Collaborative Research: Oxidant Isotopic Fractionation during Early Organic Matter Diagenesis in Marine Sediments

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ABSTRACTOCE-0117796In order to use either O2 or nitrogen for current or paleo isotopic studies of their respective marine cycles, it is imperative that we know their fractionation during sedimentary consumption and if possible determine the mechanisms. A few estimates of O2 and NO3- isotope fractionation during sedimentary consumption have been made which showed that fractionation was much less (approaching zero) than the theoretical ones. Because these observations were made only in the eastern North Pacific where productivity is high and mid-water oxygen levels are low, two research investigators from the University of Washington and the University of Texas Marine Science Institute plan to carry out a laboratory methods development and field experimentation exercise to address two questions. Question number one asks whether the near absence of isotopic fractionation observed for both oxygen consumption and denitrification in highly respiring sediments of the Pacific Northwest continental margin a ubiquitous characteristic of shelf and upper slope sediments. The second question asks to what extent N2 production and the lack of N and O isotopic fractionation is related to abiotic or microbially mediated reoxidation of reduced products of metabolism. The PIs will conduct a field program in the Gulf of Mexico to (1) measure oxidant isotopic fractionation of O2 and NO3-, as well as related species such as N2 and NH4+ in incubated sediments, both on deck and in-situ; (2) conduct on-deck core manipulation experiments with added substrates (NO3-, 15NO3-, HN4+, 15HN4+, organic material and reducing substances) and inhibitors to elucidate possible mechanisms of isotopic fractionation and (3) make porewater and benthic chamber measurements of diagenesis rates to help elucidate the processes responsible for fractionation or lack thereof.