The chromium isotopic composition of seawater and marine carbonates

Chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past. However, Cr is a trace metal in seawater and its low concentrations make isotopic measurements challenging. Here we report the first determinations of delta Cr-53 for seawater from open ocean (Argentine Basin) and coastal (Southampton Water) settings, using a double-spike technique. The total chromium concentration in seawater from Southampton Water is 1.85 nM, whereas the Cr content of Argentine Basin samples is 5.8-6.6 nM. The delta Cr-53 value of seawater from the Argentine Basin is 0.491-0.556 parts per thousand in intermediate and deep waters, and varies between 0.412 and 0.664 parts per thousand in surface waters (<150 m). The delta Cr-53 value of Southampton Water seawater is 1.505 parts per thousand, which may reflect in situ reduction of Cr(VI) to Cr(III). All of our seawater samples have higher delta Cr-53 than crustal and mantle silicates, and mass balance modelling demonstrates that river water must also be enriched in heavy Cr isotopes, indicating that Cr isotopes are fractionated during weathering and/or during transport to the oceans. We also show that the Cr isotopic composition of modern non-skeletal marine carbonates (0.640-0.745 parts per thousand) encompasses the range that we measure for Argentine Basin seawater. Thus, fractionation of Cr isotopes during precipitation of these marine carbonates is likely to be small (<0.2 parts per thousand), and they have the potential to provide a record of the Cr isotopic composition of ancient seawater. Phanerozoic carbonates are also characterised by heavy delta Cr-53 and a correlation between delta Cr-53 and Ce/Ce* suggests that the Cr and Ce cycles in the ocean are linked. (C) 2013 Elsevier B.V. All rights reserved.