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Abstract
The ocean's biological pump strips nutrients out of the surface waters and exports
them into the thermocline and deep waters. If there were no return path of nutrients
from deep waters, the biological pump would eventually deplete the surface waters
and thermocline of nutrients; surface biological productivity would plummet. Here
we make use of the combined distributions of silicic acid and nitrate to trace the
main nutrient return path from deep waters by upwelling in the Southern Ocean and
subsequent entrainment into subantarctic mode water. We show that the subantarctic
mode water, which spreads throughout the entire Southern Hemisphere and North Atlantic
Ocean, is the main source of nutrients for the thermocline. We also find that an additional
return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical
mixing, perhaps driven by tides, brings abyssal nutrients to the surface and supplies
them to the thermocline of the North Pacific. Our analysis has important implications
for our understanding of large-scale controls on the nature and magnitude of low-latitude
biological productivity and its sensitivity to climate change.