Chromium consists of four stable isotopes (50Cr, 52Cr, 53Cr and 54Cr) with natural abundances of 4.35%, 83.79%, 9.50% and 2.36%, respectively (Rossman and Taylor 1998). Among these four isotopes, 50Cr, 52Cr and 54Cr are non-radiogenic, whereas 53Cr is a radiogenic product of the extinct nuclide 53Mn, which has a half-life of 3.7 Myr (Honda and Imamura 1971). Chromium isotope systems have a wide range of applications in geochemistry and cosmochemistry. They have been used to study early solar system processes (e.g., Rotaru et al. 1992); the oxidation/reduction (redox) potential of underground systems, which governs the transport and fate of many contaminants (e.g., Ellis et al. 2002); and more recently, the redox evolution of Earth’s early ocean-atmosphere system, which is intimately linked to the evolution of life (Frei et al. 2009; Crowe et al. 2013; Planavsky et al. 2014; Cole et al. 2016). ### Chemical properties of Cr Chromium is redox-sensitive. In Earth’s near-surface environments, Cr has two main valence states, +3 and + 6, which are expressed as Cr(III) and Cr(VI), respectively. The valence state of Cr is controlled by the prevailing redox potential (Eh) and pH conditions (Fig. 1). Cr(VI) is always bound with O2− to form the oxyanion species CrO42− (chromate), HCrO4− (bichromate), and Cr2O72−(dichromate), all of which are water-soluble. In contrast, Cr3+ usually forms oxyhydroxides or oxides, which are insoluble and immobile in the natural pH range. During oxidative weathering, Cr(III) in minerals can be oxidized by O2 to Cr(VI), a process that is catalyzed by manganese oxides (Fendorf and Zasoski 1992; Economou-Eliopoulos et al. 2014). The Cr(VI) migrates to rivers and eventually to the ocean. In the modern ocean, Cr occurs as both Cr(VI) and …