Non-traditinoal stable isotopes of redox sensitive metals (Cr, U, Mo, Fe, etc.) are emerging new geochemical tools for studying biogeochemical cycling in Earth’s surface environments. My current research focuses on U and Cr isotope systems applied to tracing the evolution of earth’s early ocean-atmosphere. The valence states of these elements depend on the reduction-oxidation (redox) state of the environment. Large isotope fractionation (238U/235U and 53Cr/52Cr) occurs during reduction of U(VI) and Cr(VI) to U(IV) and Cr(III), respectively. Based on this framework, U and Cr isotopes are emerging powerful tools to constrain the redox state of environments ranging from Earth’s surface to its interior. Specifically, U and Cr isotopes have been used to indicate and quantify the reductive attenuation of groundwater contamination by Cr(VI) and U(VI). In addition, U and Cr isotope record in sedimentary rocks provide very important information regarding the redox state of Earth’s early history, which is tied to the earliest evolution of oxygenic photosynthesis in the Archean and macroscopic animal life in the Neoproterozoic.