Microbial Controls on the Size, Shape and Porosity of Hot Spring CaCO3 Mineralization

Microbial communities inhabiting vent drainage systems at Mammoth Hot Springs in Yellowstone National Park, exert strong controls on the size, shape and porosity of CaCO3 (travertine) mineral deposits at a nanometer to micron scale. Our controlled in situ kinetic experiment (ISKA) has shown that: (1) the natural steady state aragonite precipitation rate is more than twice that when microbial biomass is depleted by 80% via 0.2 um filtration; and (2) ultraviolet (UV) irradiation only slightly reduces the mean aragonite precipitation rate, while keeping all other experimental parameters held constant. An integrated petrographic (plane-light and cathodoluminscence [CL]) and high-resolution inverted fluorescence microscopy (200 nm resolution petrography) were completed on travertine samples collected from the ISKA experiment. Results indicate that, under all experimental conditions, an initial layer of dogtooth to blocky calcite cement (less than 30um) was followed by aragonite needle cement growth (10-50um). The untreated natural control experiment produced an initial layer of small calcite crystals interspersed with densely packed clusters of aragonite needles, which significantly reduced the porosity in this first layer of authigenic crystal growth. The ensuing aragonite needle clusters in the natural control were also present, but in lower concentrations, in the UV- irradiated experiments. The filtration experiment produced the largest calcite crystals with few to no aragonite needles. However, the aragonite needles were on average 40 µm long and are not arranged in dense clusters, producing a higher porosity layer composed of lower crystal densities. Early calcite crystals in both the filtration and UV-irradiation samples exhibit bright orange concentric crystal zonations under CL that were absent in the natural control. Aragonite needles in all samples exhibit no CL. Further, no clear petrographic evidence of diagenesis or mineralogical inversion were observed in the ISKA samples. These results suggest that the presence of microbial communities influenced the size, shape and resulting porosity of travertine crystallization in these hot spring systems.