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

氧化还原敏感钒的稳定同位素提供了一个特别的机会，可以系统地研究包括河流在内的缺乏约束的海洋V循环，从而解锁V同位素作为氧化还原和水化学变化的潜在强大示踪剂。除了对精确和准确的同位素分析的仪器要求外，测量海洋和河流中极低V浓度（<2 μg/L）对获得足够的V进行同位素测量构成了主要的实验室挑战。在第一期项目中，为了更快地从大量海水（2L）中提取足够的V， V的分离得到了显著改善。结果表明，海水标准（δ51VAA=+0.30‰）和北海标准（δ51VAA=+0.03‰）的V同位素组成与近期公布的海洋同位素数据吻合良好。然而，南极海水样品的δ51V值非常低，约为-1.9‰，表明海洋环境中的V特征是非均匀的。此外，本文还发现，长江流域主支流溶解V组分的V同位素特征从小河流到大河流向高δ51V值演化，（ii）在长江这样的大河中，δ51V值向河口方向不断增大，（iii） V对颗粒铁氧化物的吸附会显著影响溶解V池的V同位素组成。有趣的是，这些长江流域水体中颗粒结合V的δ51V值与Fe-Mn结核的δ51V值重叠，表明相似的V同位素分馏过程。此外，黑海沉积物的第一δ51V值表明，在850 ~ ~ 2000 m深度，δ51V值从-3.1 ~ + 0.3‰稳步增加，而在150 ~ 380m（即趋化斜向）深度，δ51V值增加了约-1.9 ~ -1.1‰。在整个采样深度，δ238U特征仅在0.03 ~ 0.17‰范围内略有变化，且与V同位素组成也不耦合。V同位素可能记录了未被U同位素记录的氧化还原过程。为了更好地了解V同位素特征，在项目的第二阶段，将对其他海洋下沉（热液铁氧化物、碳酸盐、铁锰结核、还原沉积物）以及大量海水样品的V同位素进行研究。简单的实验将进一步阐明铁氧化物吸附和沉淀对V同位素的分馏作用。最后，V同位素，连同V物种分析（III-IV-V），或与其他氧化还原指标（如U）结合，可以详细了解早期海洋的氧化还原演化，显示缺氧和富氧环境之间的差异，甚至表明生物特征。

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.