Leeuwin current dynamics in the SE Indian Ocean and implications for regional surface hydrography since the latest Miocene: Results from ODP Site 763A

Abstract

This study examines the relationship between the ocean circulation dynamics in the eastern Indian Ocean and climate variability in Northwest (NW) Australia over the last 6.1 million years. The vertical water column of the southeastern Indian Ocean was analyzed using planktic foraminifera assemblages, and oxygen and carbon isotope ratios of surface and subsurface dwelling foraminifera from Ocean Drilling Program (ODP) Site 763A. The X-ray fluorescence data of bulk sediment from ODP Site 763A was also used to understand variability in relative humidity and aridity in NW Australia associated with surface ocean warming. The findings suggest a pronounced change in surface oceanography and climate in the region from 5.4 to 5 Ma, indicating a weak Leeuwin Current (LC) and reduction in mixed layer thickness. Subsequently, between 5.0 and 3.6 Ma, a strongly stratified water column with a thick mixed layer and deep thermocline emerged owing to the intensified Indonesian Throughflow (ITF). This led to a strengthened LC and diminished surface productivity, corresponding to a wet phase in NW Australia. The closure of the Indonesian Seaway during the Middle Pliocene (3.6–3.2 Ma) restricted the flow of West Pacific Warm Water into the Indian Ocean, weakened the LC and intensified the dry phase in NW Australia. Significant shifts in planktic foraminifera abundances are observed in the early Pleistocene (2.6–1.2 Ma) perhaps linked to major changes in surface ocean circulation. During the Mid-Pleistocene Transition (1.2–0.6 Ma), the West Australian Current exerted a significant influence over Site 763A.