Philippe Sorrel
email: philippe.sorrel@univ-lyon1.fr

Kevin Jacq

Antonin Van Exem

Gilles Escarguel

Benjamin Dietre

Maxime Debret

Suzanne Mcgowan

Ducept Jules

Emilie Gauthier

Hedi Oberhänsli

Kévin Jacq

Jules Ducept

Evidence for centennial-scale Mid-Holocene episodes of hypolimnetic anoxia in a high-altitude lake system from central Tian Shan (Kyrgyzstan)

Keywords: Hyperspectral imaging, Paleolimnology, Paleoclimate, Lake stratification, Hypolimnetic anoxia, Chemocline, Central Asia, Lake Son Kol, Holocene

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Anthropogenic global warming is changing high-altitude ecosystems and environmental dynamics (Solomon et al., 2007;IPCC 2013). Regional model outputs for Central Asia predict that seasonal surface temperatures in the region will increase from 3 °C up to 11.4 °C on average, coeval with a decline in the amount of precipitation [START_REF] Ozturk | Simulation of temperature and precipitation climatology for the Central Asia CORDEX domain using RegCM 4.0[END_REF], although uncertainties in hydrological/rainfall projections are existing. High elevation regions of Central Asia (e.g., Tian Shan and Pamirs mountains, Tibetan Plateau) are particularly vulnerable to warmer climate conditions (e.g., Christensen et al., 2007). In high-altitude areas, a consequence of global warming may be shorter and wetter (and milder) winters along with longer and drier summers (Christensen et al., 2007;IPCC, 2013). Increases in solid winter precipitation, spring/summer temperatures, and (summer) rainstorms are likely to increase flooding of snowmelt-fed lake basins. Warmer temperatures are also expected to lead to eutrophication of lakes, depletion of oxygen in deep waters [START_REF] Jenny | A spatiotemporal investigation of varved sediments highlights the dynamics of hypolimnetic hypoxia in a large hard-water lake over the last 150 years[END_REF], and longer and stronger temperature stratification in summer (e.g., [START_REF] Adrian | A long-term study of the Heiligensee (1975-1992). Evidence for effects of climatic change on the dynamics of eutrophied lake ecosystems[END_REF][START_REF] Livingstone | Impact of secular ckimate change on the thermal structure of a large temperate central European lake[END_REF][START_REF] Foley | Long term changes in oxygen depletion in a small temperate lake: effects of climate change and eutrophication[END_REF]. Incomplete lake overturning may result in the development of a chemocline [START_REF] Davison | Iron and Manganese in lakes[END_REF] with an anoxic hypolimnion underneath [START_REF] Butz | Bacteriopheophytin a as an indicator of meromixis in sediments of Lake Jaczno, north-east Poland, CE 1891-2010[END_REF], which may last for prolonged periods (meromixis). Meromixis implies persistent lake stratification over multiple years caused by chemical differentiation of the bottom waters, (cf. seasonal thermal stratification which is common in deep temperate lakes and may occur above the chemocline in a meromictic lake). Investigations of long-term trends in oxygen depletion in lakes, and their relationship with climatic changes on timescales beyond the instrumental period, are still scarce in high Central Asia.

Located at the intercept between the influences of the Westerlies and the Siberian Anticyclone [START_REF] Chen | Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history[END_REF]2010), the Tian Shan holds key information about past climate variability and particularly temperature and hydrological changes in western Central Asia (e.g., [START_REF] Rousseau | Palaeoenvironmental change and glacier fluctuations in the high Tien Shan Mountains during the last millennium based on sediments from Lake Ala Kol, Kyrgyzstan[END_REF]. A number of valuable and high-quality lake sediment studies have examined Holocene climate development in the Tian Shan and Pamir mountains (e.g., [START_REF] Aichner | High-resolution leaf wax carbon and hydrogen isotopic record of the late Holocene paleoclimate in arid Central Asia[END_REF][START_REF] Beer | Vegetation history, fire history and lake development recorded for 6300 years by pollen, charcoal, loss on ignition and chironomids at a small lake in southern Kyrgyzstan (Alay Range, Central Asia)[END_REF][START_REF] Li | Moisture dynamics in central Asia for the last 15 kyr: New evidence from Yili Valley, Xinjiang, NW China[END_REF][START_REF] Mischke | Modern hydrology and late Holocene history of Lake Karakul, eastern Pamirs (Tajikistan): A reconnaissance study[END_REF][START_REF] Chen | Changes in Central Asia's water tower: Past, present and future[END_REF][START_REF] Heinecke | Late Pleistocene to Holocene climate and limnological changes ar Lake Karakul (Pamir Mountains, Tajikistan)[END_REF][START_REF] Heinecke | Climatic and limnological changes at Lake Karakul (Tajikistan) during the last ~29 cal ka[END_REF][START_REF] Heinecke | Vegetation change in the eastern Pamir Mountains, Tajikistan, inferred from Lake Karakul pollen spectra of the last 28 kyr[END_REF][START_REF] Rousseau | Palaeoenvironmental change and glacier fluctuations in the high Tien Shan Mountains during the last millennium based on sediments from Lake Ala Kol, Kyrgyzstan[END_REF], including several at Lake Son Kol (or Son Kul) in Kyrgyzstan [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF][START_REF] Pacton | Sedimentary facies analyses from nano-to millemetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)[END_REF][START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF][START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF][START_REF] Wolff | Precipitation evolution of Central Asia during the last 5000 years. The Holocene[END_REF][START_REF] Laug | Ecosystem shifts at two Mid-Holocene tipping points in the alpine Lake Son Kol (Kyrgyzstan, Central Asia)[END_REF]. The focus of these works has been on the Mid-to Late Holocene, with however limited palaeoenvironmental reconstructions between 8500 and 6000 cal. yr BP. Thus there are virtually no high-resolution palaeolimnological records of long-term lake mixing conditions from this region yet, which could provide information on the complex links between environmental, climatic and anthropogenic changes in high-altitude settings.

Here we combine hyperspectral imaging (HSI) and X-Ray fluorescence (XRF) analyses of a new core from Lake Son Kol (SK06) to obtain an unprecedented record of sedimentary pigment changes at ultra-high sampling resolution (μm-scale; sub-annual resolution).

Building upon the work of [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF], who inferred that most of the annual precipitation switched from the summer to the cold (winter) season between the Mid-and Late Holocene, we used sedimentary pigments and geochemical data to (i) decipher lake trophic status during the last 8500 years, (ii) investigate when anoxic conditions developed in the hypolimnion, and (iii) assess whether the occurrence of hypolimnetic anoxia can be linked to climatic changes in Central Tian Shan. We further evaluate whether phase boundaries identified in the multi-proxy dataset can be attributed to event-based phase transitions or more gradual shifts using multivariate statistical analyses. Comparisons with other palaeolimnological data from Lake Son Kol and regional records will be used to understand regional climate development in the Tian Shan, as well as implications for lake stratification dynamics during the Holocene.

Study site

Geological, environmental and lacustrine setting

The geology, geomorphology, climatic and lacustrine characteristics of Lake Son Kol have been extensively studied [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF][START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF].

Son Kol (41°50′52″N, 75°07′45″E, 3016 m a.s.l.) is the second largest natural lake in Kyrgyzstan, within the central Tian Shan mountain ranges (Fig. 1). It fills the central depression of an intramontane basin located between two generally east-west stretching mountain ranges, which are the Son Kol Too and Baidula Ranges (3200-3500 m a.s.l.) in the north and the Boor-Albas Range (3800-4000 m a.s.l.) in the south (Shitnikov, 1980). These mountain ranges are composed of Carboniferous sedimentary rocks and Permian granitoids (east and south) and Cambro-Ordovician metamorphic granodioritic and granitic gneisses, as well as strongly folded sedimentary rocks in the north (http://portal.onegeology.org/OnegeologyGlobal/). The modern equilibrium line altitude (ELA) of glaciers at north-facing slopes in the Central Tian Shan is ~3850-4100 m a.s.l., [START_REF] Narama | OSL dating of glacial deposits during the last glacial in the Terskey-Alatoo range, Kyrgyz Republic[END_REF][START_REF] Koppes | Late Quaternary glaciation in the Kyrgyz Tien Shan[END_REF], hence there are no glaciers in the lake watershed today and, based on ELA reconstructions, seem unlikely to have fed Lake Son Kol during the Holocene [START_REF] Savoskul | Late Holocene glacier variations in the frontal and inner ranges of the Tien Shan, central Asia[END_REF]. The widespread pasture-covered plains surrounding the lake are made up of Quaternary eroded material arranged in old Pleistocene moraines [START_REF] Abramowski | Pleistocene glaciations of Central Asia: results from 10 Be surface exposure ages of erratic boulders from the Pamir (Tajikistan) and the Alay-Turkestan range (Kyrgyzstan)[END_REF][START_REF] Narama | Record of a glacier variations during the last glacial in the Turkestan Range of Pamir-Alay, Kyrgyz Republic[END_REF] and inactive fan deltas. A large fluvial fan-delta nourished by cirque valleys of the Boor Albas range progrades from the southern shore into the lake. Along the southeastern and northeastern shore, large coastal sedimentary features such as sand spits and associated open lagoons have been developed by the prevailing westerly winds, indicating a stable lake level during at least the Late Holocene.

Today, Lake Son Kol is located above the treeline, on a high plateau surrounded by mountain meadows. Because of pronounced altitudinal changes over short distances, the vegetation rapidly changes within a few kilometers with the development of a lush alpine meadow in the lake catchment at ~3000 m a.s.l., while conversely steppe to semi-desert environments prevail in open lowlands (below 2000 m a.s.l) with sagebrush steppe (Artemisia) and Chenopodiaceae [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF].

Lake Son Kol is ~29-km long and ~18-km wide, with a surface area of ~275 km 2 and a catchment of ~1130 km 2 . The lake water volume is ~2.82 km 3 and the lake is presently ~13-m deep [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF]. According to the lake stratification classifications of [START_REF] Kalff | Limnology: Inland Water Ecosystems[END_REF], the depth and climatic location of Lake Son Kol should make it susceptible to shifts in mixing regime. The lake has a single outlet at its southern end, which discharges into the Naryn River and then to the Syr Darya down to the Aral Sea basin. Apart from direct precipitation on the lake surface, most surface inflow is from several small mostly episodic rivers fed by rainfall, groundwater and snowmelt from neighbouring mountains [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF]. A few perennial rivers supply the lake from the west where they braid in a small alluvial plain at the shore. Today, however, most of the inflow is contributed by groundwater (Beloglasov and Smirnova, 1987). Average lake water temperature fluctuates between 16 °C (14.5 °C in summer 2007;[START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF] and 0-2 °C in winter, with almost no stable thermal stratification throughout the year [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF]. Earlier monitoring data reported average summer water temperatures of 12-13 °C (Beloglasov and Smirnova, 1987), with maxima of 20-23 °C during warmer years. Freezing starts in October and ice break-up may begin as early as March (up to late April) depending on short-term climatic variability [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF].

The water column of Son Kol is well mixed and oxygenated to the lake bottom in the summer, though a small oxygen minimum has been noted at a water depth of 1.5 m revealing respiration in the epilimnion (July 2007;[START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF]. Monitoring data from the 1960s and 1970s similarly reported a well-oxygenated water body throughout the whole year, but [START_REF] Beloglasova | Atlas Kirgizskoj SSR[END_REF] observed oxygen concentrations of 0.8-1.8 mg.l -1 or less in the shallow water zone during winter. The strong fetch (maximum effective fetch of ~17 km; [START_REF] Håkanson | Principles of lake sedimentology[END_REF] and the high wind frequency (wind speeds up to 20 km.h -1 in summer and fall) impart efficient and complete mixing of the shallow water body. According to previous surveys the weather is calm only during 11% of the ice-free season (Beloglasov and Smirnova, 1987). In line with this, surface/intermediate flow currents reaching 2-8 cm.s -1 across the water body (and up to 10 cm.s -1 in the deepest part of the basin) reflect a highly dynamic lacustrine system (Beloglasov and Smirnova, 1987). Such conditions guarantee for a steady mixing of the whole water column and thus makes Son Kol mostly a polymictic lake nowadays (apart from under ice in winter). Son Kol is a moderate hard-water lake (4.56-4.96 mg -1 ) with a mineralisation of 465 mg.l -1 (summer) and 306 mg.l -1 (winter) (Beloglasov and Smirnova, 1987). Accordingly, today it is a bicarbonate lake with weak magnesium dominance. Conductivity throughout the water column ranged between 515 and 530 mS.cm -1 in the summer of 2012 [START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF].

Climatic setting

Local climatic conditions lead to short temperate summers and severe cold winters, with snow cover between November and April. Mean annual air temperature is -3.5 °C (mean of 11 °C in summer and -20 °C in winter), based on the nearest meteorological station (Dolon, 41.8° N, 75.8° E, 3040 m a.s.l.) [START_REF] Shnitnikov | Lakes of the Tian Shan and their history[END_REF]. Mean annual precipitation is 500-600 mm at Lake Son Kol, with a maximum in summer and a minimum in winter (Academy of Science of the Kyrgyz SSR, 1987). The current precipitation regime in the Tian Shan Mountains surrounding Son Kol is mainly regulated by the interaction between cyclonic activity driven by the mid-troposphere Westerlies and the southwestern branch of the Siberian anticyclonic circulation [START_REF] Aizen | Climatic and hydrologic changes in the Tien Shan, Central Asia[END_REF]. Atmospheric dust concentrations (and loess) are maximal below 3000 m a.s.l in Central Asia, with thickest loess accumulations occurring between 1000 and 1300 m a.s.l in the piedmont regions of the Tian Shan [START_REF] Machalett | Aeolian dust dynamics in Central Asia during the Pleistocene: driven by the long-term migration, seasonality and permanency of the Asiatic polar front[END_REF]. Different airflows govern convective surface winds in western Central Asia and thus, the regional distribution of remote dust particles over the year [START_REF] Bugaev | Sinopticheskie Protsessy Sredney Azii (Synoptic processes in Central Asia)[END_REF]Shettler et al., 2014).

During the summer northwestern, western and northern air intrusions and northeast anticyclonic flows control up to 83% of the local wind directions. Dust monitoring near Bishkek reveals besides that wet deposition is a substantial process for aerosol deposition in mountain areas (Shettler et al., 2014). Nonetheless, backwards trajectories calculated from Son Kol document that remote air masses likely also come from southern (Afghanistan) and further western (Mediterranean) and northern (North Atlantic) sources (Shettler et al., 2014).

Material and methods

Lake coring and cross-core correlation

Four sediment cores, including two sets of parallel cores, were extracted with a Uwitec piston corer in July 2007 from two different sites in the southeastern part of Lake Son Kol (Fig. 1).

Core SK04 (133cm) was obtained at ca. 1.15 km from the southern shore (41°46'56.6"N, 75°08'18.2"E, 10.5 m water depth) while a second set of parallel cores, SK06 and SK07 was retrieved at 1.6 km northeast of core SK04 (41°47'45.2"N, 75°08'22"E, 12.5m water depth) (Fig. 1). Although cores SK04 and SK07 have been extensively analyzed recently [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF][START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Pacton | Sedimentary facies analyses from nano-to millemetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)[END_REF], no integrative study has been undertaken on core SK06 yet, although it represents the longest lacustrine archive ever retrieved in Lake Son Kol. In spite of the significant distance between the two coring sites, the cores exhibit very similar sedimentary successions (and lithological marker layers), thus allowing a robust core-to-core correlation (Fig. 2a), as also demonstrated by the good match between magnetic susceptibility records for cores SK04, SK06 and SK07 (Fig. S1). Core SK06 was split in half lengthwise and photographed directly after core opening. A detailed macroscopic description established the different sedimentary facies (Units I-VI) as well as the stratigraphic correlations between cores SK04, SK06 and SK07 (Fig. 2a). No compaction and/or disturbance was observed in SK06.

Age-depth model

Owing to the robust core-to-core correlation between cores SK06, SK04 and SK07 (Figs. 2 andS1) and SK06 and SONK_11_D1/2 from [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF] (Fig. S2), the chronology of core SK06 (Fig. 2b) was established using available radiocarbon (AMS 14 C) dating of both terrestrial plant macroremains and bulk sediment organic matter (OM), complemented by one short-lived radionuclide measurement ( 241 Am). We selected and reappraised 8 bulk sediment samples and one terrestrial leaf from cores SK04 and SK07 [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF] along with one terrestrial plant remains from core SONK_11_D1/2 [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF] (Table 1). 14 C age anchor points were transferred to core SK06 using a detailed correlation of both lithological marker layers and magnetic susceptibility between core SK06 and SK04 and SK07 (Figs. 2a and S1) and between cores SK06 and SONK_11_D1/2 (Fig. S2). For the latter, the correlation at 33 cm in core SK06 of Poz-44073 date (at 33 cm in SONK_11_D1/2; et al. (2014).

In order to constrain the age of the topmost sediments in core SK06, we used the close agreement of lithological markers in cores SK06 and SONK_11D1/2 and assumed that the notable increase in 241 Am activity at 5.5 cm in SONK_11_D1/2 occurs at 2 cm in core SK06 (Fig. S2), and corresponded to the Northern Hemisphere nuclear weapons testing maximum in 1963 CE [START_REF] Appleby | 241 Am dating of lake sediments[END_REF]. The topmost ca. 2 cm of SONK_11_D1/2 are missing in core SK06. Finally, the age-depth model for core SK06 was established using 10 corrected conventional radiocarbon ages (Table 1), as well as the 241 Am anchor point at 2 cm sediment depth (most likely 1963 CE), as input parameters for a P_Sequence deposition model implemented in Oxcal 4.4 [START_REF] Ramsey | Deposition models for chronological records[END_REF] (Fig. 2b). The k parameter was set to 1 (as in [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF] and the radiocarbon ages were converted to calendar years by using the IntCal20 terrestrial calibration curve of [START_REF] Reimer | The IntCal20 Northern Hemisphere radiocarbon age calibration curve (0-55 cal kBP)[END_REF]. The high agreement index Amodel of 89.8% indicates that the chronology of core SK06 is reliable [START_REF] Ramsey | Radiocarbon calibration and analysis of stratigraphy: The OxCal program[END_REF][START_REF] Ramsey | Development of the radiocarbon calibration program[END_REF].

Geochemistry

Major elements in the sediments were measured by XRF at high-resolution (500-μm step size) on the split-core surface with an Avaatech Core Scanner (EDYTEM Laboratory). Prior to measuring, the core surface was cleaned and foiled to prevent contamination and core desiccation. The X-ray beam was generated with a rhodium anode and a 125-μm beryllium window, which allows a voltage range of 7-50 kV and a current range of 0-2 mA.

Geochemical data were obtained with different settings according to the elements analyzed.

They were adjusted at 10 kV and 1 mA for 10 s to detect Si, Ca, Al, Fe, Ti, K, Mn and S. For heavier elements (i.e., Sr, Rb, Zr, Br and Pb), measurements were performed at 30 kV and 9.75 mA for 30 s. XRF data are displayed as element intensities in count per second (cps) (Fig. S3). Because no major element concentration was conducted on core SK06, we rely here on XRF core-scanner data available on core SK07 (unpublished data) in order to calibrate XRF measurements in core SK06. Major element concentrations on core SK07 were previously measured on 1.5-cm thick discrete samples using XRF at the University Lyon1.

The sampling was conducted at a step of about 3-5 cm to allow for one calibration measurement every 3-5 core scanner measurement in core SK07.

Hyperspectral imaging

Hyperspectral imaging is a visible and near infra-red spectroscopic method measuring the reflectance of the sediment surface exposed to an incident light, and thus the sediment composition [START_REF] Barranco | Quantitative reassessment of brick red lutitesevidence from reflectance spectrophotometry[END_REF][START_REF] Balsam | Determining the composition of Late Quaternary marine sediments from NUV, VIS, and NIR diffuse reflectance spectra[END_REF][START_REF] Balsam | Comparison of shipboard vs. shorebased spectral data from Amazon Fan cores: implications for interpreting sediment composition[END_REF].

Prior to analysis, the thin, superficial layer was scraped from the sediment to ensure a uniform surface. Reflectance spectroscopy was performed on a core logger bench equipped with two hyperspectral cameras (VNIR-PFD and SWIR, SPECIM®), which offers a short sampling time and allows for high spatial resolution (several dozens of μm; [START_REF] Jacq | Hyperspectral core-logger image acquisition[END_REF]. The distance between the studied sample/core and the camera lens was 130 mm, with an acquisition angle of 90°. The surface of the sediment is indirectly illuminated by 18 halogen bulbs homogeneously distributed around the lens. The acquisition speed of the core logger was 0.5 mm.s -1 . Pixels in the resulting Visible and Near Infrared (VNIR) image each cover an area of 47×47 μm 2 and contain raw spectral data from 400 to 1000 nm at 6.5 nm spectral resolution (given in digit number, DN), whereas the Short Wave-Infrared (SWIR) image pixels have a resolution of 150×150 μm 2 and cover a spectral range from 1000 nm to 2500 nm at 10 nm spectral resolution. Therefore, each pixel contains spectral information related to sediment composition. The depth of field of the imagers is ca. 2 mm. A normalization step is required to convert raw data (8 bits) to % reflectance. At each acquisition, a 100% reflective SPECTRALON standard was measured and used as a reference to express the data as % reflectance. Data processing was carried out with the ENVI (v. 5.3) and IDL 8.0 softwares.

Pigment composition and concentrations were inferred from area indices (Fig. 3), which directly measure the surface area (here referred to as relative absorption band area or RABA; [START_REF] Trachsel | Scanning reflectance spectroscopy (380-730 nm): a novel method for quantitative high-resolution climate reconstructions from minerogenic lake sediments[END_REF] between the continuum and the reflectance spectrum at the absorption band targeted (for instance for the chlorophyll-a and its derivatives at 670 nm). The sum of the reflectance bands located throughout the absorption band is subtracted from the area under a line connecting the beginning and the end of the depression formed by the absorption band.

In this study, area indices are followed by wavelengths indicating the width of the absorption band considered (e.g. RABA630-700). The main absorptions are determined by continuum removal, which also allows estimating the most interesting wavelength interval. In this study, we have defined four of them centered on 670 nm, 615 nm, 844 nm and 1670 nm (Fig. 3).

Coprophilous fungal spore analysis

Catchment erosion processes in high-altitude landscapes may be re-shaped by livestock grazing related to pastoral activity [START_REF] Giguet-Covex | Changes in erosion patterns during the Holocene in a currently treeless subalpine catchment inferred from lake sediment geochemistry (Lake Anterne, 2063 m.a.s.l, NW France): the role of climate and human activities[END_REF]. The variability of grazing pressure and associated soil erosion in a catchment area can be reconstructed from the sedimentary abundance of coprophilous fungal ascospores in lake sediments, as a proxy for the density of wild and domestic herbivores (Davies and Schaefer, 2006;[START_REF] Gauthier | Pollen and non-pollen palynomoprh evidence of medieval farming activities in southwestern Greenland[END_REF][START_REF] Etienne | Influence of sample location and livestock numbers on Sporormiella concentrations and accumulation rates in surface sediments of Lake Allos, French Alps[END_REF]. Son Kol catchment is still an area of transhumance today; therefore, domestic herbivores are assumed to be the primary source of coprophilous fungal spores in lake sediments, and wild herbivores are regarded to be a trivial source. To evaluate the impact of grazing activity on soil erosion in the catchment, abundances of coprophilous fungi ascospores were measured in 37 samples of core SK06 (for the last 6000 years). The resolution is approximately 100 years from 6000 to 5000 cal. yr BP, and about 250 years from 5000 to 1500 cal. yr BP. Samples were chemically prepared according to the procedure of [START_REF] Faegri | Textbook of pollen analysus[END_REF]. Lycopodium clavatum tablets (averaging 20,848 spores per tablet)

were added to each sample [START_REF] Stockmarr | Tablets with spores used in absolute pollen analysis[END_REF] to express the results as influx or accumulation rates (number.cm -2 .yr -1 ). Coprophilous ascospores were identified and counted using a photonic microscope following pictures and descriptions in [START_REF] Van Geel | Fossil ascospores of the parasitic fungus Ustulina deusta in Eemian deposits in Denmark[END_REF], [START_REF] Van Geel | Environmental reconstruction of a Roman Period settlement site in Uitgeest (The Netherlands), with special reference to coprophilous fungi[END_REF], Van Geel andAptroot, 2006, Feeser and[START_REF] Feeser | Fresh insights into long-term changes in flora, vegetation, land use and soil erosion in the karstic environment of the Burren, western Ireland[END_REF], [START_REF] Cugny | Modern and fossil non-pollen palynomorphs from the Basque mountains (western Pyrenees, France): the use of coprophilous fungi to reconstruct pastoral activity[END_REF] and Gelorini et al. (2012). Total coprophilous fungi sensu stricto include Delitschia (UG-1066), Neurospora sp. (HdV-1), Podospora-type (HdV-368), Sordaria-type (HdV-55A), Sporormiella (HdV-113), and Trichodelitschia (HdV-546).

Data analysis

Linear dimension reduction was performed by applying a correlation matrix-based principal component analysis (PCA) using the PAST v. 4.02 software [START_REF] Hammer | PAST: Paleontological Statistics Software Package for Education and Data Analysis[END_REF]. The PCA was supplied with XRF (major elements) and hyperspectral data from core SK06 (Fig. 4).

Due to very high levels of non-normality and heteroscedasticity of the original descriptors, data were first log-transformed and then separately centered on a per-sample basis in order to remove overall-count effects and to focus on relative variations among samples. Also using the PAST software, time series corresponding to the resulting PC1 and PC2 were further investigated using Gallagher et al.'s (2011) Bayesian "changepoint modeling" algorithm for identifying abrupt regime shifts in time series, often referred as tipping points in geosystems [START_REF] Lenton | Tipping elements in the Earth"s climate system[END_REF][START_REF] Lenton | Early warming of climate tipping points[END_REF][START_REF] Scheffer | Anticipating critical transitions[END_REF]. Number of MCMC iterations required by Gallagher et al.'s method was set to 1,000,000 with a burn-in phase of 20,000 iterations; up to 300 changepoints were allowed a priori in order to generate fullyunconstrained changepoint models, resulting in an average number of changepoints of 74 and 61 for the PC1 and PC2 time series, respectively.

Results and interpretation

Sedimentology

Sediments mostly consist of alternating yellowish and light-brownish calcareous muds in the upper 1.22 m of core SK06, whereas dark organic-rich muds alternating with yellow-brown calcareous muds and white carbonate layers predominate between 1.22 and 1.91 m (Fig. 2).

The transition between the basal organic-rich sediments and the upper 1.22 m alternating calcareous muds is characterized by homogeneous greyish calcareous clays between 0.94 and 1.22 m. Apart from this transition, the sediments are finely laminated in core SK06, and especially in dark organic sediments. XRF core scanning data allows deciphering the distinct geochemical composition of the three main lithologies (Fig. S3). Dark organic sediments are typified by their high abundances in Br and S (Figs. S3 and5), whereas the intervening carbonate layers exhibit an enrichment in Ca and Sr (Fig. S3). In contrast, the transition interval (greyish homogenous clays) is best described by high abundances of Ti, Rb, Zr, Fe and Si, emphasizing their high mineral content. The upper alternations between yellowish and light-brownish sediments are characterized by a mixed geochemical composition and exhibit fluctuating values of Ti, Rb and Zr, but also highly variable abundances of Ca and Sr (Fig. S3).

VNIR-SWIR proxy data

Chlorophyll a (Chl a) and its derivative products [START_REF] Rein | In-situ reflectance spectroscopy-analysing techniques for highresolution pigment logging in sediment cores[END_REF][START_REF] Das | Inferring lake sediment chlorophyll concentrations with reflectance spectroscopy: a novel approach to reconstructing trophic status changes in alpine lakes[END_REF][START_REF] Wolfe | Experimental calibration of lakesediment spectral reflectance to Chlorophyll a concentrations: methodology and paleolimnological validation[END_REF][START_REF] Michelutti | Do spectrally inferred determinations of chlorophyll a reflect trends in lake trophic status?[END_REF] was measured as the area index between 630 and 700 nm (RABA630-700) normalized by the absorption maximum at 670 nm (R670) (Fig. 3) as defined by [START_REF] Wolfe | Experimental calibration of lakesediment spectral reflectance to Chlorophyll a concentrations: methodology and paleolimnological validation[END_REF]. Although Chl b may contribute in the absorption range between 630 and 700 nm (i.e., at 652 nm, Bidigare et al., 1990;[START_REF] Yacobi | Phycocyanin-specific absorption coefficient: eliminating the effect of chlorophyll absorption[END_REF], it is not observed in our spectral data (Fig. 3). We therefore assume that the measured absorption peak Phycocyanin is a water-soluble accessory pigment to chlorophyll exhibiting a notable absorbance trough around 610-620 nm for C-phycocyanin [START_REF] Glazer | Spectroscopic properties of C-Phycocyanin and of its α and β subunits[END_REF][START_REF] Yacobi | Phycocyanin-specific absorption coefficient: eliminating the effect of chlorophyll absorption[END_REF]. For this study, we propose to screen the phycocyanin content by measuring the area index between 600 and 630 nm (RABA600-630) normalized by the absorption peak at 615 nm (R615) as described by [START_REF] Yacobi | Phycocyanin-specific absorption coefficient: eliminating the effect of chlorophyll absorption[END_REF] (Fig. 3). It must be recalled, however, that the absorption of Chl a and accessory chlorophylls Chl b and Chl c partly overlap the range of optical activity of phycocyanins, although weakly [START_REF] Yacobi | Phycocyanin-specific absorption coefficient: eliminating the effect of chlorophyll absorption[END_REF]. However, the RABA630-700 and RABA600-630 curves exhibit different patterns (Fig. 5), implying that a genuine signal does exist apart from the Chl a plus derivatives. This also suggests that phycocyanin predominates in some intervals where Chl a plus derivatives do not (Fig. 5), thus allowing their respective trends to be distinguished and analyzed separately in core SK06.

Phycocyanins are common in cyanobacteria [START_REF] Williams | Cyanobacterial phycobilisomes[END_REF], and their concentration in shallow lacustrine systems is used to estimate the presence and relative abundance of cyanobacteria (Horva th et al., 2013). Phycocyanin has been reported in lacustrine sediments of Lake Qinghai, Central China [START_REF] Ji | Centennial blooming of anoxygenic phototrophic bacteria in Qinghai Lake linked to solar and monsoon activities during the last 18,000 years[END_REF] and used to indicate cyanobacterial blooms in the Baltic Sea [START_REF] Woźniak | Empirical model for phycocyanin concentration estimation as an indicator of cyanobacterial bloom in the optically complex coastal waters of the Baltic Sea[END_REF]. Chl a plus derivatives and phycocyanin contents are thus interpreted here as a proxy for oxygenic phototrophs, which usually indicate aerobic conditions. Total Bacteriopheophytin a (Bphe a) is a stable degradation product of Bacteriochlorophyll a or Bchl a with a characteristic spectral absorbance in the infrared range around 750-770 nm in ethanol extracts [START_REF] Coolen | Analysis of subfossil molecular remains of purle sulphur bacteria in a lake sediment[END_REF][START_REF] Rogozin | Ecology of purple sulfur bacteria in the highly stratified meromictic Lake Shunet (Siberia, Khakassia) in 2002-2009[END_REF] and 800-900 nm in sediments [START_REF] Scheer | An overview of chlorophylls and bacteriochlorophylls : biochemistry, biophysics, functions and applications[END_REF][START_REF] Ji | Centennial blooming of anoxygenic phototrophic bacteria in Qinghai Lake linked to solar and monsoon activities during the last 18,000 years[END_REF][START_REF] Blankenship | Molecular mechanisms of photosynthesis[END_REF]. Bphe a was determined from the area index between 750 and 900 nm (RABA750-900) normalized by the absorption maximum at 844 nm (R844) as defined by [START_REF] Butz | Hyperspectral imaging spectroscopy: a promising method for the biogeochemical analysis of lake sediments[END_REF] (Fig. 3). Because no other known bacterial pigment absorbs at this wavelength, the relative abundance of Bchl a (and its derivative by-products Bphe a) can be evaluated by measuring the Bphe a absorption trough between 750 and 900 nm normalized by the absorption peak at R844. Although Bacteriopheophytin b (Bphe b) may also contribute at 820 nm [START_REF] Oren | Characterization of pigments of prokaryotes and their use in taxonomy and classification[END_REF], no absorption peak is observed at that wavelength in our data (Fig. 3). We therefore assume that the measured absorption peak of particles at 844 nm represents exclusively the optical activity of Bphe a. Bphe a is produced by planktonic anoxygenic phototrophic bacteria (APBs) such as purple sulphur bacteria (Chromatiales, Rhodospirillales; [START_REF] Schmidt | Impact of late glacial climate variations on stratification and trophic state of the meromictic lake Längsee (Austria): validation of a conceptual model by multi-proxy studies[END_REF][START_REF] Rogozin | Ecology of purple sulfur bacteria in the highly stratified meromictic Lake Shunet (Siberia, Khakassia) in 2002-2009[END_REF] which commonly live in the chemocline of stratified meromictic lakes, where reduced sulphur species are very abundant and oxygen levels are low, if sufficient light is available for photosynthesis [START_REF] Butz | Bacteriopheophytin a as an indicator of meromixis in sediments of Lake Jaczno, north-east Poland, CE 1891-2010[END_REF]. Hence Bphe a is a diagnostic pigment for photosynthesis in anaerobic conditions and the presence of that pigment in the sediment may help infer longterm lake stratification, and the presence of a chemocline (Fig. 5). Aromatic organic matter content (ARO) has a characteristic absorption near 1670 nm [START_REF] Wheeler | Near infrared spectra of organic coumpounds[END_REF] and it seems that no other compound absorbs at this wavelength, so we assume that an index in this spectral range can infer this property (Fig. 5). Nevertheless, we are aware that SWIR measurements may be influenced by sediment moistness and grain size, for which no known correction can be applied yet. For this study we propose the area index between 1660 and 1690 nm (RABA1660-1690), normalized by the absorption maximum at 1670 nm (R1670).

ARO values are interpreted as a proxy for tracking the inputs of terrestrial organic matter from the shore (e.g., soils) and related catchment, with high (low) ARO corresponding to increased (decreased) inputs of detrital organic matter.

PCA analysis

XRF and hyperspectral (VNIR, SWIR) data, with a temporal resolution ranging between 3 yr.mm -1 and 7-8 yr.mm -1 , were jointly analyzed by PCA in order to characterize sediment end-members and detect abrupt shifts in the multivariate time series (Fig. 4a). The XRF corescanner of Ca, Fe and Ti was previously calibrated using XRF conducted on discrete samples in core SK07 (28 samples), exhibiting an excellent match between the two methods (Fig. 4b).

PC1, PC2 and PC3 together explain about half of the total variance (Figs. 4a andS4), allowing for the identification of different end-members in the lake system: (i) high Ti, Rb, Fe and RABA1660-1690 values for PC1 > 0 (which are related to organic and minerogenic inputs from the catchment), while Sr, Ca (which indicate endogenic carbonate precipitation)

and to a lesser extent Zr (coarse detrital input) drive PC1 < 0, making PC1 a proxy for allochthonous vs autochthonous products in the lake system; (ii) high Mn, RABA600-630 and RABA630-700 values for PC2 > 0 (which are related to oxic conditions in the water column) vs. high Br, S (and to a lesser extent Zr) and RABA750-900 values (which are linked to organic matter content and infer reducing and sulphidic conditions in bottom waters) for PC2 < 0, making PC2 a proxy for oxygenated vs. euxinic bottom water conditions in Lake Son Kol. Based on this result, prominent environmental change occurred around 8400, 8200-7800, 7700-7500, 7300-7000, 6500-6100, 6000-5700 and 5500-5250 cal. yr BP (Fig. S5), when increased influx of detritus (PC1 > 0) is in close correspondence with the development of euxinic conditions in the hypolimnion (and thus of a chemocline; PC2 < 0). The Si content appears to slightly differ from other terrigenous (river-borne) elements (PC1 and also PC3

[14.4%], whose palaeoenvironmental signature is more difficult to interpret and is not discussed here) (Fig. S4). Si-value is high in the silt fraction and is, in addition to its weak riverine contribution, mostly linked with aeolian inputs into the lake [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF].

Judging from the lack of any conspicuous link to the other elements (Fig. 4a) and given the biochemical reactivity of this element, Mn is used to decipher bottom-water redox processes, as already reported in other lacustrine settings [START_REF] Wirth | Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps)[END_REF][START_REF] Davies | Micro-XRF core scanning in palaeolimnology: recent developments[END_REF].

Time series of hyperspectral and geochemical data

Between 8500 and 7000 cal. yr BP (191-164 cm), phycocyanin (from cyanobacteria) has the highest abundances downcore, probably related to the development of benthic cyanobacterial mats in a shallow water body (Fig. 5). Chl a plus derivatives and Bphe a also oscillate at very high frequency. Bphe a and ARO maxima occur repeatedly around 8500, 8400, 8200-7800, 7700-7500 and 7300-7000 cal. yr BP, corresponding to dark organic-rich sediments along with high Br and S content. No match was found, however, between phycocyanin and Chl a.

Subsequently, Chl a plus derivatives and phycocyanin exhibit a pronounced decline between 7000 and 6500 cal. yr BP. Phycocyanin abundances drop from 7000 cal. yr BP onwards and never recover to former levels. Bphe a and ARO values also exhibit a drastic decrease in this time interval, as also do Br and S content.

From 6500-5250 cal. yr BP, Chl a plus derivatives values increase and reach their highest abundances downcore, with however considerable fluctuations. Bphe a content recovers to high levels and even dominates between 5750 and 5250 cal. yr BP, but dramatically decreases and readily disappears from 5250 cal. yr BP onwards (Fig. 5). Coevally, Br, S and ARO values are also high between 5750 and 5250 cal. yr BP (corresponding to the deposition of dark organic sediments), but rapidly drop after 5250 cal. yr BP. Chl a plus derivatives and phycocyanin exhibit a prominent decline in their abundances between 5000 and 3850 cal. yr BP. Judging from XRF core-scanner data, enhanced detrital inputs occurred in this interval (Fig. S3). Subsequently, Chl a plus derivatives and phycocyanin increase from 3850-3100 cal. yr BP, whereas Bphe a still oscillates at very low levels. Higher Br, along with enhanced ARO, also parallels the occurrence of dark organic-rich sediments in this interval. Over the course of the last 3000 years, oxygenic phototrophs fluctuate at lower levels, whereas Bphe a, Br and ARO remain low (Fig. 5).

Abrupt regime shifts

Abrupt regime shifts were identified based on slope variations of the average changepoint model calculated separately for PC1 and PC2. For each principal component, average changepoint model's slope (absolute value) is plotted against time, with 99% and 95% percentiles taken as cut-off values (Figs. 6 and7). Time intervals with average changepoint model's slope values above these cut-offs indicate abrupt changes in the time series, and can therefore be regarded as tipping points. For PC1, tipping points are recognized as 10-20 yr intervals (beyond the 99% percentile value) at ca. 3670, 5050-5080 (highest slope value), 5250,5370,5850,6150,6220,6900,7030,7980,8230,8280 and 8340 cal. yr BP (Fig. 6).

For PC2, tipping points are similarly identified as 10-20 yr intervals (beyond the 99% percentile value) around 5250 (highest slope value), 5950, 6040, 6230, 6280, 6380, 6670, 20 6700, 6900, 7060, 7750, 7800, 8320 and 8380 cal. yr BP (Fig. 7). Hence the time intervals around 5050-5250, 6220-6240 and 8200-8400 cal. yr BP are detected for both PC1 and PC2, and probably correspond to abrupt regime shifts in the lacustrine system. Interestingly, low PC2 values match with a strong peak of magnetic susceptibility (Figs. S1 and S5) around 8200-8400 cal. yr BP, suggesting the occurrence of abrupt changes in the lake and surrounding catchment that are tied with the 8200 cal. yr BP cold event widely recognized in the Northern Hemisphere [START_REF] Alley | Holocene climate instability: a large event 8000-8400 years ago[END_REF][START_REF] Rohling | Centennial-scale climate cooling with a sudden cold event around 8,200 years ago[END_REF].

Discussion

Between ca. 8500 and 5250 cal. yr BP anaerobic conditions (and the penetration of light deep in the lake to support photosynthesis for APB) are inferred from high Bphe a and Br (PC2 < 0), along with the occurrence of dark organic layers (Fig. 5). Coevally, high ARO and minerogenics content (PC1 > 0) suggest enhanced inputs of detrital organic matter and a link between terrestrial influx, sedimentary dynamics and the advent of hypolimnetic anoxia in the earlier part of the record. After 5250 cal. yr BP, the near absence of Bphe a, lower ARO values but persistently high abundances of oxygenic phototrophs suggest prolonged aerobic conditions within a well-mixed water body (Fig. 5).

Episodes of increased snowmelt and indicators of lake-level changes

Grain size and geochemical/isotopic (TC, TN, TOC, C/N, δ 13 Corg and δ 18 O) data can be useful in supporting interpretations from hyperspectral and XRF data. Here we compare previously unpublished and available data from SK07 [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF] to help interpret the 8500-5000 cal. yr BP time interval during which dark organic intervals are recurrently interbedded with yellow-brown calcareous muds (Fig. 7). Altogether, these parameters help to distinguish changes in onshore and in-lake environmental conditions such as sheet-wash from the shore, productivity and nutrient availability, and the possible mechanisms at work.

Generally, the timing of changes in geochemical and isotopic proxies in core SK07 agree with those of XRF and hyperspectral data from core SK06 (Fig. 8). Dark organic intervals with Br and ARO in core SK06 have elevated TOC, TC, TN and C/N values but strongly depleted δ 13 Corg and δ 18 O values in core SK07. Such prominent δ 13 Corg depletions might indicate the predominance of APB during phases of lake stratification because pronounced carbon isotope fractionation towards lighter δ 13 C has been associated with purple sulphur bacteria in meromictic lakes [START_REF] Zyakun | Fractionation of stable carbon isotopes by photoautrophically growing anoxygenic purple and green sulfur bacteria[END_REF]. Alternatively, short-lasting depletions in δ 18 O can indicate seasonal shifts in the hydrological regime; i.e., increased winter precipitation and/or increased meltwater discharges linked to erosive runoff during spring snow thaw, but also increased temperatures [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF] that would have fueled meltwater runoff. The dark organic-rich and δ 18 O-depleted intervals co-occur with coarser (e.g., silt-and sand-sized) minerogenic and organic (e.g. phytoclasts) inputs into the lake (Figs. 8 and S6), whereas the intervening yellow-brown calcareous muds are enriched in finer (clay-sized) detritus with low sand content (Fig. 8). Erosive runoff after thawing during spring [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF], and/or higher winter snowfall [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF], can account for coarse detritus discharge into the lake. Interestingly, the PCA analysis reveals that higher detrital inputs (PC1 > 0) are not associated with elevated Zr content in core SK06 (Fig. 4); instead, dark organic layers exhibit low Zr content (Figs. 8 andS3). Zr is usually more concentrated in sand-size lacustrine sediments (e.g., [START_REF] Cuven | Estimation of grain size variability with micro Xray fluorescence in laminated lacustrine sediments, Cape Bounty, Canadian High Arctic[END_REF][START_REF] Davies | Micro-XRF core scanning in palaeolimnology: recent developments[END_REF] and so, this Zr depletion in a few coarse dark organic intervals is puzzling, since otherwise all detritics (Ti, Rb and Zr) exhibit similar trends in the intervening yellow-brown calcareous muds (Fig. S3). Therefore, enhanced levels of Ti and Rb along with lower Zr (thus PC1 > 0)

in the Br-enriched (PC2 < 0) dark organic layers either suggest the dilution of Zr input by Ti-and Rb-laden terrestrial organic matter delivered during snow thaw (as inferred from high ARO values), and/or the dilution of Zr (associated with the coarse detrital fraction) during relatively high lake water level phases. Zr mainly originates from erosion processes in the upper catchment of Lake Son Kol because Zr is abundant in local Paleozoic intrusives (granites and granodiorites), but these tend to be covered by Quaternary sediments and soils (colluvial and glacifluvium deposits) on the shoreline belt and further up-valley. Although Zr is linked to Br and S on PC2, Zr is also significantly associated with elevated Sr and Ca (PC1 < 0; Fig. 4), suggesting that Zr is enhanced during low lake water levels, which promotes endogenic carbonate precipitation [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF]. Hence higher Zr infers the increasing influence of riverine discharges at relatively low lake levels and the reduced distance between the coring site and proximal detrital sources. Dilution of Zr most probably occurs during short-lived increases in lake levels, which in turn enhance the formation of organic-rich deposits in Lake Son Kol when elevated terrestrial inputs from snowmelt strengthened the export of dissolved organic matter into the lake. The most probable sources of material during snowmelt discharges (e.g., floods) are the abundant fan delta/fluvial material at the southern edge of the lake, and/or superficial sediments on the slopes (e.g., colluvium) directly adjacent to Lake Son Kol. In contrast, higher Ti, Rb and Zr occur coevally during lower lake level phases, which correspond to the deposition of grey calcareous muds enriched in fine siliciclastics (Fig. S3). Hence higher Zr, Sr and Ca values indicate lower lake water levels and coincide with the deposition of yellow-brown calcareous muds. In contrast, dark organic deposits enriched in Br, S, C/N and TOC are associated with the injection of large volumes of sediment-laden sheet-wash from the shores (high ARO values) during intensified winter precipitation and/or stronger snowmelt [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF], which are likely to occur under warmer climate conditions.

Redox conditions and oxygenation of the hypolimnion

The geochemical behavior of Mn-enriched phases can be used to infer past redox conditions and thus help diagnose oxic and sulphidic redox regimes in the water column (Fig. 9), as reported from Lake Cadagno [START_REF] Wirth | Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps)[END_REF]. Based on elevated Mn content, oxic conditions (PC2 > 0) prevailed around 8300-8200, 7400-7250, 5500-5400 and in particular between 5000 and 3800 cal. yr BP in Son Kol when Mn was flushed to the lake with detrital input, and probably occurred in the water column as dissolved unstable Mn 2+ or as oxidized, particulate Mn-(oxyhydr)oxides. However, no Mn-(oxyhydr)oxides were found in SK06, possibly due to a reduction of Mn 2+ (leading to a further recycling of Mn into the water column) if the sediment-interface and/or the sediments were reducing. Oxic water column conditions were accompanied by the precipitation of various carbonate minerals judging from the regular occurrences of carbonate layers [START_REF] Pacton | Sedimentary facies analyses from nano-to millemetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)[END_REF], as well as high XRF-Ca content in the yellow-brown calcareous muds separating dark organic-rich sediments, thus reflecting the high alkalinity of lake waters during those time intervals. The regular occurrence of sediment-dwelling ostracods and molluscs (which require well-oxygenated bottom waters) further supports that a well-mixed (polymictic) water body, with oxic bottom waters, has been established during the last 5000 years [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF], in accordance with recent monitoring data [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF].

In contrast, there is compelling evidence that the hypolimnion recurrently turned to a sulphidic (euxinic) redox regime between 8500 and 5250 cal. yr BP, involving a stabilization of the water-column stratification and the full development of a chemocline. Low sedimentary Mn concentrations (Fig. 9), the absence of carbonate phases and the drastic increases in Br and S content (PC2 < 0) characterize several phases of euxinic bottom waters between 8500 and 5250 cal. yr BP. Coevally, high Bphe a values (inferring that purple sulphur bacteria thrived in the water column), and the rising TOC content in dark organic-rich sediments (Fig. 8), are good indicators of reducing bottom water conditions. Although such a hypothesis was ruled out by [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF] based on the absence of varves and/or vivianite crystals (the latter indicating anoxic conditions in a non-suphidic environment during sediment deposition, which is obviously not the case here) in dark organic-rich sediments, several lines of evidence advocate for the recurrence of hypolimnetic euxinia (and thus the development of a chemocline) in Lake Son Kol before 5250 cal. yr BP. Prolonged lake stratification and reducing oxygen conditions around 8500, 8400, 8200-7800, 7700-7500, 7300-7000, 6500-6100, 6000-5700 and 5500-5250 cal. yr BP (Fig. 9) are inferred from (1) high TOC contents (up to 16%) in dark organic-rich layers (Fig. 8) that imply both reducing bottom water conditions which prevents organic matter degradation and especially of photosynthetic pigments (Figs. 7 and9); (2) the unusual preservation of numerous colonies of coccoid prokaryotes altogether with the preserved intracellular content of cyanobacteria that requires anoxic conditions [START_REF] Pacton | Sedimentary facies analyses from nano-to millemetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)[END_REF] and in particular, (3) the regular occurrence of Bphe a pigments that indicate photosynthesis in anaerobic conditions and thus hypolimnetic anoxia between 8500 and 5250 cal. yr BP.

Origin and driving mechanisms of detrital-rich organic layers in Lake Son Kol

As developed previously, the onset of euxinic bottom water conditions in Lake Son Kol between 8500 and 5250 cal. yr BP most likely occurred under warmer climate conditions that fostered meltwater runoff. Climate warming possibly fostered permafrost melting, which activated the subaquatic springs that in turn fed the lake with solute-rich waters. Such conditions are highly favorable for the sustainment of stable lake stratification and the development of a chemocline, as observed in other high-altitude lake settings [START_REF] Wirth | Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps)[END_REF]. However, this seems to be inconsistent with the regular occurrence of coarse material in dark organic-rich sediments, which implies the transport of organic and minerogenic detritus over long distances in Lake Son Kol. In meromictic lakes, the deposition of coarse detritics in a coring site today located more than 3 km from the shore could be attributed to hyperpycnal or mesopycnal currents [START_REF] Mulder | Flood deposits in continental and marine environments: Character and significance[END_REF], depending on the density contrast between inflowing and lake waters. Hypopycnal flows are not considered here because hypopycnal flows in lakes generally supply small quantities of sediments and would require further reconcentration processes in the lake (over a 3-km distance in the case of Lake Son Kol). Aeolian deposition of coarse-grained detritics (including large phytoclasts, Fig. S6)

is unlikely as we would expect much finer grain sizes in the case of loess deposits (i.e., silts) [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF], and loess deposits are generally restricted to lower altitudes in the piedmont of the Tian Shan Mountains [START_REF] Machalett | The loess sequence from Remisowka (northern boundary of the Tien Shan Mountains (Kazakhstan). Part I: luminescence dating[END_REF][START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF].

Finally, increased coastal wave erosion driven by winds can be rejected as well because higher wave activity would also have broken down the chemocline. Moreover, hyperpycnal deposits are often typified by sharp-based lag rich in organic debris [START_REF] Mulder | Flood deposits in continental and marine environments: Character and significance[END_REF], which is never encountered in core SK06 where coarse detritics are rather homogenously distributed in the organic-rich layers (apart from the occurrence of a few normal graded beds; Fig. S6). Hence, the most reasonable candidates for the deposition of coarse minerogenic and organic detritus at the coring site are homopycnal or mesopycnal flows. Homopycnal flows result from a small density contrast between the entering flow and the surrounding water [START_REF] Bates | Rational theory of delta formation[END_REF] and can export suspended sediment load either in stratified of non-stratified lake basins [START_REF] Mulder | Flood deposits in continental and marine environments: Character and significance[END_REF]. Alternatively, mesopycnal flows are partially buoyant mesopycnal flows (interflows) of turbid plumes forming along each pycnocline (that separates each water mass) [START_REF] Van Rensbergen | High-resolution seismic stratigraphy of late Quaternary fill of Lake Annecy (Northwestern Alps): evolution from glacial to interglacial sedimentary processes[END_REF], 1999;[START_REF] Schröder | Acoustic facies and depositional structures of Lake Constance[END_REF][START_REF] Chapron | Fluid-escape features as a precursor of a large sublacustrine sediment slide in Lake Le Bourget, NWAlps, France[END_REF][START_REF] Chapron | Rhône River flood deposits in Lake Le Bourget: a proxy for Holocene environmental changes in the NW Alps, France[END_REF][START_REF] Støren | Identifying the sedimentary imprint of highfrequency Holocene river floods in lake sediments: development and application of a new method[END_REF][START_REF] Mulder | Flood deposits in continental and marine environments: Character and significance[END_REF].

Formation of mesopycnal currents is promoted when the inflowing waters enter more saline lake waters, as occurred between (at least) 6000 and 5000 cal. yr BP when higher conductivities (very low lake levels) were inferred in Lake Son Kol [START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF].

Modern conductivity of Lake Son Kol (515-530 mS.cm -1 ) is about twice that of meromictic Lake Bourget in France (300-350 mS.cm -1 ). Therefore, mesopycnal (or homopycnal) flow deposits occurred during periods of strong snowmelt (and/or increased temperatures), indicated by the strong depletion of δ 18 O in dark organic layers (Fig. 8), thus implying the remobilization of sediments from the lake catchment whenever sufficient stream discharge occurred to form a turbid plume. Two main factors control floods triggered by snowmelt: the snowmelt potential and the actual melting. Snowmelt depends on the amount of seasonal solid winter precipitation (e.g., snow) in the catchment while the intensity of melting is mostly controlled by spring/summer temperature [START_REF] Støren | Identifying the sedimentary imprint of highfrequency Holocene river floods in lake sediments: development and application of a new method[END_REF]. During the warm Mid-Holocene in Central Asia [START_REF] Chen | Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history[END_REF], periods of higher solid winter precipitation coupled with relatively cool springs and warm summers may have favored increased availability of snow when early summer temperatures started to rise in the mountains, intensifying spring melt floods and the amount of terrestrial material washed into the lake.

Convective summer rainstorms during warm summers could also lead to lower lake water δ 18 O values during the Holocene thermal maximum. Intensive rainstorms in the summer would increase hillslope erosion and stream discharge at a time when the lake was probably more saline, stratified and shallow due to more intensive evaporation. Increased intensities of rainstorms and snowmelt are therefore both possible drivers of mesopycnal or homopycnal flows (and coarse sediment deposition) in Lake Son Kol. Alternatively, hypolimnetic anoxia could have developed seasonally, by occurring under ice during the winter when organic matter decomposes (if there is a limited snow cover that enables photosynthesis) and breaking down towards the end of summer. However, this hypothesis is unlikely as (1) very few graded beds related to ice melt are observed (i.e., resulting from the settling of fine-grained aeolian particles deposited on ice to the lake bottom), (2) the regular occurrence of large phytoclasts in dark organic layers with coarse minerogenic detritus (Fig. S6) implies a soil erosion from the catchment, and thus is not compatible with aeolian depositional processes and, (3) Bphe a data exhibit multi-annual periods of stratification (i.e., meromixis), rather than a seasonal signal (Fig. 10).

Holocene chemocline development

The prominent changes in oxygenation conditions in the lake water column led to distinctive changes in Lake Son Kol autotrophs (Figs. 9 and 10). Bphe a is produced by anoxygenic phototrophic bacteria (APBs) which commonly live in the chemocline (oxic/anoxic transition) of meromictic lakes [START_REF] Overmann | Characterization of a dense, purple sulfur bacteria layer in a meromictic salt lake[END_REF][START_REF] Rogozin | Ecology of purple sulfur bacteria in the highly stratified meromictic Lake Shunet (Siberia, Khakassia) in 2002-2009[END_REF][START_REF] Blankenship | Molecular mechanisms of photosynthesis[END_REF][START_REF] Butz | Bacteriopheophytin a as an indicator of meromixis in sediments of Lake Jaczno, north-east Poland, CE 1891-2010[END_REF]. APBs require nutrients, sulphides and light to flourish, as provided by stable chemocline [START_REF] Parkin | Photosynthetic bacterial production in lakes: the effects of light intensity[END_REF][START_REF] Van Gemerden | Anoxygenic photosynthetic bacteria[END_REF]Mas, 1995, Hodgson et al., 1998;[START_REF] Madigan | Anoxygenic phototrophic bacteria from extreme environments[END_REF], and thus APBs may form dense populations in meromictic lakes [START_REF] Van Gemerden | Anoxygenic photosynthetic bacteria[END_REF][START_REF] Mcgowan | Autotrophic response to lake age, conductivity and temperature in two West Greenland lakes[END_REF]. Such conditions usually develop when lakes permanently stratify, but oxygen drawdown could also develop under ice with respiration of organic matter, potentially leading to anoxic conditions during certain times of the year when light is available and APB could flourish. Since dark organic-rich sediments represent more than a single year in duration (Figs. 5 and9), Bphe a suggest that the stability of stratification increased and that there may have been periods of prolonged hypolimnetic anoxia in Lake Kol during the Mid-Holocene. Recurring Bphe a, Br, S and ARO positive anomalies (i.e., PC2 < 0) at 8500, 8400, 8200-7800, 7700-7500, 7300-7000, 6500-6100, 6000-5700 and 5500-5250 cal. yr BP thus imply periods characterized by the development of a chemocline, which are tied to increased terrestrial inputs into the lake (PC1 > 0) (Fig. 9). A very similar assemblage of spectral signatures (i.e., Bphe a and phycocyanin) was reported from Lake Qinghai (China), where higher Bphe a concentrations are tied to elevated freshwater inputs and increased warmth between ca. 8200 and 5500 cal. yr BP [START_REF] Ji | Centennial blooming of anoxygenic phototrophic bacteria in Qinghai Lake linked to solar and monsoon activities during the last 18,000 years[END_REF] (Fig. 10). This also coincides well with higher (alkenone-inferred) summer temperatures in Lake Balikun and Lake Qinghai at 8500-5000 cal. yr BP [START_REF] Hou | Large Holocene summer temperature oscillations and impact on the peopling of the northeastern Tibetan Plateau[END_REF]Zhai et al., 2017).

Warmer summers in Lake Jaczno (Poland) were also linked to the onset of lake meromixis [START_REF] Butz | Bacteriopheophytin a as an indicator of meromixis in sediments of Lake Jaczno, north-east Poland, CE 1891-2010[END_REF]. Besides, vegetation expansion in lake catchments during warm periods (and more specifically the expansion of broad-leaved trees, such as birch) is known to increase dissolved organic matter (DOM) fluxes to lakes and decrease UV light penetration [START_REF] Sommaruga | Ultraviolet radiation in a high mountain lake of the Austrian Alps: Air and underwater measurements[END_REF][START_REF] Schmidt | Impact of late glacial climate variations on stratification and trophic state of the meromictic lake Längsee (Austria): validation of a conceptual model by multi-proxy studies[END_REF]. At Lake Son Kol, the presence of trees in the catchment and around the lake probably influenced lake trophic status and mixing conditions by (1) stimulating pedogenesis, which fueled oxygen consumption rates into the water body due to enhanced organic compounds exported to the lake and (2) sheltering the lake surface from wind-induced mixing (i.e., reduced wind exposure) thus favoring a prolonged time of stratification. Interestingly, the initiation of euxinic conditions in the hypolimnion (coeval to the deposition of dark organic-rich layers) followed the mass expansion of birch and arboreal taxa in the catchment between 8500 and 5000 cal. yr BP (Fig. 10), which enhanced terrestrial inputs of DOM and thus organic matter decomposition in the lake. Therefore pollen, geochemical and sedimentological proxies all suggest that hypolimnetic euxinia developed during warmer summers, and probably milder winters/springs. As proposed by [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF], centennial-scale episodes of weakened seasonality indeed occurred between 8500 and 5000 cal. yr BP, alongside increased winter temperatures and warm summers hence promoting a longer growing season for the trees. Our findings also agree with other speleothem records from Central Asia which indicate increased moisture transport by the Westerlies, and higher winter/spring precipitation in the early part of the Holocene (Fig. 10; [START_REF] Chen | Changes in Central Asia's water tower: Past, present and future[END_REF][START_REF] Cai | Holocene moisture changes in western China, Central Asia, inferred from stalagmites[END_REF][START_REF] Liu | Holocene solar activity imprint on centennial-to multidecadal-scale hydroclimatic oscillations in Arid Central Asia[END_REF]. In contrast, the breakdown of lake stratification at 7500-7300 and 7000-6500 cal. yr BP probably coincides with cooler climate conditions (and longer and thicker ice cover) linked to the strengthening and southward position of the Siberian High in arid Central Asia. Increased influence of the Siberian High would weaken the mid-latitude Westerlies and/or push their tracks further to the south, resulting in comparably drier conditions in the Tian Shan [START_REF] Aichner | High-resolution leaf wax carbon and hydrogen isotopic record of the late Holocene paleoclimate in arid Central Asia[END_REF][START_REF] Wolff | Precipitation evolution of Central Asia during the last 5000 years. The Holocene[END_REF].

Enhanced nutrient concentrations (linked with higher soil development DOM supply to the lake) are associated with a decrease in lake water transparency between 8500 and 7000 cal. yr BP, judging from unstable values of oxygen phototrophs (Fig. 9). At roughly the same time, phycocyanin abundances rapidly oscillate at high levels, suggesting that cyanobacteria predominated over Chl a and derivatives. A similar pattern was reported from shallow lakes in southern Sweden [START_REF] Randsalu-Wendrup | Combining limnology and palaeolimnology to investigate recent regime shifts in a shallow, eutrophic lake[END_REF] and northern Germany [START_REF] Dressler | Multi-proxy reconstruction of trophic state, hypolimnetic anoxia and phototrophic sulphur bacteria abundance in a dimictic lake in Northern Germany over the past 80 years[END_REF]. Therefore, cyanobacteria, which have the ability to sustain relatively high growth rates at low light intensities [START_REF] Mur | Cyanobacteria in the environment[END_REF], likely overtook oxygen phototrophs between 8500 and 7000 cal. yr BP, concurrently with the occurrence of APB blooms at the chemocline (Fig. 9). Before 5000 cal. yr BP, Lake Son Kol was a shallow and closed lacustrine system (Fig. 9; [START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF] and it is likely that a substantial proportion of the oxygen phototrophs could derive from benthic algae (e.g., benthic diatoms and cyanobacteria). This also agrees well with the occurrence of biofilms and/or microbial mat structures between 8500 and 5000 cal. yr BP [START_REF] Pacton | Sedimentary facies analyses from nano-to millemetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)[END_REF].

A major and abrupt change in lake regime is recorded around 5250 cal. yr BP in Lake Son Kol, with a drastic decline of Bphe a concentrations that occurred within a decade or so (Figs.

9, 10). The abrupt decline in APB pigments indicates a rapid collapse of stratified conditions and the breakdown of anoxic conditions in the hypolimnion after 5250 cal. yr BP, although high Br and S are still recorded until ca. 5050 cal. yr BP. Because this event tipped the system over a threshold (as observed in both PC1 and PC2 average model slopes; Figs. 6 and7) into another stable state from which former trophic and mixing conditions never recovered, we consider this time interval as a tipping point in Lake Son Kol. A similar abrupt ecosystem threshold was reported at 4900 cal. yr BP based on chironomid remains, and attributed to the increased influence of the Westerlies causing enhanced precipitation and higher windiness at Lake Son Kol [START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF][START_REF] Laug | Ecosystem shifts at two Mid-Holocene tipping points in the alpine Lake Son Kol (Kyrgyzstan, Central Asia)[END_REF]. Drastic environmental change in the lake may also be triggered by land-use processes in the catchment, which are mainly driven by changes in grazing pressure at high altitudes. Abundance of coprophilous fungal ascospores is assumed to reflect changes in grazing activity [START_REF] Gauthier | Pollen and non-pollen palynomoprh evidence of medieval farming activities in southwestern Greenland[END_REF][START_REF] Etienne | Influence of sample location and livestock numbers on Sporormiella concentrations and accumulation rates in surface sediments of Lake Allos, French Alps[END_REF]. Influxes of coprophilous fungi sensu stricto fluctuated between 0 and 10 spores.cm -2 .yr -1 between 6000 and 4000 cal. yr BP and during the last 2500 years (Fig. 10),

suggesting the predominance of wild animals in the Lake Son Kol catchment. Nevertheless the presence of domestic herbivores cannot be excluded during 6000-4000 cal. yr BP, which would match the occurrence of human fecal biomarkers in Lake Chatyr Kol sediments around 6000-5000 cal. yr BP [START_REF] Schroeter | Biomolecular evidence of early human occupation of a high-altitude site in Western Central Asia during the Holocene[END_REF]. The imprint of domesticated livestock and human occupation was likely highest around 3700 and 2500 cal. yr BP, where influxes reached their maxima. Interestingly, influx values were generally stable during the period 5500-5000 cal. yr BP, when an abrupt ecosystem shift occurred in Lake Son Kol (Figs. 9, 10), thus implying that pastoralism apparently did not have a significant impact in the catchment and that climate is the main driver of the abrupt regime shift identified around 5250 cal. yr BP.

After 5050 cal. yr BP, the lake became persistently well-mixed and the chemocline no longer existed in the lake water column, as indicated by pervasive PC2 > 0 values (Fig. 9) and elevated Mn content between 5000 and 3850 cal. yr BP that argue for oxic conditions in the lake. Surprisingly, the advent of well-mixed conditions is closely related to a strong decline of oxygen phototrophs (including cyanobacteria) between 5050 and 3850 cal. yr BP, with strong negative anomalies around 4400-3800 cal. yr BP, coeval to high Mn anomalies (Fig. 9).

Because this phase corresponds to a noticeable decline in summer humidity and low lake levels (PC1 < 0; Fig. 9), we posit that cooler summer temperatures and longer lasting and thicker snow cover may have prevented the growth of oxygen phototrophs in the water column (as well as the establishment of a stable stratification in summer after ice cover).

Alternatively, regular wind-driven lake mixing resulting from strengthened winds during the spring ice break-up season (inducing stronger vertical water circulation) may have caused low pigment deposition and preservation in the sediments [START_REF] Cuddington | An individual-based mode of pigment flux in lakes: implications for organic biogeochemistry and paleoecology[END_REF]. The disappearance of the chemocline after 5050 cal. yr BP coincides with a decrease in terrestrial DOM inputs into the lake, as catchment vegetation opened up with a decline of birch and other arboreal taxa and a rise in Artemisia (Fig. 10; [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF], similarly as in Lake

Qinghai [START_REF] Ji | Centennial blooming of anoxygenic phototrophic bacteria in Qinghai Lake linked to solar and monsoon activities during the last 18,000 years[END_REF]. Tree removal around the lake thus probably led to higher wind exposure of the open-water area (due to increased wind fetch), inducing in turn a better mixing of the water column. Lake levels increased around 3700 cal. yr BP, corresponding to another main tipping point based on high PC1 values (Fig. 6), in line with increased planktonic diatom abundances and lower conductivities (Fig. 9; [START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF]. High PC2 (and low Bphe a) values from 3800 cal. yr BP onwards infer that repetitive mixing conditions prevented the establishment of a chemocline by oxygenating the hypolimnion [START_REF] Butz | Bacteriopheophytin a as an indicator of meromixis in sediments of Lake Jaczno, north-east Poland, CE 1891-2010[END_REF], which hampers the onset of stable temperature stratification during summer. Such conditions promoted nutrient cycling in the photic zone to support oxygenic phototrophs in the epilimnion, which diminished light penetration to the chemocline and led to the destruction of a suitable lake habitat for APB growth. Hence, all proxy data suggest that Lake Son Kol has had persistently high lake levels with well-mixed and oxygenated conditions for the last 5000 years, in accordance with its modern polymictic regime.

Conclusions and perspectives

Ultra-high temporal resolution geochemical and hyperspectral palaeolimnology data can yield a more complete understanding of changing ecological patterns and redox processes in shallow lacustrine systems. As a biomarker of planktonic APB at the chemocline of lakes, the presence/absence and variability of Bphe a in Lake Son Kol sediments provides crucial information about the timing and forcing mechanisms that control the advent and vanishing of lake stratification over the last thousands of years. Increased Bphe a, Br, S abundances along with higher terrestrial inputs from the catchment are recorded at 8500, 8400, 8200-7800, 7700-7500, 7300-7000, 6500-6100, 6000-5700 and 5500-5250 cal. yr BP, and involve recurring episodes of hypolimnetic anoxia and euxinic bottom water conditions coeval to the deposition of dark organic sediments on the lake floor. The presence of a chemocline and the associated deposition of coarse minerogenic detritus transported by concentrated flows appears to be dependent on recurrent seasonal snowmelt episodes, and thus on the quantity of snow and spring/summer temperatures that regulate the potential for meltwater events. Our data thus suggest that low lake levels and warmer climate conditions regularly prevailed during 8500-5250 cal. yr BP, and promoted massive APB blooms in a well-stratified lake system. After 5250 cal. yr BP, Son Kol water level recovered and the lake became persistently well mixed, leading to the disappearance of the chemocline. This abrupt ecosystem shift was associated with cooler conditions and strengthened wind forcing, when the influence of the Westerlies increased from 5250 cal. yr BP onwards. Intensified wind forcing also most probably coincided with a significant increase of wave activity at the lake shoreline acting as an erosive agent. The imprint of wind forcing on coastal sedimentary dynamics (e.g., littoral sand drift) and geomorphological processes can be observed based on the diagnostic sedimentary features along the northeastern and southeastern shores of the lake, where sand spits, wave-dominated beaches, barrier spits and associated open lagoons have developed, hence suggesting that Lake Son Kol is presently a wind-driven water body (as defined by Nutz et al., 2018). Further studies are required in order to properly date such geomorphological features and assess the relationships between wind forcing, past lake levels and climatic trends during the Mid-to Late Holocene. , 1987). Orange points refer to sediment core SK06, while the two yellow and the white points indicate cores SK07 and SK04 [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF][START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF][START_REF] Pacton | Sedimentary facies analyses from nano-to millemetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)[END_REF] and SONK_11_D1/2 [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF], respectively. established using Oxcal 4.4 [START_REF] Ramsey | Radiocarbon calibration and analysis of stratigraphy: The OxCal program[END_REF](Ramsey, , 1998[START_REF] Ramsey | Development of the radiocarbon calibration program[END_REF]. The blue envelope corresponds to the 2σ probability range. See text for detail. Purple-filled curves correspond to abrupt shifts identified in both PC1 and PC2. Purple-filled curves correspond to abrupt shifts identified in both PC1 and PC2.

Figure 8: Close-up on XRF and hyperspectral data for the interval 131.5-151.5 cm in core SK06 (ca. 6500-5600 cal. yr BP), plotted against grain size (clay, sand), stable isotope (δ 18 O, δ 13 Corg), total organic matter (TOC), total carbon (TC), total nitrogen (TN) and C/N data from core SK07 in Lake Son Kol [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF]unpublished data). Yellow shadings refer to intervals characterized by dark organic layers with coarser minerogenic detritus, high Br, S, Bphe a, ARO in core SK06, as well as high TOC (and TC, TN and C/N) and depleted δ 18 O and δ 13 Corg values in core SK07.

Figure 9: Time series of hyperspectral, geochemical and PCA data for Lake Son Kol (core SK06), plotted with proxy data from core SONK_11_D1/2 from Lake Son Kol [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF][START_REF] Schwarz | Mid-to Late Holocene climate-driven regime shifts inferred from diatom, ostracod and stable isotope records from Lake Son Kol (Central Tien Shan, Kyrgyzstan)[END_REF]. XRF (9-point running means) and hyperspectral data (15point running means) for core SK06 are represented as standardized values (mean = 0, standard deviation = 1). Yellow shadings outline periods for which hypolimnetic euxinia prevailed in Lake Son Kol. See text for detail. record from Lake Qinghai, China [START_REF] Ji | Centennial blooming of anoxygenic phototrophic bacteria in Qinghai Lake linked to solar and monsoon activities during the last 18,000 years[END_REF]; (b) alkenone-inferred temperature record at Lake Qinghai [START_REF] Hou | Large Holocene summer temperature oscillations and impact on the peopling of the northeastern Tibetan Plateau[END_REF]; (c) speleothem δ 18 O record from Tonnel'naya (Ton) Cave, Uzbekistan [START_REF] Chen | Changes in Central Asia's water tower: Past, present and future[END_REF]; (d) speleothem δ 18 O record from Baluk Cave, China [START_REF] Liu | Holocene solar activity imprint on centennial-to multidecadal-scale hydroclimatic oscillations in Arid Central Asia[END_REF]; (e) influx of coprophilous fungi sensu stricto (core SK06, this study); (f) arboreal pollen (%) record from Lake Son Kol, Kyrgyzstan [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF]; (g) total content of Betulaceae (%) from Lake Son Kol, Kyrgyzstan [START_REF] Mathis | Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)[END_REF] and (h) Bphe a record (15-point running means) from Lake Son Kol (core SK06, this study). Table 1: AMS 14 C dates obtained on terrestrial plan remains and bulk sediment OM (with unknown proportion of aquatic OM) in cores SK07 and SK04 [START_REF] Huang | Hydrological changes in western Central Asia (Kyrgyzstan) during the Holocene: Results of a paleolimnological study from Son Kul[END_REF] and core SONK_11_D1/2 [START_REF] Lauterbach | Climatic imprint of the mid-latitude Westerlies in the Central Tien Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years[END_REF]. Published AMS 14 C dates were transposed to core SK06 using lithological and magnetic susceptibility-inferred correlations (Figs. 2a,S1 and S2). Conventional 14 C ages were calibrated using Oxcal 4.4 with the IntCal20 calibration dataset [START_REF] Reimer | The IntCal20 Northern Hemisphere radiocarbon age calibration curve (0-55 cal kBP)[END_REF]. A reservoir correction of 150 years was applied before calibration to conventional 14 C ages. Only the samples with asterisks (*) were considered for age modeling (see text for detail).

  of particles at 670 nm almost exclusively represents the optical activity of Chl a, apart from the slight interference by the accessory chlorophylls Chl b and Chl c.

Figure 1 :

 1 Figure captions:

Figure 2 :

 2 Figure 2: (a) Detailed stratigraphic correlation between cores SK06 (this study) and cores

Figure 3 :

 3 Figure 3: (a) Visible signatures and (b) continuum removal spectra with associated main

Figure 4 :

 4 Figure 4: (a) PCA biplot of major elements obtained by XRF core scanner and hyperspectral

Figure 5 :

 5 Figure 5: Hyperspectral image and spectral indices (relative absorption band area or RABA)

Figure 6 :

 6 Figure 6: Average changepoint model's slope calculated for PC1. The green curve in the

Figure 7 :

 7 Figure 7: Average changepoint model's slope calculated for PC2. The green curve in the

Figure 10 :

 10 Figure 10: Comparison of geochemical and palynological proxies from Lake Son Kol to

Figure S1 :

 S1 Figure S1: Core-to-core correlation between sediment cores SK06, SK04 and SK06 using

Figure S2 :

 S2 Figure S2: Correlation of core SK06 (this study) with core SONK_11_D1/2 from Lauterbach

Figure S3 :

 S3 Figure S3: 500-μm resolution (grey curves) XRF core-scanner data exhibiting Br, S, Ti, Rb,

Figure S4 :

 S4 Figure S4: PCA biplots of major elements obtained by XRF core scanner and hyperspectral

Figure S5 :

 S5 Figure S5: Average changepoint models of PC1 and PC2 during the last 8500 years in core

Figure S6 :

 S6 Figure S6: Thin-section images (cross-polarized light) of the detrital layer microfacies types

Acknowledgments

We are grateful to Anne-Lise Develle (EDYTEM) for conducting XRF core-scanner analyses on core SK06. The hyperspectral imaging acquisition was conducted at the University of Normandie-Rouen. This research was funded by the Laboratory of Geology of Lyon (LGL-TPE). All data from Son Kol core SK06 are available as supplementary data via the QSR network.