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

海冰通过增加海表温度、干扰海-气热交换和驱动海洋热盐环流等途径影响地球气候系统。本研究的目的是调查鄂霍次克海海冰的历史变化，这是现在位于世界上最低纬度。首先，我们开发了鄂霍次克海冰的古海洋学代理，并将其应用于沉积物芯。我们尝试使用两种代理。一组是硅藻（冰藻的主要种类）中的特征生物标志物--高分支类异戊二烯（HBI），另一组是由海冰搬运的陆源碎屑物质--冰筏碎屑（IRD）。不幸的是，我们不能应用HBI重建鄂霍次克海冰，因为HBI没有发现在鄂霍次克海的沉积物。然而，沉积物收集器收集的沉降颗粒的时间序列数据和卫星观测的季节性海冰分布之间的比较可以证明，IRD实际上只在海冰融化期间下降，IRD可以应用于重建过去的变化在鄂霍次克海的海冰的生产和分布。我们应用IRD和许多其他古海洋环境的替代品在鄂霍次克海的沉积岩芯，并获得了以下结果。1)在冰川时期，海冰经常根据全球变冷事件（海因里希事件）而扩张。2)在全新世早期和中期的温暖时期（9000-6000年BP），海冰比今天更丰富。微体古生物和地球化学资料表明，全新世早、中期硅藻以颗石藻为主，环境温暖，盐度较低。这些事实表明，在全新世鄂霍次克海冰扩大的可能性，而不是冷却，但由于海水的清新。

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.