Evaluating the Accuracy of Biogeochemical Cycling Rates from Transient Tracers

University Of Washington, Seattle WA

Investigators

Abstract

The ability to predict future oceanic uptake of carbon dioxide and, consequently, the response of the coupled ocean/land/atmosphere system to climate forcing, requires an understanding of both how the physical, chemical, and biological systems presently function and how they are likely to respond to predicted environmental changes. Oxygen consumption/utilization (OUR) and nutrient regeneration (NRR) rates in the ocean interior provide an attractive bottom-up approach to infer marine productivity in the sunlit surface waters. Oxygen utilization rates are often estimated using the distributions of "transient tracers,' compounds such as chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) that are introduced in to the atmosphere and oceans by human activity. Tracer-based estimates are subject to uncertainties based on several factors including the time history of the tracers and ocean mixing. Investigators at the University of Washington plan to use ocean models to examine the sources of these uncertainties and determine where in the oceans the tracer-based estimates agree best (and worst) with the actual rates. The investigation of biogeochemical cycling rates and surface ocean productivity based on tracer techniques will be put into context with those from other measurement systems such as satellite or Argo floats and will provide an improved view of biogeochemical cycling in the ocean. Recommendations on where the technique works will be important for the interpretation of results from future hydrographic cruises and can guide strategies for future tracer measurements. The investigators will include undergraduate summer students in the research, and participate in outreach programs in local schools. The investigators will address the biases and uncertainties in the tracer-based OURs and NRRs using a multi-model approach. Analysis of existing tracer model output, including oxygen, phosphate, CFCs, SF6, ideal ages and transit time distributions (TTDs), will reveal in which regions and during which times transient tracer ages (combined with oxygen fields) give the best estimation of the known oxygen consumption terms in the model. They will expand this work using the National Center for Atmospheric Research's Parallel Ocean Program model output (ocean-only configuration) which contains a much larger variety of biogeochemical parameters and more complex biogeochemistry (though no SF6, TTDs or ideal age spun up to steady state). In addition to OURs, this allows investigation of the accuracy of tracer-inferred NRRs, including denitrification rates, silicate production rates, and calcium carbonate dissolution, as well as regeneration rates of dissolved organic nutrients. Finally, a planned, near-future addition of multi-biogeochemical tracer algorithms to the current offline will enable the investigators to look at complex biogeochemistry and the full spectrum of transient tracer, ideal, and TTD ages simultaneously. Finally they will address the robustness of apparent changes in OURs/NRRs observed during recent and upcoming Climate Variability and Prediction/Global Ocean Ship-based Hydrographic Investigations Program (CLIVAR/GO-SHIP) Repeat Hydrography (RH) sections.