Evaluating the Ocean's Biological Carbon Pump

Abstract:

The flux of biologically-produced organic carbon from the surface ocean (the biological pump) plays an important role in maintaining the pCO₂ of the atmosphere and the oxygen content within the ocean interior. Evaluating this flux and the mechanisms controlling it are necessary to determine marine carbon cycle feedback to climate change. The three primary methods for determining the biological pump: field measurements, satellite remote sensing, and global climate models suggest different geographic distributions of the flux. The accuracy of model and remote sensing results can be judged against field measurements with the assumption that annual net community production (ANCP) is equal to biological carbon export on time scales of a year or more. Three time-series test cases comparing: satellite-derived carbon export, ANCP determined from oxygen and inorganic carbon mass balances, and sediment trap fluxes, demonstrate the complexities of interpreting satellite data. Field determinations indicate much less geographic variability of the biological pump than satellite-derived fluxes, but annual measurements are not presently sufficient in number or geographic distribution to provide credible calibration. One approach for expanding the number of field estimates of the biological pump is to follow decades of oxygen mass balance studies at ocean time-series stations in which ANCP has been stoichiometrically related to annual net biological oxygen production calculated from models of time-series O₂ measurements. Unmanned platforms provide a feasible, affordable method for increasing the number of oxygen time-series locations. About ten percent of the profiling floats of the Argo program have oxygen sensors, but until recently the accuracy of O₂ measurements has not been sufficient to determine the air-sea oxygen flux, which is the major term in the upper ocean mass balance. Recent in situ calibration methods for O₂ sensors suggest it is now feasible to measure accurate oxygen concentrations that can be used to determine the net annual biological oxygen production, and hence the biological carbon pump, in a wide variety of productivity regimens of the world’s oceans.