Gulf Stream rings as a source of iron to the North Atlantic subtropical gyre

Abstract

Substantial amounts of nitrogen fixation occur in the North Atlantic subtropical gyre, due to the activity of cyanobacteria with high iron requirements. Iron is delivered to this region by dust from the Sahara Desert. However, this dust deposition is typically localized and episodic. Therefore, other sources of iron may also be important. Here, we report observations of dissolved iron concentrations in a Gulf Stream cold-core ring, which transported iron-rich water from near the continental slope into the subtropical gyre. We find that iron concentrations were elevated in the ring compared with subtropical waters, reflecting its source waters. Using iron data from these source waters and the identification of ring activity in satellite data, we estimate that cold-core rings provide a net flux of 0.3 ± 0.17 × 108 mol Fe yr−1 across the northwestern gyre edge, on the order of 15% of our median estimates of gyre-wide supply of iron by dust deposition. We suggest that iron supply from cold-core rings is an important source of iron to the northwestern gyre edge. We conclude that mesoscale ocean circulation features may play an important role in subtropical nutrient and carbon cycling.

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Fig. 1: GA03 Fe station sampling locations20, Gulf Stream and cold-core ring in satellite altimetry.
Fig. 2: A cold-core ring observed in the GA03 section.
Fig. 3: GA03 temperature–salinity diagram, with Fe concentrations10 in colour.
Fig. 4: Ring-driven dissolved Fe supply compared with atmospheric dissolved Fe deposition.

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Acknowledgements

We thank all those who contributed to the US GEOTRACES GA03 cruises; scientists from the US GEOTRACES Program and the Ocean Data Facility who measured the trace metals, nutrients and physical parameters for the USGT11 cruise used in this study; N. Mahowald, S. Albani, A. Ito and R. Wang for making model output available; and P. Sedwick, W. Landing, N. Mahowald, C. Measures and D. Vance for useful discussions. T.M.C. acknowledges support from the University of South Florida; J.B.P. acknowledges support from the University of Rhode Island; G.F.d.S. is supported by the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement #708407.

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All authors contributed equally to this work. T.M.C. and G.F.d.S. conceived the idea, J.B.P. carried out the ring-driven Fe transport calculations and G.F.d.S. carried out the atmospheric deposition calculations.

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Correspondence to Tim M. Conway.

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Conway, T.M., Palter, J.B. & de Souza, G.F. Gulf Stream rings as a source of iron to the North Atlantic subtropical gyre. Nature Geosci 11, 594–598 (2018). https://doi.org/10.1038/s41561-018-0162-0

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