THERMOPHILIC MICROBIAL COMMUNITIES PRESERVED IN SILICEOUS SINTER DEPOSITS AT MICKEY HOT SPRINGS, OREGON

Investigation of geothermal systems worldwide has shown that the combination of hydrothermal fluids supersaturated with silica and the presence of thermophilic microbial communities result in the preservation of biosignatures such as biofabrics and morphological microfossils. The biosignatures can be used to determine the paleoenvironment of an old sinter horizon when there is a correlation between distinct microbial communities (biofacies) and the microbially influenced sinter deposits (lithofacies). This study aims to determine whether evidence of the paleoenvironment of thermal spring ecosystems are preserved in the sinter deposits of Mickey Hot Springs located in southeastern Oregon. The biofabrics and microfossils of sinter deposits at extinct vents were compared to the microbial mats and microorganisms found in the main active thermal spring.

The main hot spring pool and outflow channel contain four main biofacies zones, which were determined in the field by the color and morphology of the microbial mats. Samples were collected from each zone, fixed in 2.5% gluteraldehyde in the field, and prepared in the laboratory for analysis by various microscopy techniques including the optical light microscope (OLM), scanning electron microscopy (SEM), and transmission electron microscope (TEM). Microscopic examination revealed that each of the four biofacies zones are dominated by well-defined taxa whose distribution is constrained to a specific temperature range, making it possible to identify the approximate water temperature of the spring in any one biofacies zone. The diversity of organisms within each of the zones increased as the temperature decreased. Furthermore, the morphology of the mats of the microbial community may reflect the relative hydrodynamics of the water (i.e. stationary or flowing), which results in the production of distinct sinter lithofacies if the mat biofabrics are silicified.

SEM analysis revealed incipient silicification of the biofilm matrix and the organisms of the microbial mats from the active spring. SEM analysis also indicated the presence of microfossils in the recent sinters from extinct vents in the area. The distribution, orientation, fabric, and morphology of the microfossils allude to past conditions of the hydrothermal system in which they were formed.