Keywords:tellurium, isotope fractionation, Fe-Mn oxides, ocean environment
Marine ferromanganese crusts (Fe-Mn crusts) are Fe-Mn oxide deposits widely distributed on the seafloor, and are important materials for (paleo)oceanographic studies (e.g. Hein et al., 2014). Tellurium (Te) is one of the highly enriched elements in the Fe-Mn crusts by more than ten thousand times relative to that in the continental crusts (Hein et al., 2010). Variation of concentration and isotopic compositions of Te in the Fe-Mn crusts could be a potential indicator for changes in redox condition of seawater, because of the existence of two valences of Te in ocean environments (+4 and +6; Lee and Edmond, 1985). In this study, we performed the measurement for concentration and isotopic compositions of Te in the surface layer of the Fe-Mn crusts, which directly contacted with ambient seawater, to investigate the relationship between ambient water environments and the compositions of Fe-Mn oxide deposits, such as the concentration and isotopic compositions. The Fe-Mn crust samples were collected from two seamounts located in the Northwest Pacific, which possess different depth profile of the dissolved oxygen concentration in seawater. Furthermore, laboratory experiments were performed to investigate the isotope fractionation associated with Te adsorption on ferromanganese oxides.
The results show that the correlation between the Te concentration and isotopic compositions in the surface layer of the Fe-Mn crusts changes as the water depth increases. With increasing depth, the Te concentration decreases more drastically at shallower water depth than at deeper water depth, and changes in isotopic fractionation were also observed at similar water depths. Moreover, between two seamounts with different depth profile of dissolved oxygen in seawater, the water depth where the concentration gradient of Te in the Fe-Mn crusts changes is different. For laboratory experiments, it is revealed that when tetravalent Te is adsorbed on iron oxide, isotope composition of the liquid phase becomes lighter, and when hexavalent Te adsorbs it becomes heavier. These results suggest that Te isotope fractionation in ferromanganese oxide deposits in the ocean may correspond to the valence of tellurium in the ocean.