Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul, Tajikistan

Accepted Manuscript Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul, Tajikistan Steffen Mischke, Zhongping Lai, Bernhard Aichner, Liv Heinecke, Zafar Mahmoudov, Marie Kuessner, Ulrike Herzschuh PII:

S1871-1014(17)30028-6

DOI:

10.1016/j.quageo.2017.05.008

Reference:

QUAGEO 848

To appear in:

Quaternary Geochronology

Received Date: 9 February 2017 Accepted Date: 25 May 2017

Please cite this article as: Mischke, S., Lai, Z., Aichner, B., Heinecke, L., Mahmoudov, Z., Kuessner, M., Herzschuh, U., Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul, Tajikistan, Quaternary Geochronology (2017), doi: 10.1016/j.quageo.2017.05.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul,

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Tajikistan

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Steffen Mischke1*, Zhongping Lai2, Bernhard Aichner3, Liv Heinecke4, Zafar Mahmoudov5,

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Marie Kuessner6, Ulrike Herzschuh4

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Faculty of Earth Sciences, University of Iceland, Iceland

School of Earth Sciences, China University of Geosciences, China

Institute of Earth and Environmental Science, University of Potsdam, Germany

Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Research Unit Potsdam, Germany

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State Administration for Hydrometeorology of the Committee for Environmental Protection,

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Tajikistan 6

Department of Geological Sciences, Free University of Berlin, Germany

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Steffen Mischke, Faculty of Earth Sciences, University of Iceland, Sturlugata 7, Askja, 101

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Reykjavík, Iceland

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Email: [email protected]

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ACCEPTED MANUSCRIPT Abstract

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Lake Karakul in the eastern Pamirs is a large and closed-basin lake in a partly glaciated

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catchment. Two parallel sediment cores were collected from 12 m water depth. The cores

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were correlated using XRF analysis and dated using radiocarbon and OSL techniques. The

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age results of the two dating methods are generally in agreement. The correlated composite

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core of 12.26 m length represents continuous accumulation of sediments in the lake basin

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since 31 ka. The lake reservoir effect (LRE) remained relatively constant over this period.

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High sediment accumulation rates (SedARs) were recorded before 23 ka and after 6.5 ka.

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The relatively close position of the coring location near the eastern shore of the lake implies

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that high SedARs resulted from low lake levels. Thus, high SedARs and lower lake levels

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before 23 ka probably reflect cold and dry climate conditions that inhibited the arrival of moist

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air at high elevation in the eastern Pamirs. Low lake levels after 6.5 ka were probably caused

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by declining temperatures after the warmer early Holocene, which had caused a reduction in

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water resources stored as snow, ice and frozen ground in the catchment. Low SedARs

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during 23-6.5 ka suggest increased lake levels in Lake Karakul. A short-lived increase of

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SedARs at 15 ka probably corresponds to the rapid melting of glaciers in the Karakul

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catchment during the Greenland Interstadial 1e, shortly after glaciers in the catchment had

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reached their maximum extents. The sediment cores from Lake Karakul represent an

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important climate archive with robust chronology for the last glacial-interglacial cycle from

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Central Asia.

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Keywords

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Radiocarbon and OSL dating; Lake sediments; Pamir Mountains; Late Pleistocene;

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Holocene

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1. Introduction

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Lake sediment records are well-established Quaternary climate archives due to their often

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continuous accumulation, their presence in regions where other important climate archives

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such as ice cores or tree rings are not available, and because they can contain a large

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variety of useful proxies for palaeoenvironmental and palaeoclimate reconstructions (Birks

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and Birks, 2006). In alpine and arid regions such as Central Asia, long-term lake records

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representing the complete last glacial and interglacial cycle (Marine Isotope Stages [MIS] 2

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and 1) are rare, as many modern lake basins were formed by glaciers during the global last

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glacial maximum (gLGM) during MIS 2 and many lake records from the mountain ranges and

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their forelands are often incomplete as a result of intermittent desiccation periods (for

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example, records from Lake Balikun, Lake Bosten and the Aral Sea; Boomer et al., 2000;

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Zhang and Mischke, 2009; An et al., 2011; Opitz et al., 2015). The logistical difficulty of

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ACCEPTED MANUSCRIPT working in remote regions may be another reason that many lake cores recovered from high

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altitude basins do not contain records that predate the mid-Holocene (Ricketts et al., 2001;

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Beer et al., 2007; Mischke et al., 2010, Zhang et al., 2012; Huang et al., 2014). However,

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older, continuous sedimentary records from Central Asia are required to assess the long-

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term dynamics of local water resources and climate, including the melting of glaciers and

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disappearance of permafrost during periods of rapid warming.

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We obtained two parallel lake sediment cores from Tajikistan’s largest lake, Lake Karakul, to

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test its potential as a long-term, continuous climate archive.

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2. Regional setting

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Lake Karakul fills a graben structure in Tajikistan’s eastern Pamir Mountains (Strecker et al.,

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1995). The closed-basin lake has an area of 388 km² and lies within a 4464 km² catchment.

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The catchment geology is dominated by granites and Carboniferous to Permian/Triassic

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metaclastic and metavolcanic rocks and the highest peaks in the catchment reach 6780 m

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(Vlasov et al., 1991; Komatsu and Tsukamoto, 2015). An island and a south-north stretching

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peninsula divide the lake basin into two sub-basins, of which the western basin is 242 m

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deep whilst the eastern sub-basin has a depth of ca. 20 m (Molchanov, 1929; Fig. 1). The

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2012 lake surface lies at an altitude of ca. 3915 m, but ancient shorelines up to 205 m above

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the modern lake level show that the lake responded sensitively to past climate changes in

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the region (Komatsu et al., 2010). Glacier moraines were mapped in the close vicinity of the

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lake and are regarded as remnants of advances ca. 15 ka ago (Komatsu and Tsukamoto,

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2015).

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The westerlies control the climate of the eastern Pamirs. High mountains shield the lake

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basin from precipitation which amounts to an annual mean of 82 mm at the Karakul

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meteorological station on the eastern shore of the lake (Karakul station data). Slightly higher

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monthly values of ca. 10 mm are recorded between March and July. Precipitation in the

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surrounding mountain regions is presumably higher (Miehe et al., 2001; Williams and

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Konovalov, 2008). Mean January, July and annual temperatures are -18.1, 8.5 and -4.0 °C,

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respectively. Vegetation is mostly confined to the wetlands and alluvial plains around the lake

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consisting of Ceratoides krascheninnikovia, Artemisia pamirica, A. korshinskyi, Ajania

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tibetica, Stipa glareosa and Oxytropis immerse (Safarov, 2003). Although one village lies on

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Lake Karakul’s eastern shore (Karakul village), human impact on the lake is probably

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negligible as the lake is not used for fishing and no boats are present.

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3. Materials and methods

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3.1 Collection and correlation of cores

ACCEPTED MANUSCRIPT An UWITEC piston corer “Niederreiter 60” was used to recover two sediment cores from ice-

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covered Lake Karakul at 12 m water depth in April 2012 (Fig. 1). The exact location of the

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cores is 39.01760 °N and 73.53275 °E; this position is only 1.8 km from the eastern shore

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but melting ice and open cracks impeded coring in a more central location. Two parallel

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cores (KK12-1 and KK12-2) were collected 10 m apart to enable the establishment of a

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gapless synthetic master core resulting from the correlation of both cores (Fig. 2). Coring

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was finished at the two sites at depths of 1087 and 1226 cm below the lake floor,

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respectively. The sediments in the plastic core liners were transported to Dushanbe and

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stored in a room without cooling for two months before shipment to Potsdam. Storage in

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Potsdam was conducted in a dark room at 4 °C.

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Core KK12-1 was split into halves and sediments were described in December 2012 whilst

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core KK12-2 was opened and described in September 2013. Relative element

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concentrations were analyzed for both cores with an AVAATECH XRF Core Scanner with Rh

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X-ray tube at 1 mA, 30 s counting time and 2 mm scan resolution. Elements from Al to Rh

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were measured at 10 kV without a filter for major elements. The elements from Cu to Bi were

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measured at 30 kV with a „Pd thick‟ filter for minor elements. The sediment surface was

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cleaned and smoothed with razor blades and covered with a 4 µm Ultralene® film prior to

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scanning. XRF intensities in counts per second were converted to element peak areas as a

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measure of relative element concentration with the software WinAxil.

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The parallel cores were correlated based on the XRF data for Br, Ca, Fe, Si, Sr and Ti. In the

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first step, major similarities in element peak-area patterns were visually identified and

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stratigraphic levels evaluated to be corresponding were connected as first-order markers

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(FOM; Figs. 3 and S1-5). In the second step, less obvious tie points were identified as

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second-order markers (SOM; Figs. 3 and S1-5). No clear relationship appears to exist

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between Si peaks measured in the two cores, therefore FOMs were not identified on the

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basis of relative Si concentration. The resulting correlation scheme consists of 12 FOMs for

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five elements and 98 SOMs for six elements. FOMs are not intersecting each other whilst the

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set of SOMs includes some intersecting markers. A minimum of 20 SOMs must be deleted to

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result in a consistent pattern of stratigraphically corresponding levels, regarded as the

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optimal correlation of the parallel cores (Fig. 4).

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3.2 Radiocarbon dating

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Twenty-two samples for radiocarbon dating were collected from both cores. Terrestrial plant

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remains were not found in the sediments and cannot be expected given the sparsely

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vegetated catchment of the lake. Thus, macro-remains of aquatic plants and bulk sediment

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had to be used for radiocarbon dating. The lake reservoir effect (LRE) was determined for a

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living charophyte collected with a short corer near the center of the eastern sub-basin at

ACCEPTED MANUSCRIPT 39.02495 °N and 73.48758 °E. The obtained F14C (fraction modern; Reimer et al., 2004) of

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0.8914 is equivalent to a LRE of 1.315 ka which is comparable with the previously reported

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LRE of 1.420 ka for Lake Karakul (Mischke et al., 2010). The average of both determined

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LREs of 1.368 ka was applied for a correction of the original radiocarbon age data assuming

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that the LRE did not change through time. The resulting corrected radiocarbon ages were

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calibrated to calendar years using OxCal 4.2 and IntCal13 (Bronk Ramsey and Lee, 2013;

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Reimer et al., 2013).

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3.3 Optically stimulated luminescence (OSL) dating

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Ten samples for OSL dating were obtained from core KK12-2. The core was opened in a

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dark room under red light. Sediment for OSL dating was carefully collected only from the

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central part of the sediment column that was assumed to be protected from light by

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surrounding sediment near the liner walls. Separate samples were collected for water

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content measurements. Wet and dry sample weights were determined and the water content

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of samples calculated. The water content shows a general increase towards the top of the

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core with values greater than 30 % apart from two samples (Table 2). Three samples,

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however, yielded anomalously low values (as low as 14.8 %) in comparison to samples

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collected from below and above (Table 2). Water loss during core storage without cooling in

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Dushanbe was assumed for the affected core segments. Therefore, we have estimated the

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original water contents of these three samples by fitting an exponential regression to the

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water content data of the samples regarded as unbiased (Table 2).

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In the luminescence laboratory OSL samples were treated with 10 % HCl and 30 % H2O2 to

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remove carbonates and organics, respectively. Samples were then wet sieved to extract the

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38-63 µm and 90-125 µm grain fractions. Only one sample (KK12-2 OSL09) had sufficient

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grains in the 90-125 µm fraction. For pure quartz extraction, the 38-63 µm grains were

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etched using 35 % H2SiF6 for about two weeks to remove feldspars (Lai and Wintle, 2006).

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Grains of the 90-125 µm fraction were etched using 40 % hydrofluoric acid for 40 min, and

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then washed with 10 % HCl to remove acid-soluble fluoride precipitates. The purity of quartz

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grains was checked by infrared stimulation, and samples with obvious infrared stimulated

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luminescence (IRSL) signals were re-treated with H2SiF6 to avoid equivalent dose (De)

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underestimation (Lai and Brückner, 2008). Pure quartz samples were then mounted onto the

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0.7 cm (large aliquot) diameter centres of the 0.97 cm diameter stainless steel discs using

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silicone oil.

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OSL measurements were carried out on an automated Risø TL/OSL-DA-20 reader.

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Stimulation was done using blue LEDs at 130 ºC for 40 s, and detection was performed

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through a 7.5 mm thick U-340 filter. Preheat was done using 260 ºC for 10 s, and cut-heat

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performed at 220 ºC for 10 s, determined after preheat plateau and dose recovery tests

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ACCEPTED MANUSCRIPT (Murray and Wintle, 2000; 2003). Signals of the first 0.64 s stimulation were integrated for

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growth curve construction after background subtraction using the last 25 channels in the

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shine-down curve.

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Concentrations of uranium, thorium and potassium were obtained by neutron activation

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analysis. For the 38-63 µm grains, the alpha efficiency value was taken as 0.035±0.003 (Lai

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et al., 2008). The cosmic-ray dose rate was estimated for each sample as a function of

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depth, altitude and geomagnetic latitude (Prescott and Hutton, 1994). The elemental

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concentrations were converted into annual dose rates according to Aitken (1998). The dose

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rates are shown in Table 2. De was determined using the combination of the single aliquot

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regenerative dose (SAR; Murray and Wintle, 2000) and the standardized growth curve (SGC)

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protocols (Roberts and Duller, 2004; Lai et al., 2007), i.e. the SAR-SGC method. For each

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sample, six aliquots were measured using the SAR protocol to construct a SGC, then 10-15

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additional aliquots were measured for natural luminescence (Ln) and test dose luminescence

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(Tn). For the additional aliquots, sensitivity-corrected natural OSL (Ln/Tn) was projected on

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the SGC to yield a De. The De results determined by the SGC are in agreement with those by

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the SAR protocol within 10 % for all samples. For all samples, the final De is the mean of all

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SAR Des and SGC Des. The recuperation of all samples is negligible.

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3.4 Establishment of age-depth model

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The age-depth model was established with OxCal 4.2. In a first run, an outlier analysis was

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conducted with the complete set of OSL and radiocarbon dates (Bronk Ramsey, 2009). A

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general outlier model using a Student’s t-distribution with an outlier probability of 5 % was

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applied. Three dates with outlier probabilities of 100% (KK12-2 OSL04, KK12-2 OSL07,

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KK12-1 538) were identified as outliers. Similarly, two additional dates with outlier

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probabilities >60 % (KK12-2 OSL01: 75 %, KK12-2 OSL05: 67%) were treated as outliers in

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subsequent age-depth modelling (Table S1).

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In a second run, the five dates identified above were manually excluded from the model. A

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deposition mediated by a random Poisson process (P-sequence with flexible Poisson

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parameter within OxCal; Bronk Ramsey and Lee, 2013) was assumed. The outlier analysis

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of this run revealed slightly enhanced outlier probabilities for samples KK12-1 1019 (39 %),

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KK12-1 851 (37 %) and KK12-2 812 (32 %) (Table S1). However, these dates were not

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excluded from the model due to the expected low total organic carbon concentrations in the

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pre-Holocene sections of the cores and the related higher uncertainty of radiocarbon age

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data.

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3.5 Investigation of exposed lake sediments

ACCEPTED MANUSCRIPT A section (KK13-S1) of relatively fine-grained sediments at 39.06448 °N and 73.59615 °E

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and ca. 19 m above the modern lake level was investigated in August 2013. The sediments

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were exposed as the result of the incision and lateral erosion of a stream flowing into Lake

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Karakul (Fig. 1). The sediments were described and 14 samples collected for ostracod

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(micro-crustacean) analysis to examine the depositional setting (Fig. 2). Sub-samples of 20 g

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were treated with 3% H2O2 for 48 hours and washed through 100 and 250 µm sieves. All

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ostracod valves were picked under a low-power binocular microscope. In addition, two sub-

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samples were used and treated for radiocarbon dating in a similar way as samples from the

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lake sediment cores.

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217 4. Results

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The sediments recovered from Lake Karakul are dominated by silt and contain abundant

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macrophyte remains in the upper halves of the two parallel cores KK12-1 and KK12-2 (Fig.

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2). Macrophyte remains are embedded in thin horizontal layers resulting in laminae from less

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than 1 to 3 mm thick. The silty sediments generally contain a higher percentage sand in the

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lower core halves. In addition, sandy plane beds or lenses of a few mm to cm thickness and

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with sharp lower and upper boundaries are intercalated in the lower parts of the cores (Fig.

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2). Shells of the gastropod Radix were recorded above 397 cm in core KK12-1 and above

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460 cm in KK12-2. The colours of the wet sediments are mostly bluish to dark or very dark

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bluish gray (GLEY 2 5/1 5PB, GLEY 2 4/1 10B or GLEY 2 3/1 10B, respectively; Munsell

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Color, 2009).

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Semi-quantitative element concentrations of Sr, Ca, Si, Ti, Fe and Br are shown in Figs. 3

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and S1-S5. The concentrations of Si, Ti and Fe show relatively large fluctuations in the lower

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part of the cores and minor fluctuations in the upper part (Figs. S2-S4). The Br

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concentrations are highly variable in the upper halves of the cores (Fig. S5).

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The F14C value for a modern charophyte sample of 0.8914 represents a LRE of 1.315 ka for

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the year of sampling (2012). The 14C age data for the sediment cores range from 1.615 ±

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0.030 to 27.91 ± 0.27 14C ka BP (Tables 1 and S1). The 13 radiocarbon age data for core

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KK12-1 are in a stratigraphic sequence apart from sample KK12-1 538 collected at 559-560

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cm core depth. The nine radiocarbon age data for core KK12-2 are in a stratigraphic

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sequence. The ten OSL ages for core KK12-2 range from 0.6 ± 0.06 to 37.4 ± 3.0 ka (Tables

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2 and S1). Among the ten samples, two dating results (KK12-2 OSL 04 and 07) are not in a

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stratigraphic sequence.

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The sediments of the exposed section KK13-S1 are mostly alternating layers of silt and

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sandy silt (Fig. 2). Sediments in the lower part of the section display cross and ripple bedding

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structures. The silts and sandy silts in the middle part are discordantly overlain by gravels. A

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sediment sample from a thin silt layer in the lowermost part of the section contained a single

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juvenile candonid ostracod valve. All other samples yielded abundant valves of Leucocythere

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dorsotuberosa and Candona sp. 1 and sp. 2. In addition, a few valves of Tonnacypris sp.

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were recorded in five of the uppermost seven samples from the section (Fig. 2). The

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radiocarbon age data for two samples from the section are 22.46 ± 0.25 14C ka BP for the

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lower sample and 11.84 ± 0.06 14C ka BP for the upper sample (Table 1; Fig. 2).

250 5. Discussion

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5.1 Recovered sediments and correlation of cores from Lake Karakul

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The recovered sediments from Lake Karakul are relatively homogenous, fine-grained silty

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sediments containing macrophyte remains in the upper part and more sand and sand

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intercalations in the lower part. The elongated, thin and mostly ca. 1.5 mm wide macrophyte

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remains resemble the leaves of Stuckenia cf. pamiricus (better known as Potamogeton

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pamiricus), which was described as Stuckenia pamirica from Lake Karakul as lectotype

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(Kaplan, 2008). However, leaf remains with larger widths possibly originate from other

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Potamogetonaceae species. Stuckenia cf. pamiricus is currently very abundant in Lake

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Karakul between 2.7 and 13.8 m water depth (Mischke et al., 2010). The most obvious

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changes in the lithology of the cores occur at 481, 354 and 322 cm depth in core KK12-2

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where the abundance of macrophyte remains in the sediments changes (Fig. 2). The lack of

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significantly coarser sediments or lag deposits suggests that sediments were continuously

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accumulated at the core sites.

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The relatively homogenous, macrophyte-rich sediments in the upper parts of the cores and

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the differing number of thin sand beds and lenses observed in cores KK12-1 (15) and KK12-

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2 (35) did not allow a correlation of the cores based on visual comparison. Thus, XRF scan

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data were used in the correlation of the cores. The identification of similar major and minor

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maxima and minima (FOMs and SOMs) in the relative concentration data for six selected

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elements provided six independent correlation approaches (Figs. 3 and S1-S5) which were

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overlain to produce a robust, unified correlation scheme. The more obvious FOMs provided a

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consistent correlation basis made of 12 FOMs for the different elements without crosscutting

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(conflicting) FOMs. The overlay of the SOM patterns for the six elements contained

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crosscutting SOMs, indicative of inconsistencies between different SOM patterns. From the

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98 original SOMs, it was necessary to remove a minimum of 20 SOMs in order to produce a

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consistent correlation scheme. The resulting scheme consisting of 12 FOMs and 78 SOMs is

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regarded as robust best-choice correlation approach based on the highest number of

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markers for correlation included (Fig. 4). It was used to “transfer” the radiocarbon age data

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for core KK12-1 to the depth scale of KK12-2, and to establish a unified age-depth model for

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both cores. The resulting depth data for the radiocarbon samples from core KK12-1 was

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used for the establishment of an age-depth model including all data for both cores (Table 1,

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Fig. 5).

283 5.2 Age-depth model, and assessment of radiocarbon and OSL age data

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The established age-depth model is based on 20 radiocarbon and six OSL dating results due

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to the identification of one radiocarbon dating result and four OSL dating results as outliers

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(Figs. 5 and S6, Table S1). The radiocarbon sample KK12-1 538 was erroneously taken from

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559-560 cm depth of the KK12-1 core although reworked material was recovered in the

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upper part of the site’s fourth core drive as a result of untimely opening of the coring barrel

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(Fig. 2). Consequently, sedimentary material which had filled the hole of the previous core

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drive entered the coring barrel before the target depth for the new coring drive was reached.

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Thus, the significantly younger dating result confirms the recovery of reworked material but

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cannot be included in the construction of an age-depth model.

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The age of KK12-2 OSL07 from 929 cm depth is significantly older than all other OSL and

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radiocarbon dating results (Figs. 5 and S6). A larger OSL dating result cannot be explained

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by unreliable radiocarbon data biased by LREs. Overestimation of the OSL age may result

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from 1) incomplete bleaching of quartz grains during transport to the lake and deposition at

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the core location, and/or 2) resuspension and re-deposition of older sediments following a

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slope failure in the lake basin possibly triggered by unstable sediments of a Gilbert delta or

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an earthquake. In contrast, the ages of KK12-2 OSL01, 04 and 05 are younger than the age

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data for samples from similar stratigraphic levels or sediments below or above (Fig. 5).

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Questioning our assumption that the LRE did not change significantly through time, the three

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younger OSL ages could indicate that the LRE was significantly larger than at present.

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However, the OSL dating samples KK12-2 OSL02, 03, 06, 08, 09 and 10 correspond very

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well to the obtained radiocarbon dating results corrected by a constant LRE. Thus, the

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majority of the OSL age data support the assumption of a relatively constant LRE during the

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last ca. 31 ka (Figs. 5 and S6). The three younger OSL dating results are apparently too

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young for reasons that are unknown. Contamination by younger sand grains by penetrating

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roots of aquatic plants is probably not a realistic scenario.

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The six OSL dating results KK12-2 OSL02, 03, 06 and 08-10 and the radiocarbon age data

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for similar stratigraphic levels indicate that the established age-depth relationship is robust

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and that the assumption of a relatively constant LRE is appropriate for this record from Lake

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Karakul. Few case studies have been conducted in Central Asia to assess possible

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variations of the LRE through time. Long et al. (2011) applied radiocarbon and OSL dating to

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a 15 ka record from Lake Zhuyeze in the Chinese part of the Gobi Desert and identified a

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small and relatively constant LRE. Morrill et al. (2006) were able to extract terrestrial and

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aquatic organic matter for radiocarbon dating of a 9-4 ka lake sequence from the Tibetan

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ACCEPTED MANUSCRIPT Plateau. They concluded that the LRE was small (<600-700 a) and relatively constant

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through time. In contrast, a study of radiocarbon and OSL age data for lake sediments from

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Ulaan Lake in Mongolia demonstrated that LREs may vary significantly over time (Lee et al.,

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2011). The study of Zhang et al. (2012) from Lop Nur Lake in the Tarim Basin (China)

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showed that radiocarbon dating of lake sediments may provide stratigraphically inconsistent

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and unreliable data in comparison to OSL age data. Thus, an examination of radiocarbon

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dating results in the light of independently applied dating techniques is indispensable for the

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establishment of robust chronologies if non-terrestrial organic matter is to be used for

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radiocarbon dating.

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5.3 Significance of exposed sediments at section KK13-S1

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Cross-bedding of sediments below 235 cm depth and a single candonid ostracod valve in a

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sample from a thin intercalated silt layer show that the basal sediments of the section were

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probably deposited in the running waters of a stream and in short-lived water bodies such as

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ephemeral pools in the stream bed. In contrast, the sediments with ripple-bedding or

333

horizontal layering above contain abundant valves of Leucocythere dorsotuberosa and

334

Candona sp. 1 and sp. 2. This ostracod assemblage is similar to the modern ostracods found

335

in Lake Karakul (Mischke et al., 2010). Thus, sediments between 235-50 cm depth were

336

accumulated in the lake. Very shallow waters are inferred from ripple bedding in the lower

337

part and cross-bedding at 170 cm depth. Higher water levels are reconstructed for the silt or

338

alternating silt-sandy silt layers above 163 cm depth that contain a few valves of Tonnacypris

339

sp. in addition to those found below. The presence of Tonnacypris sp. indicates less brackish

340

conditions and higher inflows to the lake. The large age difference of ca. 13 ka between the

341

ripple-bedded sediments in the lower part of the section and silts in the middle of the section

342

suggests that the sequence may include a hiatus which is possibly represented by the abrupt

343

transition from silts to medium sands with cross bedding at 178 cm (Fig. 2). The massive unit

344

of gravels on top of silts in the uppermost part of the section represents fluvial sediments

345

accumulated on an alluvial fan when the lake level was probably significantly below the

346

section position.

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348

5.4 Implications of the chronological data

349

The age-depth data for the two parallel cores from Lake Karakul suggest that the lake

350

generally received sediments at relatively high SedAR with an intervening period of lower

351

SedAR. The SedAR is ca. 0.6 mm/a before 23 ka (below 7.45 m depth in core KK12-2), 0.2

352

mm/a between 23 and 6.5 ka (7.45-4.81 m) and 0.8 mm/a after 6.5 ka (above 4.81 m; Figs.

353

5, 6). In comparison, sediments of a late Holocene core from the centre of the eastern sub-

354

basin were accumulated at a SedAR of 0.25 mm/a (Mischke et al., 2010).

ACCEPTED MANUSCRIPT 355 The period from 31 to 23 ka:

357

The period of high SedAR from 31-23 ka is characterized by relatively abundant sand

358

intercalations within the silty sediments and rare macrophyte remains. The high SedAR of

359

0.6 mm/a probably results from the accumulation of river-transported sediments from nearby

360

delta regions that were further distributed over the lake floor by wind and wave-driven

361

currents at shallow water depth. Relatively dry conditions are generally inferred from the

362

relatively high SedARs and from an assumed low lake level. Heinecke et al. (in review)

363

inferred relatively low levels of Lake Karakul between 29 and 19.5 ka from total inorganic

364

carbon (TIC) contents and Fe/Mn ratios of the KK12-1 core sediments (Fig. 6). The Guliya

365

ice core from the Kunlun Mountains in the west of the Pamirs, regarded as a sensitive and

366

reliable temperature record, yielded lowest δ18O values before 24 ka, implying cold and dry

367

conditions in Central Asia (Thompson et al., 1997; Yang et al., 2004; Fig. 6). However, the

368

SedARs display large fluctuations during the first section of the cores between 31-23 ka.

369

Relatively low SedARs occur between 31 and 27 ka and again at ca. 25 ka when shallow

370

water sediments were accumulated 19 m above the modern lake level at the KK13-S1

371

section location. The period of relatively low SedARs from 31-27 ka corresponds to a period

372

of glacier advances at 30 ± 3 ka in the western Himalayan-Tibetan region termed Semi-arid

373

Western Himalayan-Tibetan Stage (SWHTS) 2F by Dortch et al. (2013; Fig. 6). The results of

374

Heinecke et al. (in review) indicate slightly higher lake levels at 27 ka too (Fig. 6). The

375

coincidence between assumed slightly higher lake levels between 31-27 ka and the SWHTS

376

2F supports the inference of Komatsu and Tsukamoto (2015) who suggested the

377

simultaneous occurrence of high lake levels and glacier advances in the Karakul region.

378

However, the modelled age data before 26 ka show a relatively large probability range and

379

significant influence of the relatively old age of sample KK12-2 1086 (Figs. 5 and S6). Thus,

380

the timing of sediment accumulation in Lake Karakul before 26 ka is not well-constrained and

381

regional comparisons remain relatively uncertain.

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383

The period from 23 to 6.5 ka:

384

Low SedARs, less abundant sand intercalations and generally rare macrophyte remains in

385

the silty sediments from 23-6.5 ka indicate a higher lake level during the gLGM, the late

386

glacial and early Holocene. The low SedAR during this period is comparable to the rate of

387

0.25 mm/a determined for the late Holocene sediment core from the centre of the eastern

388

sub-basin (Mischke et al., 2010). Thus, a larger shoreline distance and higher lake level than

389

today is implied by the low SedARs between 23-6.5 ka. Correspondingly, higher lake levels

390

were also reconstructed by Heinecke et al. (in review) for the period from ca. 19.5 to 6.6 ka

391

based on TIC and Fe/Mn ratios of core KK12-1 (Fig. 6). This period was also identified as a

ACCEPTED MANUSCRIPT time of higher than present lake levels by Komatsu and Tsukamoto (2015) who applied OSL

393

dating of exposed on-shore lake and shoreline deposits. They reconstructed rising lake

394

levels since 19 ka which culminated in a 35-m above present highstand at 15 ka, followed by

395

declining-though still higher than modern lake levels until ca. 10 ka. The period of low SedAR

396

at Lake Karakul between 23-6.5 ka corresponds to a period of frequent glacier advances in

397

the western Himalaya and Tibetan Plateau region. A total of seven SWHTS were recorded

398

by Dortch et al. (2013) between 20 ± 2 ka (SWHTS 2E) and 6.9 ± 0.2 ka (SWHTS 1D). Thus,

399

the coincidence of glacier advances and high lake levels suggested by Komatsu and

400

Tsukamoto (2015) is confirmed by low SedARs in the lake between 23-6.5 ka. Increased

401

δ18O values at the Guliya ice cap at ca. 23 ka and in the NGRIP record, with the peak of the

402

Greenland Interstadial 2 (GI-2), suggest that a slight temperature increase at high elevations

403

gave way to increased precipitation and glacier growths (Thompson et al., 1997; North

404

Greenland Ice Core Project members, 2004; Fig. 6). Low evaporation due to a short ice-free

405

season probably contributed to the low SedARs and high level of Lake Karakul before and

406

during the gLGM. Komatsu and Tsukamoto (2015) identified the largest glacier advance in

407

the Karakul catchment at 15 ka shortly before GI-1e (i.e., the period of the Bølling in the

408

northern Atlantic and northern Europe). At Guliya, significantly increasing temperatures were

409

recorded between the gLGM and 15 ka, suggesting that the growth of glaciers resulted from

410

higher air humidity and snow accumulation at high altitudes. A short-lived increase in

411

SedARs at 14 ka corresponds to highest δ18O values for the last 31 ka of the Guliya ice core

412

record (Fig. 6), possibly suggesting that the glaciers in the Karakul catchment melted rapidly

413

during the GI-1 shortly after reaching their maximum extents. Following the GI-1, Komatsu

414

and Tsukamoto (2015) reconstructed declining lake levels, however, consistently low

415

SedARs in our cores from GI-1 until 6.5 ka indicate that the lake level remained significantly

416

higher than it is today. Relatively high lake levels were probably maintained during the late

417

glacial and early to mid Holocene by sufficient meltwater inflows and precipitation.

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The period since 6.5 ka:

420

The accumulation of abundant aquatic plant remains in sediments of Lake Karakul since 6.5

421

ka indicates that, around this time, water levels had dropped to a level low enough to enable

422

the establishment of a dense macrophyte cover on the lake floor, similar to the macrophyte

423

cover present in the shallower eastern sub-basin today. Alternatively, this may represent the

424

first time that the suspension load of diminishing glaciers in the catchment was low enough to

425

support more intense macrophyte growth on the lake floor as water transparency increased.

426

Either way, we interpret the high SedAR to reflect the enhanced production and preservation

427

of organic matter in the sediments as opposed to an increase in the deposition of clastic

428

sediment particles in an environment nearer to the palaeoshoreline. A lowered dispersal of

ACCEPTED MANUSCRIPT river-transported sediments over the lake floor is expected as result of the dense plant cover.

430

A lower lake level since 6.6 ka was also inferred by Heinecke et al. (in review). Only three

431

relatively short regional glacial stages were observed since 6.5 ka by Dortch et al. (2013)

432

further supporting the interpretation that low levels of Lake Karakul were accompanied by

433

diminished glaciers in the lake’s catchment (Fig. 6). The first occurrence of abundant

434

macrophyte remains in the sediments of Lake Karakul at 6.5 ka coincides with a reversal in

435

the long-term δ18O increase of the Guliya ice core record since the gLGM (Fig. 6). Lower

436

temperatures in the mid and late Holocene probably caused diminished meltwater inflows to

437

the lake and lower air humidity and precipitation in the catchment.

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438 6. Conclusions

440

The chronological study of two parallel sediment cores from Lake Karakul in the eastern

441

Pamirs revealed that:

1) Sediment accumulation at the core position has been continuous over the last 31 ka.

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443

Therefore, the cores represent valuable climate records for the last glacial-interglacial

444

cycle from a poorly-studied region.

446 447 448 449

2) The combined application of radiocarbon and OSL dating provided a robust chronology for the sediments from the lake.

3) The modern LRE of 1.368 ka for two macro-algae collected in 2008 and 2012 does not appear to have changed significantly over time.

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4) Low SedARs were recorded between 23 and 6.5 ka indicating the period of highest

450

lake levels and encompassing the largest glacier advance at ca. 15 ka in the Karakul

451

catchment.

5) High lake levels were maintained during different climate conditions: during cold

EP

452 453

conditions before and during the gLGM, as well as during the late glacial warming

454

and early-to-mid Holocene.

456 457

6) The present state of the lake with relatively low levels and a dense macrophyte cover

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on the lake floor was established at around 6.5 ka, during the mid Holocene.

458

Acknowledgements

459

We thank Karsten Adler and Tim Jonas for help during core drilling in April 2012 and Paolo

460

Ballato for help with section sampling in August 2013. We are indebted to Matthias Röhl for

461

assistance during core opening and sediment description, to Ilhomjon Rajabov from

462

Tajikistan’s State Administration for Hydrometeorology of the Committee for Environmental

463

Protection for logistical support, and to Elizabeth Bunin for helpful comments on the

464

manuscript draft. Funding was provided by the German Research Foundation DFG (grants

465

Mi 730/15-1 and Ai 134/2-1).

ACCEPTED MANUSCRIPT 466 References

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470 471 472 473

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474 475 476

Beer, R., Heiri, O., Tinner, W., 2007. 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). The Holocene 17, 977-985.

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Birks, H.H., Birks, H.J.B., 2006. Multi-proxy studies in palaeolimnology. Vegetation History and Archaeobotany 15, 235-251.

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Bronk Ramsey, C., 2009. Dealing with outliers and offsets in radiocarbon dating. Radiocarbon 51, 1023-1045.

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Bronk Ramsey, C., Lee, S., 2013. Recent and planned developments of the program OxCal. Radiocarbon 55, 720-730.

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Dortch, J.M., Owen, L.A., Caffee, M.W., 2013. Timing and climatic drivers for glaciation across semi-arid western Himalayan-Tibetan orogeny. Quaternary Science Reviews 78, 188-208.

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Heinecke, L., Mischke, S., Adler, K., Barth, A., Biskaborn, B.K., Plessen, B., Nitze, I., Kuhn, G., Rajabov, I., Herzschuh, U., in review. Climatic and limnological changes at Lake Karakul (Pamir Mountains) during the last 29 cal kyr BP. Journal of Paleolimnology.

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538 539 540 541

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579 580 581

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Zhang, J.-F., Liu, C.-L., Wu, X.-H., Liu, K.-X., Zhou, L.-P., 2012. Optically stimulated luminescence and radiocarbon dating of sediments from Lop Nur (Lop Nor), China. Quaternary Geochronology 10, 150-155.

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Tables

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588 Table 1

590 591 592 593 594 595

Radiocarbon age data for the cores (KK12-1 and KK12-2), for exposed sediments (KK13-S1) from Lake Karakul and for a macro-algae collected alive. F14C is given for modern sample (Reimer et al., 2004; LRE lake reservoir effect) 1 Midpoint depth is estimated for depth scale of core KK12-2; 2corrected for LRE of 1.368 ka as the mean of a previously reported result of Mischke et al. (2010) and the new result, and calibrated (2σ)

SC

589

597 598 599 600 601

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OSL dating results for samples from core KK12-2 water content values in brackets were originally measured but were regarded as too low due to assumed water loss during sediment core storage, more details are provided in section 3.3

1

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Figure captions

604 Fig. 1

606

Core location in Lake Karakul in the eastern Pamirs (circle). The cross marks the position of

607

section KK13-S1 ca. 19 m above the lake. Solid white lines indicate the positions of terminal

608

moraines near the lake. Dotted lines indicate the positions of lateral moraines. Inset shows

609

the location of Lake Karakul in Central Asia (dot) and in Eurasia (arrow).

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605

610 Fig. 2

612

Lithology and images (A-D) of recovered sediments from Lake Karakul. Cores KK12-1 and

613

KK12-2 were obtained 10 m apart to produce a gapless composite core. KK13-S1 is a

614

section of exposed sediments ca. 19 m above the lake.

615

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Fig. 3

617

Example of core correlation using XRF data for Sr by defining first-order and second-order

618

markers (FOMs and SOMs). Two SOMs (marked with arrows) were deleted afterwards to

619

produce a consistent correlation scheme including FOMs and SOMs of all six elements (Fig.

620

4).

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616

621 Fig. 4

623

Resulting correlation of cores KK12-1 and KK12-2 using FOMs (solid lines) and SOMs

624

(broken lines) for six elements, and positions of dating samples. The dark grey and crossed

625

sediment section in KK12-1 is reworked material resulting from an untimely release of the

626

piston before planned drill depth was reached.

627

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Fig. 5

629

Age-depth model for the composite core. Five outliers (marked in red) were excluded prior to

630

modelling. Original calibrated age data with 2σ error shaded in light grey and modelled ages

631

in dark grey (Fig. S6). White dots and brackets mark µ and 2σ of modelled ages. The blue

632

shaded area indicates the 2σ-range (95.4% probability) of modelled ages.

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633 634

Fig. 6

635

(A) Sediment accumulation rate (as running mean for 10 cm wide intervals) for the sediments

636

from Lake Karakul; (B) age data for shoreline (dots) and off-shore (triangles) deposits (black:

637

Komatsu and Tsukamoto, 2015; red: this study) shown relative to the present lake level

638

(indicated by the dotted grey line). Glacier advance in the Karakul catchment (grey horizontal

639

bar) and inferred lake level curve (blue) and from Komatsu and Tsukamoto (2015), extended

ACCEPTED MANUSCRIPT by the 25 ka level; (C) axis 2 principal component analysis (PCA) sample scores of Heinecke

641

et al. (in review) as lake level indicator based on PCA of five proxies representing lake-

642

internal processes (TIC concentrations, Fe/Mn ratios, etc.) of core KK12-1 plotted on the age

643

scale for both cores presented here; δ18O records from the Guliya ice core (D; Thompson et

644

al., 1997) and from NGRIP (E) in Greenland (North Greenland Ice Core Project members,

645

2004); (F) regional glacial stages across the semi-arid western Himalayan-Tibetan orogen

646

(SWHTS) of Dortch et al. (2013).

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Supplementary information

650 651

Fig. S1

652

Core correlation using XRF data for Ca by defining FOMs and SOMs. Four SOMs were

653

deleted afterwards to produce a consistent correlation scheme including FOMs and SOMs of

654

all six elements (Fig. 4).

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655 Fig. S2

657

Core correlation using XRF data for Si by defining FOMs and SOMs. Five SOMs were

658

deleted afterwards to produce a consistent correlation scheme including FOMs and SOMs of

659

all six elements (Fig. 4).

SC

656

660 Fig. S3

662

Core correlation using XRF data for Ti by defining FOMs and SOMs. Two SOMs were

663

deleted afterwards to produce a consistent correlation scheme including FOMs and SOMs of

664

all six elements (Fig. 4).

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665 Fig. S4

667

Core correlation using XRF data for Fe by defining FOMs and SOMs. Five SOMs were

668

deleted afterwards to produce a consistent correlation scheme including FOMs and SOMs of

669

all six elements (Fig. 4).

670

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Fig. S5

672

Core correlation using XRF data for Br by defining FOMs and SOMs. Two SOMs were

673

deleted afterwards to produce a consistent correlation scheme including FOMs and SOMs of

674

all six elements (Fig. 4).

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676

Fig. S6

677

Comparison of calibrated (light grey) and modelled (dark grey) age data for the two cores

678

from Lake Karakul. Manually removed outliers in brackets and red colour. The data are

679

vertically arranged according to their relative stratigraphic order.

680 681 682

Table S1

683

Outlier probabilities for radiocarbon and OSL age data from two OxCal runs. Samples with

684

probabilities >60% (bold) were manually removed before the second run which was used to

685

generate the final age-depth model.

ACCEPTED MANUSCRIPT

Table 1

14

7530 ± 2430 ± 11900 ± 13550 ±

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Cal age (a BP) LRE: 1315 a n.d. 1266- 1357 2808- 2997 4297- 4527 6950- 7167 927- 1053 12187-12676 13837-14250 20720-21365 23806-24392 23668-24247 25844-26403 29603-30715 2718- 2841 4844- 5039 5320- 5581 13740-14080 15976-16520 23127-23806 25875-26565 27099-27684 30243-31172 12117-12572 25092-25769

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C age (a BP) 14 F C: 0.8914 1615 ± 30 2755 ± 35 4175 ± 30 5340 ± 35 40 30 60 60

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Material living charophyte Potamogeton remains Potamogeton remains Potamogeton remains Potamogeton remains TOC Potamogeton remains TOC TOC aquatic moss Potamogeton remains Potamogeton remains Potamogeton remains TOC TOC TOC TOC TOC TOC TOC TOC TOC TOC TOC TOC

18790 ± 100 21400 ± 110 21280 ± 100 23230 ± 130

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Comp. depth (cm) 0.0 0.8 113.1 269.2 392.7 492.8 539.9 571.6 664.3 735.3 827.6 908.9 1022.4 1116.5

EP

Depth (cm) 0 01 114- 115 249- 250 371- 372 469- 470 559- 560 581- 582 646- 647 737- 738 823- 824 899- 900 1002-1003 1086-1087 225- 226 422- 423 460- 461 599- 600 689- 690 745- 746 925- 926 1114-1115 1213-1214 145- 147 225- 230

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Sample No KK12-10 KK12-1 01 KK12-1 115 KK12-1 250 KK12-1 370 KK12-1 468 KK12-1 538 KK12-1 560 KK12-1 625 KK12-1 689 KK12-1 775 KK12-1 851 KK12-1 935 KK12-1 1019 KK12-2 226 KK12-2 411 KK12-2 430 KK12-2 569 KK12-2 617 KK12-2 673 KK12-2 812 KK12-2 987 KK12-2 1086 KK13-S1 103 KK13-S1 20

27300 ± 200 3995 ± 35 5720 ± 40 6070 ± 40 13400 ± 70 14850 ± 80 20870 ± 100 23320 ± 160 24470 ± 170 27910 ± 270 11840 ± 60 22460 ± 250

2

Lab. ID Poz-53829 Poz-53820 Poz-56643 Poz-53822 Poz-56644 Poz-53823 Poz-53826 Poz-56556 Poz-56555 Poz-53827 Poz-56648 Poz-53828 Poz-56649 Poz-53821 Poz-65011 Poz-65013 Poz-69039 Poz-66255 Poz-77813 Poz-69038 Poz-69262 Poz-69261 Poz-66467 Poz-64629 Poz-64627

Radiocarbon age data for the cores (KK12-1 and KK12-2) and exposed sediments (KK13-S1) from Lake Karakul and a macro-algae collected alive. 14 F C given for modern sample (Reimer et al., 2004; LRE lake reservoir effect).

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Midpoint depth estimated for depth scale of core KK12-2; corrected for lake-reservoir effect of 1368 years as the mean of a previously reported result of Mischke et al. (2010) and the new result, and calibrated (2σ)

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K (%)

Th (ppm)

U (ppm)

2.15 ± 0.06 1.96 ± 0.06 1.55 ± 0.05 1.60 ± 0.05 1.94 ± 0.06 2.01 ± 0.06 1.54 ± 0.05 1.68 ± 0.06 2.12 ± 0.06 2.19 ± 0.06

9.40 ± 0.27 8.62 ± 0.26 7.86 ± 0.24 7.06 ± 0.23 11.60 ± 0.32 10.90 ± 0.31 5.14 ± 0.17 9.26 ± 0.27 9.50 ± 0.28 9.45 ± 0.27

12.70 ± 0.28 9.39 ± 0.22 8.30 ± 0.21 12.10 ± 0.27 6.04 ± 0.16 2.93 ± 0.10 1.80 ± 0.08 3.87 ± 0.12 3.94 ± 0.12 5.22 ± 0.14

Water content 1 (%) 55.9 44.7 43.5 (39.0) 42.9 37.1 35.0 (20.8) 33.0 (14.8) 30.5 30.7 34.4

Table 2

Dose rate (Gy/ka) 2.80 ± 0.22 2.90 ± 0.21 2.54 ± 0.19 3.13 ± 0.23 2.84 ± 0.20 2.38 ± 0.17 1.64 ± 0.12 2.42 ± 0.18 2.49 ± 0.17 2.83 ± 0.21

De (Gy)

OSL age (a)

1.69 ± 0.09 8.24 ± 0.44 9.00 ± 0.48 6.18 ± 0.47 24.31 ± 0.41 56.90 ± 2.30 61.40 ± 1.60 70.20 ± 1.80 67.30 ± 2.30 77.00 ± 1.70

600 ± 60 2840 ± 260 3550 ± 320 1970 ± 210 8600 ± 600 23900 ± 2000 37400 ± 3000 29100 ± 2200 27000 ± 2000 27200 ± 2100

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Aliquot no. 14 14 17 14 16 15 16 17 20 17

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KK12-2 OSL01 KK12-2 OSL02 KK12-2 OSL03 KK12-2 OSL04 KK12-2 OSL05 KK12-2 OSL06 KK12-2 OSL07 KK12-2 OSL08 KK12-2 OSL09 KK12-2 OSL10

Depth (cm) 115 230 347 427 603 751 929 1118 1210 1224

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OSL dating results for samples from core KK12-2 1 water content values in brackets were originally measured but were regarded as too low due to assumed water loss during sediment core storage, section 3.3 provides more details

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