Origin of the Holocene Sediments in the Ninetyeast Ridge of the Equatorial Indian Ocean

The long-term evolution of the South Asian monsoon system and its influence on the Bay of Bengal (BOB) is of great interest to climate scientists. A number of climate forcings trigger the changes of the Indian summer monsoon (ISM) precipitation centroid, while the ISM rainfall projected by climate models shows a large discrepancy in local precipitation patterns. Moreover, the continuous recovery of paleoceanographic records in the BOB is often a struggle due to the presence of the fan-dominated depositional regime of the Bengal Fan. In this study, we present multi-proxy records of the last 13 kyrs from a sediment core (HI1710-MC1) at the Ninetyeast Ridge (NER) in the southern BOB, which is prevented from turbidite deposition. Our result suggests that the surface ocean environment and detrital provenance at the NER have not responded sensitively to the ISM variation and largely remained stable for the last 13 kyrs. The biogenic fraction (CaCO 3 , total organic carbon, and total nitrogen contents) has remained relatively constant regardless of the Indian monsoon variability during the Holocene. The radiogenic isotope ( ε Nd and 87 Sr/ 86 Sr) and clay mineral compositions of the detrital sediments indicate that the two major sources (the Himalaya through the Ganges–Brahmaputra–Meghna River system with a minor contribution from the Indo-Burma Ranges via the Irrawaddy-Salween River system) have played a primary role in delivering sediments to the study site. Our results imply that the longer sediment records preserved at the NER can be used to reconstruct the relative changes of runoff in the two major river systems. The Holocene record at the NER, thus, provides a basis for the study of the Late Quaternary variability in the Indian monsoon precipitation patterns and resultant runoff to the BOB.