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2008 Annual Science Report

Indiana University, Bloomington Reporting  |  JUL 2007 – JUN 2008

Radiolytic Oxidation of Sulfide Minerals as a Source of Sulfate and Hydrogen to Sustain Microbial Metabolism

Project Summary

Microbial ecosystems have been discovered in crustal environments up to 2.8 kilometers below the surface of Earth. Life in this extreme environment apapears to be sustained by high concentrations of dissolved sulfate and hydrogen. Splitting of water molecules by radiation from uranium can produce oxidation gradients that result in simple ionic products usable for maintenance and growth of microbial organisms. A set of experiments exposing water and common sulfide minerals to radiation in a laboratory reactor were conducted to test this hypothesis.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

Lithoautotrophic microbial ecosystems have been documented in crustal environments up to 2.8 kilometers below the surface of Earth reportedly sustained by high concentrations of dissolved sulfate and hydrogen, however the origin of these metabolic resources remains enigmatic. Radiolysis of water coupled to oxidation of sulfide minerals in uranium-bearing rocks could produce gradients of hydrogen and partially to fully oxidized sulfur species in suitable proportions for maintenance and growth of microbial organisms. A set of experiments exposing deoxygenated water and pyrite mixtures to gamma radiation were previously conducted to test this hypothesis. Molecular hydrogen was found to be the dominant gaseous species (highest yields with high water to pyrite ratio experiments), with molecular oxygen two orders of magnitude lower than hydrogen. Aqueous sulfate and elemental sulfur (S0) were the only sulfur bearing products recovered from these radiolysis experiments and their sulfur isotopic compositions measured. Additional quantitative understanding of reactions resulting from radiation of water with other sulfide minerals is needed to recognize and predict products of radiolysis on Earth and other rocky planets.

A new set of radiolysis experiments will soon be carried out using the mineral pyrrhotite and deoxygenated water in varying ratios. X-ray diffraction was used to characterize a pyrrhotite sample from a massive sulfide ore deposit in Labrador, Canada. The sample is predominantly composed of pyrrhotite (78%) and troilite (18%) with about 4% magnetite (Fig. 1). This pyrrhotite/troilite sample is appropriate as an analog for common sulfide minerals in basalts on Earth and andesitic basalts inferred on Mars. Quartz-glass reaction vessels have been constructed for these experiments to recover gaseous, liquid, and solid products for identification and isotopic measurement. Specifically the vessels have been designed to capture any hydrogen gas product for isotopic measurement. Additionally, determining the ratio of reduced (Fe2+) to oxidized (Fe3+) iron in solution will be critical for understanding the role of iron species as oxidants in sulfate production and reactants in back reactions that limit the formation of iron oxides. The Fe2+/ Fe3+ratio will be determined using ion chromatography and a visual wavelength detector.

Through further assessment of reactions resulting from radiolysis, isotopic fingerprints for sulfur, oxygen, and hydrogen can be identified for a fundamental geochemical process that is significantly understudied as a source of energy for life. Radiolysis of water coupled to oxidation of metallic sulfides could be a significant source of reduced and oxidized sulfur and iron species in crustal environments of rocky planetary bodies with levels of radioactivity less than half of the average value on Earth.

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    Lisa Pratt Lisa Pratt
    Project Investigator
    Edward Ripley Edward Ripley
    Project Investigator
    Seth Young
    Project Investigator
    Liliana Lefticariu

    Objective 1.1
    Models of formation and evolution of habitable planets

    Objective 2.1
    Mars exploration

    Objective 3.3
    Origins of energy transduction

    Objective 4.1
    Earth's early biosphere

    Objective 5.1
    Environment-dependent, molecular evolution in microorganisms

    Objective 5.3
    Biochemical adaptation to extreme environments

    Objective 6.2
    Adaptation and evolution of life beyond Earth