Monsoon related changes in sea surface productivity and water column denitrification in the Eastern Arabian Sea during the last glacial cycle

Sea surface salinity reconstruction for the Holocene and the Last Glacial Maximum (LGM) suggests that the LGM-Eastern Arabian Sea was more saline (by ∼ 1.5 psu) than the Holocene due to weakened freshwater flux to the region, and intensified winter monsoons. Time-series of sedimentary organic-carbon (Corg), ∑alkenone and δ13Corg for the last 100 kyr, together, indicate increased glacial productivity. Significantly reduced sedimentary-δ15N during the LGM indicates, however, diminished water column denitrification in spite of increased productivity. The distinct decoupling of denitrification from productivity during the LGM can be explained by vigorous ventilation of the thermocline as a result of simultaneously intensified formation of high-salinity water and deep-winter mixing in northern-Arabian Sea related to the extreme cold climate. The closely comparable time-series δ15N records across the Arabian Sea suggest basin-wide homogenisation of the isotopic signal. This probably resulted from combined monsoon-dependent surface water mixing, due to changes in the relative strength of reversing surface circulations, and intermediate water mixing, due to changes in north–south salinity gradient.