[M-IS18] 古気候・古海洋変動

[MIS18-P11] Interannual change of diatom fragment composition for the mid-Pleistocene in the southern Bering Sea

*小野寺 丈尚太郎¹、Kemp Alan²、Pearce Richard²、堀川 恵司³、高橋 孝三⁴ (1.海洋研究開発機構、2.University of Southampton、3.富山大学、4.九州大学)

Diatoms are major primary producers in the central Bering Sea and are important for the marine ecosystem and biogeochemical cycles in the region. Some diatom taxa can be applied as a paleoceanographic proxy. For comparison of modern and past oceanographic environments, diatom fragments in laminated sediment on the eastern Bowers Ridge obtained at Site IODP 323-U1340 were observed at the annual time scale. The estimated age of studied samples is the early Marine Isotope Stage (MIS) 14 (~558 ka) and Termination IX (~786 ka) from MIS 20 to 19. Compared to modern diatom flora in sinking particles studied in the southeastern Aleutian Basin of the Bering Sea, major diatom taxa in each laminated sediment were much various from oceanic taxa such as Neodenticula seminae common in modern flora to the dominance of Thalassiosira and Chaetoceros resting spores showing more productive condition, and Thalassiothrix suggesting stratigraphic condition. Some parts of diatom fragment composition periodically changed in several years to the decadal time scale, which probably reflects cyclicity in paleoclimate conditions. Based on dry bulk density of similar laminated sediments obtained on top of Bowers Ridge and the mean thickness of studied laminated sediment, reconstructed total mass flux was at least 5 times higher than total mass flux at the southeastern Aleutian Basin in the modern Bering Sea. The lithogenic matter was mainly supplied from the Bering Sea shelf as estimated by REE stable isotope analysis. The continuous formation of laminated sediments for decades to centennials probably reflects the advective ocean current condition from shelf to basin, and it kept a high sinking rate of diatom particle with the lithogenic matter.