Application of chromium isotopes as a proxy for redox processes in present and ancient environments: promises and pitfalls

Chromium (Cr) is a redox-sensitive element, and significant Cr isotope fractionation occurs during redox transformations among its different oxidation states. As a result, the Cr isotopic system has been widely applied as a tracer for redox processes in both present and ancient environments, such as the reductive remediation of hexavalent Cr contamination and the reconstruction of oxygen dynamics in Earth’s atmosphere and oceans. Recent studies, however, suggest that the Cr isotopic compositions of aquatic systems and sediments are influenced by more complex processes than previously thought, introducing additional uncertainties in the interpretation of Cr isotopic data. In this review, we first summarize the mechanisms of Cr isotopic fractionation associated with low-temperature processes and the current understanding of the Cr cycle and its isotopic behavior in Earth’s surface environments. We then examine the key processes controlling the Cr isotopic compositions of major environmental reservoirs, with a focus on the geochemical interpretation of sedimentary Cr isotope records, their paleoenvironmental implications, and potential directions for future research.