Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as delta Cr-53) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth's atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of delta Cr-53 values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and delta Cr-53 values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, delta Cr-53 approximate to 0.99 parts per thousand) and Mono Lake (USA, approximate to 0.78 parts per thousand) display significantly higher delta Cr-53 values compared with ancient microbialites from the Andree Land Group in Greenland (720 Ma, approximate to 0.36 parts per thousand) and the Bitter Springs Formation in Australia (800 Ma, approximate to-0.12 parts per thousand). The delta Cr-53 values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable delta Cr-53 values in skeletal carbonates, do not induce variability in delta Cr-53 values in microbialites. Together with stable C and O isotope compositions and REY patterns, delta Cr-53 values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and delta Cr-53 results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the delta Cr-53 values of the waters from which they precipitated. However, it remains unclear if these delta Cr-53 values record (paleo-) redox conditions or rather result from other physico-chemical parameters.