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Illuminating biosignatures with synchrotron light: stepping toward an unambiguous proxy for life

Tuesday, April 29, 2014

Post-impact hydrothermal systems are one of the most favourable environments in which to search for evidence of life beyond Earth. Interpreting in situ enigmatic features as indicators of biological activity is a notoriously difficult task. The goal of characterizing biosignatures is not only to identify attributes as uniquely produced by biological processes, but also to recognize these attributes as unambiguous indicators of life. Transition metals, such as Fe, play a significant role in microbial metabolism. Autotrophic microorganisms exploit redox disequilibrium gaining energy required for growth through cascades of oxidation-reduction reactions. These biological processes geochemically alter the microbial substrate resulting in a ‘metabolic signature’ with the potential to be preserved in the rock record.  Synchrotron near edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with scanning transmission X-ray microscopy (STXM) is a powerful tool to search for such microbially induced geochemical changes. Using putative trace fossils preserved in hydrothermally altered impact glass as a case study, I will explore the potential of NEXAFS spectroscopy to investigate and characterize putative biosignatures. The high spectral resolution of NEXAFS spectroscopy combined with the high spatial resolution of STXM can be used to speciate transition metals providing a spatially correlated map of oxido-reduction patterns and organically bonded carbon on a sub-micrometer scale. Gaining a better understanding of the biogeochemical processes that yield specific Fe and S redox patterns could lead to the identification of the first unambiguous proxy for life. Establishing transition metal speciation biosignatures in impactite substrates has profound implications for the recognition of biological processes both from early Earth and on other terrestrial planets.

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