Tuesday, 6 September 2005 - 3:40 PM

This presentation is part of: Biogeochemistry and Environmental Geochemistry

Change of carbon and nitrogen isotopic compositions in an AMS 14C dated sediment of Lake Hovsgol (Mongolia)

Fumiko Nara1, Takahiro Watanabe2, Yukinori Tani3, Yuko Soma3, Mitsuyuki Soma3, Kazuho Horiuchi4, Akio Imai1, Toshio Nakamura2, and Takayoshi Kawai5. (1) National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Japan, (2) Center for Chronological Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan, (3) Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Shizuoka, Japan, (4) Departments of Earth and Environmental Science, Hirosaki University, Bunkyo-cho, Hirosaki, 036-8561, Japan, (5) Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan

Vertical profiles of stable isotopic compositions of total organic carbon (d13CTOC) and total nitrogen (d15NTN) in short sediment core (X106) collected from Lake Hovsgol (Mongolia) were determined, and were discussed based on the sedimentation age determined by AMS 14C, along with compositional changes in the sedimentary steryl chlorin esters (SCEs). SCEs are sterol esters of pheophorbide or pyropheophorbide, and formed from chlorophyll and sterols during the grazing of phytoplankton by zooplankton. Thus, the distribution of sterols in SCEs in sediments is considered to reflect the original sterols in phytoplankton assemblage at the time of deposition. AMS 14C measurement shows that the base of core X106 corresponded to the 23 14C ka. The d13CTOC and d15NTN values in core X106 shifted from -23 ‰ to -27 ‰ and from 5 ‰ to 3 ‰, respectively, during 23-15 14C ka, and were nearly constant after 15 14C ka, indicating significant change in the isotopic compositions of organic carbon and nitrogen in the sediment during the later stage of the last glacial period. Before 15 14C ka, total organic carbon (TOC) negatively correlated with d13CTOC and d15NTN (R = 0.944, 0.889, respectively, n = 38). Thus, the increase of TOC during the later stage of the last glacial period would result in the negative shift in d13CTOC and d15NTN values in Lake Hovsgol. Since total SCEs, representing autochthonous (phytoplanktonic) productivity, correlated with TOC (R = 0.942, n = 73), the enhanced primary productivity of phytoplankton in the lake were responsible for the increase in TOC. Interestingly, the change in composition of SCEs, that is, sterol composition in SCEs, indicated by HPLC analysis, accompanied the shift in d13CTOC , and d15NTN . The change of phytoplankton assemblage, increase in autochthonous productivity and the shift in d13CTOC and d15NTN therefore occurred concurrently in this period of Lake Hovsgol.

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