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Bioturbation and the d56Fe Signature of Dissolved Iron Fluxes from Marine Sediments
  • Sebastiaan J. Van de Velde,
  • Andrew Willliam Dale,
  • Sandra Arndt
Sebastiaan J. Van de Velde
Université Libre de Bruxelles

Corresponding Author:sebastiaan.van.de.velde@ulb.be

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Andrew Willliam Dale
GEOMAR Helmholtz Centre for Ocean Research Kiel
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Sandra Arndt
Universit ́e Libre de Bruxelles
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Iron is a key limiting nutrient for phytoplankton. Continental shelf and slope sediments are important sources of dissolved iron (DFe). Stable iron isotopes (d56Fe) are a particularly useful tool to quantify the DFe sources and sinks in the ocean. The isotopic signature of the sedimentary DFe source is controlled by environmental factors such as bottom water redox conditions, carbon oxidation and bioturbation by burrowing fauna, but the exact relation on a global scale is poorly understood. We developed a reaction-transport model capable of tracing dissolved iron isotope fractionation in marine sediments to quantify the isotopic signature of benthic DFe fluxes under a wide range of environmental conditions. We derived fractionation factors for iron reduction (-1.3 permille), iron oxidation (+0.4 permille), iron sulphide precipitation (+0.5 permille and dissolution (-0.5 permille and pyrite precipitation (-0.7 permille) that were in line with existing literature. At bottom-water oxygen concentrations >50 µM, bioturbation increased the benthic DFe flux and increased the d56Fe signature. In contrast, at bottom-water oxygen concentrations <50 µM, a reduction in bioturbation led to a decrease in the benthic DFe flux and its d56Fe value. On a global scale, a model simulation without bioturbation decreased the sedimentary DFe release from ~158 Gmol DFe yr-1 to ~70 Gmol DFe yr-1, and decreased the variability in the d56Fe signature of the DFe flux. Finally, we find that a decrease in ocean oxygen content by 40 µM can increase global sedimentary DFe release by up to 103 Gmol DFe yr-1.