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殊的机会，系统地调查约束不良的海洋V循环，包括河流，从而解锁V同位素作为一个潜在的强大的示踪剂的氧化还原和水化学变化。除了精确和准确的Visotope分析的仪器要求外，测量海洋和河流沃茨中极低的V浓度（<2 μg/L）对获得足够的V进行同位素测量提出了重大的实验室挑战。在项目的第一阶段，钒分离得到了显著改进，以便更快地从大量海水（2升）中提取足够的钒。强化测试表明，海水标准（δ 51 VAA =+0.30‰）和北海（δ 51 VAA =+0.03‰）的V同位素组成与最近公布的海洋同位素数据非常一致。然而，南极海水样品的δ 51 V值非常低，约为-1.9‰，表明海洋环境中存在非均匀的V特征。此外，还发现长江干流和支流溶解态V的δ 51 V同位素特征由小河流向大河流逐渐向高δ 51 V值方向演化。（ii）在长江这样的大河流中，δ 51 V值向河口不断增加，颗粒状铁氧化物对溶解态钒库的钒同位素组成有显著影响。有趣的是，这些长江流域沃茨中颗粒结合态V的δ 51 V值与铁锰结核的δ 51 V值重叠，表明它们具有相似的V同位素分馏过程。此外，黑海沉积物的δ ~（51）V值表明，从850 ~ 2000 m深度，δ ~（51）V值在-3.1 ‰ ~+0.3 ‰之间稳定增加，而从150 ~ 380 m（即向化学跃层方向），δ ~（51）V值增加约为-1.9 ‰ ~-1.1‰。在整个采样深度范围内，δ 238 U的变化范围仅为0.03 ‰ ~ 0.17‰，且与V同位素组成无关。潜在的V同位素记录的氧化还原过程，没有存档的U同位素。为了更好地了解钒同位素特征，在项目第二阶段，将调查其他大洋汇（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钒同位素。简单的实验将有助于更清楚地了解铁氧化物吸附和沉淀对钒同位素分馏的影响。最后，钒同位素，沿着钒物种分析（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，显示缺氧和缺氧环境之间的差异，甚至指示生物特征。

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.