Assessing Coastal Benthic Denitrification using High Precision Water Column N2/Ar

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ABSTRACTProposal No.: OCE-0623199Benthic denitrification on continental margins is thought to exceed water column denitrification, but its magnitude is less certain due in part to the large range in difficult, short-term rate measurements made at relatively few locations. Improved estimates of benthic denitrification will advance our fundamental understanding of marine N dynamics and the state of the oceanic N budget. In this proposal, a PI from University of Massachusetts - Dartmouth, will conduct a geochemical assessment of coastal benthic denitrification by measuring the biogenic N2 that accumulates in the overlying water column against the large background of atmospheric N2 through greatly improved N2/Ar determination. A major advantage of these measurements will be the large space/time scales of integration as determined by water mass residence time and contact area with the sediments.To accomplish this goal, he will develop very high precision and high throughput analytical capability using lab-based isotope ratio mass spectrometer (IRMS) and sea-going quadrupole mass spectrometer (QMS) instruments. In the laboratory, a dissolved gas extraction system will be developed and optimized through coupled operation of the IRMS and QMS. Sample collection and handling protocols will be tested by dockside sampling through a seasonal cycle as well by analysis of samples collected on monthly water-quality assessment surveys of Long Island Sound (LIS). Sea trials of the DG-QMS system will then be carried out on two short cruises to LIS with its large expected biogenic N2/Ar anomalies. Ancillary measurements for quality control determination of stoichiometric relationships include O2/Ar, 15N-N2, 18O-O2, 36Ar, DIC, 13C-DIC, and nutrient concentrations. In addition, via a subcontractor at University of Rhode Island (URI), deployment of a new profiling gas tension device will test the potential of this technology for denitrification studies in conjunction with high precision gas ratio analysis.The broader impacts of this proposal include developing capability for addressing major questions that have recently arisen regarding the ocean's N cycle which as a major biogeochemical cycle in turn impacts global carbon cycling. Denitrification also has an important role in ameliorating coastal eutrophication stimulated by anthropogenic N input, and the LIS component of this project has direct relevance in this regard. The proposed work will create technology important for future studies of marine denitrification as well as conduct the first geochemical assessment of benthic denitrification using high precision water column N2/Ar. The proposed research will expand the scope of the new University of Massachusetts Intercampus Graduate School and will foster the exchange of ideas and results through collaborations at URI. During the course of this work, undergraduate and graduate students will be exposed to marine biogeochemical and oceanographic research for the first time through their involvement in this project. The project will specifically support a M.S. or Ph.D. student to examine the LIS time-series samples.