Development of Paleoceanographic Proxy for Sea Ice and Analysis of Historical Variation in Okhotsk Sea Ice.

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Sea ice influences climate system on Earth through several ways such as increasing the surface albedo, interfering air-sea heat exchanges and driving thermo-haline circulations in ocean. The purpose of this study is to investigate historical variation in the sea ice of the Sea of Okhotsk, which is now located at lowest latitude in the world. Firstly, we had developed paleoceanographic proxies for Okhotsk sea ice, and applied them to sediment cores. We attempted to utilize two kinds of proxies. One was a group of characteristic biomarkers in diatom (main species of ice algae), Highly Branched Isoprenoids (HBI), and the other was terrigenous clastic materials transported by sea ice, Ice Rafted Debris (IRD). Unfortunately, we could not apply HBI for the reconstruction of Okhotsk sea ice because HBI was not found in sediments of the Sea of Okhotsk. However, comparison between time-series data of sinking particles collected by sediment traps and satellite observations of seasonal sea ice distribution could demonstrate that the IRD actually drops only during the period of sea ice melting, and IRD could be applied to reconstruct past changes in production and distribution of sea ice in the Sea of Okhotsk. We applied the IRD and many other proxies for paleoceanographic environments to sediment cores in the Sea of Okhotsk, and have obtained following results. 1) In glacial periods, sea ice expanded frequently according to global cooling events known as Heinrich Events. 2) During a warm period of early and middle Holocene (9000-6000 yrs BP), sea ice was more abundant rather than today. Micropaleontological and geochemical data clarified that during early and middle Holocene Coccolithopholid dominated diatom under the warm and less saline circumstance at that time. These facts suggest a possibility that in Holocene the Okhotsk sea ice has expanded not by cooling but due to freshening of seawater.