The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Sedimentary ²³¹Pa/²³⁰Th serves as a pivotal paleoceanographic proxy for delineating changes in the strength and geometry of ocean circulation as well as marine productivity throughout the last glacial-interglacial cycle. Its application builds on the fact that both Meridional Overturning Circulation (MOC) and scavenging by marine particles can shape the distribution of ²³¹Pa and ²³⁰Th in the ocean. Previous studies have addressed the crucial role of MOC on the ²³⁰Th and ²³¹Pa profiles and sedimentary ²³¹Pa/²³⁰Th in the Atlantic Ocean and Pacific Ocean, and the influence of boundary scavenging especially in the Pacific Ocean. However, how the MOC and scavenging processes exert their impact on sedimentary ²³¹Pa/²³⁰Th in the Indian Ocean remains unresolved. In this study, a two-dimensional model embedded with reversible scavenging and prescribed overturning schemes was employed to simulate the features of distribution of ²³⁰Th and ²³¹Pa within the water column and sediments in the Indian Ocean. In particular, the convex configuration deviating from the theoretical linear pattern for both ²³⁰Th and ²³¹Pa could be attributed to the deep convection, advection and upwelling of the deep water, highlighting the critical control of MOC. Concurrently, both bottom scavenging induced by re-suspended particles within the nepheloid layer and boundary scavenging facilitates the significant depletion of ²³¹Pa in depths in specific regions like the Madagascar Basin. The depth-profiles of sedimentary ²³¹Pa/²³⁰Th below ~1,500 m in the central basin can be identified as a result of MOC as the primary controlling factor, while boundary scavenging contributes to systematically lower the ²³¹Pa/²³⁰Th. This finding reaffirms the efficacy of sedimentary ²³¹Pa/²³⁰Th as a proxy for studying the alteration of deep ocean circulation in the Indian Ocean.