Measuring the Oceans Biological Pump

Determining the annual mass balance of oxygen in the upper ocean

Adequate calibration of the satellite remote sensing observations will require increasing the number of locations where the biological pump is determined by upper ocean mass balance.

by Steve Emerson    Published: December 22, 2014

About half of the photosynthesis on Earth takes place in the euphotic zone of the oceans.  This process uses energy from the sun to produce organic matter and oxygen from carbon dioxide and water.   The ratio of oxygen to organic carbon produced is about 1.45 and appears to be roughly constant over time and space.  The net flux of photosynthetically-produced organic carbon from the upper ocean to the interior removes CO2 from the atmosphere and provides the substrate for respiration reactions that consume oxygen and produces metabolites in the deep sea.   We measure this flux, called the ocean’s biological pump, by determining the annual mass balance of oxygen in the upper ocean.

There have been a series of efforts to evaluate the biological pump from satellite remote sensing of ocean color and optical backscatter.  However, in the few locations were both mass-balance and satellite-determined fluxes have been determined they do not agree very well.  Adequate calibration of the satellite remote sensing observations will require increasing the number of locations where the biological pump is determined by upper ocean mass balance.  This can be done by making accurate oxygen measurements over a period of at last a year on autonomous platforms like moorings, gliders and profiling floats.  Two examples of this effort, using well-calibrated oxygen sensors on profiling floats and gliders, are presented here in a movie created by the Center for Environmental Visualization in the School of Oceanography from data collected in two separate projects.  Persons involve in this research are graduate students Seth Bushinsky and Noel Pelland along with Professors Steven Emerson, Stephen Riser and Charlie Eriksen.  This movie was presented at the American Geophysical Union fall meeting (2014) in the Sverdrup lecture by Emerson.

The movies show upper-ocean oxygen results from the North Pacific at Ocean Station Papa (OSP) from well-calibrated Aanderaa oxygen sensors deployed for two years on an Argo float, and a little more than one year on a sea glider.  The profiling float spends most of its life at 1000 meters where it drifts freely and profiles to the surface about once every week.  The sea glider was programed to dive to 1000 m and then reassend 2-3 times per day while navigating a butterfly pattern approximately 50 km on a side.  Both movies follow the degree of oxygen supersaturation in percent, {([O2]measured – [O2]atm. sat.) /[O2]atm. sat.} x 100, as a function of time and ocean depth in the upper 200 meters.  The white line in the movie follows the depth of the mixed layer determined from temperature and salinity measurements. The two-dimensional section below the movies of the float and glider profiles indicates the surface-ocean oxygen supersaturation as a function of time.

These results are used in an upper-ocean model that takes into account all physical processes that cause oxygen supersaturation--temperature change, air-sea exchange by diffusion and bubbles, mixed layer entrainment, horizontal and vertical advection and diapycnal mixing at the base of the wintertime mixed layer (~110 m).   The differences between the measured and calculated oxygen distributions represent the biologically produced oxygen flux.   Assuming this flux is stoichiometrically related to organic carbon export yields a biological pump of 0 – 1.0 mole C m­­-2 yr-1 in this area of the ocean during the period June 2013 to June 2014.  This value is lower than previous mass-balance estimates, because we now have the data necessary to determine the upper ocean processes during the winter months.  If data similar to that presented here at OSP can be derived from other areas of the ocean, it will be possible to calibrate satellite-based global estimates of the biological pump.