[M-IS02] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS02-P03] Japan Sea paleoceanography and faunal turnover during the late Miocene global cooling reconstructed by radiolarian fossil assemblage

*Mizuki Tojima¹, Kenji Matsuzaki², Masayuki Ikeda¹ (1.The University of Toyko, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:late Miocene, radiolarian fossil assemblage, Japan Sea

The late Miocene global cooling (LMGC) was accompanied by global environmental changes including major changes in monsoon intensity and ocean circulation, and also by marine and terrestrial biotic overturn. The Japan Sea has been an East Asian monsoon region and was greatly affected by ocean circulation during the LMGC because it was connected to the deep water of the North Pacific in the early late Miocene. Recent studies have revealed drastic environmental changes in the Japan Sea during the LMGC. A dominant endemic radiolarian Cycladophora nakasekoi became extinct and more organic-poor diatom-rich sediment started to deposit, suggesting increased productivity and bottom-water oxygenation. Analysis of radiolarian fossil assemblage, which can be used as proxies for variable water masses, suggested increased inflow of Pacific Central Water (PCW) into the Japan Sea and southward shift of the subarctic front under the intensified East Asian winter monsoon (EAWM) (Matsuzaki et al., 2022). They showed eccentricity-paced variations of radiolarian flux; however, other climatic proxies are mainly paced by the 41 kyr obliquity cycle, which they failed to detect possibly due to lower sampling resolution. Orbital-scale variations were only weakly detected especially at the transition stage; thus the cause of the change in radiolarian assemblage remains unclear. In this study, we conducted a higher-resolution radiolarian fossil assemblage analysis on sediment core samples collected from site U1425 in the central part of the Japan Sea drilled during the Integrated Ocean Drilling Program (IODP) Expedition 346. Combining with other proxies, we tried to reconstruct the paleoceanography of the Japan Sea and identify the causes of the faunal shift. We detected 40 kyr cycles in absolute abundances of environmental proxy species, which were not detected in Matsuzaki et al. (2022). A ~40 kyr cycle in Tricolocapsa papillosa abundance, a proxy for the PCW, together with the prominent 40 kyr cycle in the benthic oxygen isotope at that time (Holbourn et al., 2018), suggests that 40 kyr paced variations in the volume of the Antarctic ice sheet controlled the intensity of the Pacific Meridional Overturning Circulation (PMOC) and PCW inflow into the Japan Sea. Absolute abundance of Cycladophora sphaeris, subarctic shallow water radiolaria, also showed a 40 kyr signal. A 40 kyr cycle is also prominent in the quartz grain size variation in the Chinese Loess Plateau, a proxy for the wind intensity of the EAWM, at 7 Ma (Sun et al., 2010); thus probably the EAWM was paced by a 40 kyr cycle also during the studied interval (7.6-7.3 Ma) and its strong wind promoted inflow of subarctic shallow water into the Japan Sea in a 40 kyr cycle. On the other hand, abundance of C. nakasekoi, a dominant endemic radiolarian before the LMGC, which shows a similar decreasing trend with T. papillisa and became extinct, only showed a ~100 kyr signal, suggesting that C. nakasekoi was not directly related to the PCW. The 100 kyr cycle is observed in an East Asian summer monsoon (EASM) proxy from lacustrine sediment in China. Hence, it is possible that C. nakasekoi abundance was related to the EASM intensity variations. C. nakasekoi seems to have become extinct as a result of the environmental change in the Japan Sea during the LMGC under decreased inflow of the PCW, weakened EASM, and intensified EAWM.