1. Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey 08544, USA
2. IGPP and Department of Atmospheric Sciences, University of California at Los Angeles, Los Angeles, California 90095, USA
3. Department of Ecology, Evolution and Marine Biology and the Marine Science Institute, University of California, Santa Barbara, California 93106, USA
4. NOAA/Geophysical Fluid Dynamics Laboratory, PO Box 308, Forrestal Campus B Site, Princeton, New Jersey 08542, USA

Correspondence to: J. L. Sarmiento¹ Email: jls@princeton.edu

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 Ocean¹ and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere^{2, 3} and North Atlantic Ocean³, 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 tides⁴, 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.

1. Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey 08544, USA
2. IGPP and Department of Atmospheric Sciences, University of California at Los Angeles, Los Angeles, California 90095, USA
3. Department of Ecology, Evolution and Marine Biology and the Marine Science Institute, University of California, Santa Barbara, California 93106, USA
4. NOAA/Geophysical Fluid Dynamics Laboratory, PO Box 308, Forrestal Campus B Site, Princeton, New Jersey 08542, USA

Correspondence to: J. L. Sarmiento¹ Email: jls@princeton.edu

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