The 3He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site
Other literature type
Stanley, R. H. R.
Jenkins, W. J.
Doney, S. C.
Lott III, D. E.
Significant rates of primary production occur in the oligotrophic ocean,
without any measurable nutrients present in the mixed layer, fueling a
scientific paradox that has lasted for decades. Here, we provide a new
determination of the annual mean physical supply of nitrate to the euphotic
zone in the western subtropical North Atlantic. We combine a 3-year time
series of measurements of tritiugenic <sup>3</sup>He from 2003 to 2006 in the
surface ocean at the Bermuda Atlantic Time-series Study (BATS) site with a
sophisticated noble gas calibrated air–sea gas exchange model to constrain
the <sup>3</sup>He flux across the sea–air interface, which must closely mirror
the upward <sup>3</sup>He flux into the euphotic zone. The product of the <sup>3</sup>He
flux and the observed subsurface nitrate–<sup>3</sup>He relationship provides an
estimate of the minimum rate of new production in the BATS region. We also
apply the gas model to an earlier time series of <sup>3</sup>He measurements at
BATS in order to recalculate new production fluxes for the 1985 to 1988 time
period. The observations, despite an almost 3-fold difference in the
nitrate–<sup>3</sup>He relationship, yield a roughly consistent estimate of
nitrate flux. In particular, the nitrate flux from 2003 to 2006 is estimated
to be 0.65 ± 0.14 mol m<sup>−2</sup> yr<sup>−1</sup>, which is ~40 %
smaller than the calculated flux for the period from 1985 to 1988. The
difference in nitrate flux between the time periods may be signifying a real
difference in new production resulting from changes in subtropical mode water
formation. Overall, the nitrate flux is larger than most estimates of export
fluxes or net community production fluxes made locally for the BATS site,
which is likely a reflection of the larger spatial scale covered by the
<sup>3</sup>He technique and potentially also by the decoupling of <sup>3</sup>He and
nitrate during the obduction of water masses from the main thermocline into
the upper ocean. The upward nitrate flux is certainly large enough to support
observed rates of primary production at BATS and more generally in the
oligotrophic subtropical ocean.