An ostracod-inferred large Middle Pleistocene freshwater lake in the presently hyper-arid Qaidam Basin (NW China)

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 sediments 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 luminescence 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 230Th/U ages scattering between 100 and 400 ka, a Middle Pleistocene age between 120 and 400 ka is assumed for the lake formation.