MICROBIAL BIOSIGNATURE CAPTURE AND PRESERVATION IN SULFATE EVAPORITES: IMPLICATIONS FOR MARS EXPLORATION

Previous research on microbial fossilization has emphasized preservation in carbonates, cherts, phosphorites and shales, with only limited work on sulfates and other evaporites. Given the importance of sulfates on Mars as targets for future surface robotic missions, it seemed prudent to undertake a detailed study of microbial biosignature capture and preservation in sulfates, with a particular emphasis on the retention of biosignatures during diagenesis. Primary sulfate evaporites may be classified into three basic subfacies: Water column-nucleated, Bottom-nucleated and Displacive Growth (see Warner 1999). A preliminary study was conducted to evaluate the potential for biosignature capture in each of these three primary sulfate subfacies. Applying a combination of mineralogical (X-ray power diffraction, SEM, petrographic thin section-fluorescence microscopy, plus laser Raman) and elemental analysis (total carbon and nitrogen, total organic carbon and d13C) methods, sulfate samples from ~25 localities (worldwide) were screened for evidence of morphological and organic biosignatures. While morphological biosignatures were found to be rare, autofluorescence of in situ organics in thin sections allowed the targeting of samples for more specific biogeochemical analyses. Sulfate evaporites are currently of great interest for the future astrobiological exploration of Mars, having been detected both from orbit and on the Martian surface. Our results suggest that terrestrial sulfates from all three primary evaporite subfacies retain low abundances of entrapped carbonaceous organic materials (0.03 – 0.96 wt% C_org and 0 - .091wt% N) within mineral and/or sediment matrixes, and at abundances that are detectable by standard biogeochemical methods. Associated d13C values (-25.54 to +2.20) support a biological origin for the carbon. However, our study also revealed that mineral fluorescence in sulfates may present a major obstacle for organics detection using remote spectral methods, such as laser Raman. These results hold important implications for the development of future payloads for the exopaleontological exploration of Mars.