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

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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.