Influences of summer monsoon variations on the terrigenous influx, bioproductivity and early diagenetic changes in the southwest Bay of Bengal during Late Quaternary

Abstract

The sediment core (MGS11/02), collected from the lower boundary of the Bay of Bengal (BoB) oxygen minimum zone (890 m) is studied to understand the influences of summer monsoon variations on the terrigenous sediment influx and its relation to bioproductivity and early diagenetic changes in the southwest BoB during the Late Quaternary. The high content of terrigenous controlled metals increased, kaolinite/chlorite increased, and K/Rb decreased after 19 cal kyr BP, indicating a simultaneous increase in the terrigenous influx and intensity of chemical weathering during this period. The variations in summer monsoon and land influx observed in the core sediments are also reflected in the sources of organic matter. High values of δ13C observed at 40 to 19 cal kyr BP suggests higher influence of marine organic matter and/or presence of C4 terrestrial plant. The negative shift of δ13C between 19 and 6 cal kyr BP could be the result of the change in terrestrial plant biomass to C3 photosynthesis and increased marine organic matter. Rock magnetic properties and redox-sensitive metals distribution suggests the prevalence of reduced environment during 52 to 19 cal kyr BP and oxic condition during 19 cal kyr BP to present. The low sedimentation during glacial period allowed sufficient time for pyritization which resulted in the low magnetic signals during 52 to 20 cal kyr BP and led to reducing conditions in the sediment interval. The δ15N values in the core sediments are suggesting the absence of denitrification in the water column during last 52 cal kyr BP. Previous studies suggest that the BoB is a geochemical ‘tipping point’ because the slight changes in water column denitrification by anthropogenic or climatic impact can accelerate BoB nitrogen contribution to the global nitrogen budget. This study indicates the absence of water column denitrification signals during last 52 cal kyr BP even though drastic climatic changes happened in the BoB in the Late Quaternary.