TEXTURAL CLUES AND MICROBIAL PROCESSES IN ASSOCIATION WITH PRECIPITATION OF SILICEOUS SINTER: CASTLE GEYSER, YELLOWSTONE NATIONAL PARK

Recognizing biotic signatures preserved in rock is central to understanding the origins of life, reconstruction of geologic history and extra-planetary exploration. Hydrothermal systems are ideal localities to enhance understanding of the interplay of microbial communities and mineral precipitation as: (1) Hydrothermal systems are consistent with the geologic interpretation of Archean environments. (2) The first organisms were almost certainly chemosynthetic, thermophilic microbes. (3) Hydrothermal processes are linked to planetary formation and evolution and are likely to have existed on other planets. (4) Meteorites show high temperature, aqueous alteration of minerals and contain biologically related amino acids.

Precisely how microbes affect the precipitation of silica is speculative. In general, bacteria are not recognized for building silica tests as diatoms and radiolarians are known to do. In contrast, metabolic processes influence the microenvironment and may indirectly cause precipitation in response to changes in pH or other factors. In the least, microbes supply nucleation sites for mineral precipitation and as a result become fossilized in the process. Recognition of fossilization created textures and fabrics in modern sinters provides a foundation for interpretation of ancient sinters. Furthermore, biofacies models provide a structure for targeting biological exploration of ancient systems.

Research of Castle Geyser sinters dated at 928 +/- 40BP focused on four facies: pool margin, vent, proximal vent, and discharge channel. SEM, EDS, optical petrography and fluorescence microscopy analysis of samples confirms the presence of microbe forms (filaments, rods, spores and cocci) and related textures (microbe/algal mats, micro-stromatolites, and “bacterial shrubs,”) in all facies studied. Although the frequency of microbial remnants increased with greater distance from the vent, there was evidence of carbon and microbial fabrics even within the interior of the geyser cone. The presence of both biotic and abiotic structures within the vent cone may reflect cyclic geyser activity and suggests that the geyser activity may need consideration when modeling hydrothermal systems.