Physical Control of Carbon Export in the Subarctic Pacific

University Of Washington, Seattle WA

Investigators

Abstract

Intellectual Merit: One of the largest outstanding and ongoing research problems in the physical and biogeochemical oceanographic community concerns the fate of organic carbon generated from primary production in the surface ocean. Though carbon export exerts a profound control on CO2 concentrations in the world?s atmosphere, in-situ measurements of production rates are not fully reconciled with those inferred from the global annual average, and the spectrum of variability of carbon export in a variety of oceanic environments has yet to be fully determined. A popular method for evaluating ocean metabolism is the formulation of an oxygen mass balance, in which an oxygen budget is constructed in the mixed layer or euphotic zone, the effects of physical processes are removed, and the residual oxygen production is stoichiometrically related to the export of carbon to depth. While ship-based oxygen mass balance studies are hindered largely by expense and the availability of resources, recent efforts using remote measurement of oxygen from moored sensors display considerable potential for cost-effective and geographically widespread diagnostics of the biological pump. These techniques have been expanded to mobile autonomous platforms such as profiling Argo floats and ocean gliders, which offer an enhanced, highly-resolved vertical and horizontal picture of major physical processes controlling primary production. Continuing the evolution of studies of net biological oxygen production from autonomous platforms, the project will apply a comprehensive physical and oxygen mass balance assessment to a remote-sensing time series of unprecedented duration and resolution: the University of Washington Seaglider Ocean Station P time series in the southern Gulf of Alaska. Three separate Seaglider deployments orbited the well-instrumented National Oceanic and Atmospheric administration Pacific Marine Environmental Laboratory (NOAA/PMEL) Station P mooring from June 2008 to January 2010, collecting detailed vertical and horizontal profiles of basic physical properties, bio-optical variables, and dissolved oxygen. During extended portions of this time, the Station P mooring observed atmospheric variables and corresponding physical parameters within the near surface ocean; taken together these datasets represent a powerful tool for adding to the established picture of physical variability and net oxygen production in the central subarctic Pacific Ocean. In the course of our proposed analysis, we hope to: 1) better constrain horizontal advection, vertical advection, and diapycnal mixing processes, as they apply to the budget of an active tracer; 2) obtain a robust estimate of net oxygen production in the mixed layer during the course of the time series; 3) evaluate respiration (oxygen consumption) below the mixed layer and above the permanent halocline; and 4) estimate the dependence with depth of respiration below the pycnocline. Broader Impacts: This project will expand the knowledge base of the physical dynamics controlling the biological pump in the Gulf of Alaska as well as add to the length of the time series which has been analyzed in a carbon export context. A better understanding of advection in the surface layer would allow comparison to previous residual estimates of this term and its relative importance at each phase of the seasonal cycle. Exploring the dependence in time and depth of diapycnal diffusivity would improve and clarify our knowledge of its role in vertical transport of oxygen at Station P. Additionally, analysis of a combined moored and autonomous vehicle time series should provide a foundation of techniques to be used in similar future deployments in difficult-to-observe regions of the world ocean.