Chromium Isotope Systematics in Modern and Ancient Microbialites

• Main Authors: Sylvie Bruggmann, Alexandra S. Rodler, Robert M. Klaebe, Steven Goderis, Robert Frei
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Minerals
• Subjects: Cr isotopes; C isotopes; O isotopes; rare earth elements; Proterozoic; carbonates
• Online Access: https://www.mdpi.com/2075-163X/10/10/928

Changes in stable chromium isotopes (denoted as δ⁵³Cr) 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 δ⁵³Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ⁵³Cr 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, δ⁵³Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ⁵³Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ⁵³Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ⁵³Cr values in skeletal carbonates, do not induce variability in δ⁵³Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ⁵³Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ⁵³Cr 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 δ⁵³Cr values of the waters from which they precipitated. However, it remains unclear if these δ⁵³Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.