The chromium isotope fractionation factor in seawater

Abstract

The chromium (Cr) stable isotope system is an increasingly used redox proxy for studying the oxygenation history of Earth's ocean-atmosphere system, but understanding of Cr isotope fractionation in modern seawater is still nascent. A linear relationship has been found between the isotope composition and the logarithmic concentration of total dissolved Cr in the global ocean. Such relationship was used to extract an ‘effective’ Cr isotope fractionation factor of ca. 0.7‰ associated with the removal of Cr from seawater. However, there is a statistically significant proportion of data that deviate from the global linear array, and the proportion of data plotting below the array is higher than that plotting above the array. Such scattering and skewing patterns are present even after excluding data that may have been caused by freshwater influences and potential inconsistency in methods used to purify Cr from seawater. A simple numerical exercise was conducted to demonstrate that the observed data scattering and skewing could be explained by parallel removal pathways with ‘intrinsic’ Cr isotope fractionation factors ranging from −0.8‰ to −1.6‰ combined with the presence of variable amount of dissolved Cr(III) in seawater. These results based on global seawater Cr isotope data are consistent with previous conclusions based on individual oceanographic sites.