R. H. R. Stanley, W. J. Jenkins, S. C. Doney, and D. E. Lott, III,
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, Biogeosciences, submitted
We provide a new determination of the annual mean physical supply of nitrate to the euphotic zone in the western subtropical North Atlantic based on a three year time-series of measurements of tritiugenic 3He from 2003 to 2006 in the surface ocean at the Bermuda Atlantic Time-series Study (BATS) site. We combine the 3He data with a sophisticated noble gas calibrated air-sea gas exchange model to constrain the 3He flux across the sea-air interface, which must closely balance the upward 3He flux into the euphotic zone. The product of the 3He flux and the observed subsurface nitrate-3He relationship provides an estimate of the minimum rate of new production in the BATS region. We also applied the gas model to an earlier time series of 3He measurements at BATS in order to recalculate new production fluxes for the 1985 to 1988 time period. The observations, despite an almost three-fold difference in the nitrate-3He relationship, yield a roughly consistent estimate of nitrate flux. In particular, the nitrate flux from 2003-2006 is estimated to be 0.65 ± 0.3 mol m-2 y-1, which is ~40% smaller than the calculated flux for the period from 1985-1988. The difference between the time periods, which is barely significant, may be due to 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 BATS site, which is likely a reflection of the larger spatial scale covered by the 3He technique and potentially also by decoupling of 3He and nitrate during obduction of water masses from the main thermocline into the upper ocean.