Morphogenesis of tufa microfabrics at a very cold spring in Alberta, western Canada
摘要
The tufa mound at the very cold water Jackknife Springs in west-central Alberta, western Canada, resulted from calcite-encrusted mosses, fossilizing the plants in upright growth positions. Spar preferentially developed on the surfaces of more rigid moss plant stems, whereas smaller acicular crystallites accumulated across the more fragile upper and lower surfaces of the leaves. These tufa microfabrics provide insight into the calcite precipitation processes when spring water temperatures are below a 6–9 °C threshold. This environment necessitates degassing to control the saturation level as the dominant process for calcite precipitation. Nucleation and growth of larger spar crystals occurred along organic molecules of the microbial gels that were responsive to organic molecules sourced from moss flora because the very cold, 3–5 °C, water temperature range constrained the availability of biofilms with sufficiently abundant extracellular polymeric substances. Morphogenesis of the tufa microfabrics was also strongly influenced by the unusually high manganese concentration. The adsorption Mn2+ ions into the calcium carbonate lattice exerted significant controls on crystal growth habits, favoring acicular crystallite development and precluding coalesce of crystallite arrays into a subhedral or euhedral crystal, a microfabric domain that commonly occurs in tufa and travertine elsewhere. Micro-conical structures, 200–400 µm long, consisting of calcite and amorphous calcium carbonate developed as drapes over moss stem protuberances. Their tips extend outward with claw-like curvatures oriented in the direction of microcurrent flow along the porosity network, resulting in microstructures unique to western Canada tufa deposits.