nai

Project Progress

Ion microprobe analyses carried out in the Keck Center for Isotope Geochemistry
(T. Mark Harrison and Kevin D. McKeegan, Directors) continue to provide highest
quality data for studies of the early Earth and Mars. Building on a pilot study
aimed at finding the oldest terrestrial materials (Mojzsis et al., 2001), some
5,300 individual zircon crystals from the Jack Hills conglomerate in Western Australia
have now been dated. Two percent of these are 4 billion years old or older, thus
providing a source for geochemical evidence that can be used to constrain models
of the Earth’s earliest atmosphere and hydrosphere.

Collaborator James Farquhar’s discovery of substantial mass-independent fractionations of sulfur isotopes in rocks older than about 2.3 billion years (Farquhar et al., 2000) and his demonstration that these effects are produced in gas-phase reactions (Farquhar et al., 2001) implicate the atmosphere in sulfur cycling on the early Earth (Runnegar et al., submitted). Understanding these mass-independent isotopic effects requires knowledge of
a simpler and better-understood modern system. UCLA photochemist James Lyons has
developed a quantitative model for the behavior of mass-independently fractionated
oxygen in the lower atmosphere and the stratosphere (Lyons, 2001). This work will
lead to models and experiments aimed at a better understanding of the more complex
sulfur system.

The possibility that organic matter on the early Earth was made inorganically is being investigated experimentally in a cross-team collaboration with the Penn State team (Craig E. Manning, Tracey Herrera, Christopher H. House). They are also exploring the interaction of microbes with olivine and other mineral substrates. Preliminary results indicate that microbial activity greatly increases certain inorganic reactions by using reaction products for metabolic purposes.

In a remarkable example of the “emergent” properties of the NASA Astrobiology
Institute, involving both cross-team and cross-discipline collaborations, Victoria
J. Orphan and her coauthors provided stunning isotopic and phylogenetic evidence
for the anaerobic use of methane by highly organized microbial consortia composed
of sulfate-reducing bacteria and methane-consuming archaea (Orphan et al., 2001;
Zimmer, 2001). In a follow-up study (Orphan et al., 2002), they used the same
techniques to expose the complexity of microbial communities involved in methane
oxidation in an anoxic cold seep environment.