Chromium stable isotope geochemistry in the Mobile Bay Estuary

The chromium (Cr) isotope system is an important redox proxy for understanding Earth's oxygenation history. However, quantitative application of this proxy is hindered by a lack of clear understanding of the global Cr isotope mass balance. Estuarine environments potentially play important roles in influencing the global oceanic Cr mass balance because riverine Cr derived from continental weathering must first travel through estuaries before entering the open ocean. We collected water, suspended particulate matter, and sediment samples across a salinity gradient in the Mobile Bay Estuary located in the Northern Gulf of Mexico, with a purpose of evaluating whether Cr behaves conservatively in estuarine environments. A simple conservative mixing model between the fresh Mobile River water and Gulf of Mexico seawater alone was insufficient to explain the Cr concentration and isotope data, suggesting that Cr behaves non-conservatively in this estuary system. A model incorporating conservative mixing, partial Cr(VI) reduction and partial scavenging of the produced Cr(III) is developed to reproduce the observed data. Our data and modeling exercise suggest that up to 60% of Cr(VI) is reduced to Cr(III) in the Mobile Bay Estuary with a Cr isotope fractionation factor of −1.2‰, and 40% of the produced Cr(III) is removed from solution. Although these parameters may not be the sole solution to the system of equations involved, they are consistent with previous estimates. Future redox-specific Cr isotope data in estuarine environments are needed to place finer constraints on these parameters in estuaries, which is critical for estimating global marine Cr isotope mass balance using riverine Cr concentration and isotope data.