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The impact of ocean deoxygenation on iron release from continental margin sediments

Nature Geoscience volume 7, pages 433–437 (2014)Cite this article

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Abstract

In the oceans’ high-nitrate–low-chlorophyll regions, such as the Peru/Humboldt Current system and the adjacent eastern equatorial Pacific1, primary productivity is limited by the micronutrient iron. Within the Peruvian upwelling area, bioavailable iron is released from the reducing continental margin sediments2. The magnitude of this seafloor source could change with fluctuations in the extension or intensity of the oxygen minimum zones3,4. Here we show that measurements of molybdenum, uranium and iron concentrations can be used as a proxy for sedimentary iron release, and use this proxy to assess iron release from the sea floor beneath the Peru upwelling system during the past 140,000 years. We observe a coupling between levels of denitrification, as indicated by nitrogen isotopes, trace metal proxies for oxygenation, and sedimentary iron concentrations. Specifically, periods with poor upper ocean oxygenation are characterized by more efficient iron retention in the sediment and a diminished iron supply to the water column. We attribute efficient iron retention under more reducing conditions to widespread sulphidic conditions in the surface sediment and concomitant precipitation of iron sulphides. We argue that iron release from continental margin sediments is most effective in a narrow redox window where neither oxygen nor sulphide is present. We therefore suggest that future deoxygenation in the Peru upwelling area would be unlikely to result in increased iron availability, whereas in weaker oxygen minimum zones partial deoxygenation may enhance the iron supply.

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Figure 1: Relationship between water column oxygenation and iron release from continental margin sediments.
Figure 2: Study area and biogeochemical context.
Figure 3: Age model.
Figure 4: Redox proxies.

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Acknowledgements

Financial support for this study was provided by the 7th Framework Program of the European Union (Marie Curie IOF to F.S., BICYCLE, Project No. 300648) and the German Research Foundation through Collaborative Research Centre 754 ‘Climate-Biogeochemistry Interactions in the Tropical Ocean’ (www.sfb754.de/en). The US National Science Foundation supported J.M. (Grant No. 1029889) and A.C.M. (Grant No. 1131834). We thank A. Bleyer, B. Domeyer, J. McKay and A. Ungerer for assistance in the laboratory and A. Schmittner for insightful comments on the manuscript.

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Authors and Affiliations

  1. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon 97331-5503, USA

    Florian Scholz & Alan C. Mix

  2. GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1–3, 24148 Kiel, Germany

    Florian Scholz & Christian Hensen

  3. Department of Geoscience, University of Akron, Akron, Ohio 44325, USA

    James McManus

  4. Institute of Geosciences, Kiel University, Ludewig-Meyn Straße 10, 24118 Kiel, Germany

    Ralph R. Schneider

Authors
  1. Florian Scholz
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  2. James McManus
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  3. Alan C. Mix
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  4. Christian Hensen
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  5. Ralph R. Schneider
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Contributions

F.S., J.M., A.C.M. and C.H. designed the study; F.S. carried out the laboratory work; R.R.S. guided the coring campaign and contributed radiocarbon dates; F.S. wrote the manuscript with contributions from all co-authors.

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Correspondence to Florian Scholz.

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Scholz, F., McManus, J., Mix, A. et al. The impact of ocean deoxygenation on iron release from continental margin sediments. Nature Geosci 7, 433–437 (2014). https://doi.org/10.1038/ngeo2162

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