Evidence of Microbial Influence on the Barstow Formation Lacustrine Tufa Deposits

Microbially-mediated calcium carbonate precipitation of the middle Miocene Barstow Formation tufa deposits has previously been inferred, however, convincing evidence of biological influence has not been demonstrated.  In this study, a three-level scale of analysis was used in order to assess their potential biogenicity.  Samples were analyzed at a 1. macrostructural scale, revealing morphological features at a scale of tens of centimeters, 2. mesostructural scale showing internal textures at a centimeter scale (e.g., lamination, concentric growth, evidence for sediment binding beyond the angle of repose), and 3. a microstructural scale distinguishing microscopic features based on thin sections (e.g., evidence of microfossils, crystal precipitation, etc.).  Microscopic analyses reveal the presence of filamentous and round colonies of calcified microfossils. Filaments are aligned perpendicular to the growth direction of the tufa, indicative of a phototactic response. Episodes of fibrous calcite laminae are separated by clumps of round microbial colonies suggesting that microbial colonies served as sites for crystal nucleation and growth. Evidence is also recorded for the growth of calcite laminae beyond the angle of repose. This study confirms that microorganisms influenced the formation of these tufa precipitates and has important implications for the study of early life on Earth and the possibility for life elsewhere in our solar system.