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Project Progress

Radiolysis of water can accelerate water/rock interaction through production of radicals (e.g., hydrogen, hydroperoxyl, hydroxyl), ions (e.g., superoxide, protons, hydroxide), and reactive molecules (e.g., hydrogen, hydrogen peroxide, oxygen). Radiolytic oxidation can be observed in modern groundwater associated with uranium ore bodies and can be inferred for ancient groundwater based on the composition and isotopic signature of minerals. Prior to development of an oxygen-rich atmosphere on Earth, radiolytically generated oxidants could have reacted with pyrite and provided local plumes of partially to fully oxidized sulfur species suitable for microbial metabolism.

We evaluated sulfur isotope effects associated with reactions between pyrite and radiolytic oxidants using a series of sealed-quartz-tube experiments run with 60 mg of acid-cleaned pyrite, 10 milliliters of deoxygenated water, and concentrations of hydrogen peroxide (H₂O₂ ) at millimolar levels (Figure 1). Experiments ranged in temperature from 4 to 150 degrees Celsius with durations from 1 to 10 days.

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In initial experiments, primary oxidation products were dissolved sulfate, elemental sulfur, iron sulfate minerals, and iron oxyhydroxide minerals. X-ray diffraction patterns and images from scanning electron microscopy reveal anhedral to subhedral hydrated iron sulfates in globular clusters of about 10-30 micrometers in diameter forming on pyrite surfaces (Figure 2). Sulfur isotopic compositions remained unchanged for pyrite but showed distinct enrichment of ³⁴S in produced sulfate and elemental sulphur. The isotopic difference between sulfate and pyrite was 0.5-1 permil and between elemental sulfur and pyrite was 1-2 permil. Our results indicate that pyrite oxidation by H₂O₂ induces greater fractionation that has been recognized in previous studies. Although difference in isotopic composition for produced sulfate and elemental sulfur compared to starting pyrite are not large but compensating depletion of ³⁴S in undetected products could be substantial if proportional yields are small. Preliminary isotope results from high-temperature experiments indicate that the missing ³⁴S-depleted fraction might be held in iron sulfate and/or iron oxyhydroxides. Similar fractionation in ancient terrestrial samples or in extraterrestrial samples could be mistaken as evidence for microbial activity.

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Highlights:

• 
  Recently published paper by Lin and others (Geochemistry Geophysics, and Geosystems, 2005) presents an exciting and controversial hypothesis for the crucial of radiolytic hydrogen production in sustaining deep-subsurface microbial communities on Earth and, potentially, on other planets.
• Laboratory experiments simulating radiolysis in the deep subsurface yield distinctive non-biogenic isotope signatures for sulfate and elemental sulfur produced by oxidation of pyrite.
• Radiolysis of groundwater in uranium-rich deposits can provide oxidized and reduced chemical species suitable for sustaining diverse communities of microbes.