Vanadium isotopic fractionation during the formation of marine ferromanganese crusts and nodules

Abstract

Vanadium (V) isotopes can be significantly fractionated during the delivery, cycling, and burial to the ocean. The utilization of V isotopes might provide a useful geochemical tracer for the evolution of the oceans. This study provides the first detailed investigation of V isotopes in ferromanganese (Fe-Mn) crusts and nodules, which are widely distributed in the modern oxic ocean. Our results show a large variability of V isotope compositions in Fe-Mn crusts and nodules with a δ51V range from −0.89 to −1.65‰, much greater than current analytical uncertainty (±0.10‰, 2 SD). Therein, the most recent layers of hydrogenetic Fe-Mn crust and hydrogenous nodules have a narrower V isotope range (−0.89‰ to −1.25‰), with no correlation to their collection location or water depth. Thus, our results suggest a relatively homogeneous V isotope composition of marine hydrogenetic Fe-Mn crusts and nodules, with an average δ51V of −1.05 ± 0.16‰ (2 SD). The hydrogenetic Fe-Mn crusts and nodules are depleted in 51V compared with the recently reported seawater value (0.2 ± 0.15‰) by ∼1.2 ± 0.2‰ (2 SD). This can be explained by isotope fractionation during the adsorption of V onto Fe-Mn oxyhydroxides. The various δ51V in Fe-Mn nodules (−0.98‰ to −1.65‰) might be caused by diagenetic precipitation of V from pore fluids with lighter isotope composition compared to seawater, thus driving a negative V isotope shift recorded in Fe-Mn nodules with a diagenetic imprint. Additionally, a depth profiles across an Fe-Mn crusts reveals systematic changes in δ51V from −1.04 ± 0.13‰ (2 SD) in upper layers to 1.32 ± 0.06‰ (2 SD) in lower layers representing the relatively older deposition. The observed δ51V depth profiles might record the changes of V isotope composition of seawater. The proposed temporal variations of δ51V in seawater could be controlled by the isotope fractionation and fluxes of various V sources and sinks to the ocean that is likely related to the global redox state of the oceans. Another potential interpretation of this isotopic shift in the crusts could be related to the modification of the primary V isotope signature due to diagenetic remobilization and reorganization. However, there is no obvious evidence that definitively documents a diagenetic control or signal. This study highlights the burial of V with Fe-Mn oxyhydroxide as an important control on the V isotope composition of seawater, and the potential application of Fe-Mn crusts to track the temporal V isotope variations of seawater.