The equatorial Indian Ocean upper water-column structure influenced by cross-basinal water exchange over the last ~40000 years

Abstract

The central equatorial Indian Ocean (CEIO) is influenced by both, summer and winter monsoon and is known for cross basin exchange of water between the Arabian Sea (AS) and the Bay of Bengal (BoB). Here, we present planktic foraminiferal assemblage record from a sediment core (SSD-044/GC-01, 5°N; 77°E) from the CEIO, which is in the pathway of this cross-basin water exchange, in order to reconstruct changes in upper water column structure during the last 43 ka, at millennial year timescales. Our data displays an increase in ratio of mixed layer eutrophic to mixed layer oligotrophic foraminifer species from MIS (Marine Isotope Stage) 3 to MIS 2 which further decreased sharply during the last deglaciation and remained low thereafter. This trend matches well with the sedimentary Corg content which is interpreted as a productivity signal, in line with other studies from this region. Furthermore, the relative abundance of thermocline foraminifer species displayed highest values during MIS 3, a decrease through the MIS 2, a sharp decrease through the early deglaciation and an increase thereafter into the Holocene. Our records suggest that the increased productivity at this site seems to be fed by nutrient injection from nutricline to the photic zone which manifests in a corresponding increase in thermocline foraminifera species. The nutrient injection to the photic zone maybe a result of increased vertical/convective mixing due to increased flow of cooler/high-salinity BoB water probably caused by decreased freshening of the BoB during glacials. Furthermore, our data suggests a warm, stratified upper water column and low surface nutrient (oligotrophic) conditions during the last deglaciation and the Holocene.