Marine ferromanganese oxide: A potentially important sink of light chromium isotopes?

Marine ferromanganese oxides are widely distributed in oxic marine sediments and are a sink for a range of metal elements derived from seawater. Their potential as a sink of chromium (Cr) isotopes has yet to be investigated and may have important implications for our understanding of the Cr-isotope system. In this study, we reported Cr-isotope data of modern oceanic Fe-Mn crusts for the first time. The δ53Cr values of the surface scrapings of the Fe-Mn crusts from the central North Pacific seamounts, range from −0.85 to −0.15‰, with an average of −0.42 ± 0.34‰ (2σ, n = 11), which are lower than published data from Pacific seawater (0.53-1.43‰ with an average of 0.79‰). These results reveal preferential removal of light Cr isotopes from seawater into Fe-Mn crusts, with an isotopic fractionation (△53Cr = δ53CrFe-Mn crust − δ53Crseawater) estimated from −2.3 to −0.7‰, averaging at ca. −1.2‰. We suggest that such significant negative isotopic fractionation may have been reached through adsorption of dissolved Cr (VI) oxyanions in the oxic water column onto the surface of Mn-oxide phases during the precipitation of Fe-Mn crusts. However, our modelling results imply that the deposition of Mn-oxide sediments would make insignificant contribution to elevation of the global seawater average δ53Cr value.