[M-IS06] Evolution and variability of the Tropical Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS06-P01] Changes in the Indonesian Throughflow since the late Miocene based on radiolarian assemblages in the western tropical Indian Ocean: Response to the Indo-Pacific Gateway

*上栗 伸一¹、松崎 賢史² (1.茨城大学教育学部、2.東京大学大気海洋研究所)

キーワード：インドネシア通過流、インド- 太平洋ゲートウェイ、中新世、放散虫

The Indo-Pacific Gateway is a narrow tropical passage linking the Pacific and Indian Oceans, and changes in its topographic configuration can alter the volume of water exchanged between both oceans through the Indonesian Throughflow (ITF). Although previous studies have explored possible historical changes in the ITF during several time intervals of the Plio-Pleistocene to estimate the timing of possible changes in the gateway topography, little is known about those of the late Miocene, an interval characterized by global sea surface cooling, winter monsoon intensification, and termed the Late Miocene Global Cooling. Here we present radiolarian assemblages data since the late Miocene at Ocean Drilling Program Site 710 in the western tropical Indian Ocean for reconstructing the history of ITF variability since the Late Miocene. Modern and Pleistocene published data suggest that radiolarian assemblages can be divided into three distinct ITF zones: 1) the surface ITF related to shallow water, 2) the thermocline ITF mainly corresponding to subsurface water, and 3) the intermediate ITF related to upper intermediate water. The intermediate ITF weakened at 12 to 10 Ma, the surface ITF significantly decreased at 7.0 Ma, and then the thermocline ITF strengthened at 4.5 Ma. We propose that such changes correspond to changes in topographic configuration of the Indo-Pacific Gateway, which likely occurred in a stepwise manner during these three. The possible closure of the Indo-Pacific Gateway might have had an important influence on local to global paleoceanography, such as cooling and a 'biogenic bloom'. We also reconstructed sea surface temperature (SST), Antarctic Intermediate Water (AAIW), and biological productivity (upwelling) changes since the late Miocene based on radiolarian assemblages. Radiolarian-based SST exhibited a significant stepwise cooling from 9.0 to 6.5 Ma but then reversed to a warming trend from 4.5 to 2.0 Ma, with a slight decrease in SST after 2.0 Ma. These patterns are largely consistent with fluctuations in Mg/Ca-based SST. AAIW has gradually been extending northward since 7 Ma and has further expanded since 3 Ma, establishing the modern AAIW.