NSF Award
1503596
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. <br/><br/>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.
