[A-OS15] Chemical and Biological Oceanography

[AOS15-P02] Paleoceanographic changes of the eastern Indian Ocean over the past 420,000 years: Implications for the long-term evolution of the Indonesian Throughflow and Leeuwin Current

*Hana Kawashima¹, Hideko Takayanagi¹, Azumi Kuroyanagi¹, Shigeyuki Wakaki², Yuna Kimoto¹, Ryota Wako¹, Tsuyoshi Ishikawa³, Yasufumi Iryu¹ (1.Tohoku University, 2.National Museum of Japanese History, 3.JAMSTEC)

Keywords:Glacial-Interglacial Cycles, Isotopes, Leeuwin Current, Indonesian Throughflow

The Indonesian Throughflow (ITF) and Leeuwin Current (LC) play crucial roles in transporting high-temperature, low-salinity water from the Western Pacific Warm Pool to the subarctic region, significantly influencing the climate, ocean environments, and coral reef ecosystems in and around Western Australia. Although recent oceanographic observations have provided insights into the seasonal to multidecadal dynamics of the ITF and LC and the factors contributing to their variability, our understanding of their long-term variability remains limited. For example, the timing of the initiation and evolution of the recent ITF-LC system still needs to be fully understood. In this study, we analyzed the oxygen isotope composition (δ18O) of four planktic foraminiferal species and the neodymium isotope composition (εNd) of bulk benthic foraminifers in carbonate sediment cores from IODP Expedition 356 sites U1460 and U1464 to delineate long-term paleoenvironmental evolution of the eastern Indian Ocean over the past 420,000 years. The four planktic foraminiferal species are surface-dwelling species Globigerinoides ruber and Trilobatus sacculifer, and intermediate thermocline species Neogloboquadrina dutertrei and Pulleniatina obliquiloculata.

Our δ18O results showed a continuous long-term decreasing trend in δ18O over the past 420,000 years. A similar long-term increasing trend of εNd values was observed at both sites. Furthermore, at Site U1464, the δ18O variation of G. ruber preceded that of the other species, suggesting that the timing of δ18O fluctuations depends on the depth at which those planktic foraminiferal species lived. These results indicate that the temperature and salinity in the eastern Indian Ocean region have gradually become warmer and less saline due to the enhanced contribution of Pacific-origin seawater, suggesting that the ITF and LC may have strengthened throughout the past 420,000 years.