Global cycling of vanadium isotopes from multiple ocean water masses and a restricted euxinic basin

Abstract

To date, there have been only six vanadium isotope data (δ51V) points for open ocean seawater samples from four locations that lack vertical or spatial constraints. To provide more robust constraints on global seawater δ51V signatures and explore the potential spatial deviations due to local/regional biogeochemical processes, we measured the V concentrations ([V]) and δ51V values of seawater samples collected in the South Atlantic Ocean (GEOTRACES 40°S transect) and in the restricted euxinic Black Sea (MedBlack GEOTRACES cruises in 2013, R/V Pelagia cruise 64PE373; “euxinic” indicates free hydrogen sulfides in the water column). The South Atlantic sampling profiles intersect surface (∼5 m) to bottom waters (∼4500 m), which cover the major global ocean water masses that participate in large-scale ocean circulations. The two seawater samples in the Black Sea were collected at 100 and 150 m, across the anoxic-to-euxinic chemocline where dissolved V is proposed to be removed from the water column by sinking particulates. The results reveal relatively constant seawater V concentrations in deep water masses below the euphotic zone (>100 m) and a minor [V] depletion of ∼6 % observed in the surface seawater samples (<100 m, within the euphotic zone) in the South Atlantic Ocean. The depletion is likely due to biological processes but is also modified by seawater mixing processes. The deep ocean water masses, except for the Antarctic Bottom Water, present relatively constant δ51V values averaged at 0.27 ‰ ± 0.14 ‰ (2SD). In contrast, the Antarctic Bottom Water, which forms from the Antarctic continental margin, presents a relatively lighter seawater δ51V value at 0.03 ‰, which may result from the local influence of the weathering inputs from the Antarctica continent but also requires further verification. Meanwhile, the surface seawater samples in the euphotic zone (<100 m) present heavier values averaged at 0.42 ‰ ± 0.08 ‰ (2SD) along with the minor depletion of seawater [V] relative to deeper water masses, which suggests a preferential uptake of 50V during biological processes. The two Black Sea seawater samples show relatively low [V] values of around 16 nmol/kg in the shallow water columns and lighter seawater δ51V values at 0.17 ‰ and −0.07 ‰, which should reflect a combination of the regional hydrological cycle and biogeochemical processes. This work now confirms the conservative nature of V isotopes in large-scale deep ocean circulations and provides a more robust framework for interpreting the marine sedimentary δ51V variations. Furthermore, the distinguishable deviations of seawater δ51V values due to the biological processes in the euphotic zone or weathering inputs in severely restricted basins analogous to the Black Sea and/or in near-coast environments also propose careful interpretation of ancient marine sedimentary δ51V values that were documented under similar depositional environments.