Impact of estuaries on fluvial Cr input into the ocean: Perspectives from the Mobile Bay Estuary, Northern Gulf of Mexico

Abstract

The chromium (Cr) stable isotope system has been recently developed as a promising redox proxy to study Earth’s oxygenation history. However, an incomplete understanding of the global marine Cr isotope mass balance hinders its quantitative application. Specifically, whether estuary environments can alter fluvial Cr flux into the global ocean, together with its isotopic composition, is still debated. Here, we report the first systematic redox-dependent stable Cr isotope data for an oxic estuary (the Mobile Bay Estuary in the northern Gulf of Mexico). Our data suggest that the Cr(VI) supplied by the Mobile River is completely reduced to Cr(III) before salinity reaches 5. A Rayleigh distillation model calculates an isotope fractionation factor of −0.1‰ to −0.3‰ (isotopically light Cr(VI) preferentially reduced). Approximately 15–30% of fluvial Cr(III) is removed from solution in the mixing zone, and this process likely preferentially removes isotopically heavy Cr(III). Therefore, if the face values of the Cr concentration and δ53Cr of the Mobile River were used as input terms for the Gulf of Mexico, Cr flux would have been overestimated by a factor of 5–7 and its δ53Cr overestimated by 0.3–0.7‰. A literature survey suggests that seasonally variable fluvial Cr flux is lost from solution in the majority of estuaries studied thus far. Estuary loss of Cr and annual variation combined add an order of magnitude uncertainty to the global fluvial Cr flux estimated previously. How fluvial δ53Cr is affected in an estuary environment is difficult to generalize because of the competition between partial Cr(VI) reduction that removes light isotopes and Cr(III) scavenging that likely removes heavy isotopes, and such competition likely varies among estuaries.