Global variability of chromium isotopes in seawater demonstrated by Pacific, Atlantic, and Arctic Ocean samples

Abstract Seawater chromium (Cr) isotope and concentration data are presented from multiple sites in the Arctic Ocean, and three locations in the Atlantic and Pacific Oceans. A 2400-m profile illustrates the heterogeneity of δ 53 Cr in the Arctic Ocean with depth and water-mass source (Pacific vs. Atlantic). The highest δ 53 Cr values occur in Pacific-sourced waters, which also have the lowest Cr concentration. Chromium concentration and δ 53 Cr data from these locations, in conjunction with published data for the South Atlantic Ocean, yield a simple logarithmic function, as predicted by Rayleigh fractionation in a closed system. The observed Cr isotope signature is hypothesized to arise from fractionation during the reduction of Cr(VI) in surface waters and oxygen minimum zones, scavenging of isotopically light Cr(III) to deeper water and sediment, and subsequent release of this seawater-derived Cr(III) back into seawater, either as organic complexes with Cr(III) or after oxidation to Cr(VI). The isotopic fractionation factor ( e ) associated with Cr cycling in seawater is estimated to be − 0.80 ± 0.03 ‰ ( 2 σ ) . Samples from the sea-ice affected Surface Mixed Layer of the Arctic Ocean (∼10 m depth) deviate from the general trend, and samples proximal to rivers illustrate geographic variation in δ 53 Cr values for continental runoff, but prompt loss of this signature away from the source.