BACTERIALLY INDUCED PRECIPITATES IN HOT SPRING SETTINGS

Bacterially induced precipitates are present in many karst depositional environments. The most distinctive morphologies are shrub and shrub-like forms (similar to garden variety shrubs or bushes). Similar features have been recognized composed of calcite, Mn- and Fe-rich minerals (e.g., pyrolusite, cryptomelane, and goethite), as well as siliceous sinter (opal-A). Shrub-like bacterial accumulations have been grown under laboratory conditions and it is well demonstrated that bacteria can induce the precipitation many different minerals. In nature, the shrubs range from those with a highly irregular shape (i.e., no crystallographic pattern) to those in which mineral habit is readily evident (e.g., dendrites). Peloids, silt-sized quasi-spherical mineralized bodies are also bacterially induced precipitates. Whereas no bacterial fossils or micropores are present in the precipitates immediately adjacent to the shrubs or peloids, shrubs and peloids have an abundance of bacterial body fossils and/or bacteria-sized micropores, holes left upon the decay of the bacteria.

The shrubs most commonly occur in layers that range from incipient microscopic forms to 4 cm in height. Generally, the shrubs do not form in the extreme proximal hot spring environments immediately adjacent to the spring vents, these settings are dominated by abiotic precipitates due to the harsh environment inhibiting organic growth and the highly supersaturated conditions of the waters. Nor do the shrubs occur at the distal ends of the deposits, these are sites at which the conditions have ameliorated and the structures are commonly dominated by higher taxa of organisms. Shrubs most commonly form between these two environmental settings.

Mn- and Fe-rich bacterially induced precipitates are presently forming associated with Artesia Geyser, Lower Geyser Basin, Yellowstone. The Mn- and Fe-rich precipitates are immature, XRD patterns do not display well developed crystallinity and, in contrast to the more common bacterially induced precipitates, they do not display a shrub-like morphology. The Mn- and Fe-rich precipitates form millimeter thick continuous laminae intercalated with centimeter thick layers of fan-shaped calcite crystals near the spring orifice (T=90^oC, pH=7.7) to thick diffuse black masses downflow (T=45^oC, pH=8.0).