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

Abstract

δ 15N values in modern sediments of the northern Indian Ocean range from < 4‰ in the northern Bay of Bengal to > 11‰ 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 δ 15N values in Arabian Sea particulate matter and surface sediments. δ 15N minima in the northern Bay of Bengal are caused by input of terrestrial organic matter as well as depleted nitrate from rivers. δ 15N is about 8‰ in the equatorial Indian Ocean as the influence of oxygen deficiency and river input ceases. δ 15N of recent sediments correlates with the weighed mean δ 15N of sediment trap material caught 500–1000 m above the sea floor, but with an enrichment of 2–3‰ between deep traps and underlying sediments. Only at two locations characterised by high sedimentation rates, there is no significant offset of δ 15N values between traps and sediments. δ 15N of sediments and trap samples from the Arabian Sea are correlated to the Degradation Index (DI) [Dauwe, B., Middelburg, J.J., Hermann, P.M.J., Heip, C.H.R., 1999. Linking diagenetic alteration of amino acids and bulk organic matter reactivity. Limnology and Oceanography, 44, 1809–1814], an indicator of amino acid degradation intensity, confirming the sensitivity of the δ 15N to degradation. These results have to be considered for the interpretation of the δ 15N record in sediment cores with large variations of diagenetic intensities.