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USGS Mendenhall Postdoctoral Research Fellowship Program

17-30. Biogeochemistry of the Sulfur Cycle as It Relates to Mineral Resources in the Midcontinent Region

The Upper Midcontinent Region is a highly prospective area for future mineral resource development. Current priority exploration and development projects include copper-nickel-cobalt-platinum group element mafic deposits (Ripley, 2014), sediment-hosted copper-silver deposits (Bornhorst and Williams, 2013), and banded iron-formation deposits (Cannon and others, 2007). In all of these deposits, mineral resources are associated with sulfide minerals, either in the ores themselves or in waste rock that will be removed to access the ores. In such settings, a detailed understanding of the sulfur cycle is essential for effective mitigation of adverse environmental effects, and ultimately sustainable development. Acid-mine drainage is the result of the weathering of a sulfide mineral, such as pyrite in the presence of oxygen and water to produce sulfuric acid and iron precipitates. The resultant sulfur-rich drainage may experience a variety of fates including dilution, evaporative concentration, oxidation, or reduction. Oxidation may result in elevated dissolved sulfate concentrations. Subsequently, dissolved sulfate may undergo bacterial sulfate reduction downstream in the anoxic sediments of wetland areas. The resulting elevated dissolved sulfide concentrations have been shown to be toxic to wild rice (Fort and others, 2014), an important subsistence food source for Native Americans in the Upper Great Lakes region. In fact, the State of Minnesota recently considered adopting a more process-oriented approach to establishing appropriate environmental guidelines particularly to protect wild rice. Further, sulfate-reducing bacteria are known to be the primary methylators of mercury in aquatic ecosystems, and their activity can be enhanced by elevated dissolved sulfate concentrations. Methylmercury, a potent neurotoxin, is the mercury species that bioaccumulates in aquatic food webs and results in fish consumption advisories, which is a major concern for sport fishermen, commercial fishing, and Native Americans in the Upper Great Lakes region. Thus, the sulfur cycle, particularly as related to mining, can have far-reaching influences on many aspects of environmental health of a region.

The formation of acid-mine drainage and subsequent processes that act upon it downstream are largely restricted to the elements of sulfur (S), oxygen (O), hydrogen (H), iron (Fe), and carbon (C). Carbon, S, and Fe can all serve as an energy sources for various downstream biogeochemical reactions. Thus, traditional light stable isotope geochemical techniques for S, O, H, and C can yield unique insights into the diverse biogeochemical processes and resultant environmental impacts, which directly influences sustainable development of mineral resources. These insights include understanding the conditions for formation of acid drainage, fingerprinting multiple anthropogenic and natural sources of sulfur within a watershed, elucidating the importance of dilution and evaporation in watersheds to overall water chemistry, and constraining the rates of bacterial sulfate reduction as they relate to mercury methylation and sulfide toxicity to wild rice. All of these topics are essential to sound decision-making related to future potential mining, as well as to the remediation of existing abandoned mines.

We invite proposals to develop a comprehensive, unbiased understanding of the sulfur cycle related to proposed mining projects in the Upper Great Lakes region, using stable isotope geochemistry as a primary tool. The goal of our research is to ultimately provide the scientific foundation for meaningful discussion that would benefit all stakeholders about the potential benefits and environmental risks of these projects. We seek a candidate with: (1) experience in the field sampling of sediment and soil profiles, surface water, and groundwater, particularly for stable isotope analysis; (2) analytical skills in the area of low-temperature stable isotope geochemistry; (3) interpretive skills in modeling complex biogeochemical phenomena; and (4) proven ability to disseminate scientific results through oral presentations and peer-reviewed scientific publications.

Our USGS lab is currently fully capable of performing all the necessary stable isotope analyses. The lab has a Thermo Delta V Plus continuous-flow isotope ratio mass spectrometer. Peripheral devices supporting the mass spectrometer include a Costech Elemental Analyzer for S, C, and N isotope analysis, a Thermo Finnigan TC/EA for oxygen isotope analysis of sulfate solids, and a gas bench for C and O analysis of carbonates. The lab also has a Picarro L2120-I water analyzer for O and H isotope analysis of water samples. Supporting geochemical analyses of water, rock, and sediment are routinely obtained from contract labs. Supporting mineral characterization facilities (petrographic microscopes, scanning electron microscope, electron microprobe) also are available.

References

Bornhorst, T.J., and Williams, W.C., 2013, The Mesoproterozoic Copperwood sedimentary rock-hosted stratiform copper deposit, Upper Peninsula, Michigan: Economic Geology, v. 108, p. 1325-1346.

Cannon, W.F., LaBerge, G.L., Klasner, J.S., and Schulz, K.J., 2007, The Gogebic iron range—a sample of the northern margin of the Penokean fold and thrust belt: U.S. Geological Survey Professional Paper 1730, 44 p.

Fort, D.J., Mathis, M.B., Walker, R., Tuominen, L.K., Hansel, M., Hall, S., Richards, R., Grattan, S.R., and Anderson, K., 2014, Toxicity of sulfate and chloride to early life stages of wild rice (Zizania palustris): Environmnetal Toxicology and Chemistry, v. 33, p. 2802-2809.

Ripley, E.M., 2014, Ni-Cu-PGE mineralization in the Partridge River, South Kawishiwi, and Eagle intrusions: A review of contrasting styles of sulfide-rich occurrences in the Midcontinent rift system: Economic Geology, v. 109, p. 309-324.

Proposed Duty Station: Reston, VA

Areas of PhD: Geology, geochemistry, hydrology, chemistry, ecology (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Chemist, Research Hydrologist, Research Physical Scientist, Research Ecologist. (This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Robert Seal, (703) 648-6290, rseal@usgs.gov.

Human Resources Office Contact: Carrie Marez, cmarez@usgs.gov, 303 236-9555


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U.S. Department of the Interior, U.S. Geological Survey
URL: http://geology.usgs.gov/postdoc/opps/2019/17-30 Seal.htm
Direct inquiries to Cara A. Campbell at ccampbell@usgs.gov
Maintained by: Mendenhall Research Fellowship Program Web Team
Last modified: 11:07:40 Fri 08 Feb 2019
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