Siliceous Shrubs in Yellowstone’s Hot Springs: Implications for Exobiological Investigations

Introduction: Potential relict hot springs have been identified on Mars and, using the Earth as an analog, Martian hot springs are postulated to be an optimal locality for recognizing preserved evidence of extraterrestrial life [1]. Distinctive organic and inorganic biomarkers are necessary to recognize preserved evidence of life in terrestrial and extraterrestrial hot spring accumulations. Hot springs in Yellowstone National Park, Wyoming, U.S.A., contain a wealth of information about primitive microbial life and associated biosignatures that may be useful for future exo-biological investigations. Numerous siliceous hot springs in Yellowstone contain abundant, centimeter-scale, spinose precipitates of opaline silica (opal-A). Although areally extensive in siliceous hot spring discharge channel fa-cies, these spinose forms have largely escaped attention. These precipitates referred to as " shrubs " , consist of porous aggregates of spinose opaline silica that superficially resemble miniature woody plants, i.e., the term shrubs. Shrubs in carbonate precipitating systems have received considerable attention [2], and represent naturally occurring biotically induced precipitates. As such, shrubs have great potential as hot spring environmental indicators and, more importantly, proxies for pre-existing microbial life. Observations: At Cistern Spring, a siliceous terraced mound accumulation, shrubby precipitates are evident in approximately 1900m 2 of the deposit. Sili-ceous shrubs occupy shallow terracettes (1-20cm deep) and can be observed throughout most of the 20m of Cistern's flowpath. The shrubs constitute a major precipitate style associated with discharge chan-nel/flowpath facies of siliceous terraces, and are found in siliceous spring waters ranging in temperature from 76.4 to 16.2 o C and pH from 6.0 to 7.4. Whereas adjacent rimstone dams tend to be characterized by smooth walls of opaline precipitates, pool bottoms tend to be bumpy and irregular and are characterized by the shrubby siliceous precipitates. Shrubs have a radiating or arborescent form, consisting of aggregates of bladed opal-A (1-5cm high). Even at the macroscale, muci-laginous microbial colonies occupy the bladed precipitate edge and vary in color from brownish-grey (higher temperature) to orange or green (lower temperature). At the thin-section scale, shrubby horizons from Cistern Spring exhibit different assemblages of microbes associated with the precipitates. At higher temperatures, microbial filaments lacking pigmentation