Stable nitrogen isotopic ratios of sinking particles and sediments from the northern Indian Ocean

Abstract

Delta sup(15)N values in modern sediments of the northern Indian Ocean range from less than 4 ppt in the northern Bay of Bengal to greater than 11 ppt in the central Arabian Sea due to the strong spatial variability of processes influencing nitrogen cycling in the region. Suboxic conditions in Arabian Sea intermediate water lead to denitrification and accumulation of isotopically enriched nitrate in the water column. Upwelling of this enriched nitrate into the euphotic zone results in high delta sup(15)N values in Arabian Sea particulate matter and surface sediments. Delta sup(15)N minima in the northern Bay of Bengal are caused by input of terrestrial organic matter as well as depleted nitrate from rivers. Delta sup(15)N is about 8 ppt in the equatorial Indian Ocean as the influence of oxygen deficiency and river input ceases. Delta sup(15)N of recent sediments correlates with the weighed mean delta sup(15)N of sediment trap material caught 500-1000 m above the sea floor, but with an enrichment of 2-3 ppt between deep traps and underlying sediments. Only at two locations characterised by high sedimentation rates, there is no significant offset of delta sup(15)N values between traps and sediments. Delta sup(15)N of sediments and trap samples from the Arabian Sea are correlated to the Degradation Index (DI), an indicator of amino acid degradation intensity, confirming the sensitivity of the delta sup(15)N to degradation. These results have to be considered for the interpretation of the delta (15)N record in sediment cores with large variations of diagenetic intensities.