CARBON-13 CONSTRAINTS ON THE SEASONAL INORGANIC CARBON BUDGET AT THE BATS SITE IN THE NORTHWESTERN SARGASSO SEA
Gruber, N., C.D. Keeling and T.F. Stocker, Carbon-13 constraints on the seasonal inorganic carbon budget at the BATS site in the northwestern Sargasso Sea, Deep-Sea Research I, 45, 673-717. 1998.
The seasonal budget of dissolved inorganic carbon (C) in the mixed layer at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site is assessed on the basis of a 4-year time series (1991-1994) of high-precision C, alkalinity and the 13C/12C ratio of C. Compared to previous studies, our budget is constrained by observed changes in 13C/12C which permit calculation of the net community production. This approach makes use of the effect that a strong isotopic fractionation occurs during the photosynthetic uptake of CO2. Uncertainties in the C budget are determined using a Monte Carlo method. We found that net community production is the dominant process in generating the observed drawdown of about 26.1 µmol/kg between April and October by removing on the average (-32.8 +/- 4.2) µmol/kg from the mixed layer. This net community production occurs in the absence of measurable nitrate and phosphate concentrations. We hypothezise that N2 fixation in combination with vertical migration may be large enough to meet the biological nutrient demand in the mixed layer during the summer/fall period. Explaining the observed summer/fall drawdown with physical processes alone requires highly unlikely horizontal advective velocities. If tentatively extrapolated to the whole euphotic layer, our estimate is well within the range of estimates based on a wide variety of methods. The region around BATS is computed to be a weak sink for atmospheric CO2 with an annual rate of (0.81 +/- 0.37) mol/m2. It is estimated that about half of this influx is associated with the uptake of anthropogenic CO2.
see related article by Gruber and Keeling