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High-latitude controls of thermocline nutrients and low latitude biological productivity

  • A Corrigendum to this article was published on 19 October 2011

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 Hemisphere2,3 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.

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Acknowledgements

We thank I. Belkin and K. Moore for making their frontal paths in the Southern Ocean available to us. This paper benefited from comments by M. Bender, B. McNeil, D. Sigman, C. Sweeney and R. Toggweiler. J.L.S. was supported by a NOAA Office of Global Programs grant to the Carbon Modeling Consortium for model development, and by an NSF grant for model and observational interpretations as part of the JGOFS Synthesis and Modeling Project. J.L.S. and J.D. were supported by a DOE Office of Science grant for the nutrient depletion scenarios. N.G. was supported by the DOE, and M.A.B. by the NSF.

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Correspondence to J. L. Sarmiento.

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Further reading

Figure 1: Polar stereographic maps of upper ocean nutrients and physics.
Figure 2: Global maps of nutrient properties mapped on the potential density surface σθ = 26.80.
Figure 3: Predicted global zonal mean of the fractional contribution of Southern Ocean nutrient supply to global export production.
Figure 4: Southern Ocean control on thermocline nutrient concentrations.

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