THE ISOTOPIC EXPRESSION OF IRON SHUTTLING IN THE EUXINIC BLACK SEA BASIN AND IMPLICATIONS FOR THE RISE OF OXYGEN IN THE EARLY ATMOSPHERE

Modern sediments deposited beneath a sulfidic (euxinic) water column, such as in the Black Sea, are typically enriched in reactive iron relative to average oxic to suboxic deposits. Similar iron enrichments are observed in sedimentary rocks from ancient euxinic basins. The mechanism commonly proposed for this iron enrichment is shuttling of benthic iron from suboxic shelf sediments to the deep basin, where it is quantitatively sequestered during Fe-sulfide precipitation in the water column. Low δ⁵⁶Fe values in porewaters from suboxic sediments are a consequence of redox cycling and indicate that the sedimentary iron flux should have a characteristically light (<-2.0 ‰) iron isotope composition. Because Fe uptake under euxinic conditions is quantitative, any additional Fe isotope fractionation during Fe-sulfide precipitation is not recorded in the sediments. Consistent with shelf-to-basin iron transport and quantitative water-column Fe sequestration, modern euxinic sediments show moderately low isotopic compositions for pyrite Fe (δ⁵⁶Fe ~ -1‰) and total Fe (δ⁵⁶Fe ~ -0.3‰) that are significantly lower than for total Fe in sediments on the suboxic shelf (δ⁵⁶Fe ~ +0.1‰). Under anoxic (but not euxinic) conditions, Fe uptake in the Fe-rich water column is not quantitative, and the Fe isotope composition of the benthic Fe source is overprinted by fractionations associated with authigenic mineral formation. Fe isotope compositions in pyrites and bulk sediments throughout the Archean are significantly lower than in pyrites from modern marine sediments, including the Black Sea. We propose that these low δ⁵⁶Fe values reflect the combined effects of shelf redox-cycling and Fe shuttling preserved in authigenic Fe-sulfide precipitates. Our interpretation implies that shallow sediments in the Archean ocean underwent redox-cycling in order to generate an isotopically light benthic iron source and that a shelf-to-basin iron shuttle operated during that time. Chemical cycling of Fe is intimately linked to the availability of oxygen in the ocean and ultimately to the oxygenation of the atmosphere. The isotopic expression of the iron enrichment mechanism could therefore provide important insights into the evolution of the iron cycle and the redox balance of the atmosphere-ocean system of the early Earth.