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

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 (53Cr) as a paleoredox proxy has required recent revision. However the basic idea behind using Cr isotopes as redox tracers is straightforwardthe 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. Critically the Cr isotope systemunlike many other trace metal proxiescan 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.