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

当天然水变得缺乏氧气时，会产生溶解的硫化物，并通过微生物硫酸盐还原积累起来。硫化条件发生在古代海洋中，并导致海洋生物群的灭绝事件。氧化还原敏感的痕量金属，如钼(Mo)和Re(Re)，在含氧水和硫化物水中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演化的理解。现代海洋中也存在氧气枯竭现象，这可能是人类活动导致的富营养化的结果。这种情况已经在全球范围内观察到，主要是在沿海地区，并已成为导致大规模鱼类死亡的主要环境问题。因此，除了提供有关地球大气层中氧气的上升(大约24亿年前)如何以及何时促进生命发展的信息外，钼和稀土还有助于评估和防止现代海洋内缺氧区的发展。然而，为了充分利用钼和稀土作为缺氧条件的指示器，这些金属在硫化物水域中经历的化学转化必须得到很好的定义，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的支柱。对此，已经启动了一项双管齐下的研究。(1)利用我们最近开发的层析方法(反相离子对层析：RP-IPC)对天然硫化物水中的实际Mo和Re形态进行定量。研究人员的目标是(A)将他们目前的RP-IPC方法与电感耦合等离子体质谱(ICP-MS)结合起来，(B)利用RP-IPC-ICP-MS来定量测定绿湖(美国纽约)水柱中的硫代钼酸根和硫代吩酸根阴离子，以及(C)定量硫代钼酸盐和硫代吩甲酸根之间过渡过程中任何稳定的钼或Re同位素分馏。(2)考察了FeMoS立方烷团簇在正辛酸条件下最终埋藏Mo和Re中的作用。他们将(A)产生和分析包含一系列初始Fe、Mo、硫化物、离子强度和pH条件以及微量Re的溶液，并(B)使用X射线吸收精细结构(XAFS)光谱来表征在测试溶液中产生的任何FeMoS固体。这项研究的影响将广泛扩展。最明显的五个成果是：(1)支持地球科学界研究生物圈演化。(2)为金属硫酸盐形态定量和同位素分馏提供了一种新的方法。(3)在拟议研究的各个方面吸收来自代表性不足的少数族裔的几名本科生研究人员参与。(4)作为国际合作的一部分培养一名博士生。(5)在适当的会议上交流研究成果，并提交几篇科学文章来传播研究成果。

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.