[M-IS32_30PM1] 地球掘削科学

[MIS32-15] IODP Expedition 346 アジアモンスーンの概要

*多田 隆治¹、Murray Richard W.²、Zarikian Carlos Alvarez³、Expedition 346 Scientists ⁴ (1.東京大学大学院理学系研究科、2.Earth and Environment, Boston University、3.Texas A&M University、4.IODP)

キーワード：IODP, Expedition 346, 日本海, 東シナ海, ダンスガードーオシュガーサイクル, 東アジアモンスーン

Alaska on August 2nd, sailed all the way to the Japan Sea/East Sea (JS/ES), drilled 7 sites in the JS/ES and 2 sites in the northern East China Sea (ECS), and ended her cruise at Pusan, Korea on September 28th. During six weeks of drilling, we recovered 6135.3 m of core, with an average recovery of 101%, which is a record of IODP. The expedition was originally aimed to test the hypothesis that Plio-Pleistocene uplift of Himalaya and Tibetan Plateau (HTP) and/or emergence and growth of the northern hemisphere ice sheets and consequent establishment of the two discrete modes of westerly jet (WJ) circulation is the cause of the millennial-scale variability of the East Asian summer monsoon (EASM) and amplification of the Dansgaard-Oeschger cycles (DOC). The expedition is also aimed to test the hypothesis that surface and deep water conditions of the JS/ES has been controlled by the nature and strength of the water influx through the Tsushima Strait which are strongly influenced by EASM precipitation, eustatic sea level changes, and EAWM cooling. In order to explore the linkage between WJ circulation and EASM precipitation, it is critical to obtain high-resolution, continuous sedimentary records that preserve proxies of both WJ and EASM. In this respect, the JS/ES is ideal because its hemipelagic sediments contain significant amount of the eolian dust transported from East Asia by the WJ, and alternations of dark and light layers that characterize Quaternary sediments of the sea record variations of EASM precipitation over South China (Tada et al., 1999). Sites are also arranged along the north-south transect to monitor the behavior of the WJ. The sites are arranged to make the depth transect to monitor the behavior of deep water through changes in calcium carbonate compensation depth and bottom water oxygenation level. Northern East China Sea is ideal to monitor changes in EASM precipitation because its surface water salinity and temperature during summer is significantly influenced by the discharge of the Yangtze River whose drainage area covers the majority of the South China where EASM precipitation is most intense (Kubota et al., 2010).Because of recent advances in drilling technology and newly developed analytical tools, we were able to collect and examine sediment records that were impossible to acquire even a few years ago. The newly engineered half piston core system (called the half APC) enabled us to recover the deepest piston core in DSDP/ODP/IODP history (490.4 m in Hole U1427A). That achievement was also the deepest continuously recovered piston cored sequence, initiated at the mudline and penetrating to the ~500 m depth solely by piston coring. These technological advances delivered a series of new surprises. Examples are pristine dark?light laminae from ~12 Ma sediment recovered by piston core from 410 m core depth below seafloor, Method A [CSF-A] at Site U1425 and from 210 m CSF-A at Site U1430.Through this expedition, we collected the geological evidence necessary to test the hypotheses described above through drilling in the JS/ES and northern part of the ECS, and are trying to 1) specify the onset timing of orbital and millennial-scale variability of EASM, EAWM and WJ and reconstruct their evolution process and spatial variation patterns, and 2) reconstruct orbital and millennial-scale paleoceanographic changes in the JS/ES to clarify the linkage between the paleoceanography of the JS/ES and EASM, EAWM and/or sea level. Comparison of the obtained results with the uplift history of HTP and/or ice volume changes will enable us to test the hypotheses.