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

海冰通过增加地表反照率、干扰海-气热量交换、驱动海洋热盐度循环等多种途径影响地球气候系统。这项研究的目的是调查鄂霍次克海海冰的历史变化，目前鄂霍次克海位于世界最低纬度。首先，我们开发了鄂霍次克海冰的古海洋学指标，并将其应用于沉积岩芯。我们尝试使用两种代理。一个是硅藻(冰藻的主要种类)、高度分枝类异戊二烯(HBI)中的一组特征生物标志物，另一个是由海冰、冰排碎屑(IRD)输送的陆源碎屑物质。不幸的是，我们不能将HBI用于重建鄂霍次克海冰，因为在鄂霍次克海的沉积物中没有发现HBI。然而，将沉积物捕捉器收集的沉降物颗粒的时间序列数据与卫星观测的季节性海冰分布进行比较，可以证明IRD实际上只在海冰融化期间下降，IRD可以用来重建鄂霍次克海海冰过去生产和分布的变化。我们将IRD和其他许多古海洋环境指标应用于鄂霍次克海的沉积岩芯，并取得了以下结果。1)在冰川时期，海冰根据被称为海因里希事件的全球变冷事件而频繁膨胀。2)早、中全新世暖期(距今9000-6000年)，海冰比今天更为丰富。微体古生物和地球化学资料表明，全新世早、中期，在温暖、盐度较低的环境下，粘虫类以硅藻为主。这些事实表明，在全新世，鄂霍次克海冰的膨胀不是通过冷却，而是由于海水的淡化。

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.