The Chromium Isotope System as a Tracer of Ocean and Atmosphere Redox

Abstract

The stable chromium (Cr) isotope system has emerged over the past decade as a new tool to track changes in the amount of oxygen in Earth’s ocean-atmosphere system. Much of the initial foundation for using Cr isotopes (δ53Cr) as a paleoredox proxy has required recent revision. However, the basic idea behind using Cr isotopes as redox tracers is straightforward—the largest isotope fractionations are redox-dependent and occur during partial reduction of Cr(VI). As such, Cr isotopic signatures can provide novel insights into Cr redox cycling in both marine and terrestrial settings. Significant Cr isotope fractionations occur during oxidation and reduction processes in soils and rivers, making it difficult to estimate the δ53Cr value of the Cr flux to the oceans or gauge how this fractionation has varied through Earth’s history. In seawater there is also extensive variability in δ53Cr values, which is likely predominantly tied to Cr(VI) reduction in low oxygen water masses and biotic reduction in shallow waters. Nevertheless, shifts in reconstructed marine δ53Cr values can potentially be used to shed light on the distribution of suboxic and anoxic marine environments. Critically, the Cr isotope system—unlike many other trace metal proxies—can respond to short-term redox perturbations (e.g., on timescales characteristic of Pleistocene glacial-interglacial cycles). The Cr isotope system can also be used to probe the Earth’s long-term atmospheric oxygenation, pointing towards low but likely dynamic oxygen levels for the majority of Earth’s history.