Achit-Nuur Lake (49°30'N, 90°36'E, 1435 m a.s.l.) lies in a broad intermountain depression in the northeastern part of the Mongolian Altai. The lake has an area of 297 km², a maximum length of 28 km and maximum width of 16 km. Achit-Nuur has an average depth of ca. 2 m and a maximum depth of 5 m. This freshwater lake has inflows from the Huv-Usny-Gol River and the Mogen-Buren River, which flow into the northern part of the lake, and an outflow via the Usun-Holoi into the Kobdo River and thence into the closed Hara-Us-Nuur Lake. The lake was probably formed by tectonic processes in the middle Pleistocene which resulted in the damming of the Mogen-Buren and Huv-Usny-Gol rivers. Achit-Nuur has low, swampy northern and southern shores with salt-marsh vegetation, and high eastern and western shores, formed by granite rock and covered by desert steppe vegetation. Macrophyte vegetation (Chara and Potamogeton) is widespread in the shallow zone.
A 200cm (Achit 2010-1) and another 234cm (Achit 2010-2) core was obtained from the lake in August of 2010. The cores are made up of carbonate-rich lake mud in the upper part, silty sediments in the mid part and sand in the bottom part. Three plant leaf layers were recorded at the 108-111cm, 127-128cm and 158-160cm intervals in core Achit 2010-1. They represent a very shallow water level and a shift to a bog environment. The high carbonate content and high TOC content in the upper part of the core represent a relatively high lake level and probably relatively warm conditions resulting in high organic carbon influx from aquatic and possibly terrestrial plants. Chronological and geochemical data will be presented and the westerly-derived precipitation history in western Mongolia discussed.