Gruber, N., C. D. Keeling, and T. F. Stocker, 1998:
Carbon-13 constraints on the seasonal inorganic carbon budget at the
BATS site in the northwestern Sargasso Sea. Deep-Sea Research I,
45(4-5), 673-717. |
Abstract: 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-yr 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. We are thus able
to quantitatively separate all processes that contribute to the observed
seasonal cycle in the mixed layer. Uncertainties in the C budget
are determined using a Monte Carlo method. We found that net community
production is mainly responsible in generating the observed C drawdown
of about 26 µmol kg -1 between April
and October by removing on the average -40 ± 4 µmol kg-1
from the mixed layer. This net community production occurs in the absence
of measurable nitrate and phosphate concentrations. We hypothesize that
N2 fixation in combination with vertical
migration may be large enough to meet the biological nutrient demand in
the mixed layer during this summer/fall period. Physical processes do not
explain the observed summer/fall drawdown except when very improbably large
horizontal advective velocities are assumed. Annual net community production
of 2.3 ± 0.9 mol m-2 is not well
constrained by 13C/12C,
but if tentatively extrapolated to the whole euphotic layer, it is well
within the range of estimates based on a wide variety of methods. The region
around BATS is computed to be a moderate sink for atmospheric CO2
with an annual uptake of 1.8 ± 0.5 mol m-2,
with about 25% of this influx associated with the uptake of anthropogenic
CO2. |