Nitrogen Cycling in Marine Sediments: Isotope Fraction- ation Effects

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A pilot study is propoosed to evaluate the usefulness of 15/N/14N ratios for studying the sedimentary nitrogen cycle. Denitrification in marine sediments, particularly on continental shelves, is a principal sink for combined nitrogen from the ocean and in addition to nitrification is known to strongly fractionate nitrogen isotopes. For sediments, coupled nitrification-denitrification and transport control of fluxes are likely to strongly modify these fractionation effects. Initial objectives will be to determine whether nitrogen isotopic signals are sufficiently large such that they are both coherent and interpretable. Model analysis suggests that this will be the case. Sites in Buzzards Bay-MA, along a transect between the Gulf of Maine and Bermuda will be studied. 15N values for pore-water NO3-, NH4+, and sediment PN will be determined downcore and flux chamber experiments will be carried out to determine the net isotopic effect with respect to the water column. Incubation experiments of core slices will be carried out to determine microbial fractionation factors.