Holocene Sedimentary Environments and Relative Sea Levels at a Site along the Coast of Lake Tofutsu in the Eastern Part of Hokkaido, Japan

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We obtained the core sediment containing abundant molluscan shells at a site along the coast of Lake Tofutsu in the eastern part of Hokkaido, Japan. In order to clarify changes in sedimentary environments at this site, we analysed molluscan and diatom assemblages, and carried out sulfur analysis of sediments. These were then used along with three radiocarbon dates for molluscan shells to determine the marine limit (ML), which provides a relative sea-level (RSL) record approximating the height of the high water levels.Intertidal molluscan shells are abundant in the sediments with the altitude below -0.23m. Crassostrea gigas is dominant accompanied by a warm-water species Trapezium (Neotrapezium) liratum below -1.33m, while Littorina (Neritrema) sitkana and Batillaria cumingii predominantly occurred between -1.33m and -0.23m. A shell of T. (N.) liratum at -1.40m showed the 14C age of 3, 320±60yrs BP, yielding an additional Holocene record of a warmwater species that lived in the coastal region along the Okhotsk Sea at that time. Diatom assembalges and sedimentary sulfur content more than 0.3% indicate marine influence for the range below+0.30m. The ML at +0.30m altitude identified by diatom and sulfur analyses is higher than that at -0.23m by the mollusca analysis. An intertidal diatom Pseudopodosira kosugii occurred between -0.23m and +0.02m, and Terpsinoe americana also occurred chracteristically between -2.08m and -1.33m, yielding reasonable RSLs for identifying the paleo-mean sea level (PMSL). These PMSL records with altitudes of -0.23m (1, 290±60yrs BP) and -1.40m (3, 320±60yrs BP) suggest relative subsidence during the late Holocene at the coast of Lake Tofutsu.

Holocene Sedimentary Environments and Relative Sea Levels at a Site along the Coast of Lake Tofutsu in the Eastern Part of Hokkaido, Japan

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