Abstract
We present a data‐assimilated model of the ocean's phosphorus cycle that is constrained by climatological phosphate, temperature, salinity, sea‐surface height, surface heat and freshwater fluxes, as well as chlorofluorocarbon‐11(CFC‐11) and natural Δ14C. Export production is estimated to be 5.8±2.0×1012 mol P/yr of which (26±6)% originates in the Southern Ocean (SO) south of 40°S. The biological pump efficiency, defined as the proportion of the ocean's phosphate inventory that is regenerated, is (39±7)%. Dividing the SO south of 40°S into a sub‐Antarctic zone (SANTZ) and an Antarctic zone (ANTZ) separated by the latitude of maximum Ekman divergence, we estimate that the SANTZ and ANTZ account, respectively, for (23±5)% and (3±1)% of global export production, (17±4)% and (3±1)% of the regenerated nutrient inventory, and (31±1)% and (43±5)% of the preformed nutrient inventory. Idealized SO nutrient depletion experiments reveal a large‐scale transfer of nutrients into circumpolar and deep waters and from the preformed to the regenerated pool. In accord with the concept of the biogeochemical divide, we find that nutrient drawdown in the ANTZ is more effective than in the SANTZ for increasing the efficiency of the biological pump, while having a smaller impact on production in regions north of 40°S. Complete SO nutrient drawdown would allow the biological pump to operate at 94% efficiency by short circuiting the transport of nutrients in northward Ekman currents, leading to a trapping of nutrients in circumpolar and deep waters that would decrease production outside the SO by approximately 44% while increasing it in the SO by more than 725%.
Highlights
[2] The Southern Ocean (SO) ventilates a large fraction of the ocean, both through the formation of newly ventilated water masses and through the exposure to the atmosphere of old water masses [Toggweiler and Samuels, 1993; Primeau, 2005; Primeau and Holzer, 2006; DeVries and Primeau, 2011]
[3] Three-dimensional ocean general circulation model (GCM) simulations of SO nutrient drawdown have demonstrated that increasing biological production and nutrient drawdown in the SO would greatly increase the strength of the biological pump, but they have shown that biological production in the rest of the ocean tends to decrease with increased nutrient depletion in the SO [Sarmiento and Orr, 1991; Sarmiento et al, 2004; Marinov et al, 2006; Oschlies et al, 2010]
Nutrients regenerated in North Atlantic deep water enter the SO at depth where they are either entrained into Antarctic bottom water, perhaps after upwelling to the Antarctic zone (ANTZ) surface or are upwelled into the sub-Antarctic zone (SANTZ), where they are again stripped out by biological production. This results in a gradual accumulation of phosphate into circumpolar and deep waters that must eventually be balanced by eddy diffusive fluxes out of ANTZ-ventilated waters (Figure 8)
Summary
[2] The Southern Ocean (SO) ventilates a large fraction of the ocean, both through the formation of newly ventilated water masses and through the exposure to the atmosphere of old water masses [Toggweiler and Samuels, 1993; Primeau, 2005; Primeau and Holzer, 2006; DeVries and Primeau, 2011]. PRIMEAU ET AL.: SOUTHERN OCEAN NUTRIENT TRAPPING [4] DeVries et al [2012] coupled a data-assimilated ocean circulation model to a simple phosphorus cycling model in which biological production was parameterized as being proportional to the concentration of phosphate in the euphotic layer, with no spatial variations in the proportionality (rate) constant, and conducted a sequence of simulations in which they varied the rate constant for the biological phosphate uptake. Their idealized experiments showed that SO export production increased dramatically as the rate constant increased. The geological restoring term, being extremely small, is neglected in all diagnostic expressions presented below
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