Collaborative Research: Unraveling Molybdenum and Rhenium speciation: Identifying the burial pathways of redox proxies in sulfidic settings

当天然水体缺氧时，就会产生溶解的硫化物，并通过微生物硫酸盐还原作用积累起来。硫化条件发生在古代海洋中，并导致了海洋生物群的灭绝事件。氧化还原敏感的微量金属，如钼（Mo）和铼（Re），在氧化水和硫化水之间表现出截然不同的化学行为，因此可以作为工具来增强我们对地球氧化历史和生命进化的理解。氧气消耗也发生在现代海洋中，这可能是人为引起的富营养化的结果。这种情况在全球都有，主要是在沿海地区，并已成为一个主要的环境问题，导致大量鱼类死亡。因此，除了提供关于地球大气中氧气的增加（大约- 24亿年前）如何以及何时促进了生命的发展的信息外，Mo和Re还有助于评估和防止现代海洋中缺氧区的发展。然而，为了充分利用Mo和Re作为缺氧条件的指标，必须很好地定义这些金属在硫化物水中所经历的化学转化，以及控制它们最终埋藏的机制。填补这些知识空白是这项研究的支柱，也是它将产生的未来工作。为此，开展了一项双管齐下的研究。(1)利用我们最近开发的色谱方法（反相离子对色谱法：RP-IPC）定量天然含硫水体中Mo和Re的实际形态。研究人员的目标是(a)将他们目前的RP-IPC方法与ICP-MS相结合，(b)利用RP-IPC-ICP-MS来量化绿湖（美国纽约）水柱中的单个硫钼酸盐和硫过石榴酸盐阴离子，这是一个以永久趋化cline为特征的湖泊，(c)量化硫钼酸盐和硫过石榴酸盐之间转变过程中任何稳定的Mo或Re同位素分异。(2)考察了富氧条件下，femo cubane团簇在Mo和Re最终埋藏中的作用。他们将(a)生产和分析由一系列初始铁、钼、硫化物、离子强度、pH条件以及微量Re组成的溶液，(b)使用x射线吸收精细结构（XAFS）光谱来表征测试溶液中产生的任何FeMoS固体。这项研究的影响将广泛延伸。最突出的五个成果是：(1)支持地球科学界研究生物圈演化。(2)为定量测定硫金属酸盐形态和同位素分馏提供了新的方法。(3)让几名来自代表性不足的少数民族的本科研究人员参与拟议研究的各个方面。(4)培养一名博士生，作为国际合作的一部分。(5)在适当的会议上交流研究成果，并提交几篇科学论文以传播研究成果。

When natural waters become devoid of oxygen, dissolved sulfide is produced and can accumulate via microbial sulfate reduction. Sulfidic conditions occurred within ancient oceans, and lead to extinction events in marine biota. Redox sensitive trace metals, like molybdenum (Mo) and rhenium (Re), display contrasting chemical behavior between oxygenated and sulfidic waters and therefore can be used as tools to enhance our understanding of the Earth's oxygenation history and the evolution of life. Oxygen depletion also occurs within modern oceans, probably as a result of anthropogenically-induced eutrophication. Such conditions have been observed globally, predominately in coastal areas, and have become a major environmental issue leading to massive fish kills. Thus, in addition to providing information on how and when the rise of oxygen (approximately ­2.4 billion years ago) in Earth's atmosphere facilitated the development of life, Mo and Re can contribute to assessing and preventing the development of oxygen-depleted zones within modern oceans. However, to fully exploit Mo and Re as indicators of oxygen-depleted conditions, the chemical transformations these metals undergo in sulfidic waters must be well defined, as well as the mechanisms governing their ultimate burial. Filling these gaps in knowledge is the backbone of this study as well as the future work it will spawn.In response, a two-pronged study has been initiated. (1) Quantify actual Mo and Re speciation in natural sulfidic waters utilizing our recently developed chromatographic method (reverse phase ion pair chromatography: RP-IPC). Investigators aim to (a) couple their present RP-IPC method with ICP-MS, (b) utilize RP-IPC-ICP-MS to quantify individual thiomolybdate and thioperrhenate anions in the water column of Green Lake (New York, USA), a lake characterized by a permanent chemocline and (c) quantify any stable Mo or Re isotopic fractionations during transitions among thiomolybdates and thioperrhenates. (2) Survey the role FeMoS cubane clusters play in the ultimate burial of Mo and Re under euxinic conditions. They will (a) produce and analyze solutions comprising an array of initial Fe, Mo, sulfide, ionic strength, and pH conditions as well as trace quantities of Re, and (b) use X-Ray Absorption Fine Structure (XAFS) spectroscopy to characterize any FeMoS solid produced in the test solutions. The impact of this study will extend widely. Five outcomes stand out most clearly: (1) Support the geoscientific community studying biospheric evolution. (2) Provide a new method for quantifying thiometallate speciation and isotopic fractionation. (3) Involve several undergraduate researchers from underrepresented minorities in all facets of the proposed research. (4) Train a Ph.D. student as a part of an international collaboration. (5) Communicate research results at appropriate conferences, as well as submit several scientific articles to disseminate findings.