Natural variations of the 51V/50V isotope composition: A new redox tracer?

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The stable isotopes of the redox sensitive vanadium offer a particular opportunity to investigate systematically the poorly constrained marine V cycle including rivers, and thus to unlock V isotopes as a potentially powerful tracer of redox and water chemistry variations. Inaddition to the instrumental requirement for precise and accurate Visotope analysis, the measurement of extremely low V concentrations (<2 μg/L) in ocean and river waters poses a major laboratory challenge in obtaining enough V for an isotope measurement. In the first project phase, the V separation was significantly improved in order to faster extract sufficient V from large amounts of seawater (2L). Intensive testing shows an excellent agreement of the V isotopecomposition of a seawater standard (δ51VAA=+0.30‰) and the North Sea (δ51VAA=+0.03‰) with recently published ocean isotope data. However, very low δ51V valuesof approximately -1.9‰ for an Antarctic seawater sample indicate heterogeneous V signatures in the marine environment. In addition, I showed that the V isotopic signatures of the dissolved V fraction in the main and tributary rivers of the Yangtze River Basin evolve toward to higher δ51V values from the smaller rivers to the large streams, (ii) within a large river like the Yangtze, the δ51V values increase continuously towards the estuary, and (iii) the V isotope composition of the dissolved V pool can be significantly influenced by V adsorption to particulate Fe oxides. Interestingly, the δ51V values of the particulate bound V in These Yangtze Basin river waters overlap those of Fe-Mn nodules, suggesting comparable V isotope fractionation processes. Additionally, the first δ51V values of sediments of the Black Sea indicate a steady increase of δ51V values from -3.1 to + 0.3‰ from a depth of 850 to ~2,000m, while from 150 to 380m (i.e. towards the chemocline) an increase of approximately -1.9 to -1.1‰ was observed. For the whole sampling depth, the δ238U signatures vary only slightly from 0.03 to 0.17‰, and are also decoupled from the V isotope composition. Potentially the V isotopes record redoxprocesses that were not archived by the U isotopes. In order to better understand the V isotope signatures, in the second project phase the V isotopes of the other oceanic sinks (hydrothermal Fe oxides, carbonates, Fe-Mn nodules, reduced sediments) as well as a large suite of seawater samples will be investigated. Simple Experiments will shed more light on V isotope fractionation by adsorption and precipitation by Fe oxides. Finally, V isotopes, along with V species analyses (III-IV-V), or in combination with other redox indicators such as U, may allow a detailed view of the redox evolution of the early oceans, showing differences between anoxic and euxinic environments, or even indicating biosignatures.