Biogeochemical cycle of chromium isotopes at the modern Earth's surface and its applications as a paleo-environment proxy

Chromium (Cr) is a redox-sensitive metal element with two natural oxidation states, commonly as the soluble Cr (VI) oxyanion (CrO42- or HCrO4-) and the insoluble Cr (III)-compounds. Over past two decades, developments in mass spectrometric techniques make mass-dependent Cr isotope fractionation during Cr redox reactions, Cr (III) oxidation and abiotic/biotic Cr (VI) reduction, as well as other redox-independent processes detectable. This could help identify a range of magnitudes of Cr isotope fractionation observed during terrestrial Cr mobilization, riverine transport of Cr into estuary systems, oceanic Cr cycling, and Cr sinks into sediments, and understand how Cr isotopes biogeochemically cycle at the modern Earth's surface. Based on this framework, the stable Cr isotope system is increasingly used as a paleo-environmental proxy to trace the oxygenation history of the atmosphere-hydrosphere system. In this contribution, we focus on reviewing analytical methods of stable Cr isotopes, mass-dependent Cr isotope fractionation factors determined by laboratory experiments, the biogeochemical cycle of Cr isotopes at the modern Earth's surface, and its applications as a paleo-environmental proxy, in the nearly twenty-year development. Despite several outstanding uncertainties, the stable Cr isotope system shows great promise as a powerful monitor of atmospheric and oceanic oxygenation.