Tectonically driven adjustments within ocean gateways and their impacts on deep water production, thermohaline circulation, and nutrient distribution are well constrained. With no deep water formation in the modern northern Indian Ocean, this study aims to reconstruct the possible source and pathways of deep circulation since the late Oligocene, altering the water mass properties at study sites in the eastern Indian Ocean. Our benthic foraminifera results suggest that tectonic gateways influenced the deep water masses in the eastern Indian Ocean. Significant changes in deep water masses at the studied sites commenced with a gradual reduction in corrosive deep water in the eastern equatorial Indian Ocean (EIO) due to the inflow of relatively warmer and less corrosive Tethyan Overflow Water (TOW) through the Tethyan Gateway in the late Oligocene (similar to 24 Ma). The EIO gradually became less corrosive from the late Oligocene to the middle Miocene (similar to 24 to 14 Ma). This turnover reflects the intrusion of southward-flowing TOW and a reduction in the Antarctic Bottom Water (AABW) between similar to 24 and similar to 14.5 Ma in the Indian Ocean. Hereafter, a significant switch in deep water mass was established, contemporaneous with the expansion of Antarctic ice sheets that led to the enhanced production of cold and corrosive AABW that replaced the warm and less corrosive TOW at similar to 14 Ma. Furthermore, between similar to 12 and 8 Ma the closure of the Tethys Seaway caused a significant hydrological reorganization, replacing the TOW with the Pacific Deep Water (PDW) and North Component Water (NCW). At similar to 8 Ma, the contribution of the PDW diminished and AABW flow increased again after a reduction between 12 and 8 Ma, leading to a gradual increase in corrosive deep water. Since 8 Ma, a circulation pattern resembling the modern framework was established with the intrusion of the North Atlantic Deep Water into the CDW, and this pattern was developed by similar to 5.7 Ma. Furthermore, the final closure of the Central American Seaway increased the formation of the NADW and its intrusion into the CDW after similar to 3.2 Ma.
