The Use of 15NH3 to Estimate Fractionation of Nitrogen Isotopes during Diagenesis of Organic Matter

氮的生物地球化学循环非常容易受到环境条件变化的影响。了解海洋氮循环现在和过去对自然和人为变化的响应，对于我们预测未来环境变化对海洋初级生产力的潜在影响，从而对全球和区域海洋生态系统的影响，具有重要意义。解释海洋氮循环过去波动的最有力工具之一是保存在海洋沉积物中的有机物质的15N同位素组成(D15N)。在各种环境中，该示踪剂可提供关于海洋生产力和营养物质利用程度的可变性的信息，或关于固氮或反硝化作用的相对重要性的信息。然而，要准确解释古海洋记录，必须回答的一个关键问题是估计成岩过程中氮同位素分馏的程度。在这项研究中，南加州大学的研究人员将测量加州边境和加利福尼亚州海湾几个近缺氧盆地沉积物的孔隙水中氨的D15N。还将分析大量有机氮和总可水解氨基酸的固相丰度和d15N。由于研究地点没有灌溉效应，通过将扩散-平流-反应模型与孔隙水数据进行拟合，将有可能建立固相成岩过程中损失的15N和14N的质量平衡。将这些模型计算与沉积物积累速率和固相氮数据相结合，将确定同位素分馏程度，并将有助于评估细菌对同位素轻氮的吸收是否在沉积物氮收支中发挥重要作用。为了评估在更多氧化地点进行分馏的可能性，将使用在缺氧和缺氧条件下分解的沉积物和新鲜浮游生物的混合物进行培养实验。

ABSTRACTOCE-0136500The biogeochemical cycle of nitrogen is very susceptible to changes in environmental conditions. Understanding the present and past response of the marine nitrogen cycle to natural and anthropogenic changes is important for our ability to predict the potential impact of future environmental changes on primary production in the oceans, and consequently their effect on global and regional marine ecosystems. One of the most powerful tools for deciphering past fluctuations in the marine nitrogen cycle is the 15N isotopic composition (d15N) of organic matter preserved in marine sediments. In various settings, this tracer may provide information about variability in ocean productivity and the degree of nutrient utilization, or about the relative importance of nitrogen fixation or denitrification. However, to accurately interpret the paleoceanographic record, a critical question that must be answered is to estimate the extent of nitrogen isotope fractionation during diagenesis. In this study, researchers at the University of Southern California will measure the d15 N of ammonia in pore waters of sediments from several near-anoxic basins in the California Borderland and the Gulf of California. Analyses of the solid phase abundance and d15N of bulk organic nitrogen and THAA (total hydrolyzable amino acids)will also be made. Because irrigation effects are absent from the study sites, it will be possible to construct mass balances for 15N and 14N lost during diagenesis of solid phases by fitting diffusion-advection-reaction models to pore water data. The combination of these model calculations with information on sediment accumulation rate and solid phase nitrogen data will establish the degree of isotope fractionation, and will also be useful for evaluating whether bacterial uptake of isotopically light nitrogen plays a significant role in the sedimentary nitrogen budget. To evaluate the potential for fractionation at more oxic sites, incubation experiments will be run using mixtures of sediment and fresh plankton decomposing under both oxic and anoxic conditions.