A LONG RECORD OF PALEOCLIMATES FROM NORTH MONGOLIA: SEDIMENTS OF LAKE KHUBSUGUL

Lake Khubsugul (North Mongolia) fills a depression in the Baikal Rift Zone. Unlike its sister Lake Baikal, itl is almost saturated with CaCO₃, because its major loss of water is by evaporation (80%). The outlet is a small Egerin River. Atmospheric precipitation is 10-50 mm in winter, 300 mm in summer. Moisture is mostly delivered by cyclones born in the North Atlantic. The lake is at the far extent of their tracks, and hence its sediments are of special interest for paleoclimatology.

Recent studies have shown that Lake Khubsugul in the Last Glaciation had no outlet, was almost dessicated, and has been only filled to its present outlet in the Holocene. Signs of recent paleo-deltas at ca. 200 m below the present water surface were found by seismic profiling. A few gravity cores were studied, and one dated by ¹⁴C AMS to 12 ky BP. Sediment accumulation rate in the Holocene was 4 cm/ky. The sediment of this time is a diatomite (80 mln frustules/g). Prior to the Bølling, diatoms were either absent or non-preserved. Ostracods were highly abundant. Carbonates and at times gypsum (proxy of brackish water) precipitated to the bottom. Sediments of the Upper Pleistocene, compared to those of the Holocene, are enriched in Ca, Sr, Mg, C_inorg and show lower concentrations of Sc, Ni, Y, U, W, and REEs (all extractable with weak nitric acid). Transition from the Upper Pleistocene to the Holocene resulted in an increase of organic matter from < 1 to > 6 %, and of BiSi from 1 to 20%. Abrupt climate oscillations associated with Bølling-Allerød - Younger Dryas are clearly revealed by these and other proxies. Moreover, the proxies reveal a short-term arid episode centered between 6 and 5 ky BP, contemporary with the desertification of Sahara.

In March of 2003, a 50 m drilling core from Lake Khubsugul was obtained. The core contains a record of paleoclimates of the last few hundred thousand years. The first results of its studies will be reported.