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2006 Annual Science Report

Indiana University, Bloomington Reporting  |  JUL 2005 – JUN 2006

Experimental Study of Radiolytic Oxidation of Pyrite: Implications for Mars-Relevant Crustal Processes

Project Summary

In subsurface environments, radiolysis can produce gradients of both electron acceptors and electron donors that are possible sources of metabolic energy [2]. Radiation-induced chemical reactions have particular significance in geologic environments where molecular oxygen derived from the atmosphere is a negligible input.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

Radiolytic dissociation of water produces a highly reactive combination of oxidizing (e.g., H2O2, OH radicals, and O2) and reducing (e.g., H atoms and H2) species [1]. In subsurface environments, radiolysis can produce gradients of both electron acceptors and electron donors that are possible sources of metabolic energy [2]. Radiation-induced chemical reactions have particular significance in geologic environments where molecular oxygen derived from the atmosphere is a negligible input. Results from geochemical studies of unconformity-related uranium deposits indicate that radiolysis is considerably under recognized as a naturally occurring source of chemical energy for biotic and abiotic reactions. In particular, radiolysis of water coupled to oxidation of sulfide minerals [3] or elemental sulfur [4] can produce gradients of partially to fully oxidized sulfur species that might be suitable for microbial metabolism.

In order to evaluate the efficiency of radiolytic sulfide oxidation in the production of sulfate gradients and the stable isotope signatures of sulfur products, L. Lefticariu performed a series of radiation experiments using Park City pyrite and DI water. Radiation experiments were carried out in collaboration with J. Laverne at Notre Dame using a 60Co gamma source at the Radiation Laboratory. The dose rate was of 11.3 krad/min (113 Gy/min), as determined by the Fricke dosimeter. Water used in these experiments was de-oxygenated to minimize competing reactions with O2. These water/pyrite mixtures were degassed and flame sealed in quartz tubes of 2 cm diameter and 10 cm length. The tubes were irradiated from 1 to 14 hours. After irradiation, samples were analyzed for gaseous, aqueous, and solid species produced during radiolysis.

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Molecular hydrogen was the dominant gas collected at the end of pyrite-water irradiation experiment. Molecular oxygen was more than an order of magnitude lower than the H2. The γ radiolysis of liquid water in closed systems leads to a low, constant H2 concentration due to back reactions [8]. The initial experiments showed that the radiation chemical yield of molecular hydrogen increases with increase in the total irradiation dose. For the same total irradiation dose, the yield of H2 increases with decrease in the initial amount of water. The dominant sulfur products collected during the initial experiments were aqueous sulfate and trace amounts of gaseous sulfur dioxide. Sulfate was the only aqueous sulfur species detected by ion chromatography methods in the final solution.

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In addition to studies of pyrite, we investigated whether an oxidation pathway exists with converts NH3 to NO3. An undergraduate at Princeton, R. Raymond, carried out these experiments in collaboration with D. Sigman at Princeton University and L. Lefticariu at Indiana University. Initial results verify oxidation of NH3 to NO3 by γ irradiation. The isotopic composition of the NO3 produced by irradiation is distinct from that of contaminating NO3.

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  • PROJECT INVESTIGATORS:
    Lisa Pratt Lisa Pratt
    Project Investigator
  • PROJECT MEMBERS:
    Edward Ripley
    Co-Investigator

    David Bish
    Collaborator

    Gary Hieftje
    Collaborator

    David Finkelstein
    Postdoc

    Liliana Lefticariu
    Postdoc

    Irene Arango
    Doctoral Student

  • RELATED OBJECTIVES:
    Objective 3.3
    Origins of energy transduction

    Objective 4.1
    Earth's early biosphere

    Objective 4.2
    Foundations of complex life

    Objective 5.1
    Environment-dependent, molecular evolution in microorganisms

    Objective 5.2
    Co-evolution of microbial communities

    Objective 5.3
    Biochemical adaptation to extreme environments

    Objective 6.1
    Environmental changes and the cycling of elements by the biota, communities, and ecosystems

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems