Bacterial shrubs, crystal shrubs, and ray-crystal shrubs: bacterial vs. abiotic precipitation

Hot-water travertine deposits commonly contain shrub-like morphologies, on the centimeter to meter scale, that range from highly irregular forms (bacterial shrubs) to features that display regular geometric patterns (crystal shrubs and ray-crystal crusts). The bacterial shrubs have been previously recognized as the product of bacterially induced precipitation whereas the ray-crystal crusts have been described as due to dominantly abiotic precipitation. The bacterial shrubs have a highly h-regular morphology, similar to woody plants, in contrast, end member crystal shrubs display distinct crystal habits and regular repeating morphologies, i.e., they commonly have attributes of noncrystallographic as well as crystallographic dendrites. There is a complete gradational relationship between the bacterial shrubs and crystal shrubs. Ray-crystal crusts are large fan-shaped features composed of subparallel, extremely coarse bladed crystals of calcite. All three types (bacterial shrubs, crystal shrubs, and ray-crystal crusts) contain either a dense tangle of bacterial body fossils and/or micron-sized pores. The micropores are bacterial molds. The immediately enveloping spar crystals around all three morphologic types are devoid of bacterial body fossils and micropores. The bacterial shrubs, crystal shrubs, and ray-crystal crusts are, to differing degrees, the product of bacterially induced precipitation as well as abiotic mineral precipitation. The differences in morphology are: due to the relative contribution of bacterially induced precipitation as compared with that of abiotic mineral precipitation. Bacterially induced precipitates can form in environments too chemically harsh or otherwise inhospitable for other taxa to thrive. Epiphyton and Renalcis commonly formed in occult environments under conditions inhospitable to other taxa. Consideration of the published descriptions and interpretations concerning Epiphyton and Renalcis, and by analogy the origin of bacterial shrubs, have led to the conclusion that Epiphyton and Renalcis also are bacterially induced precipitates. (C) 1999 Elsevier Science B.V. All rights reserved.