2007 Annual Science Report
Indiana University, Bloomington Reporting | JUL 2006 – JUN 2007
Experimental Study of Radiolytic Oxidation of Pyrite as a Source of Sulfate and Hydrogen to Sustain Microbial Metabolism
Dissolved sulfate and hydrogen sustain lithotrophic ecosystems in crustal environments up to 2.8 kilometers below the surface of Earth but the origin of these metabolic resources is enigmatic. It has been hypothesized that 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. This reaction pathway is important in oxygen-limited environments, such as deep subsurface, where molecular oxygen, the main oxidant at earth’s surface is negligible in input. Unusual geochemical signatures such as wide ranges in the sulfur isotope values of pyrite and of sulfate in groundwater associated with roll-front-type uranium deposits are probably related to a radiolytic process of sulfide oxidation, followed by disproportionation of intermediate sulfur species. No experimental or field studies, however, have produced a quantitative understanding of the interaction of radiated water with sulfide minerals. Understanding these types of secondary radiation reactions is crucial in predicting the production of both reduced and oxidizing species in deep crustal environments.
To quantify the products of radiolytic pyrite oxidation, we performed a series of sealed tube experiments in which anaerobic pyrite-water mixtures were exposed to gamma radiation. Gaseous, liquid, and solid products have been recovered, identified, and measured for their isotopic composition. Molecular hydrogen is the main gaseous species recovered in our experiments. Yields of molecular oxygen, a likely gaseous product of water radiolysis, are two orders of magnitude lower than molecular hydrogen. No gaseous sulfur species were detected in these experiments.
Aqueous sulfate was the sole sulfur product identified in solution. Concentrations of aqueous sulfate were measured in solution after pyrite-water mixtures were exposed to different radiation doses, and a linear correlation between the yields of aqueous sulfate and logarithmic increase in radiation dose was observed. Elemental sulfur (S0), polysulfides (FeSn), and metal deficient sulfide (Fe1-xS2), known as S0-like or sulfur-rich species were recovered by sequential extraction using organic solvents. Enrichment in sulfur on the surface of reacted pyrite is inferred to be an early step in a series of reactions in which sulfur-bond disulfide is stepwise oxidized to sulfate.
Radiolysis of water is likely to be widespread in continental crust on Earth and other rocky planets with a hydrated lithosphere and parts-per-million or higher levels of long-lived radionuclides. Reactions driven by radiolysis are significantly underestimated as a source of energy for life.
PROJECT INVESTIGATORS:Lisa Pratt
PROJECT MEMBERS:Edward Ripley
RELATED OBJECTIVES:Objective 2.1
Outer Solar System exploration
Sources of prebiotic materials and catalysts
Origins of energy transduction
Biochemical adaptation to extreme environments
Environmental changes and the cycling of elements by the biota, communities, and ecosystems
Adaptation and evolution of life beyond Earth