The Ocean’s Biological Pump Determined from In Situ Oxygen Measurements on Profiling Floats

The flux of organic carbon from the surface ocean to the interior, the ocean’s biological organic carbon pump, is the centerpiece of the marine organic carbon cycle, influencing the pCO₂ of the atmosphere and the oxygen deficit in the deep ocean. The global value of the flux has been determined by satellite remote sensing and by ocean general circulation models, but these values have not been verified by observations. A time-series of measurements that can resolve the seasonal cycle is required to do this, which limits the number of locations where verification is possible. We resolve this problem by observing the upper ocean oxygen budget using in situ measurements on profiling floats. We have deployed 45 Argo floats with Aanderaa oxygen sensors that are calibrated against atmospheric pO₂, in the subtropical and subarctic gyres, and near the equatorial Pacific. From a full year of oxygen data and a model of upper ocean oxygen and carbon fluxes, we are beginning to observe the global mosaic of the biological pump. There are two striking results emerging from our measurements: (1) The open-ocean variation of the biological pump in the northern hemisphere is much more subtle geographically than suggested by ocean GCMs and satellite remote sensing. The observed carbon flux in the locations of the northern-hemisphere subtropical and subarctic oceans that we have studied is 2.5 ± 1 mole C m^-2 yr^-1 whereas the global models would suggest variability of up to a factor of 5. (2) As we begin to receive data from the southern hemisphere we find that air-sea biological O₂ fluxes in the Pacific and Indian subtropical areas are lower than in the northern hemisphere, and in some cases into rather than out of the ocean, which implies net heterotrophy. This result suggests that horizontal transport of nutrients and dissolved organic carbon (DOC) from nutrient rich areas plays an important role in supporting the biological organic carbon pump in these locations.