FOSSILIZATION PROCESSES IN MODERN THERMAL SPRINGS: CLUES FOR ASSESSING THE BIOGENICITY OF ANCIENT HYDROTHERMAL DEPOSITS. Jack

Hydrothermal systems have been cited as important targets in the search for evidence of an ancient biosphere on Mars (1,2). Indeed, such environments appear to have been widespread on Mars earlier in its history (3), while subsurface hydrothermal systems may have provided clement environments for life throughout the subsequent history of planet. Comparative studies of modern and ancient hydrothermal systems on Earth have the potential to provide important constraints on longterm evolutionary trends in hydrothermal ecosystems (4). And the results of such studies also assist in refining strategies to explore Mars for a fossil record of ancient hydrothermal life. To create a comparative framework for interpreting the fossil record of ancient hydrothermal deposits on the Earth, and possibly Mars, we have carried out parallel studies of the microbial biosedimentology, taphonomy and geochemistry of active hydrothermal environments in Yellowstone National Park. One goal of the research is the development of highly integrated biosedimentological and paleontological models for siliceous (5,6), carbonate (7) and Fe-oxideprecipitating springs (8, and Farmer et al., work in progress). In this report we emphasize fossilization processes in subaerial siliceous thermal springs over a broad range of temperatures. Although such studies are primarily intended to provide a basis for evaluating the microbial contributions to the fossil record of ancient hydrothermal deposits on Earth, they also have implications for the biogenicity of suspect “nanofossils” in Martian meteorite ALH84001 (9).