DETECTION OF TEXTURAL, GEOCHEMICAL, AND MOLECULAR BIOMARKERS IN VOLCANIC ROCKS AS A PROXY FOR STUDIES OF EXTRATERRESTRIAL MATERIAL

We discuss the use of petrological, geochemical, and microbiological methods for the detection of microbial activity within volcanic rocks and glass from a variety of terrestrial settings, including modern marine basalts and volcaniclastic rocks, ophiolitic terranes, and desert basalts. By using this novel combination of methods we are able to convincingly demonstrate the presence of microbial activity. These techniques could be easily applied to samples returned from Mars and other extraterrestrial bodies where liquid water and conditions suitable for life may have existed.

Two textural types of microbial alteration in volcanic glass are commonly observed: tubular and granular. Tubular textures are characterized by micron-scale, tubular to vermicular, channel-like features and branching bodies extending into fresh glass. Granular textures appear as solid bands, semicircles or irregular patches of individual and/or coalesced spherical bodies with irregular protrusions into fresh glass. We have conducted detailed SEM imaging of these features in thin sections and on grain mounts of freshly exposed surfaces using iridium as the coating material. Iridium provides an exceptionally thin, stable, and conductive film that allows intricate structures to be resolved, particularly within the channels. We have imaged delicate filament-like structures and material resembling desiccated biofilm that might otherwise be obscured by thicker coatings. X-ray element maps collected by electron microprobe show elevated levels of C, N, P, and K associated with the microbial alteration features. X-ray maps also indicate the presence of elevated levels of sulfur associated with some microbial alteration features, possibly resulting from reduction of sulfate by microbes. Bulk-rock carbon isotope ratios of carbonates in samples of microbially altered volcanic glass are commonly variably depleted by as much as -20 per mil, suggesting biologic fractionation. We have also treated samples with various nucleic acid stains that specifically bind to DNA and RNA. Our technique combines commonly accepted nucleic acid staining techniques with laser scanning confocal microscopy to produce images of exceptional quality that provide spatial information on the presence of DNA/RNA associated with areas of suspected microbial alteration.