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

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 >= 2 L) of seawater samples, we employ the Bio-Rad (R) 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 delta V-51(AA) value of +0.27 parts per thousand +/- 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 delta V-51(AA) signature of around +0.02 parts per thousand +/- 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 6.0 mu g/L) and particulate-bound V (ca. 0.03 to 17 mu g/L) was found for the sample suite. The obtained delta(51)V(AA )values of the dissolved V pool span a range of -0.76 parts per thousand (+/- 0.18; 2s.d.) to -0.10 parts per thousand ( +/- 0.22, 2s.d.), whereas particulate-bound V extends to lower 6 51 V signatures between -2.13 parts per thousand ( +/- 0.30, 2s.d.) and -0.11 parts per thousand ( +/- 0.11, 2s.d.). Notably, dissolved V from the river sources and small tributaries scatters between ca. - 0.4 parts per thousand to - 0.7 parts per thousand, and agrees well with the predicted average delta(51)VAA value of - 0.6 parts per thousand +/- 0.3 for continental run-off. For the lower Yangtze River, however, the dissolved delta(51)VAA signatures increase from the Three-Gorges Dam towards the estuary from - 0.76 parts per thousand to - 0.10 parts per thousand, suggesting V isotope fractionation due to adsorption to abundant particulate Fe oxides, but may also reflect an input of anthropogenic V. The low delta(51)VAA 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.