Climatic changes during the Holocene and late Pleistocene recorded in Ulaan Lake sediments, Mongolia

Abstract

Lake Ulaan, located in Omnogovi Province, southern Mongolia, is the easternmost lake in the Gobi Valley near Gobi-Altai Mountains. Ulaan Lake was fed by both the Ongin River which drains the Khangai Mountains and outlet of ancient large lake upslope which was drained by meltwaters from glaciers of Mongolian Altai. This lake had a maximum areal extent of 65 km2, and the altitude of this lake is about 1,000 m. The lake was semi-permanent in 1990s, but it is dried out completely. The study area is located far beyond the modern northern limit of the Asian summer monsoon, but it was located between the maximum extent of summer monsoon and winter monsoon during most of the Holocene. Therefore, sediments of Ulaan Lake have a record of the change of monsoon climate. Three sets of core sediment were collected at three sites in Ulaan Lake for delineation of the Quaternary paleoclimatic variations through integration of sedimentological, mineralogical and geochemical studies. The sediments were dated using an OSL (Optically Stimulated Luminescence) method. Grain size distribution and magnetic susceptibility were measured and total nitrogen and carbon, TIC (Total Inorganic Carbon), and geochemical compositions were analyzed for examining core sediments. According to OSL dates, a 584 cm-long core sediment have a record for ca. 16000 years. In this core, sediment of the top 100 cm interval is composed dominantly of clay and is homogeneous in color (reddish brown), whereas core interval between 100 cm and 400 cm is largely composed of silt and contains mottled features and several cm-thick grayish layers. Sediment below 400 cm depth is alternated by sand and silt. Bimodal grain size distribution (3~4φ, 11~12φ) in the whole core sediments indicates that this lake sediment was transported by two different processes, e.g. riverine and eolian. Using TOC and CaCO3 as proxy variables for productivity, the period of summer monsoon maximum in the study area is estimated to be ca. 8300 ~