17:15 〜 18:45
[MIS12-P22] Fluctuation in silicoflagellate assemblages in the Japan Sea sediments off Wakasa
Silicoflagellate is a marine phytoplankton with biogenic opal skeletons. There are two major silicoflagellate genera, Dictyocha and Stephanocha. Genus Dictyocha dominates tropical to temperate regions, whereas Stephanocha dominates polar to subpolar regions. The ratio of the relative abundances of the two genera has been used as a sea-surface temperature (SST) proxy. The four straits connecting the Japan Sea to the surrounding seas are shallower than 130 m. The Tsushima Warm Current (TWC) from the East China Sea through the Tsushima Strait is the sole warm current flowing into the Japan Sea. TWC supplies heat and salt to the Japan Sea. During the last Glacial Maximum, ~130 m sea level drop significantly reduced the TWC inflow, leading to the cooling and freshening of the Japan Sea surface water.
In this study, to reconstruct the Japan Sea SST change at millennial time scales, we investigated the silicoflagellate assemblages in the WB6 piston core (36°29.30'N, 135°25.62'E, water depth 845 m, total core length 13.675 m) in the Japan Sea off Wakasa during the KR15-10 cruise. The silicoflagellate classification was based on Poelchou (1976). Dictyocha messanensis has two morphotypes: forma messanensis and forma spinosa. Because there were intermediate forms between forma messanensis and forma spinosa, it is necessary to distinguish them. The basal ring diameters of the encountered Dictyocha messanensis skeletons in the WB6 core were measured. The size distribution histogram showed bimodal patterns with 23-27 μm and 30-37 μm peaks. We classified the former as D. messanensis f. messanensis and the latter as D. messanensis f. spinosa. The distribution of size is different from the sample. D. messanensis f. spinosa increases before the Last Glacial Maximum. Dictyocha/Stephanocha fluctuation synchronized with Mg/Ca SST (Sagawa et al., 2023) fluctuation, just millennial scale but small-scale change during 36.5-39.5 ka. Therefore, fluctuation in silicoflagellate assemblages can be used as a glacial sea-surface temperature proxy.
In this study, to reconstruct the Japan Sea SST change at millennial time scales, we investigated the silicoflagellate assemblages in the WB6 piston core (36°29.30'N, 135°25.62'E, water depth 845 m, total core length 13.675 m) in the Japan Sea off Wakasa during the KR15-10 cruise. The silicoflagellate classification was based on Poelchou (1976). Dictyocha messanensis has two morphotypes: forma messanensis and forma spinosa. Because there were intermediate forms between forma messanensis and forma spinosa, it is necessary to distinguish them. The basal ring diameters of the encountered Dictyocha messanensis skeletons in the WB6 core were measured. The size distribution histogram showed bimodal patterns with 23-27 μm and 30-37 μm peaks. We classified the former as D. messanensis f. messanensis and the latter as D. messanensis f. spinosa. The distribution of size is different from the sample. D. messanensis f. spinosa increases before the Last Glacial Maximum. Dictyocha/Stephanocha fluctuation synchronized with Mg/Ca SST (Sagawa et al., 2023) fluctuation, just millennial scale but small-scale change during 36.5-39.5 ka. Therefore, fluctuation in silicoflagellate assemblages can be used as a glacial sea-surface temperature proxy.