[M-GI28] Drilling Earth Science

[MGI28-05] Variability of the Indonesian Throughflow and Australian Monsoon across the Mid Pleistocene Transition (IODP 363, Site U1483)

*Kenji Matsuzaki¹, Ann Holbourn², Wolfgang Kuhnt², Li Gong², Masayuki Ikeda³ (1.Atmosphere and Ocean Research Institute, The university of Tokyo, 2.Christian-Albrechts University of Kiel, Institute of Geosciences, 3.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo)

The Mid Pleistocene Transition between 1400 and 600 ka represents a major shift in global climate, as it was marked by a change from ~41 to ~100 kyr glacial/interglacial periodicity, resulting in higher amplitude sea-level variations and intensified glacial cooling. The Indonesian Throughflow (ITF), which controls the exchange of heat between the Pacific and Indian Oceans, is a major component of the global climate system. Indeed, the heat transported by the ITF is related to the Asian-Australian Monsoon dynamics and the strength of the warm surface flow and cool thermocline is controlled by the seasonal reversal of monsoonal winds (Gordon, 2005). Recent studies showed that changes in sea-level also influence the ITF because most of the seas feeding the ITF are very shallow. Therefore, reconstruction of ITF variability during the MPT has the potential to clarify the impact of glacio-eustatic sea-level and monsoonal climate dynamics on the hydrology of Northwestern Australia.
The International Discovery Program (IODP) Expedition 363 retrieved at Site U1483 (13°5.24′S, 121°48.25′E), drilled in 1733 m water depth on the Scott Plateau, off Northwestern Australia, an extended, continuous hemipelagic sediment succession spanning the past two million years. In addition, around 30 core top samples were obtained along the Northwest Australian Margin during Sonne Expedition 257 in summer 2017. In this study, we analyzed radiolarian assemblages from 25 core top samples retrieved during Sonne 257 and 190 samples from IODP Site U1483 to monitor the variability of the thermocline during the MPT in order to determine ITF dynamics in relation with glacio-eustatic sea-level variations and tropical monsoon strength.
To assess the variability of the ITF and local thermocline at Site U1483, we mainly rely on statistical analysis of radiolarian assemblages from Sonne 257 core top samples. Indeed, because radiolarian species do not follow a Gaussian distribution, we conducted spearman’s rank correlation for non-parametric data between radiolarian species and sea water temperature at 0 m to identify species, which are useful temperature indicators at this depths. We then conducted a multiple regression analysis to derive an equation for paleo-temperature estimates at 0, 100 and 300 m. Based on assemblage census data from Site U1483 and the transfer function from the Sonne 257 dataset, we estimate SST between 450 and 1600 ka at IODP Site U1483. We will review these results and evaluate their potential to reconstruct the evolution of the thermocline off Northwestern Australia in relation to ITF variability during the MPT.