. Methods Previous results on a sediment core from the same lake based on a combination of different organic fractions for AMS dating (Mischke 3.1. Coring and surface sampling et al., 2010) were based on a much lower mean RE and thus pro- vided a chronology which differed from the KH17 record. The final Sediment coring in Kuhai Lake was performed by a UWITEC chronology of core KH17 converted to calibrated ages was derived short coring device using 3 m long PVC liners and a UWITEC90 from the R algorithm in Bacon (Blaauw and Christen, 2011) and is coring system in overlapping (1 m) drilling technique from a reported in ka. floating platform including a fixed ground plate with funnel that guarantees coring within the same borehole and a fixed position for 3.3. Stable isotopes (d18O, dD, d13C) unleashing the piston. In total, 6 sediment cores were obtained from various sites and water depth within the lake. Cores KH14 and Water samples for the determination of stable isotopes (d18O, KH17 were taken from the deepest part of the lake (20.6 m water dD) were analysed at the School of Geological and Engineering depth) at the same site (Fig. 1B, Table S3, supplement). They were Sciences, Nanjing University and NIGLAS (Fig. 4). cut lengthwise and sampled at 1 cm-intervals for further analysis. Measurement of the d18O values from bulk carbonate in the Core KH14 (2.84 m length) and KH17 (2 m individual lengths) were KH17 sediments was performed at the Leibniz Laboratory for spliced toward a composite core by visual inspection and high- Radiometric Dating and Stable Isotope Research, CAU Kiel. Car- resolution (1 cm) geochemical data (Fig. S1, supplement) with a bonates identified by XRD analyses were used to adjust the reaction final length of 6.84 m, free of any sediment gap. Further sampling time of individual samples for isotopic determination. Possible and analyses also included overlapping parts to evaluate the ac- allochthonous carbonates from the catchment remained on a very curacy of splicing but final results of proxies refer to the composite low level (<5%) as revealed for fluvial sediments and aeolian de- core KH17. In addition, 102 surface samples from the entire lake posits in off-shore regions of the lake and thus did not significantly were collected by a grab or short coring device. Only the upper 2 cm affect the overall isotopic variations. of sediment was subject to measurements of modern spatial grain Based on semi-quantitative X-ray diffractometric (XRD) results size distribution, geochemical compounds, minerals, stable iso- from the core (2 cm resolution on average), several carbonate topes and ostracod assemblages. Here we only report the spatial mineral phases could be identified: calcite, monohydrocalcite 18 distribution pattern of stable oxygen isotopes (d O). Measure- (MHC), high-magnesian calcite (>4 mole% MgCO3: HMC), dolomite ments of inflowing river water, ponds from the catchment, rain/ (Dol) and aragonite (Ara). The former two minerals dominated the snowfall events and lake water during the field seasons in bulk samples, while aragonite only occurred occasionally in low 2014e2016 included vertical profiles of dissolved oxygen (DO), amounts, except for the period between 570 and 410 cm depth water temperature, pH, electric conductivity, total dissolved solids (Fig. 5, unit 2: ca.11.5e8 ka). The reported d18O values however, are (TDS) and stable isotopes (d18O, dD). based on bulk carbonate. All carbonate minerals reacted completely during the measurement procedure. 3.2. Dating Bulk d18Oisreflected by mass balance of the d18O value for each mineral phase and its percentage contribution to the total car- Three sets of radionuclide 210Pb/137Cs dating for the upper bonate. However, as discussed for three scenarios below, on 20e25 cm sediment of cores KH13, KH15 and KH16 were per- average, the measured bulk d18O of KH17 does represent similar formed at Nanjing Institute of Geography and Limnology, Chinese d18O values to calcite. The oxygen isotope fractionation for precip- Academy of Sciences (NIGLAS). As the upper 10 cm sediment itating calcite, aragonite and MHC is very similar, differing at B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 69 identical ambient temperature by less than 0.5‰ (Friedman and Yan et al. (2017), identified under a stereoscopic microscope and O'Neil, 1977; Jimenez-L opez et al., 2001). On average, these min- scanning electron microscope (SEM) at Department of Earth Sci- erals represent 70.4 ± 18.9% of the carbonates in KH17, while ence, Freie Universitat€ Berlin, here reported in abundances (ln dolomite and high-magnesian calcite occupy the remaining part. [sumþ1]). Compared to calcite at identical ambient temperature, the ox- ygen isotope fractionation for precipitating dolomite or HMC is 4. Results higher by about 2.5‰ and 1.5‰, respectively (Jimenez-L opez et al., 2004; Vasconcelos et al., 2005). 4.1. Lithology and chronology Scenario 1): In the (unlikely) case where all minerals were precipitated contemporaneously, measured bulk d18O would differ Sediment composition of core KH17 revealed laminated clayey- in KH17 by only about 0.5‰ (on average) from d18O of calcite as silty mud in most parts of the core. Variations in lamina thickness calculated by mass balance. including distinct carbonate layers and non-laminated sequences in Scenario 2): Assuming that dolomite and high-magnesian the lower part as reported by Mischke et al. (2010) also occurred in calcite were precipitated at higher ambient temperature but still our core. Increased silty to sandy fractions were found between 240 from the same lake water as the other carbonates, the d18O values of and roughly 300 cm and from 526 cm down-core (Figs. 2 and 5). dolomite and HMC will be lower, and hence the difference from the Carbonate-rich laminated mud and partly crumbly carbonates were other carbonates will decrease. At a 10 C higher temperature for found at 230e260 cm and at 420e526 cm core depth. Median grain dolomite precipitation, the d18O value of dolomite will reach the size distribution confirms the majority of fine fractions between 10 same d18O value as calcite. Overall, this scenario indicates that bulk and 20 mm in the upper 5 m of the core, while significantly coarser d18O would have similar d18O values to calcite. On average, the fractions up to 120 mm (median) dominated the lower part (Fig. 2A). difference will be < 0.5‰, comparable with reported offsets for Our age model refers to radionuclide dating (Fig. 2B) and AMS aragonite and Mg-calcite (Leng and Marshall, 2004) and dolomite radiocarbon ages (Fig. 2A) from different fractions. The uppermost (Morrill et al., 2006). dated samples in the core provided too old ages in comparison with calculated 210Pb/137Cs dating results at 10 cm sediment depth. 3.4. Carbonates Hence, AMS ages were affected by various processes (e.g., incor- poration of old/dead carbon; limited exchange with atmospheric Total carbonates reported as CO3 and organic matter (OM) were CO2) resulting in certain reservoir errors, not uniform within the retrieved by loss on ignition (LOI), following the procedures entire lake, similar to Lake Qinghai (Henderson et al., 2010; An described by Heiri et al. (2001) and measured at Nanjing University. et al., 2012). As the quantity of both components are influenced by grain size in Those errors are very common in lacustrine sediments from the bulk samples we corrected the results against the grain size frac- TP and adjacent regions, thus subject to discussions about potential tions <63 mm, assuming that fine particulate OM and carbonate reasons of contamination and their spatio-temporal differences in mainly occur in fractions smaller than sand. As both components sediment records (Hou et al., 2012; Mischke et al., 2013). The RE in are considered a part of a compositional dataset we applied an the core KH17 was calculated by interpolation between ages and additive log ratio (alr) transformation to avoid spurious correlation the set age of zero BP at 10 cm core depth yielding a mean RE of according to Aitchison (1986). 2495 years. Linear regressions of core sequences as used in a X-ray diffraction (XRD) was performed on powdered samples Qinghai Lake record (An et al., 2012) however, were not applied (2 cm resolution) using a Rigaku Miniflex 600 with a copper Ka here because a continuous linear sedimentation rate was consid- tube in the 3e80 (2q) range with steps of 0.02 (2q), at the Physical ered unlikely, according to grain size variations. Notably the ages in Geography Laboratory, Department of Earth Sciences, Freie Uni- the lower part of the core (Fig. 2a) did not allow applying a linear versitat€ Berlin. Identification of minerals was based on X'Pert 1.01 regression due to expected much higher sedimentation rates of software. All identified minerals (main reflexes) were transferred sandy sequences. Hence, those ages, consistent with increased into a relative percentage of each mineral with respect to the sum sedimentation rate were applied, while the other ages were dis- of all identified main reflexes (counts per second: cps) carded from the age model (Fig. 2, Table S1). Both methods how- (Wünnemann et al., 2010). ever, cannot fully resolve age uncertainties as the RE error might Geochemical elements were identified by X-ray fluorescence have been not constant over time. Hence, we consider our age (XRF) line scans on the second half of each KH17 core segment in model as the best approximation towards a reasonable chronology. 5 mm resolution using an Avaatech XRF core scanner at Yunnan A previously published age model from the lake (Mischke et al., Normal University, Kunming, China. Preliminary results were 2010) revealed a rather different chronology mainly due to a published by Hu et al. (in press). Here we refer to the results of much lower calculated RE. The reported age model for the last titanium (Ti) as a representative of allochthonous flux. roughly 1000-yr sedimentation history reported by Li et al. (2017) was based on a RE close to our record and thus matches our 3.5. Grain size chronology quite well.

Grain size analysis in 1 cm resolution was performed at Nanjing 4.2. Spatio-temporal distribution of selected proxies University, using a Malvern Mastersizer 2000 analyser. Sample preparation included removal of carbonate and organic matter by 4.2.1. Stable isotopes in modern surface sediments and water 18 10% hydrochloric acid and 30% H2O2 according to ISO Norm Modern distribution of d O in surface samples of Lake Kuhai 14688e1 (2011). Median grain size data was mainly used in this (ca. 15-yr average values) display a spatially heterogeneous pattern study for adjustment of the chronology. with values ranging between 9‰ and þ2.5‰ (Fig. 3). Apparently, the distribution of lighter isotopic values is connected with 3.6. Ostracods inflowing rivers that transport isotopically light rain/snow and meltwater from surface soils into the basin, similar to reported Ostracods were collected from 968 samples (~3 g dry weight) assumptions from west Tibet lakes (Gasse et al., 1991, 1996). following the procedures described by Mischke et al. (2003) and Modern isotopic composition (d18O, dD) of rain/snow, river and 70 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84

Fig. 2. Chronology of core KH17 from the center of Kuhai Lake (a). 210Pb/137Cs age calculation from cores KH15 (b). Sediment succession in core KH14/17 for the upper 15 cm was 137 210 almost identical to core KH15. The age zero BP (1950 AD) occurred at 9e10 cm sediment depth, according to Cs peaks. Mean sedimentation rate by Pbex according to the 1 137 210 equation of Van Eaton et al. (2010) was calculated to 8.8 yr cm , pointing to a difference of 27 years between Cs and Pbex-calculation for the upper 10 cm sediment, probably 210 due to a calculated mean sedimentation rate by Pbex without considering very high water content in the upper 10 cm sediment and related compaction through depth.

18 Fig. 3. Spatial distribution patterns of d Ocarb in surface samples of Lake Kuhai (inverse distance weighted interpolation). Red stars mark the location of retrieved sediment cores with core numbers and mean d18O values at the core sites. Black dots indicate the location of surface samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 71

Fig. 4. Modern stable isotopes (d18O, dD) from rain, snow, river and lake water of Kuhai and adjacent regions for different seasons. The local evaporative line (LEL) differs remarkably from the global meteoric water line (GMWL), due to evaporation effects. Off-monsoon and summer monsoon precipitation overlap, while lake water is significantly enriched by > 10‰.

Fig. 5. Proxy records from Lake Kuhai KH17 core, lithology and calibrated 14C ages, plotted against sediment depth. Relative abundances of ostracods; median grain size variations in core KH17; determined mineral phases by XRD, comprising calcite (green), aragonite (Ara, pink), monohydrocalcite (MHC, blue) and high-Mg calcite/dolomite (HMC/Dol, brown); total carbonate (additive log ratio) derived from loss on ignition; d18O isotope record (blue line) and d13C isotope record (green line) from carbonates. Dotted lines mark boundaries between units 1e6 according to variations in oxygen isotopes, carbonates and titanium. Mid-point ages refer to the Bacon age-depth model (Fig. 2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 72 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 lake water in Kuhai and adjacent regions (Fig. 4, Weynell et al., unsolved phenomenon, although not a singularity compared to 2016) displays strong seasonal differences with generally lightest other lakes (e.g., Talbot, 1990; Horton et al., 2016). Hence, this values during the colder seasons and significant enrichment of aspect was excluded from further discussion. 18 d Orain accompanied by high deuterium excess between May and June (June-early July) and Oct./Nov. (Ren et al., 2013). Rainfall 4.2.3. Mineral phases and total carbonate events during early July 2016 in the neighbouring Donggi Cona Lake X-ray diffraction analyses (XRD) from KH17 sediments revealed 18 (onset of summer monsoon) revealed heavy d Op between 0‰ different carbonate mineral phases that contributed to the total and 5‰ (Fig. 4), similar to observations in the distant Maduo amount of carbonate (Fig. 5). Stoichiometric calcite occurred with County (Ren et al., 2013). Rain/snowfall at Kuhai Lake in early June relatively constant amounts of ~10% throughout the record. MHC 2015 revealed very light values between 10‰ and 12‰. Lake was present from unit 3 up-core with highest values of up to 15% in water however, was already isotopically enriched by about 8e10‰ unit 3, with generally lower and fluctuating amounts (0e~5%) in 18 in June/July (Fig. 4). This is comparable with 15-yr averaged d Ocarb units 4e6. This highly unstable mineral readily converts to calcite in surface samples. Occasional summer monsoon-related rainfall or aragonite in presence of Mg (Winland, 1969; Fukushi et al., 2011; events with isotopically light values however, may have also Rodriguez-Blanco et al., 2014), while perhaps colder and deeper contributed to the overall isotopic composition in Kuhai Lake but water conditions and potentially fast sedimentation rate are able to were counterbalanced by mixing processes in the lake and high preserve this mineral (Hull and Turnbull, 1973; Li et al., 2008). MHC evaporation during the summer period. was found as a precursor or intermediate product during calcite formation (Fukushi et al., 2011). Surface samples from the entire 4.2.2. Proxies in core KH17: stable isotopes (d18O, d13C) Lake Kuhai revealed the majority of MHC in greater water depth The sedimentary record of core KH17 (Fig. 5) is divided into 6 below the thermocline. units based on major changes in mineral composition, carbonates, The metastable aragonite, commonly indicating increased oxygen isotopes and titanium through depth. salinity of evaporative shallow marine water bodies in presence of 18 d Ocarb ranges between 3.59 and 8.85‰ (mean: 2.67‰ for the Mg (Burton and Walter, 1987) also forms in supersaturated warm units 2e6, of the entire Holocene) with lightest values in unit 1, and shallow waters of lacustrine systems (Gasse et al., 1991; Roeser interrupted by significantly heavier peaks in the middle part of this et al., 2016). Aragonite was mainly formed during unit 2 exceeding 18 fi unit. During unit 2 d Ocarb increases rapidly towards heavier values 20% of all identi ed minerals and occurred only sporadically af- but displays frequent short-term fluctuations of about 4e5‰ dur- terwards. High-Mg calcite (>4 mole % Mg) and dolomite experi- ing early and late parts of this unit. In comparison with the previous enced several strong peaks in unit 4 (up to 50%). Slightly elevated unit 2 fluctuations in unit 3 remain less pronounced with a mean of amounts were also found in unit 6 (~15%). In modern sediments of about 1.8‰ between stronger excursions (light values) at the Lake Kuhai, these minerals only occurred at few locations in beginning and the end of this unit. Frequent fluctuations with shallow water environments above the thermocline, likely in as- 18 amplitudes of about 10‰ in d Ocarb are recorded in unit 4. Varia- sociation with microbial mediated processes as reported from tions throughout units 5 and 6 remain less pronounced compared Qinghai Lake (Deng et al., 2010). to the preceding period and fluctuate around the modern mean Low carbonate content in core KH17 (here reported as CO3, value of 0.96‰ by about 3e5‰. Stronger excursions (lighter Fig. 5) was observed in units 1, 5 and 6 (mean: 6.5% CO3). After a values) occurred at the beginning of unit 6. rapid increase in unit 2 with peak values of ~45% CO3 in the second 13 Carbon isotopes (d Ccarb, Fig. 5) vary between 4.19 and 6.44‰ half on this unit, a general decreasing trend in unit 3 and extreme 18 fl fl (mean: 0.1‰) and only partly follow d Ocarb variations. Lightest uctuations thereafter in unit 4 are recorded. Slight uctuations in values occurred in the middle part of unit 1 and in unit 4. Stronger units 5 and 6 remain on a relatively low level (2e9%, mean: ~6% 18 fluctuations with a general increasing trend (heavier values) are CO3). Carbonate generally co-varies with d Ocarb (Table 1). recorded in unit 2, followed by relatively stabilized heavier values in unit 3 and a general declining trend thereafter peaking in unit 4. 4.2.4. Grain size composition Units 5 and 6 are characterized by low-amplitude fluctuations with Highly variable median grain size composition (Fig. 5) with a slight increasing trend until the top of the core. Co-variance be- dominance of coarsest fractions (sand: 80e120 mm) occurred in 18 13 fl tween d Ocarb and d Ccarb is insignificant for the entire Holocene unit 1 and the early part of unit 2. Stronger uctuations between record, even for individual chrono-sequences except for the Late- coarse and fine fractions occurred simultaneously with heavier 18 13 glacial period (Table 1, Figs. 5 and 6). This relationship is remarkably d Ocarb, slightly higher carbonate and depleted d C in the middle different from other closed lake systems, where both isotopes part of this unit. During the succeeding units median values of commonly co-vary (Talbot, 1990; Fontes et al., 1996; Li and Ku, 10e20 mm remained the dominant fractions with slightly coarser 1997; Leng and Marshall, 2004; Horton et al., 2016), although the components in units 4 (middle part) and 6 (early part). 13 18 general trends of d Ccarb follow those of d Ocarb (Fig. 7). The overall weak correlation within the closed lake at site KH17 except 4.2.5. Allochthonous sediments (titanium) for the lower part of the core (13.5e11.6 ka, Table 1) remains an Titanium is considered a representative component for

Table 1 Correlation (r) between proxies. Bold values refer to >99% confidence level.

Time period (ka BP) Correlation (r)

18 13 18 18 d Ocarb/d Ccarb Carbonate/d Ocarb d Ocarb/Ti Carbonate/Ti 0e13.6 0.04 0.52 ¡0.56 ¡0.69 0e2.9 0.36 0.64 ¡0.65 ¡0.59 2.9e5.1 0.18 0.92 ¡0.72 ¡0.72 5.1e7.5 0.11 0.80 ¡0.75 ¡0.67 7.5e11.5 0.09 0.70 ¡0.60 0.49 11.5e13.6 0.80 0.53 0.40 0.37 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 73

18 Fig. 6. Chrono-sequences of KH17 record for d Ocarb (blue), carbonate (red) and titanium (black) with respective correlation factors. A: 13.6e7.5 ka. The shaded area marks a period 18 18 of weak correlation between d Ocarb and carbonate/Ti. Important negative spikes of d Ocarb are assigned to periods of possible prolonged cold seasons. B:7.6e5.1 ka; C:5.1e2.9 ka; D: 2.9 ka ePresent. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 74 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84

18 13 Fig. 7. Comparison of Kuhai KH17 isotope records (d Ocarb, d Ccarb, including 10-point running average) with the composite speleothem record from Dongge cave (Cheng et al., 2016), Dole effect (Severinghaus et al., 2009) with ages of excursions, North Atlantic hematite-stained grains (HSG, Bond et al., 2001), Insolation at 40N(Berger and Loutre, 1991) and reconstructed lake level variations during the last 14 ka. Shaded grey areas mark HSG maxima; light blue areas and dotted line in the lake level record are estimated variations. Arrows show opposite trends between cave records and lake records (Kuhai Lake) in the westerly-dominated domains of the semi-arid/arid regions on the TP. YD: Younger Dryas; DACP: Dark Ages Cold Period; MCA: Medieval Climate Anomaly; LIA: Little Ice Age. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) allochthonous sediment supply to the lake. Relatively high and 4.2.6. Ostracods constant proportions occurred during unit 1, interrupted by Relative abundances of ostracods in Lake Kuhai referred to three stronger fluctuations in the middle part of this unit, contempora- species, Limnocythere inopinata (Baird, 1843), Eucypris mareotica neous with fluctuations in grain size, d18O, d13C and carbonate. (Fischer, 1855) and Ilyocypris sp., of which the latter one only Similar to the aforementioned proxies, Ti experienced strong fluc- occurred in few samples (Yan, 2017). Leucocythere sp. found tuations from low to high amounts in units 2 and 4, less pro- together with E. mareotica in the lower part of a sediment core from nounced in unit 3 and relatively stabilized higher input during units the same lake as well as occasional occurrences of Cyprideis torosa, 18 5 and 6. Ti is negatively correlated with d Ocarb and carbonate Fabaeformiscandona danielopoli and Tonnacypris edlundi (Mischke through all units and chrono-sequences (Fig. 5, Table 1). et al., 2010) were not recorded in KH17 core. According to Fig. 5, B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 75 ostracods occurred in high abundances in units 2, 4 and 6 but only precipitation dependent on the overall P/E ratio. With respect to occasionally or were completely absent in the other units 1, 3 and 5. the summer monsoon domains in China Zhang et al. (2011) argued Under modern conditions those ostracods only occurred in Lake that the amount-weighted average isotopic composition of input Kuhai above 10e12 m water depth, of which L. inopinata is known water from precipitation masks seasonal differences between as a typical shallow water species (Meisch, 2000; Mischke et al., summer and winter precipitation which can be decoupled by 18 2008). Also E. mareotica was not found below 12 m water depth considering the composition of d Olw against the ratios of summer in surface samples of Kuhai Lake, although it is known that this to winter precipitation. This method however, is applicable for species can live in greater water depth as reported from other Ti- modern times with reference to instrumental precipitation data as betan lakes (e.g., Mischke et al., 2003, 2008; Yan and Wünnemann, done for many sites across the Tibetan Plateau (e.g., Yao et al., 2013; 2014). The reason for spatially limited presence of ostracods re- Ren et al., 2013; Li and Garcione 2017), but less applicable for past mains unclear but may be related to locally unfavourable living scenarios as reliable seasonal precipitation data is lacking. conditions in the lake, especially in the profundal zone. Being aware of this aspect, we propose that the relation between 18 authigenic/biogenic carbonates and their isotopic signal (d Ocarb) 5. Discussion could be an appropriate measure for the detection of seasonal differences in source water supply under variable P/E conditions. The high-resolution isotopic record from the Kuhai Lake at the Given the fact that authigenic (fine fraction) and biogenic carbon- transitional domain of summer monsoon influence may serve as a ates preferably form in surface waters under warmer water tem- paradigm for the interpretation of regional hydro-climatic condi- peratures, lower pCO2 and in response to photosynthesis of tions in response to the interplay between westerly-derived macroscopic plants and algae (Talbot, 1990; Fontes et al., 1996; Leng climate and summer monsoon impact during the last 14 ka. In and Marshall, 2004), they reflect hydrodynamic conditions of the contrast to large-sized closed lake systems such as Qinghai Lake, for lake during summer, mainly associated with maximum phyto- example, which generally provide subdued signals homogenized plankton productivity (Teranes and McKenzie, 2001; Leng and by the buffering of the large lake volume (Leng and Marshall, 2004), Marshall, 2004). This fully applies to the Kuhai Lake and suggests smaller endorheic (hydrologically closed) lakes like Kuhai are able that d18O in those carbonates reflect the isotopic composition of the to better reflect past variations in water balance and related iso- lake water during the warm season of summer monsoon impact. 18 topic effects preserved in lake carbonates. Even relatively short- Hence, evaporative enrichment causes heavier d Ocarb leading to term intervals are detectable if the temporal resolution of a re- co-variance with carbonate amount, given that bi-carbonate and cord is high enough. respective cations are sufficiently available. During the “winter season”, here denoted to the period between mid of September and 5.1. Hydro-climatic processes controlling d18O April, low temperatures, photosynthetic activity and carbonate precipitation are significantly reduced. Concurrent with low air/ Complex isotopic fractionation processes in water bodies were water temperatures and reduced evaporation isotopically lighter discussed in a larger number of publications (e.g., Talbot, 1990; rain/snowfall, snowmelt and thawing of frozen ground (He et al., Fontes et al., 1996; Li and Ku, 1997; Leng and Marshall, 2004; 2016) during spring (April/June) promote the prevalence of ligh- Horton et al., 2016; Qiang et al., 2017). They all highlight the ter d18O in lake water. Modern precipitation data reveal ca. 43% of importance of evaporative enrichment in water bodies as a ratio of annual rain/snowfall during these seasons. Hence, low amount of 18 precipitation over evaporation (P/E). The isotopic composition of carbonate precipitation co-varies with light d Ocarb, taking into lake carbonates which typically precipitate in the upper water account that non-summer rain/snowfall and related runoff in the columns of a lake system however, reflect multiple processes northern parts of the TP and adjacent regions is isotopically related to temperature, disequilibrium effects (vital effects) and the depleted (Tian et al., 2001; Yao et al., 2013). This pattern differs initial isotopic composition of the water body (Leng and Marshall, from the summer monsoon affected domains on the TP. 2004). The latter is influenced by local precipitation, river/ Conversely, if amount-controlled isotopically light summer groundwater discharge and e most important for the Kuhai record monsoon rainfall in this region would have exceeded the evapo- - the timing (season) of the source water supply. Isotopic compo- ration effect, the resulting lighter lake water during summers sition of rain/snowfall however, depends on air temperature, should show a negative correlation with carbonate amount, as moisture source and rainfall amount (Zhang et al., 2011). indirectly inferred from Holocene isotopic records on ostracods in Carbonate from the catchment transported by fluvial and Qinghai Lake sediments (Lister et al., 1991; Liu et al., 2007; An et al., aeolian processes revealed allochthonous carbonate content 2012) and Koucha Lake (Mischke et al., 2008). These relationships generally below 5% of dry bulk samples (dolomite: <1%) and sug- however, were subject to modification, if other sources than local gest that almost all identified carbonate phases are of authigenic/ rain/snowfall (glacier meltwater and/or groundwater inflow, for endogenic/biogenic origin. example) contributed to the overall water budget. Changes in lake A strong evaporative enrichment of lake water by > 10‰ level (water volume) are expected to accelerate or mute those re- observed in many closed lake systems worldwide (Horton et al., lationships as assumed for the Holocene Sumxi Co lake record in 2016) also applies to Kuhai Lake, both in modern lake water and western Tibet (Gasse et al., 1991). in lake carbonates of the past 14 ka. Similar to other lakes on the TP and adjacent regions (Morrill et al., 2006; Zhang et al., 2011; Qiang 5.2. Seasonal effects and westerly influence during the last 14 ka et al., 2017) temperature dependent equilibrium fractionation be- tween water and calcite by a gradient of about 0.24‰/C(Craig, Transferring the aforementioned relationships to the KH17 lake 1965) is rather unlikely for the KH17 record and cannot explain record (Fig. 6AeD), our data reveal high carbonate content that 18 the large variations solely. always co-varies with heavy d Ocarb over the entire 14 ka. Corre- In general, the preserved isotopic signals in lake carbonates lation factors for individual time sequences range between r ¼ 0.53 18 18 reflect the averaged d O of lake water (d Olw) derived from the (p < 0.01) and r ¼ 0.92 (p < 0.01) (Table 1). Those significant cor- different proportions of year-round rainfall and related water relations are evidence that carbonate precipitation always 18 supply. For the KH17 record the measured d Ocarb value in each increased with isotopic enrichment through evaporation, which sample provides a mixed signal of at least 20 years (mean) should be a common process. One exception from this covariance 76 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 falls into the period between 11.0 and 10.9 ka where heavier (p < 0.001) to 0.75 (p < 0.001) for the Holocene. Weaker correla- 18 d Ocarb was accompanied by low carbonate content, possibly due tion was calculated for the Lateglacial period (Table 1). Similar to the input of fine-grained carbonate-poor aeolian sediments. factors of r ¼0.59 to r ¼0.72 (both p < 0.001) apply to Ti/carb 18 Light d Ocarb values never corresponded with high carbonate and indicate that increased allochthonous flux occurred earlier 18 content which should have been the case if isotopically light rain than the onset of the summer monsoon season when light d Ocarb 18 (d Op) related to the amount effect of summer monsoon domi- and low carbonate were detected. This relationship can be expected 18 nated the d Olw as indirectly inferred from other records on the because a larger amount of sediment transport through aeolian northern TP (Bangong Co: Gasse et al., 1996; Qinghai Lake:, Colman processes is related to the winter and spring seasons (e.g., Yan et al., et al., 2007; An et al., 2012; Lake Genggahai: Qiang et al., 2017; 2017). Moreover, snowmelt and the decay of frozen surface soils in Kuhai Lake: Li et al., 2017), at least for certain time intervals such as combination with periglacial processes such as soil wasting is the early and middle Holocene, for example. This assumption does highly active in this high-elevated region during spring and delivers not exclude a possible influence of lighter summer precipitation higher proportions of weathered fine clastic material along with (amount effect) in that region but it may have been masked by the runoff from snowmelt (He et al., 2016) to the basin. The isotopic overall high evaporation, persisting low P/E ratio and recycled composition of the frozen soil water and snow (light d18O) is mainly water vapour. Several studies meanwhile demonstrate that modern that of non-monsoon precipitation. Overall sediment flux during 18 light d Op during the summer season is not the only contributor to the summer season including local sheet flooding processes after the lake's water budget but more likely a mixture of different occasional heavy rainfall events seems to have been comparatively rainfall sources with different isotopic signatures (Tian et al., 2001, lower. 2007; Yao et al., 2013; Ren et al., 2013; Yang and Yao, 2016), and local convective rainfall events (Curio et al., 2015; Curio and 5.3. Chronology of hydro-climatic variations Scherer, 2016). They obviously counterbalance potential amount- weighted summer monsoon rainfall in the semiarid/arid regions Variations in the isotopic composition of the KH17 record of the TP. Simulations based on the isotopic atmospheric circulation through time are discussed in combination with selected proxy model LMDZiso however, indicate that continental recycling is one data that monitor hydrologic variations of the water budget within of the main moisture sources for the TP during both winter and the uncertainty of our age model. summer with highest effect during summer (Yao et al., 2013). Those recycling processes over land cause generally heavier isotopic 5.3.1. Younger Dryas (YD: 12.8e11.5 ka) 18 values in local rainfall (Tian et al., 2001; Yu et al., 2007). Most depleted d Ocarb and low carbonate content (calcite) 18 Consequently, we can argue that significantly lighter d Ocarb in dominated the YD interval between 12.8 and 11.5 ka, interrupted by association with low carbonate content in the KH17 record cannot a series of enriched values between 12.5 and 12.0 ka (Fig. 6A). be assigned to the summer monsoon amount effect but is more Frequent fluctuations in grain size from sand to silt-clay fractions, likely associated with lower temperatures, reduced evaporation high allochthonous flux (Ti) and the sporadic occurrence of and with moisture supply that did not contribute to the hydro- ostracod valves (mainly E. mareotica, few L. inopinata and Ilyocypris logical balance during the summer season. These climatic condi- sp., Fig. 5, unit 1) indicate very shallow pond-like conditions with tions generally occurred between September and mid of April variable fluvial and likely aeolian impact (Fig. 7). According to our when air temperatures reached values close to and below zero C, interpretation model discussed above, the Kuhai region experi- contemporaneous with snowfall events delivered along the shifting enced prolonged colder climatic conditions (winter season) with pathways of the MLW with strong effect along the north-eastern limited water supply, both during summer and winter. Only short- 18 part of the TP (Schiemann et al., 2009; Nagashima et al., 2013; term spells of enhanced evaporation (heavier d Ocarb) of a still Chiang et al., 2015; Thomas et al., 2016) during the Holocene. Such shallow and perhaps slightly saline water body (occurrence of water sources are generally depleted due to lower condensation aragonite, Fig. 5, unit 1) is recorded within a roughly 500-yr in- temperature and long-distance rain-out effects (Tian et al., 2001, termediate period. These cold and mainly dry hydro-climatic con- 2007). Modelling results confirm the annually high percentage of ditions are well-known from numerous climate records in China snowfall in this region during the winter season resulting from and other parts of central Asia, generally attributed to weak or westerly-derived air masses (Maussion et al., 2014; Yang and Yao, absent summer monsoon impact and dominance of westerly- 18 2016) as also indicated by regional studies in Kyrgyzstan derived climate. Thus, our argument that depleted d Ocarb in the (Lauterbach et al., 2014) and the north-eastern TP (Thomas et al., KH17 record is mainly associated with colder climatic seasons and 18 2016). Such snowfall events of isotopically light d Op in water supply from sources other than the summer monsoon is September and also in June were observed during the field seasons reasonable. Records from the modern summer monsoon domains between 2014 and 2016 (Fig. 4) and support our assumption that show opposite (heavier) isotopic signals (Fig. 7) which were inter- 18 significantly lighter d Ocarb in the Kuhai KH17 record represents preted as weak or non-summer monsoon impact during this time the contribution of water during the colder off-monsoon season interval. (e.g., Wang et al., 2001; Yuan et al., 2004;; Dykoski et al., that outweighed the summer signal for decades. Modern isotopic 2005; Liu et al., 2014), thus lacking amount-related summer composition of Hurleg Lake water in the Qaidam Basin indicates monsoon rainfall. major contribution of snowmelt from its catchment that influences the isotopic composition (He et al., 2016) in spring, very similar to 5.3.2. Early Holocene (11.5e7.5 ka, Fig. 6A) Kuhai Lake. The onset of the early Holocene climate amelioration after 11.5 18 With respect to the detection of sediment influx from external ka is documented by a rapid increase of d Ocarb towards heavier sources we selected the titanium signal as a reliable measure for the values, contemporaneous with the increase of carbonate precipi- contribution of allochthonous components to the Kuhai lake basin tation (Fig. 6A). Increasing primary productivity as revealed by a by fluvial, denudation and aeolian processes (Haug et al., 2001; trend to heavier d13C values (Fig. 5, unit 2) under a rising lake level Wünnemann et al., 2010; Hu et al., in press). This component is (Fig. 7A, C) with increasing input of suspended sediments (<30 mm), 18 significantly anti-correlated (negative co-variance) to d Ocarb and indicates rising temperatures and enhanced rainfall perhaps mainly carbonate content (Table 1; Fig. 6AeD) throughout the entire Kuhai during summer, contemporaneous with an intensified summer 18 time series. Correlation factors of Ti/d Ocarb range from r ¼0.65 monsoon (Mischke et al., 2008; An et al., 2012; Liu et al., 2014; Hou B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 77 et al., 2017). Strong fluctuations with negative spikes at around 11.3, geomorphological perspective. Following this assumption we can 11.1,10.8, 10.6 and 10.2 ka however, are indicative of rather unstable attribute this period to the Holocene hydro-climatic optimum, climate conditions, switching frequently between summer and similar in timing to records from Hala Lake (Wünnemann et al., winter dominance, while detrital flux followed these fluctuations. 2012; Yan and Wünnemann, 2014), Genggahai Lake (Qiang et al., Summer-related precipitation was obviously not enough for a 2017), Gonghai Lake (Chen et al., 2015; Rao et al., 2016a,b), Kou- continuously rising lake level, comparable with results from the cha Lake (Mischke et al., 2008) and records in arid central Asia nearby Qinghai Lake (Li and Liu, 2014; Liu et al., 2015; Chen et al., (ACA) used for discussion in respect to the general competing in- 2016a). The occurrence of aragonite and high abundances of os- fluence between the EASM and the MLW (Chen et al., 2008, tracods (Fig. 5, unit 2) during this period indicates an increased but 2016a,b; An et al., 2012). still relatively shallow saline water body which was subject to At Linggo Lake in the central-northern TP however, higher lake intensified evaporation, preventing the lake from further rise levels derived from dD variability were mainly associated with (Fig. 7A). This is very similar to reported high salinity of the shallow increased moisture supply from westerly sources, implying less Koucha Lake (Mischke et al., 2008). Aragonite and ostracods both influence of summer monsoon in this region (Hou et al., 2017). indicate that the water body of Kuhai Lake did not dramatically Isotope values and co-varying carbonate content in the Kuhai change and remained at ca. 12e14 m below its present level. record indicate that amount-related summer monsoon rainfall 18 Interestingly, the spikes in the KH17 record almost match pe- with strongly depleted d Op was also not the main supplier but riods of weakened summer monsoon influence recorded in cave more likely a mix of different water sources with an overall isotopic records (Fleitmann et al., 2003, 2008; Cheng et al., 2016) within composition closer to the middle Holocene mean isotopic value 18 dating uncertainties. The ca. 800-yr long period of heavier d Ocarb that contributed to a positive water balance. By contrast, depleted and high carbonate precipitation after ca. 10 ka indicates stabilized values for this time period and parts of the early Holocene hydrological conditions with relatively high influence of the sum- (10.6e9.4 ka) in the Genggahai Lake record north-west of Kuhai mer season rainfall and related evaporative enrichment of lake Lake however, were tentatively assigned to strengthened summer 18 water isotopes. A general trend towards lighter d Ocarb accompa- monsoon moisture supply, derived from increased lake levels nied by increasing Ti flux culminating in a strong negative peak at mainly fed through ground water (Qiang et al., 2017). 18 ca. 9.1 ka may indicate a stepwise shift from summer-to winter- Three peaks of heavier d Ocarb at around 6.6 ka, 6.4 ka and dominated hydro-climatic conditions. Three pronounced negative between 6.0 and 5.8 ka exceed the mean value by about 1.9‰ and spikes, centring at 9.1, 8.2 and 7.6 ka between extreme positive suggest intermediate lake level fluctuations in response to reduced 18 peaks (d Ocarb >0‰) occurred thereafter. The first two are assigned water supply and more effective evaporation during the summer to well-known climate (cooling) anomalies triggered by meltwater season. The influence of cold-season climate and related water pulses into the North Atlantic (Alley et al., 1997; Ellison et al., 2006; supply remained on a low level and might have been masked by Fleitmann et al., 2008) that caused a slow-down of the Atlantic stronger summer signals. meridional overturning circulation (AMOC, McManus et al., 2004) The decline to significantly lighter isotope values and reduction and its hemispheric-scale signal transfer across the TP, most likely in carbonate, accompanied by increased Ti flux after 5.5 ka (second by the MLW. Recently, this climate signal was also described from sub-unit of the middle Holocene) leads to a period of pronounced the nearby Donggi Cona lake record (Saini et al., 2017) and suggests hydro-climatic fluctuations which corroborate the decline of a significant delay and/or earlier withdrawal of the summer season, summer monsoon strength in nearly all Chinese cave records after comparable to the previous periods of negative spikes. The subse- 6 ka (e.g., Wang et al., 2005; Liu et al., 2014). 18 18 quent turn to extreme positive d Ocarb values indicates an abrupt Large-amplitude fluctuations of >10‰ for d Ocarb between 5.0 short-term recovery to summer-dominated evaporative influence and 2.9 ka, synchronous with carbonate changes (highest correla- for both periods. Andersen et al. (2017) ascribed similar successions tion) and in antiphase with Ti flux (Fig. 6C) indicate frequent al- from a central European lake record to a period of higher-than- ternations between winter- and summer-dominated seasons on a average temperatures that immediately followed the 8.2 ka event multi-decadal timescale. The re-occurrence of ostracods, alter- in response to an enhanced resumption of the AMOC. We postulate nating peaks of high-Mg calcite/dolomite, calcite and MHC, sup- that this could be also a case for the Kuhai region. plemented by slight increases in median grain size (Fig. 5, unit 4), indicate a strong fluctuating but declining trend in lake level, 5.3.3. Middle Holocene (7.5e2.9 ka, Fig. 6B and C) perhaps with short-term desiccation phases between 3.5 and 3 ka This relatively long middle Holocene period appears in the (Fig. 7A). Subaerial exposure of lake deposits that were subject to KH17record as two significantly different isotopic sub-units, drying effects are indicated by the presence of crumbly carbonates divided into an early and late stage. The early part of the middle for this period. Differently from early Holocene isotopic variations Holocene between 7.5 and 5.4 ka was characterized by generally we suppose that summer monsoon-related water supply remained stable hydro-climatic conditions as inferred from slight variations very weak or even temporarily absent to enable this strong evap- 18 18 in carbonate content and Ti flux (Fig. 6B). Mean d Ocarb of 1.91‰ orative enrichment of d Ocarb and contemporaneous lake level was heavier than the overall Holocene mean (2.67‰) but lighter decline from its highest stage to a swampy and/or pond-like envi- than the modern mean (0.96‰). The lack of ostracods, disap- ronment with periodically desiccated stages within a few centuries. pearance of aragonite and first appearance of MHC that could be Conversely, the strong negative isotope values indicate periods retained under deeper cold water columns below a developed with dominance of non-summer water supply that remained less seasonal thermocline (Hull and Turnbull, 1973; Li et al., 2008), affected by evaporation. Such dramatic fluctuations in isotopic (Fig. 5, unit 3) suggest an increased lake level probably up to its composition became accelerated due to a successively declining maximum Holocene stage (~10 m above the present level), due to lake level and water volume. The related residence time of lake increased precipitation mainly during the summer season (Fig. 7A). water was shortened mainly during the summer season. The onset of summer stratification in the water body was very It has been suggested that this period of aridification over the likely. This overall positive water balance where precipitation ex- entire TP (within the uncertainty of individual chronologies) (Gasse ceeds water loss through evaporation (P/E ratio >1), must have et al.,1991,1996; Wei and Gasse,1999; Hou et al., 2016), in India and occurred at least during the rainy periods, apparently in summer South Asia (Staubwasser et al., 2003; Demske et al., 2009; Leipe time. Loss by overflow during this period can be excluded from the et al., 2014; Dutt et al., 2018) and in the Chinese monsoon realm 78 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84

(e.g., Wang et al., 2005; Hu et al., 2008; Dong et al., 2010; Selvaraj in the monsoon realm of China (e.g., Zhang et al., 2011; Cheng et al., et al., 2011) was a widespread climate anomaly not only of 2016) cannot be simply transferred to the Kuhai Lake as its hy- regional dimension, but likely globally tied with the southward drology and isotopic composition of the lake water from which shift of the intertropical convergence zone (ITCZ) (Haug et al., 2001) carbonates precipitate was not primarily affected by precipitation and increased El Nino Southern Oscillation (ENSO) (Rein, 2007; amount but obviously by P/E ratio (dominance of evaporation), Sinha et al., 2011), both preventing the ASM from migration to temperature and source water with strong links to the MLW. northern latitudes. Glacier advances in the northern hemisphere Despite the so-called “temperature effect” (Tian et al., 2008) in the (Mayewski et al., 2004; Liu et al., 2014), colder climatic conditions arid regions of the TP, the timing and sources of water supply of Qinghai lake water derived from alkenones (Hou et al., 2016) and contributing to the net balance of Kuhai Lake seem to have been an plateau-wide cold events (Mischke and Zhang, 2010) corroborate equally important factor. We assume that during the winter season depleted d18O in the Guliya ice core record (Thompson et al., 1997) an active Asian winter monsoon (AWM) did not play a role in terms as also seen in the KH17 record for the same time interval and may of effective moisture supply due to its generally dry-cold nature indicate not only dry but also colder climatic conditions in the derived from the Siberian high pressure system (Domroes and wider region. A simultaneous weakening of the ASM in China is Peng, 1988). recorded in most of the cave records (Dykoski et al., 2005; Hu et al., The results from Kuhai Lake indicate that the influence by the 18 2008; Dong et al., 2015) though offsets in timing remain enigmatic. MLW was stronger during phases of depleted d Ocarb attributed to In summary, the shortening of ASM influence gave rise for extended prolonged “winter seasons” including spring melt processes and influence of westerly-derived climate with prolonged non-summer the delay and/or earlier withdrawal of the summer season (ASM- monsoon seasons in the Kuhai region and beyond across the related rainfall patterns). This scenario, for example, was discussed northern TP. for shorter time sequences during the late middle Holocene form isotope records of Genggahai Lake (Qiang et al., 2017) and Linggo 5.3.4. Late Holocene (ca. 2.9ka-present) Lake (Hou et al., 2017), for the LIA in the Qinghai Lake area In comparison with the preceding period, smaller-scale fluctu- (Henderson et al., 2010; Yan et al., 2017), and recently observed 18 ations around a mean value of 2.58‰ (d Ocarb) occurred during (2018) by a roughly 14 days delay of Qinghai Lake ice melt. 18 the late Holocene (Fig. 6D). Generally lower carbonate content The phases of depleted d Ocarb partly resemble periods of 18 (5e10%) though positively correlated with d Ocarb and still nega- weakened ASM influence (Fig. 7D) and even more clearly the de- tively correlated with Ti flux suggests a return to stabilized hy- viation of the d18O in air from seawater d18O, as a reasonable drologic conditions under mean temperatures similar to these of measure of ASM strength, known as Dole effect (Severinghaus et al., today. The disappearance of ostracods between ca. 2.9 and 1.5 ka, 2009, Fig. 7F). This coincidence may support the validity of our reduction/lack of aragonite and high-magnesian calcite but higher chronology within uncertainties. percentages of MHC (Fig. 5, units 5e6) indicate an increased lake Furthermore, the isotopic peaks in the Kuhai record resemble level above the modern boundary for living conditions of ostracods almost all phases of increased North Atlantic drift ice (Fig. 7G) in Kuhai Lake. Furthermore, the development of a persistent sum- indicated by hematite-stained grains (HSG) which reflect cooling mer stratification and likely better preservation of MHC after ca. 2.9 events during the last 11 ka (Bond et al., 2001). Hence, it is ka, centred at around 2 ka (Fig. 7A). This higher lake level similar to reasonable to assume that those climate signals were transmitted that of the modern time corresponds to frequent low-amplitude by the MLW across the TP and induced short-term climate de- 18 fluctuations of d Ocarb pointing to an overall increased water teriorations even at sites close to the modern boundary of ASM supply during both seasons, similar to the modern time. A declining influence. Suppose that those seasonal shifts are tied with the lake level thereafter between 1.5 and 1.0 ka, contemporaneous with competing influence of both the MLW and ASM, a prolongation of the so-called Dark Ages Cold Period (DACP), is indicated by stronger the colder season climate depended on the strength of the MLW 18 excursions of d Ocarb (alternating light and heavy values), the re- and the shifting pathway across the TP in relation to the ASM in- appearance of ostracods and increased percentages of high Mg fluence (Schiemann et al., 2009; Nagashima et al., 2013; Chiang calcite/dolomite. We assume a stronger impact of the westerly- et al., 2015) during the entire Holocene. derived cold season hydro-climate in excess of summer Taking the spatial distribution of modern annual precipitation, 18 monsoon-related rainfall. Conversely, the succeeding period of the temperature and d Op over the TP and summer monsoon Medieval Climate Anomaly (MCA) appears to have been more controlled regions in China into account (Fig. 8), arid regions with balanced in terms of P/E and related seasonal effects. Reverse generally low annual precipitation display co-variance between 18 conditions during the Little Ice Age (LIA) is assumed by the varia- temperature change and d Op (Yao et al., 2013). This applies to 18 tions in d Ocarb and carbonate content, similar to a record from locations across the central and northern part of the TP, which are Qinghai Lake (Henderson et al., 2010). considered to be dominated by westerly-derived climate (Yao et al., 2013; Maussion et al., 2014) including some sites along the modern 5.4. Spatio-temporal bi-partite patterns of climate control on the TP summer monsoon boundary (transitional zone; Fig. 1). Hence, isotopic signatures during summer are significantly enriched due to The overall inverse isotopic trends between ASM-dominated generally low P/E ratio and inland recycling processes in compari- regions in China and the Kuhai Lake record at the boundary of son to summer monsoon-controlled sites (Lhasa, Guiyang, Xi’An, modern summer monsoon influence are demonstrated in Fig. 7B for example, Fig. 8) and other regions in the monsoon realm. This 18 and E. The increasing trend of depleted d Ocarb in cave records after implies that precipitation amount in summer does not play any role the YD with the onset of Holocene climate warming and and if so for certain rainfall events they are readily counterbalanced 18 strengthened ASM corroborates the decrease of d Ocarb (towards by the overall rainfall sources as discussed above. This modern heavier values) for the same period and for the last 2 ka denoted to scenario can be applied to past periods of the Holocene by isotopic the so-called “2ka shift” of increased ASM relative to the declining signatures in available lake records across the TP. trend of the northern Hemisphere insolation (Fig. 7D, Cheng et al., Records from the northern part of the TP (Kuhai Lake, this study; 2016). Reverse relationships are detectable for the middle-to late Hala Lake: Yan and Wünnemann, 2014; Manas Lake: Rhodes et al., Holocene after ca. 6 ka. They clearly show that amount-controlled 1996; Sumxi Co: Gasse et al., 1991; Wei and Gasse, 1999) and rainfall leading to depleted d18O in speleothem and lake records Koucha Lake: Mischke et al., 2008; Fig. 9AeE) are compared with B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 79

18 Fig. 8. Selected sites of modern temperature (red line), precipitation (blue bars) and d Op (black line) according to GNIP and TNIP data (Yao et al., 2013), showing a clear discrimination between summer monsoon influenced and westerly affected regions over the Tibetan Plateau and monsoon China. Green dotted line marks the modern boundary of summer monsoon impact in accordance with the mean annual precipitation distribution over China. See also Fig. 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) records from ASM amount-affected sites in the north-eastern and TP that the ISM could not pass the Kunlun Mt. range because of south-western domains of the TP (Qinghai Lake: An et al., 2012; its topographic barrier function. It is however, an open question Bangong Co: Wei and Gasse, 1999; Seling Co: Gu et al., 1993; Wei whether summer monsoon derived effective moisture could and Gasse, 1999, Fig. 9FeH). They display remarkable differences penetrate far into the interior of the TP during the early-middle in their isotopic signatures during the last 14 ka. Despite certain Holocene as inferred from the available records (Gasse et al., differences among the records due to chronological constraints, 1991; Rhodes et al., 1996; Hou et al., 2017) and from the Guliya lower data resolution and local hydrological conditions, the records ice core Thompson et al., 1997; Yao et al., 1997). from the northern part of the TP display comparable isotopic trends A persistent increase of effective moisture after ca. 6 ka inferred as discussed for Kuhai Lake. They indicate a plateau-wide domi- from a loess-paleosol sequence in the Xinjiang region of arid central nance of P/E ratio, temperature effect and source water isotopic Asia (ACA, Chen et al., 2016b) cannot be detected in the lake records composition under a persistent influence of the MLW and its sea- on the northern TP (part of the ACA). The record from Kuhai Lake sonal effects on the isotopic composition during the entire Holo- indicates a general decrease of water volume after its lake high cene. Hence, the amount effect of ASM-related rainfall in those stand during the middle Holocene despite intermediate regions did not play a role or was even absent. An exception can be fluctuations. inferred from the Koucha Lake (marked as a dotted line in Fig. 9E; By contrast, isotopic signatures in available lake records from Mischke et al., 2008) where the isotopic signatures from ostracods the summer monsoon domain follow the isotopic trends in cave during the early Holocene seem to have been stronger influenced records (Fig. 7E). According to the modern distribution patterns of 18 by the amount effect of a strengthened ASM. The chronology for precipitation, temperature and d Op (Fig. 8) in summer this time period however, remains questionable. Ramisch et al. monsoonedominated regions it is reasonable to assume similar (2016) reported evidence from Lake Heihai on the north-western conditions during the entire Holocene in those regions. This also 80 B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84

Fig. 9. Comparison of isotope records from various lakes on the Tibetan Plateau. Westerly domain: Kuhai Lake (this study), Hala Lake (Yan and Wünnemann, 2014), Manas Lake (Rhodes et al., 1996), Sumxi Co (Gasse et al., 1991; Wei and Gasse, 1999); ASM influenced domains: Koucha Lake (Mischke et al., 2008), dotted line indicates unclear chronology; Qinghai Lake (An et al., 2012), Bangong Co (Gasse et al., 1996; Wei and Gasse, 1999), Seling Co (Gu et al., 1993; Wei and Gasse, 1999), all redrawn according to published individual chronologies and related uncertainties. Shaded areas mark unstable climatic conditions during the early Holocene and late middle Holocene. B. Wünnemann et al. / Quaternary Science Reviews 200 (2018) 65e84 81 implies that amount-controlled precipitation at the respective sites indicate frequent alternations between seasonally different hydro- prevailed during the summer season, if other factors such as climatic conditions that were dominant for decades. Most of the changes in the P/E ratio and isotopically light meltwater input from negative spikes in the Kuhai record resemble cooling events in the adjacent glaciers remained insignificant. North Atlantic realm likely transmitted by the MLW and their The isotopic records based on ostracod assemblages in Qinghai seasonal shifts across the TP. They also corresponded to periods of Lake sediments (Fig. 9F; Lister et al., 1991; Liu et al., 2007; An et al., weakened ASM in the monsoon domains and suggest a prolonga- 2012) few hundred km north of Kuhai Lake and in Koucha Lake tion of the colder winter season coupled with a delay and or earlier sediments south of Kuhai Lake (Mischke et al., 2008), differ from withdrawal of summer season hydro-climatic conditions. the Kuhai record and roughly follow the trends of the isotopic re- Sediment influx caused by eolian and fluvial processes exceeded cord from Dongge cave (Dykoski et al., 2005). This trend however, is the contribution during the summer season due to strong influence only visible in Koucha Lake for the early Holocene period (Fig. 9E) of periglacial processes and spring water snowmelt processes in based on the respective chronology. Ostracod shells however, high altitudes of the TP. Lake level fluctuations promoted acceler- preserve the isotopic signal of the ambient water within few days ated or muted isotopic signatures. Shallow pond-like conditions during moulting in spring or summer (Meisch, 2000), given that with fluvial impact existed during the Younger Dryas event, fol- adult specimen were used for isotope analysis. Hence, isotopic lowed by increasing but still lower lake level during early Holocene composition of the host water during other seasons cannot be under unstable climatic conditions. The Holocene hydro-climatic preserved as ostracods only live for short periods within the optimum with highest lake level occurred during the middle Ho- summer season. Despite this limitation respective results were al- locene, followed by a strong decline afterwards due to climate ways interpreted in terms of strong summer monsoon influence deterioration towards cold and dry conditions. The late Holocene although interacting with the MLW (An et al., 2012). Conversely, experienced medium fluctuations which accord to known climatic Henderson et al. (2010) argued that depleted d18O from authigenic oscillations during the last 2 ka. carbonates in Qinghai Lake were more likely associated with westerly-derived moisture signals during the Little Ice Age rather Author contributions than summer monsoon, thus challenging the interpretation of climate signals by stable oxygen isotopes (Zhang et al., 2011). Given BW and DY designed the project, wrote the draft version of the that summer monsoon dominance in that region was valid, the manuscript, performed the age model and analysed minerals and modern bulging shape of monsoon moisture intrusion into small ostracods; NA and FR performed stable isotope data measurement parts of the north-eastern TP (modern monsoon boundary, Fig. 1) and supported interpretation; YZ, QS, PH supported sediment an- would have existed during most periods of the Holocene, whilst the alyses (grain size, minerals), statistics and text writing. neighbouring sites (Kuhai Lake, Hala Lake, Koucha Lake after the early Holocene) remained excluded, according to the different Acknowledgements influencing factors discussed above. The isotopic record from Koucha Lake (Fig. 9E) resembles the isotopic signatures of the Kuhai The research was funded by the 1000 foreign talents pro- record quite well for the younger period after around 7.5 ka indi- gramme of the Chinese Government provided to BW. Further cating similar hydro-climatic variations as noticed in the other re- funding was related to the distinguished professor research funds cords from the westerly domain on the TP. from East China Normal University, Shanghai, Deutsche For- The spatially inverse isotopic records from lake carbonates schungsgemeinschaft (DFG, WU270-10/3) and NSFC grants reveal a clear discrimination between westerly-controlled and 40971003 and 41806105. We thank professors G. C. Cao, K. L. Chen ASM-dominated regions across the Tibetan Plateau, underpinning and G. L. Hou from Qinghai Normal University for their support in the bipartite nature of climate control on the TP. Our data indicate organising fieldwork, Dr. W.L. Xia from NIGLAS for providing that the modern domains reported by Yao et al. (2013) most likely radionuclide data and H.C. Zhang, Yunnan Normal University, for existed over the last 14 ka, even during periods of positive water enabling XRF measurements. We also thank German and Chinese balances. A penetration of the ASM deep into the interior of the students for support during field research and laboratory analyses. semiarid/arid regions of the TP therefore remains questionable. Finally we thank Dr. Bing Xu, an anonymous reviewer and the ed- Interpretation of ASM impact based on the amount effect is not itor for their suggestions to improve the manuscript. applicable for lake records from the northern part of the plateau. Appendix A. Supplementary data 6. Conclusion Supplementary data related to this article can be found at The high-resolution oxygen isotope record from Kuhai Lake https://doi.org/10.1016/j.quascirev.2018.09.040. carbonates in the north-eastern part of the Tibetan Plateau (tran- B: Lake Kuhai with locations of cores (red circles with core sitional domain) in combination with selected sediment proxies numbers), surface samples (yellow dots), water samples (blue dots) was used to identify hydrodynamic variations in response to the and catchment boundary (red line). The orange triangle marks the interplay between the ASM and MLW. Our results show that the core data published by Mischke et al. (2010). 18 covariance between d Ocarb and carbonate content is a potential tracer for deciphering seasonality influence of climate in a decadal- References to centennial-scale over the last 14 ka. Evaporative enrichment of 18 d O during the summer season dominated the isotopic composi- Aitchison, J., 1986. 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