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Modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans

¹ Department of Earth Sciences, University of California–Riverside, Riverside, California 92521, USA
² School of Earth and Space Exploration, and Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
³ College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
⁴ School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA

The increase in atmospheric oxygen ca. 2.4 Ga had a significant impact on the geochemical cycling of Fe. The history of environmental oxygenation may be recorded in the Fe isotope composition of Archean and Proterozoic sediments, but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations. Here we present Fe isotope data and iron/aluminum ratios from the Black Sea oxic shelf and euxinic basin. The isotope data demonstrate that shelf Fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin. We propose that there is net transport of isotopically light Fe from sediments of the shelf to those of the distal, anoxic basin, consistent with enrichments in reactive Fe seen in the deep basin. The low ⁵⁶Fe benthic Fe flux is generated during the coupling of microbial Fe(III) reduction or sulfidization with Fe²⁺_aq oxidation. Low ⁵⁶Fe values reported previously from Late Archean sedimentary pyrites may be an isotopic fingerprint of analogous Fe redox cycling in the Late Archean oceans. This interpretation implies shallow-water Fe redox recycling in the Late Archean. We predict that the light isotopic compositions of the Late Archean will prove to be distinct from those of the Early Archean, before Fe redox cycling became an important process, and we infer that this difference may be related to the presence of oxygen in the surface ocean.

Key Words: Fe isotopes • Black Sea • Fe shuttle • pyrite • early life

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