Fluctuations in productivity and denitrification in the Southeastern Arabian Sea during the Late Quaternary

Abstract

Sedimentological and stable isotopic characteristics of sediments have been studied in a core from the southeastern Arabian Sea containing records of the past 70 ka. Palaeoproductivity proxies such as organic carbon (C_org), total nitrogen (TN) and calcium carbonate (CaCO₃) contents, show high values at the core top and during the Last Glacial Maximum (LGM) and marine isotope stage (MIS) 4, suggesting high productivity, whereas low C_org and CaCO₃ contents are associated with the MIS ½ and mid-MIS 3, indicating reduced productivity. The δ¹⁸O values in planktonic foraminifera range between - 2.7% and - 0.1%, with a large glacial-interglacial amplitude Δδ¹⁸O of ∼2.6%, suggesting changes related to monsoonal precipitation/ runoff. The δ¹⁵N values fluctuate between 5.4% and 7.3%, signifying variation in denitrification intensity. The δ¹⁵N indicates an overall increase in denitrification intensity during MIS 1 and MIS 3 and, reduced intensity during MIS ½, LGM and mid-MIS 3. Higher primary productivity and reduced denitrification intensity during LGM and MIS 4 might be due to convective winter mixing and more oxygenated subsurface waters. Reduced primary productivity during MIS ½ and mid-MIS 3 might be the effect of enhanced precipitation associated with the intensified southwest monsoon fortifying near-surface stratification.