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Environmental and Mineralogical Controls on Fossilization: Key Elements in a Strategy for Mars Exopaleontology

Introduction: One cannot expect to land just anywhere on Mars and discover a fossil record. What are the important factors that should guide the search? While the potential range of fossil information preserved in rocks of all ages is quite broad, including actual cellular remains (1), various classes of chemofossils (2), biominerals, biofabrics, and stromatolites (3), studies of fossilization processes in modern environments on Earth, as well as the terrestrial fossil record, indicate that the preservation of biosignatures is strongly influenced by the physical, chemical and biological factors of the environment (4). Acting together, these factors determine the types of deposits and range of information preserved. Important Fossilization Processes: On Earth virtually all of the well-preserved cellular microbiotas in the Precambrian were preserved by either 1) early diagenetic infusion of silica or phosphate into organic materials and sediments, or 2) by rapid burial of organic materials in fine-grained, clay-rich shales. These two situations comprise a very narrow taphonomic window that greatly restricts opportunities for preserving cellular level biosignatures in rocks (4). Of course, there are also intrinsic properties of organisms that promote their preservation, such as the presence of recalcitrant cell walls or extracellular elements. While fossilization requires very special conditions, still we have documented an extensive Precambrian fossil record on Earth. It is crucial that such preservational constraints be considered carefully before mounting an expensive and time-consuming campaign to explore for an ancient Martian biosphere. Environments where the sedimentary processes defined above commonly operate in the presence of biology include: arid marine or lacustrine shorelines (5), evaporite basins (6), mineralizing springs (7), and subsoil hardpans (8). Long-term Preservation: Rock type (e.g. the mineralogy and texture of the host sediment) is also a fundamental factor influencing the long