Iron isotopes and the redox evolution of Ediacaran sediments

The Ediacaran age (ca. 570 Ma) Shuram excursion, a ca. $12‰$ depletion in $\delta ^{13}\mathrm{C}_{\mathrm{carb}}$, may record a dramatic oxidation of marine sediments associated with a reorganization of Earth’s carbon cycle closely preceding the rise of large metazoans. However, several geochemical indicators suggest it may instead record secondary processes affecting the sediments such as post-depositional alteration. The stable isotopic composition of iron incorporated within carbonates $(\delta ^{56}\mathrm{Fe}_{\mathrm{carb}})$ reveals an anomalous $^{56}\mathrm{Fe}$-depletion (down to $-1.05‰$) in strata containing the Shuram excursion, while the underlying and overlying strata have crustal $\delta ^{56}\mathrm{Fe}_{\mathrm{carb}}$ values. These depleted $\delta ^{56}\mathrm{Fe}_{\mathrm{carb}}$ data during the Shuram excursion reflect incomplete reduction of iron oxides, limited by low ambient organic carbon contents. This elevated pulse of sedimentary iron oxides would consume the majority of the limited pool of organic carbon and therefore would give rise to very low net organic carbon burial during a time of enhanced detrital delivery of oxidized iron to the sediments. These results imply a syndepositional origin for the Shuram excursion, which represents a shift in the redox composition of Earth’s sedimentary shell toward more oxidizing conditions, perhaps removing a long-lived buffer on atmospheric oxygen.