The <sup>3</sup>He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site

Abstract

Abstract. 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 3 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 3 He flux across the sea-air interface, which must closely mirror the upward 3 He flux into the euphotic zone. The product of the 3 He flux and the observed subsurface nitrate-3 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 3 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-3 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 −2 yr −1 , 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 3 He technique and potentially also by the decoupling of 3 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.

Heavy Noble Gas Isotopes as New Constraints on the Ventilation of the Deep Ocean

“…Due to their chemical and biological inertness, dissolved noble gases in seawater are useful tools for disentangling physical from biogeochemical processes (Hamme et al, 2017;Stanley & Jenkins, 2013). Among other applications, noble gases have been used to constrain bubble injection (e.g., Emerson & Bushinsky, 2016;Stanley et al, 2009), diapycnal mixing rates (e.g., Ito et al, 2007), oxygen production (e.g., Spitzer & Jenkins, 1989), nutrient fluxes (e.g., Stanley et al, 2015), and the strength of the carbon solubility pump (Hamme et al, 2019;Nicholson et al, 2010). In the deep ocean, solubility disequilibria of noble gas concentrations display a globally consistent pattern (Hamme & Severinghaus, 2007;Jenkins et al, 2016;Loose et al, 2016): supersaturation of light, less soluble noble gases (He and Ne), and undersaturation of heavy, more soluble noble gases (Ar, Kr, and Xe).…”

Is seasonal net community production in the South Pacific Subtropical Gyre anomalously low?

“…Data have accumulated to the point where we can now estimate seasonal or annual NCP for oceanic regions with highly contrasting biogeochemical properties. These include the North Atlantic and North Pacific subtropical gyres [ Bates , ; Emerson et al ., , ; Quay et al ., ; Stanley et al ., ], the equatorial Pacific [ Parker et al ., ; Quay et al ., ], the subarctic North Pacific [ Emerson et al ., ; Quay et al ., ], and the various zones of the Southern Ocean [ Reuer et al ., ]. Comparing NCP for different regions is challenging because different studies report rates from weekly to annual timescales.…”

Internal Tides Drive Nutrient Fluxes Into the Deep Chlorophyll Maximum Over Mid‐ocean Ridges

“…Thus, estimates of export production in the subtropical ocean require a nitrogen supply term of approximately 0.5-0.9 mol·N·m −2 ·year −1 (Jenkins, 1982;Jenkins & Doney, 2003). This is validated by geochemical estimates of the physical supply of nitrate to the euphotic zone of the subtropical North Atlantic which range between 0.7 and 0.8 mol N·m −2 ·year −1 (Jenkins & Doney, 2003;Stanley et al, 2015). These estimates exclude the biological sources of nitrogen from nitrogen fixation or zooplankton migration which would increase the supply term further (Bianchi et al, 2013;Mahaffey et al, 2005;Tuerena et al, 2015).…”