Abstract

Before anthropogenic modifications, Ohaaki Pool (Broadlands-Ohaaki) and Dragon's Mouth Geyser (Wairakei) emitted waters at temperatures of 93-98 degrees C. The siliceous sinter that precipitated around their vents has the characteristics of "geyserite", a dense laminated deposit of presumed abiogenic origin, that was precipitated from waters too hot (more than 73 degrees C) to support microbes other than thermophilic bacteria. Petrographic and SEM examinations of the sinters that surround the vent of Dragon's Mouth Geyser and form the pool-rim dam of Ohaaki Pool show that they incorporate columnar stromatolites and silicified, laminated stromatolitic mats that contain well preserved filamentous microbes. At both localities the microbes lack evidence of desiccation or shrinkage, which implies that they were silicified rapidly at or shortly after their death. Microbe preservation is commonly spectacular; even the trichome is recognizable in many specimens. Although replacement by silica preserved the cells of the trichome, cell walls and septae were not preserved, possibly because they were destroyed by autolysis before silicification. The sheath that surrounded the trichome is normally well preserved. However, late silica precipitation produced thick external encrustations of amorphous silica around many sheaths. Although boiling and very hot (more than 90 degrees C) waters were discharged, temperatures at many sites surrounding the vents remained sufficiently low and moist to support a microbial community that included thermophilic bacteria and cyanobacteria. In these cooler niches, the microbes and their biofilms served as highly favorable templates for the nucleation and growth of amorphous silica, and collectively provided a microbial framework for the laminated accretionary sinter. Silica was supplied from supersaturated spring and geyser fluids that were delivered to sites of sinter precipitation as aircooled spray, as proximal runoff, or as the result of oscillations in the surface level of the spring or geyser pool waters. In the field and in hand specimen, the resulting columnar sinter appears to be identical in morphology to that inferred to originate from abiotic silica precipitation in a sterile environment. Consequently, some columnar, and stratiform geyserites are probably not "abiotic" precipitates but are true silica stromatolites.

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