Low levels of iron limit primary productivity across much of the Southern Ocean. At the basin scale, most dissolved iron is supplied to surface waters from subsurface reservoirs, because land inputs are spatially limited. Deep mixing in winter together with year-round diffusion across density surfaces, known as diapycnal diffusion, are the main physical processes that carry iron-laden subsurface waters to the surface. Here, we analyse data on dissolved iron concentrations in the top 1,000 m of the Southern Ocean, taken from all known and available cruises to date, together with hydrographic data to determine the relative importance of deep winter mixing and diapycnal diffusion to dissolved iron fluxes at the basin scale. Using information on the vertical distribution of iron we show that deep winter mixing supplies ten times more iron to the surface ocean each year, on average, than diapycnal diffusion. Biological observations from the sub-Antarctic sector suggest that following the depletion of this wintertime iron pulse, intense iron recycling sustains productivity over the subsequent spring and summer. We conclude that winter mixing and surface-water iron recycling are important drivers of temporal variations in Southern Ocean primary production.
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We thank all observational scientists that generously shared iron data (especially M. Klunder and P. Sedwick, who did so before publication), the GEOTRACES programme (www.geotraces.org), K. Arrigo and G. van Dijken for providing iron utilization data files and A. Barton for comments on the manuscript. The Argo float data were collected and made freely available by the International Argo Program (http://www.argo.ucsd.edu). This work benefitted from the support of the French Agence Nationale de la Recherche (ANR) grant ANR-10-LABX-18-01 of the national Programme Investissements d’Avenir, the CSIR Parliamentary Grant, NRF-SANAP and the EU FP7 Marie Curie International Research Staff Exchange Scheme (IRSES) Fellowship SOCCLI (The role of Southern Ocean Carbon cycle under CLImate change), which received funding from the European Commission’s Seventh Framework Programme under grant agreement 317699. J.B.S. received support from Agence Nationale de la Recherche (ANR), ANR-12-PDOC-0001, as well as from the British Antarctic Survey as a BAS Fellow. This research was partly supported by the Australian Government Cooperative Research Centres Programme through the Antarctic Climate and Ecosystems CRC (ACE CRC), University of Tasmania Rising Stars grant no B0019024 and Australian Antarctic Science project no 2900, the New Zealand Ministry for Science and Innovation and the Institute of Marine and Antarctic Studies, University of Tasmania.
The authors declare no competing financial interests.
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Tagliabue, A., Sallée, JB., Bowie, A. et al. Surface-water iron supplies in the Southern Ocean sustained by deep winter mixing. Nature Geosci 7, 314–320 (2014). https://doi.org/10.1038/ngeo2101
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