Coupling between suboxic condition in sediments of the western Bay of Bengal and southwest monsoon intensification: A geochemical study

Abstract Reconstruction of paleo-redox conditions in a radiocarbon ( 14 C) dated sediment core (SK-218/1), covering the past 45 ka (thousand calendar years), collected from the western Bay of Bengal (Lat: 14° 02′N; Long: 82° 00′E) at a water depth of 3307 m, has been made based on redox-sensitive element geochemistry. The high U/Th ratio, Mo enrichment, Mo/U enrichment factor ratio, Ce/Ce*  We also provide the first record of the nitrogen isotopic composition (δ 15 N) of sedimentary organic matter in the western Bay of Bengal, a region where the mesopelagic oxygen minimum zone (OMZ) is just short of being suboxic (denitrifying) today. The sedimentary δ 15 N fluctuated considerably in the past, especially during the Marine Isotope Stage 3. Oscillations in δ 15 N were apparently in concert with those in organic carbon and nitrogen contents and could be related to climatic changes (Heinrich and Dansgaard–Oeschger events) in the North Atlantic. The Dansgaard–Oeschger 12 event appears to have exerted the most intense effect on Bay of Bengal biogeochemistry when surface productivity, as inferred from the organic carbon and nitrogen contents, was the highest recorded in the core, and the δ 15 N reached up to 6.3‰. Considering the probable dilution by isotopically light terrigenous organic matter, it would appear that OMZ of the Bay of Bengal had turned denitrifying. However, the absence of suboxic conditions in the sediments at this time suggests a decoupling of the benthic processes with those in the mesopelagic water column.