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

氮是调节海洋生物过程的关键元素。沉降物和表层沉积物中两种稳定氮同位素的比值(以ol5N表示)反映了上覆水中氮的来源、生物地球化学过程和营养状况。这一比率通常被用作重建过去地质历史中海洋营养循环的替代指标，对于探测气候和海洋变化所强迫的一种基本营养物质循环中的全球变化越来越重要。使用15N/I4N比值作为替代指标的可能偏差是在有氧、缺氧或缺氧条件下的沉积速率和成岩作用。通过对悬浮物和沉积物中总氮的SL5N的研究，推测了生物产品在氧化和缺氧条件下的差异降解。我们建议在全球氮循环的一个关键区域--阿拉伯海(第117站，722和724点)探索这些可能的偏差；该区域在冰川和间冰期条件下也经历了沉积速率的重大变化。为了阐明可能的成岩作用和沉积速率的影响，我们需要结合有机质质量指数(即降解状态)来确定6I5N，该指数受沉积物水界面的氧含量和沉积速率的影响。一个合适的指标是基于氨基酸比率的降解指数。这种对稳定的氮同位素比率及其主要分子来源(氨基酸)的耦合分析将使人们能够评估氮同位素比率对于重建海洋过去的生物地球化学循环的价值。

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.