Fluctuations in productivity and denitrification in the southeastern Arabian Sea during the Late Quaternary
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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 sub(org)), total nitrogen (TN) and calcium carbonate (CaCO sub(3)) 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 sub(org) and CaCO sub(3) contents are associated with the MIS 1/2 and mid-MIS 3, indicating reduced productivity. The delta sup(18)O values in planktonic foraminifera range between - 2.7 ppt and - 0.1 ppt, with a large glacial-interglacial amplitude delta sup(18)O of approx. 2.6 ppt, suggesting changes related to monsoonal precipitation/ runoff. The delta sup(15)N values fluctuate between 5.4 ppt and 7.3 ppt, signifying variation in denitrification intensity. The delta sup(15)N indicates an overall increase in denitrification intensity during MIS 1 and MIS 3 and, reduced intensity during MIS 1/2, 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 1/2 and mid-MIS 3 might be the effect of enhanced precipitation associated with the intensified southwest monsoon fortifying near-surface stratification.
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