Chromium Isotope Systematics in Modern and Ancient Microbialites

Sylvie Bruggmann,Robert Frei,Robert M Klaebe,Steven Goderis,Alexandra S Rodler

doi:10.3390/min10100928

Abstract

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.

Highlights

Sedimentary carbonates are increasingly examined for their chromium isotope composition to investigate the evolution of oxygen in the Earth’s atmosphere and oceans (e.g., [1]).Chromium mobilization from igneous rocks is dependent on oxidative weathering, and accumulation of Cr in marine sediments requires the presence of oxygen in the atmosphere (0.0003% to 0.003% of present atmospheric levels (PAL) [2])
The Cr in the microbialite samples were too low for sub-millimetric Cr isotope analysis and/or the microbialite laminae were too thin to be sampled individually, and multiple laminae were sampled at a time
BasedononCrthe knowledge on Cr isotope with fractionation we propose with a basic conceptual fora Cr incorporation isotope fractionation in microbialites, associated organics, we model propose basic conceptual and model for Cr incorporation and isotope assuming the presence of common cyanobacteria and sulfate-reducing bacteria

Summary

Introduction

Sedimentary carbonates are increasingly examined for their chromium isotope composition to investigate the evolution of oxygen in the Earth’s atmosphere and oceans (e.g., [1]).Chromium mobilization from igneous rocks is dependent on oxidative weathering (catalyzed byMnO2 ), and accumulation of Cr in marine sediments requires the presence of oxygen in the atmosphere (0.0003% to 0.003% of present atmospheric levels (PAL) [2]). Minerals 2020, 10, 928 of Cr induce isotope fractionation, where the lighter Cr isotopes are preferred in reduction reactions, resulting in redox-dependent changes of 53 Cr/52 Cr [3,4,5]. Despite the evidence of redox-dependent Cr isotope fractionation, a large range of fractionation effects can occur without any involvement of redox changes but in the presence of ligands [11,12]. Modern biogenic carbonates show a wide range in δ53 Cr values and a negative offset relative to ambient seawater (∆53 Cr ≈ 0.0%–0.9%) [13,14,15,16,17,18,19]. Despite the growing compilation of δ53 Cr values of modern carbonate materials, its large range in skeletal carbonates complicates their use as archives for paleo-reconstructions. Data on Cr isotope systematics in modern non-skeletal carbonates [15] or carbonates in microbial mats remains scarce to date

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