Chromium isotope systematics in the Connecticut River

Abstract

Limited constraints on Cr isotope fractionation during weathering and river transport is currently a gap in our understanding of the chromium (Cr) isotope system, which is an emerging proxy in environmental and paleoenvironmental studies. Here, we investigate Cr mobility and isotope fractionation from the temperate Connecticut River, USA, including Cr concentrations and isotopic compositions of river water, suspended particles, riverbed sediments, and weathering profiles. The δ53Cr values of the Connecticut River water range from −0.17‰ to +0.92‰, which are similar to or higher than the weathered rocks in the catchment (−0.08‰ to −0.29‰). We find seasonal variations in dissolved δ53Cr values in some but not all tributaries, suggesting that dissolved δ53Cr is not a simple function of seasonality but may also be influenced by sub-catchment heterogeneity in lithology. In contrast to dissolved Cr, we found consistent seasonal difference in suspended Cr concentration and δ53Cr. Suspended δ53Cr is 0.1‰ higher than the unfractionated BSE in the fall (0.01–0.13‰), but indistinguishable from the BSE in the spring (−0.11‰ to 0.00‰). The suspended Cr concentration is also lower in the spring, and with higher Al-Mn-Fe concentrations. The lower suspended Cr concentration and δ53Cr in spring may be linked to increased silicate and oxide load with depleted Cr due to stronger hydrological flux. Building from our dataset, there is not a consistent correlation with climate zones in a compilation of δ53Cr and Cr concentration data from river water and weathering profiles, suggesting that climate is not a dominating factor controlling Cr isotopic behavior during weathering, suggesting that other factors (e.g., local catchment conditions and dissolved organic matter) may also be responsible for the observed river water δ53Cr variability.