Relationship between redox condition and bottom water composition in the Mid-Ocean Ridge flank of the Northwest Indian Ocean during MIS 2 and MIS 1

Deep-water circulation in the Indian Ocean plays a vital role in global oceanic processes, and the bottom water was derived primarily from the Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW). We systematically investigated the evolution of bottom-water redox conditions and water mass composition during glacial–interglacial cycles, for which the Nd isotopes in the ferromanganese oxide phases and redox-sensitive elements in sediment cores from both sides of the Northwest Indian Ocean Ridge were analyzed in consideration of available data of MIS 1 (Marine Isotope Stage 1). The enrichment factors (EF) of sensitive elements such as U and V (U_EF, V_EF＜2), and extremely low U/Th ratios suggested that the study area was remained in an overall oxic condition since the MIS 2 (Marine Isotope Stage 2). The εNd values of the authigenic Fe-Mn oxide phase sediments ranged from −6.56 to −6.83 in MIS 2 stage, while those in MIS 1 stage were from −7.16 to −8.09. The binary mixing model calculations showed that the bottom water was consistently dominated by Antarctic Bottom Water (AABW) during both MIS 2 and MIS 1 (accounting for ＞75%). Although εNd values show a slightly negative shift after entering the interglacial period due to the variation of the source area characteristics, the difference in εNd value between glacial and interglacial stages stems mainly from the changes in the water mass end-members themselves, rather than from variations in regional water mass mixing proportions. The notable isotopic consistency on both sides of the ridge reflects efficient ventilation of the bottom water in the Northwest Indian Ocean with Circumpolar Deep Water (CDW) from the southern main passageway, occurring equally on both sides of the ridge.