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Biogenicity of Silica Precipitation Around Geysers and Hot-Spring Vents, North Island, New Zealand
Brian Jones (1), Robin W. Renaut (2), Michael R. Rosen (3)
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 (> 90°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 air-cooled
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 appear to be identical in morphology to that
inferred to originate from abiotic silica precipitation in a sterile environment.
Consequently, some columnar, spicular, and stratiform geyserites are probably
not "abiotic" precipitates, but are true silica stromatolites.
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