Collaborative Research: Applying O2/Ar, DELTA17O and 222Rn methodologies to constrain organic carbon productivity in the upper ocean of the ETSP

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Recently the National Science Foundation funded a field campaign to study the occurrence of nitrogen fixation in the eastern tropical South Pacific (ESTP-10°S-20°S, 80°W-110°W) and determine its contribution as a source of new nitrogen to primary production in the region. The proposed cruise tracks cover a wide range in primary productivity, from some of the most productive to least productive marine waters in the world. Two scientists from the University of Southern California in collaboration with a researcher from the University of California, Los Angeles will participate in the cruises to quantify the contribution of nitrogen fixation to primary and export production rates. To attain their goal, the researchers would determine Net Community Production and Gross Photosynthetic Production (NCP and GPP) by measuring biological O2 supersaturation and 17O excess of oxygen dissolved in the upper ocean. Oxygen supersaturation from biological processes can be determined from measuring O2/Ar ratios of near surface waters, which effectively removes the effect of supersaturation produced by bubble injection and other physical processes. The 17O excess permits surface water oxygen that has been produced by photosynthesis in situ to be distinguished from oxygen from the atmosphere. In addition, 222Rn profiles in the upper water column would also be obtained and used to assess the rate of air-sea gas exchange and eddy diffusivity across the thermocline. A system to measure near-surface radon while underway is also planned and should ascertain spatial/temporal variability in gas exchange and help identify recently upwelled or downwelled water masses. Results for the O2/Ar and delta 17O methodologies, coupled with areal mapping of radon deficiencies will yield an excellent data base for estimation of carbon export within the ETSP.In terms of broader impacts, the research may lead to reduced uncertainties in the gas exchange rates which could improve the precision of productivity measurements. This project represents a collaborative effort between two academic institutions in the Los Angeles area. One postdoc and two undergraduate students from the University of Southern California will be supported and trained as part of the project.