Abstract
Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the Southern Ocean surface into the global pycnocline (roughly the upper 1.2 km), as opposed to directly from the deep ocean. We estimate that 62 ± 5% of the pycnocline nitrate and phosphate originate from the Southern Ocean. Mixing, as opposed to advection, accounts for most of the gross nutrient input to the pycnocline. However, in net, mixing carries nutrients away from the pycnocline. Despite the quantitative dominance of mixing in the gross nutrient transport, the nutrient richness of the pycnocline relies on the large-scale advective flow, through which nutrient-rich water is converted to nutrient-poor surface water that eventually flows to the North Atlantic.
Highlights
The ocean mitigates global warming by absorbing fossil-fuel CO2 and heat from the atmosphere[1]. Central to this role is the exchange of water between the deep ocean and the ocean’s upper water column, with the latter communicating more directly with the atmosphere. This exchange restores to the upper ocean nutrients that have been exported by the sinking of biological material[2,3]
In the first conceptions of ocean circulation, the return of deep water to the pycnocline was by interior upwelling[4,5], including at low latitudes
Subsequent work pointed to wind-driven upwelling in the Southern Ocean, followed by subduction as Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), as the dominant route by which water and nutrients are returned from the deep ocean to the pycnocline[10,11,12,13,14]
Summary
For the PFZ-SAZ ventilating area and deep ocean, the median of the station’s weighted averages is assumed to be representative of the mean properties of each domain This approach appears to be robust because similar NO3− concentrations are reported for WOA13 (that is, annual properties) and the NO3− isotope database (Table 1). Significant isotopic gradients are observed (Extended Data Fig. 3; see Supplementary Information for a discussion of the large-scale distribution of nitrate isotopes in the pycnocline), which could induce a sampling bias in the assessment of the mean pycnocline properties These gradients are mostly driven by variable NO3− isotope budget-related fluxes and are restricted to the areas influenced by the North Atlantic Ocean and the Oxygen Minimum Zones[19] (OMZs, contributing to ~24% of the pycnocline NO3− pool).
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