The sup3/supHe flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site

摘要

Significant rates of primary production occur in the oligotrophic ocean,without any measurable nutrients present in the mixed layer, fueling ascientific paradox that has lasted for decades. Here, we provide a newdetermination of the annual mean physical supply of nitrate to the euphoticzone in the western subtropical North Atlantic. We combine a 3-year timeseries of measurements of tritiugenic 3He from 2003 to 2006 in thesurface ocean at the Bermuda Atlantic Time-series Study (BATS) site with asophisticated noble gas calibrated air–sea gas exchange model to constrainthe 3He flux across the sea–air interface, which must closely mirrorthe upward 3He flux into the euphotic zone. The product of the 3Heflux and the observed subsurface nitrate–3He relationship provides anestimate of the minimum rate of new production in the BATS region. We alsoapply the gas model to an earlier time series of 3He measurements atBATS in order to recalculate new production fluxes for the 1985 to 1988 timeperiod. The observations, despite an almost 3-fold difference in thenitrate–3He relationship, yield a roughly consistent estimate ofnitrate flux. In particular, the nitrate flux from 2003 to 2006 is estimatedto be 0.65 ± 0.14 mol m?2 yr?1, which is ~40 %smaller than the calculated flux for the period from 1985 to 1988. Thedifference in nitrate flux between the time periods may be signifying a realdifference in new production resulting from changes in subtropical mode waterformation. Overall, the nitrate flux is larger than most estimates of exportfluxes or net community production fluxes made locally for the BATS site,which is likely a reflection of the larger spatial scale covered by the3He technique and potentially also by the decoupling of 3He andnitrate during the obduction of water masses from the main thermocline intothe upper ocean. The upward nitrate flux is certainly large enough to supportobserved rates of primary production at BATS and more generally in theoligotrophic subtropical ocean.