Silicon and zinc biogeochemical cycles coupled through the Southern Ocean

Abstract

Zinc is vital for the physiology of oceanic phytoplankton. The striking similarity of the depth profiles of zinc to those of silicate suggests that the uptake of both elements into the opaline frustules of diatoms, and their regeneration from these frustules, should be coupled. However, the zinc content of diatom opal is negligible, and zinc is taken up into and regenerated from the organic parts of diatom cells. Thus, since opaline frustules dissolve deep in the water column while organic material is regenerated in the shallow subsurface ocean, there is little reason to expect the observed close similarity between zinc and silicate, and the dissimilarity between zinc and phosphate. Here we combine observations with simulations using a three-dimensional model of ocean circulation and biogeochemistry to show that the coupled distribution of zinc and silicate, as well as the decoupling of zinc and phosphate, can arise in the absence of mechanistic links between the uptake of zinc and silicate, and despite contrasting regeneration length scales. Our simulations indicate that the oceanic zinc distribution is, in fact, a natural result of the interaction between ocean biogeochemistry and the physical circulation through the Southern Ocean hub. Our analysis demonstrates the importance of uptake stoichiometry in controlling ocean biogeochemistry, and the utility of global-scale elemental covariation in the ocean in understanding these controls.

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Figure 1: Example depth profiles of Zn, Si and PO4 in the three main ocean basins.
Figure 2: Location map.
Figure 3: Contrasting variability in nutrient uptake in the Atlantic sector of the Southern Ocean.
Figure 4: Coupled major and micronutrient distributions in the global ocean.

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Acknowledgements

This research was supported by ETH Zürich and Swiss National Science Foundation (SNF) Grant 200021-153087/1 to D.V. S.H.L. is supported by a Leverhulme Early Career Fellowship and G.F.d.S. by a Marie Sklodowska-Curie Fellowship. We are very grateful for the constructive comments of B. Twining, which helped us to improve the manuscript.

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D.V. and S.H.L. conceived the study. D.V. wrote the first draft of the paper. G.F.d.S. constructed the biogeochemical model in collaboration with S.K., conceived and carried out the sensitivity simulations, and analysed the model output. M.C.L. and R.M. were responsible for the Atlantic data in the GEOTRACES Intermediate Data Product and used in the figures. All authors read and commented on the paper.

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Correspondence to Derek Vance.

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Vance, D., Little, S., de Souza, G. et al. Silicon and zinc biogeochemical cycles coupled through the Southern Ocean. Nature Geosci 10, 202–206 (2017). https://doi.org/10.1038/ngeo2890

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