SEDIMENT CORE FROM ICE-DAMMED PALEOLAKE DARHAD, MONGOLIA

Natural exposures of sediment at 1560 m amsl on the banks of the Shargyn Gol (river) in Darhad basin (51.4ºN, 99.4 ºE) consist of 11 m of varved silts unconformably overlain by distal outwash from the nearby (5 km) Pleistocene Jarai Gol glaciers and 1 m of loess reworked into lacustrine deposits and capped by 1 m of unreworked loess. Uncorrected ¹⁴C dates from clam shells from the reworked loess give ages of 11,000-13,000 ¹⁴C yr B.P.; larch twigs give ages of ~9300 ¹⁴C yr B.P. Dates for fish scales and moss fragments from the varved sediments give ages of 33,000-50,000 ¹⁴C yr B.P. LGM sediments appear to be missing from the sequence. Darhad basin contains only small lakes today, and the most obvious explanation for deep (100-250 m) late Pleistocene lakes is damming of the basin by outlet glaciers from the Sayan ice field. To elucidate the late glacial lake history, and therefore the history of maximum ice advances from the ice field, we extracted a 91-m sediment core from the lake floor at 1547 m amsl, 10 km NW of the town of Renchinlkhumbe. Preliminary analysis and dating suggest that Darhad basin was at least partially filled throughout much of the late Pleistocene, but it is not clear that the LGM lake level exceeding 1560 m, even though cosmogenic nuclide dating indicates moraines around the basin then. The lake appears to have persisted or reformed below 1560 m amsl even after the LGM glaciers retreated, possibly because of damming by outwash from the LGM outlet glacier rather than the ice itself. In western Central Asia LGM glaciers were smaller than ones earlier in the last glacial cycle, which does not appear to have been the case in Mongolia. The sediments from the Darhad paleolake have the potential to elucidate aspects of the glacial and hydrologic history more thoroughly than analysis of glacial drift directly.