Author's personal copy An ostracod-inferred large Middle Pleistocene freshwater lake in the presently hyper-arid Qaidam Basin (NW China) q Steffen Mischke a, * ,1 , Zhencheng Sun b, y , Ulrike Herzschuh c , Zizhen Qiao d , Naida Sun b a Institute of Geological Sciences, Freie Universita ¨t Berlin, Malteserstr. 74-100, 12249 Berlin, Germany b University of Petroleum, Department of Earth Science, Changping, Beijing 102200, P.R. China c Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A43,14473 Potsdam, Germany d Headquarter of Petroleum Exploration and Development of Qinghai Province, CNPC, Dunhuang 736202, P.R. China article info Article history: Available online 18 March 2009 abstract Exposed sediments of 50 m thickness were investigated in the 120,000 km 2 Qaidam Basin at the northern margin of the Tibetan Plateau. Fine-grained silty sediments constitute most of the section and were accumulated in a lake. Intercalated beds of aeolian sand, and of fluvial or littoral origin, show that the lake experienced periods of desiccation and shallow water levels at the section site. The lake sedi- ments provided surprisingly species-rich ostracod assemblages with almost all species indicating freshwater to oligohaline conditions. The species richness, the position of the section in the southern neighbourhood of a topographic rise in the Qaidam Basin and intercalated layers of fluvial and aeolian sands suggest that a relatively marginal, littoral lake setting existed throughout most of the sequence. A maximum water depth of several tens of metres may be assumed at least for some parts of the section, and a large lake covering about half the size of the Qaidam Basin may be inferred from our data. This inference is in clear contrast to the present-day wind-shaped topography with salt playas and shallow saline lakes in the lowermost depressions of the Qaidam Basin. Dating by optically stimulated lumi- nescence and uranium-series measurements did not allow the establishment of a firm chronology for the investigated section. However, based on infrared stimulated luminescence dates of around 120–140 ka obtained on feldspars and regarded as minimum ages; and 230 Th/U ages scattering between 100 and 400 ka, a Middle Pleistocene age between 120 and 400 ka is assumed for the lake formation. Ó 2009 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction Late glacial and Holocene environmental and climate change on the continents has been the subject of countless studies since the beginning of Quaternary research, with the contemporary inter- glacial and the last glacial maximum (LGM) often regarded as the opposite extremes of Quaternary climate modes. In contrast to this common view, one has to be aware that environmental variability may have experienced an even larger amplitude at certain places during earlier periods of the Quaternary, although much less work has been done on the Quaternary climate history prior to the LGM due to the difficulties in dating beyond the limit of radiocarbon ages and the restricted accessibility of climate archives. Ice sheets have been larger than during the LGM (e.g., Zheng et al., 2002; Svendsen et al., 2004), deserts displayed more distinct retreat in places (e.g., Hooghiemstra et al., 1992) and a number of lakes covered much larger areas than during MIS 2 and 1 (e.g., Causse et al., 1989; Szabo et al., 1989; Magee et al., 1995). With respect to Central Asia, few studies focus on environ- mental or climatic reconstructions of Quaternary periods prior to the LGM. There is certainly a need for more detailed studies of geological archives which pre-date the LGM to improve under- standing of the full range of the Quaternary environmental and climate variability. Published data from studies of cores from the Qaidam Basin or other sedimentary basins in western China or from the investigation of loess sections were successfully used to trace the glacial–interglacial fluctuations through the Quaternary on land (e.g. Chen et al., 1999; Guo et al., 2000; Chen et al., 2001; Jiang and q Prof. Zhencheng Sun died in February 2009 after a long illness. This paper is dedicated to his memory. * Corresponding author. Tel.: þ49 30 83870285; fax: þ49 30 83870745. E-mail addresses: smischke@zedat.fu-berlin.de (S. Mischke), uherzschuh@ awi-potsdam.de (U. Herzschuh), sunnd7@hotmail.com (N. Sun). y Deceased. 1 Present address: Limnological Research Center, Newton Horace Winchell School of Earth Sciences, University of Minnesota, 220 Pillsbury Hall, 310 Pillsbury Drive SE, Minneapolis, MN 55455-0219, USA. Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.03.002 Quaternary International 218 (2010) 74–85