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极易受到环境条件变化的影响。了解海洋氮循环对自然和人为变化的现在和过去的响应，对于我们预测未来环境变化对海洋初级生产的潜在影响，从而预测其对全球和区域海洋生态系统的影响，具有重要意义。破译海洋氮循环过去波动的最有力工具之一是海洋沉积物中保存的有机物的15N同位素组成（d15N）。在各种情况下，这种示踪剂可以提供有关海洋生产力变异性和养分利用程度的信息，或关于固氮或反硝化的相对重要性的信息。然而，为了准确地解释古海洋记录，必须回答的一个关键问题是估计成岩作用期间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来自加利福尼亚边境地区和加利福尼亚湾几个近缺氧盆地的沉积物孔隙水中氨的d15 N。还将分析固体有机氮和THAA（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，因此可以通过将扩散-平流-反应模型拟合孔隙水数据来构建固相成岩过程中损失的15N和14N的质量平衡。这些模型计算与沉积物积累速率和固相氮数据的结合将确定同位素分馏的程度，也将有助于评估细菌对同位素轻氮的吸收是否在沉积氮平衡中起重要作用。为了评估在更多含氧地点进行分馏的潜力，将使用在有氧和无氧条件下分解的沉积物和新鲜浮游生物的混合物进行孵化实验。

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.