Vanadium isotope fractionation during hydrothermal sedimentation: Implications for the vanadium cycle in the oceans

Variations of V isotope ratios (δ51V) in sedimentary rocks have been proposed as a potential proxy for paleo-oceanic redox conditions, although the marine δ51V system is not fully constrained. Hydrothermal sediments are known to be a significant sink of V from the ocean. However, the V isotope fractionation associated with the accumulation into hydrothermal sediments from seawater and its influence on the marine isotopic budget of V are still unexplored. This work presents the first V isotope study of hydrothermal sediments specifically samples from the Trans-Atlantic Geotraverse (TAG) hydrothermal field and Pacific Ocean under influence of the hydrothermal plume arising from the southern East Pacific Rise (SEPR). The studied sediments show authigenic δ51V values ranging from −0.4 to −0.1‰, which is remarkably different from that of non-hydrothermal oxic sediments (−1.2 to −0.7‰) as well as open ocean seawater (+0.2‰). The V isotope composition of these hydrothermal sediments at TAG is likely controlled by the isotope fractionation during the rapid initial uptake of V from seawater into hydrothermal Fe oxyhydroxide particulates that precipitate when reducing hydrothermal fluids mix with oxic seawater. The sediments collected from the deep south Pacific Ocean west of SEPR show authigenic δ51V values overlapping with the range observed within the TAG field, despite their distal location extending to more than 4000 km from the buoyant portion of the SEPR hydrothermal plume. As has also been observed for Fe in particulates west of the SEPR, our results indicate the persistence of V within hydrothermal particulates (likely in the form of V-rich Fe oxyhydroxides) that settle to the sediments after long-range transport, while still preserving the original V isotope signature. Combination of this work with previous studies, allows us to establish a more robust mass balance model that assesses the effect of V removal by hydrothermal precipitation on the marine elemental and isotopic budget of V. Our new results suggest that the flux of V into the hydrothermal sink accounts for ∼13% of the global marine V input. This model also indicates that the V isotope composition of seawater could potentially be altered with variations in hydrothermal activity which is in addition to changes in the global marine redox conditions but likely on very different time scales. Thus, V isotope ratios have the potential to provide new insights into ancient ocean chemistry.