Nitrogen isotopic constraints on nutrient transport to the upper ocean

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. Much of the nutrient transport from the deep ocean into the ocean's upper water column occurs through the Southern Ocean, with mixing and advection playing complementary roles, according to a box model analysis of the isotopic composition of ocean nitrate.

Automated summary

Ocean circulation plays a crucial role in supplying nutrients for global ocean productivity, with the Southern Ocean being a major source of nutrients for the upper ocean. Mixing is the main driver of nutrient input to the pycnocline, but advection plays a complementary role in nutrient transport from the deep ocean to the upper water column.