Chromium isotopes in an acidic fluvial system: Implications for modern and ancient Cr isotope records

Abstract

Chromium is an important redox-active element, with significant Cr isotope fractionations driven by redox transformations in the natural environment and quantified through laboratory experiments. Recent work has demonstrated that non-redox-driven transformations can also produce small variations in Cr isotopes, but the role of these processes in controlling environmental Cr isotope variations remains unclear. The highly acidic but biologically diverse acid rock drainage system in Río Tinto, Spain (and affiliated rivers) provides a unique opportunity to examine how non-redox-related processes can substantially affect the isotopic composition of Cr in a natural system. Our results suggest that under conditions where Cr redox transformations are obviated, Cr cycling is largely controlled by formation of Fe-oxyhydroxides and Fe-hydroxysulfates, Cr adsorption, and Cr speciation, with little apparent Cr isotope fractionation between source materials, waters, and sediments. Moderate Cr isotope shifts, however, may be the result of non-redox processes, including adsorption to organic ligands and potentially biological uptake. These results add to a growing body of evidence for the importance of non-redox transformations in contributing to environmental Cr isotope variations, with implications for the use of chromium isotopes in studies of sedimentary Cr isotope records. In particular, this study is a validation of the potential for sulfide weathering in the Proterozoic to deliver relatively unfractionated Cr isotopes to the ocean, despite rising and sustained high oxygen levels.