Stable nitrogen isotopic ratios of sediments from the Arabian Sea: Influences of water column oxygenation sedimentation rates, and diagenesis

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Nitrogen is a key element in regulating biological processes in the ocean. The ratio of the two stable N-isotopes (expressed as ol5N) in sinking particles and surface sediments reflects Nsources, biogeochemical processes, and nutrient regime in the overlying water. The ratio is commonly used as a proxy to reconstruct marine nutrient cycles in the geological past, and is gaining importance for the detection of global changes in the cycling in an essential nutrient forced by climatic and oceanographic change. Possible biases on the use of 15N/I4N ratios as a proxy are sedimentation rates and diagenesis under either oxic, suboxic, or anoxic conditions. Differential degradation of biological products under oxic and anoxic conditions has been postulated from several studies on Sl5N of bulk nitrogen in suspended matter and in sediments. We propose to explore these possible biases in a key region for the global nitrogen cycle, the Arabian Sea (Leg 117, Sites 722 and 724); the region also experienced significant changes in sedimentation rates under glacial and interglacial conditions. To elucidate possible diagenetic effects and effects of sedimentation rates requires that we determine 6I5N in conjunction with an index of organic matter quality, i.e., the state of degradation, which is affected by oxygen content at the sediment water interface and by sedimentation rate. A suitable index is a degradation index based on the ratios of amino acids. This coupled analysis of stable N-isotope ratio and its dominant molecular source (amino acids) will permit an assessment of the value of nitrogen isotope ratios for reconstructions of past biogeochemical cycles in the ocean.