Last glacial to post glacial climate changes in continental Asia inferred from multi-proxy records (geochemistry, clay mineralogy, and paleontology) from Lake Hovsgol, northwest Mongolia

摘要

Multi-proxy records (geochemistry, clay mineralogy and paleontology) from three sediment cores from Lake Hovsgol, northwest Mongolia, provide paleoenvironmental changes in continental Asia from the last glacial to the present (during the last 29 cal ka BP). This paper presents the first continuous δ~(13)C and δ~(18)O dataset for ostracod shells (Cytherissa lacustris) from Lake Hovsgol sediments. A positive shift in ostracod δ~(13)C values (from + 0.5‰ to + 3.2‰) began at ca. 21 cal ka BP, and was associated with a decrease in δ~(13)C values of total organic carbon (from - 24.7‰ to - 27.3‰) and an increase in the mass accumulation rate of total organic carbon (MAR-TOC). These results suggest that increased lake productivity led to the (13)~C-enrichment of dissolved inorganic carbon in Lake Hovsgol. In addition, decreases in clay content and the illite crystallinity index (full width at half maximum of the 10 A peak) began at ca. 21 cal ka BP, indicating an increase in surface runoff and a change in sources of clastic materials in the watershed. The highest δ~(18)O tracod values (up to - 4.8‰) at 16-15 cal ka BP indicate the increases in precipitation from the East Asian monsoon. The total pollen amount increased, and Betula, Alnus, and Salix were dominant (up to 53%, 30%, and 6%, respectively) after 16-15 cal ka BP. These findings indicate that broad-leaved trees expanded around Lake Hovsgol. In addition, fossil diatom valves appeared after 16-15 cal ka BP, suggesting climate amelioration, and increased input of soil-derived nutrients and dissolved silica. Between 10 and 5 cal ka BP (Holocene climate optimum), productivity was high in and around the lake (MAR-TOC, up to 47.3 mg/cm2 kyr; MAR-diatoms, up to 14.4×10~5 valves/cm~2 kyr; total pollen concentration, up to 9.0×10~5 grains/cm~3). Environmental changes during the last 29 kyr in and around Lake Hovsgol are clearly associated with fluctuations in summer insolation and East Asian monsoon intensity.