Late Quaternary transition from lacustrine to a fluvio-lacustrine environment in the north-western Nam Co, Tibetan Plateau, China

Holocene sediments from a 9 m high outcrop at the Gangyasang Qu's entry into Lake Nam Co, on the south-eastern Tibetan Plateau, were studied in a multiproxy approach. The lowermost part of the section is of early Holocene age and consists of varved sediments deposited in a proglacial lake. Geomorphological investigations of fossil beach ridges and fossil cliff lines in the littoral zone of Nam Co indicate a late Pleistocene lake level high of 29 m above the present level. A warming is visible in ostracode-rich silts deposited after 9.6 ka BP. An ostracode-based transfer function estimates a lake level 28 m higher than today from this time until about 5.2 ka BP. A late Pleistocene lake level high was accompanied by an increase in lake size, causing Nam Co to overflow into the lower course of Gangyasang Qu, where a bay with slack-water conditions formed. Between 4.2 and 1.3 ka BP the lake level was approximately 10 m higher than today. The progradation of the Gangyasang Qu delta into Nam Co and the decreasing lake level shifted the study site into a fluvio-lacustrine environment. The sandy character and relatively low numbers of ostracode valves in the topmost sediments indicate increased fluvial activity, possibly due to a temporary increase in flood frequency. This facies lasted at least until 1.3 ka BP, when the lake level of Nam Co dropped again and the river incised more than 10 m into the underlying fluvio-lacustrine and lacustrine sediments.