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

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 delta Cr-53 in the Arctic Ocean with depth and water-mass source (Pacific vs. Atlantic). The highest delta Cr-53 values occur in Pacific-sourced waters, which also have the lowest Cr concentration. Chromium concentration and delta Cr-53 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 (epsilon) associated with Cr cycling in seawater is estimated to be -0.80 +/- 0.03 parts per thousand (2 sigma). Samples from the sea-ice affected Surface Mixed Layer of the Arctic Ocean (similar to 10 m depth) deviate from the general trend, and samples proximal to rivers illustrate geographic variation in delta Cr-53 values for continental runoff, but prompt loss of this signature away from the source. Published by Elsevier B.V.