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

Abstract

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.