Early Initiation of Tasman Leakage and the formation of modern Indian Ocean circulation

The exchange of water between the Pacific and Indian Oceans is important in regulating planetary climate. North of Australia, this exchange plays a key role in regulating the Indo-Pacific Warm Pool with far-reaching effects via Indonesian Throughflow (ITF). The exchange south of Australia is far less understood, and much of the exchange occurs at intermediate depths through Tasman Leakage (TL). Here, we investigate Ocean Drilling Program (ODP) Site 752 which was drilled on Broken Ridge. The Site is located within the path of TL, and thus, can provide a paleoceanographic history of TL. Benthic foraminiferal (BF) assemblage is a useful tool to reconstruct paleoceanographic patterns. At ODP Site 752, BF assemblages vary over time but at no point exhibit evidence of extreme stress or oxygen deficiency. Benthic foraminiferal diversity measured with the Fischer Alpha diversity index remains between 5 and 10, indicating moderate diversity throughout the last 9 million years. Furthermore, the high abundance of epiphytic species Cibicidoides wuellerstorfi and Lobatula lobatula likely reflects a high current energy environment over Broken Ridge during this time. Based on our comprehensive benthic foraminiferal assemblage study, we suggest that the driving factor behind the benthic ecological changes on Broken Ridge since the Late Miocene has been TL intensity, distinguished by its kinetic energy. In addition, we present a ~13 Myr neodymium (Nd) isotopic record, suggesting that TL onset likely occurred sometime in the late Middle Miocene, advecting isotopically older Pacific-sourced waters into the Indian Ocean. The latter findings challenge the previously presumed onset of TL at ~7 Ma and indicate a much earlier initiation of TL between 14 – 10 Ma, that intensified during the Late Miocene when the modern-like TL was established.