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Project Progress

Our work on the chronology of Precambrian soil development was focused on Astrobiology Drilling Project cores from the Trendall Ridge and Steer Ridge, Australia. Analysis by the rubidium-strontium isotope system yielded ages in the range of 2.6-2.8 Ga, consistent with a regional metamorphic event, possibly accompanied by K-metasomatism. Isochrons from the more resistant samarium-neodymium isotope system yielded ages of ~3.5 Ga, which coincides with the likely time of soil formation. Thus these systems provide independent evidence of the antiquity (and later disturbance) of the soil systems, and they indicate the relative mobility of geochemical tracers commonly used to study the conditions (e.g., atmospheric composition) under which ancient soils develop.

Research on the pyrite “life cycle” is the primary focus of doctoral student Amy Wolfe. She has developed techniques for precipitation of stoichiometric Fe-oxyhydroxides which will be used in experiments on Fe oxide dissolution and pyrite precipitation in sedimentary environments. Part of this project will involve measuring Fe isotope fractionations associated with these processes, and Ms. Wolfe has been working on a new technique for purifying Fe for isotope analysis. These activities will be combined to identify signatures of biotic and abiotic processes in ancient sedimentary environments.

Work on dust fluxes in the Mars-like environment of the Atacama Desert, Chile, formed the basis of an honors thesis by undergraduate Justin Hynicka. In this study, Sr isotopes were used to track the source of dust in soil profiles developed in environments ranging from arid to hyperarid. We found a strong correlation between the sulfate and carbonate fractions in the soils, suggesting a similar source and pedogenic history. All profiles showed an change is isotope ratio over time, consistent with either a change in source or some redistribution of cations even under hyperarid conditions. Ongoing work on the silicate portion will help relate the source of salts to the source of silicate material.