Authigenic chromium enrichments in Proterozoic ironstones

Chromium isotopes have recently emerged as a proxy for atmospheric oxygen levels in the Precambrian. Chromium (Cr) enrichments, along with a suite of other redox sensitive metals, have also been used as marine redox proxies for decades. Despite significant interest and growing use of this system, which has been used to argue for low oxygen levels through the mid-Proterozoic leading up to the rise of animals, there is a poor understanding of the host phases of Cr in the sedimentary record. Without this context, interpretation of measured isotopic signatures and concentrations of Cr in various sedimentary archives can be ambiguous. In order to shed light on this problem, we have used electron microprobe elemental mapping to identify authigenic and detrital Cr phases. We have examined a range of ironstone samples that have been previously analyzed for Cr isotopes and find that Cr is predominantly associated with iron rich allochems instead of detrital Cr mineral phases. We did not find evidence for detrital chromite or other Cr-rich detrital minerals. Additionally, we find evidence for authigenic Ti enrichments, and decoupling of Al and Ti-elements traditionally used as detrital tracers-similar to what has been reported in other slowly accumulating metal-rich sediments (e.g., iron manganese nodules). Lastly, we have explored a range of possible sedimentary Cr isotope signals that could result from changes in the percentage of detrital versus authigenic Cr incorporated into a sample, providing clear evidence for the ambiguity in interpretations of this isotope system without petrographic context.