Vanadium and its isotope composition of river water and seawater: Analytical improvement and implications for vanadium isotope fractionation

Abstract

Investigation of redox variations in recent and paleo-oceans has been of particular scientific interest to elucidate the rise and variations of the atmospheric oxygen level by analyses of isotopic signatures of redox-sensitive elements like Fe, Mo, and U. Vanadium is another redox-sensitive metal that has become the target of stable isotope research during the last decade. Research of the oceanic V cycle revealed a rather complex interplay of riverine V as a major V source to the oceans on one side with V deposition in sediments and at hydrothermal vents as major sinks on the other. The balance between these major V pools is sensitive to the ocean water oxygen level and chemistry. The current data set of stable V isotope signatures of seawater is still very small, but indicates already subtle variation of the V isotope signatures in the marine environment. However, the V isotopes of marine sediments and particularly the riverine V isotope composition of dissolved and particulate V, i.e. the major source of V in modern marine environments, has not been constrained at all so far.In this study, we present a new method for efficient V separation from seawater that allows multiple analyses of the V isotope composition of a single sample. To separate V from large amounts (volume ≥2L) of seawater samples, we employ the Bio-Rad® Chelex-100 resin and conventional cation and anion resins to yield a high V recovery of ≥90% from an UV-irradiated sample. Non-irradiated samples were marked by lower V recovery rates of ca. 75%, which was also observed in earlier studies. Further tests however revealed that even such reduced V yields do not incur significant V isotope fractionation within analytical uncertainty. Our δ51VAA value of +0.27‰ ±0.14 (2s.d., n=3) for the NASS-6 seawater reference solution perfectly matched earlier results. In addition, seawater collected in the Wadden Sea at the German North Sea coast is marked by a δ51VAA signature of around +0.02‰ ±0.19 (2s.d., n=17), which is slightly lower than those of the great oceans, and may be related to an influx of river water, bioactivity, or a tide-induced V mobilization.To characterize the V isotope composition of the major V source to the oceans, we determined for the first time V isotope signatures of 13 selected rivers (dissolved and particulate fractions of source water, tributary rivers, and the Yangtze River) in the Yangtze River Basin, China. A large variation of dissolved V (ca. 0.07 to 6.0μg/L) and particulate-bound V (ca. 0.03 to 17μg/L) was found for the sample suite. The obtained δ51VAA values of the dissolved V pool span a range of −0.76‰ (±0.18; 2s.d.) to −0.10‰ (±0.22, 2s.d.), whereas particulate-bound V extends to lower δ51V signatures between −2.13‰ (±0.30, 2s.d.) and −0.11‰ (±0.11, 2s.d.). Notably, dissolved V from the river sources and small tributaries scatters between ca. −0.4‰ to −0.7‰, and agrees well with the predicted average δ51VAA value of −0.6‰ ±0.3 for continental run-off. For the lower Yangtze River, however, the dissolved δ51VAA signatures increase from the Three-Gorges Dam towards the estuary from −0.76‰ to −0.10‰, suggesting V isotope fractionation due to adsorption to abundant particulate Fe oxides, but may also reflect an input of anthropogenic V. The low δ51VAA of particulate V largely follow this trend, and thus indicate ongoing V isotope fractionation during riverine V transport to the ocean.Our first results of stable V isotope investigation of river waters show that V isotope signatures can indeed carry their host rock signature, but are also sensitive to adsorption-driven fractionation in oxidized environments. The latter strongly depends, as predicted from earlier theoretical calculations, on the presence of particulate Fe-(oxyhydr)oxides and highlights gradual V isotope fractionation during riverine V transport to the ocean.