Paleoclimatic Implication During the Late Quaternary From the Lake Hovsgol Sediment in Northern Mongolia

Lake Hosvgol is located in a high altitude area(1660m) of northern Mongolia. The lake occupies a southern part of the Baikal Rift Zone, which formed about 2.5-4 Ma. It is the second largest and deepest lake in East Asia following Lake Baikal - 136 km long and average 20 km wide with a maximum depth of 262 m and average depth of 139 m. At present an outlet from the lake is the Egerin river running from its southernmost bay. We collected 20 gravity cores in July, 2004 and August, 2006. Among them, HS 7(from the central part of lake) and HS 14 (from the northwestern marginal part) have been analyzed in terms of sedimentary texture, microfossil and chemical element. HS 7 is divided into three units on the basis of sediments texture, sediment color and observation of fossils. Unit A (0-10cm) is laminated mud with olive gray color. Ostracods are not observed in this interval but diatom contents are high. At Unit B (10-27cm), mud content is slightly low and lamination is well developed. It is dark greenish gray in the upper part(about 2 cm), and is alternated with dark greenish gray and light brownish gray in the lower part. Also, ostracods fragments appear at the end of Unit B. Diatom contents decrease and much less in a lower core. Unit C (27-128cm) is generally massive and is crudely stratified. Its color is dusky blue green. Over the all interval of Unit C, ostracods are well preserved, but diatoms are not present. Trace elements are analyzed for ostracods(Cytherissa lacustris) in this interval. As a result, Mg/Ca and Sr/Ca ratios increase in a lower part. From HS 14 ostracods occur over the all interval but diatom doesn't exist. The overall color is greenish gray and sands and/or boulders are interlayered. We also analyzed trace elements for ostracod(Cytherissa lacustris). The chemical pattern is similar with that of Unit C of HS 7. The results of dating suggest that HS 14 was deposited during the late Pleistocene, not Holocene. Due to no dating ages for HS 7 we compare it with HS 14's and other cores(Prokopenko et al., 2005). As a result, we assumed that the boundary between Unit A and Unit B of HS 7 is about the start of the Holocene, and the boundary between Unit B and Unit C of HS 7 is about the end of Pleistocene. Increasing diatom contents of Unit A of HS 7 indicates higher nutrient input by enhanced precipitation/runoff in Holocene than in Pleistocene. At the end of Pleistocene, decreasing Mg/Ca and Sr/Ca value in ostracods of two cores imply lowering salinity in water. Therefore at the start of Holocene, precipitation/runoff increased more than at the end of Pleistocene.