Abiotic Pyrite Formation Produces a Large Fe Isotope Fractionation

Abstract

The iron isotope composition of sedimentary pyrite has been proposed as a potential proxy to trace microbial metabolism and the redox evolution of the oceans. We demonstrate that Fe isotope fractionation accompanies abiotic pyrite formation in the absence of Fe(II) redox change. Combined fractionation factors between Fe(II)(aq), mackinawite, and pyrite permit the generation of pyrite with Fe isotope signatures that nearly encapsulate the full range of sedimentary δ(56)Fe(pyrite) recorded in Archean to modern sediments. We propose that Archean negative Fe isotope excursions reflect partial Fe(II)(aq) utilization during abiotic pyrite formation rather than microbial dissimilatory Fe(III) reduction. Late Proterozoic to modern sediments may reflect greater Fe(II)(aq) utilization and variations in source composition.

Cu isotopes in marine black shales record the Great Oxidation Event

“…2.45-2.15 Ga represent a more important proportion of the local dissolved Fe pool being deposited as iron oxides or sulfides with increasing environmental oxygenation (8); and a return to negative values under euxinic basinal conditions ca. 2.08 Ga (7) is best explained by enrichment of light isotopes into iron sulfides formed in the water column or during early diagenesis (38). Across the GOE, differences between our bulk shale δ 56 Fe values and those reported for pyrites across the same interval (8) Cu values ≤0‰ in shales after 2.3 Ga suggests that Cu isotopes captured a unidirectional shift in the oxygenation of the atmosphere−ocean system across the GOE.…”

“…Fe values from our own sample set also capture the GOE transition (Fig. 2) (8,11,31,37), whereas smaller degrees of Fe removal by iron oxide or sulfide precipitation can yield positive sedimentary iron isotope enrichments (8,38).…”

Isotope Fractionation Processes of Selected Elements

“…Larger Fe isotope fractionations occur during biogeochemical redox processes, which include dissimilatory Fe(III) reduction Icopini et al 2004;Crosby et al 2007), anaerobic photosynthetic Fe ( Fe than Fe in ironhydroxides. In summary, negative δ 56 Fe-values in sedimentary rocks may reflect ancient DIR (Yamaguchi et al 2005;Johnson et al 2008), other studies have, however, favored abiological processes for the occurrence of negative Fe isotope values (Rouxel et al 2005;Anbar and Rouxel 2007;Guilbaud et al 2011). Especially large iron isotope fractionations have been found in Proterozoic and Archean banded iron formations (BIFs) and shales (Rouxel et al 2005;Yamaguch et al 2005).…”

Response to Comment on “Abiotic Pyrite Formation Produces a Large Fe Isotope Fractionation”

“…Therefore, the amount of Fe use as pyrite should not be <2% to produce 56 Fe-depleted pyrite, as Czaja et al assert, but can be as large as 50% (2). We should clarify that in figure 2 in (2), the 10% in column B does not refer to the requirements of a model but refers to the fact that the fractionation presented was recorded in experiments in which 10% of Fe was precipitated by sulfide (8,9).…”