HIGH-RESOLUTION ANALYTICAL APPROACHES TO CHARACTERIZING BIO/LITHOFACIES IN THE MICROBIAL HABITATS OF TERRESTRIAL HOT SPRINGS

    Actively growing siliceous sinter material from Octopus Spring, Yellowstone National Park is being studied using a multi-faceted high-resolution biogeochemical approach to characterize the biological and lithological properties of structures representative of some of Earth’s earliest microbial habitats. The sinter deposit exhibits four bio/lithofacies based upon structural, textural, and chemical components. These include 1) a crust with visible algal component, 2) an altered interior, 3) light/dark banded microlaminae, and 4) colloform lower edge (structural protrusions into the pool).  X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) indicate opal-A to be the dominant mineral phase, except within the altered interior, which is composed of quartz and also contains a lithic fraction, comprising dioctahedral smectite, hematite, rutile, and pseudobrookite. The altered region is likely due to inclusion of rhyolitic fragments into the sinter material and does not derive from the sinter itself.   The altered region also varies in that it has a different texture, other regions containing very fine to wavy to concave up laminations.  Textural analysis was conducted using light, confocal, and backscatter electron imaging.  For chemical analysis, micro-Dumas combustion for C/N analysis showed the greatest percentage of carbon within the dark banded microlaminae (1.1% C).  Electron microprobe study indicated elemental ratios of Ca:Mg, Na:K, F:Cl, and Na:C > 1, consistent with the composition of geothermal waters in felsic volcanic rocks.  Si L2,3-near edge X-ray absorption spectroscopy concurs with FTIR and XRD in identifying the altered interior as primarily quartz and thus probably the oldest part of the sinter. Light-bands within the microlamellar region exhibit near-edge spectra similar to rhyolitic glass and thus having a cryptocrystalline structure. SEM identified putative microfossils within these dark bands. Native fluorescent material was also evident from confocal microscopy. XRD and FTIR evidence indicate that this is calcite. The carbonate component also explains why C-content increases from 0.26 % in the crust down through the sinter to 1.1% in the microlamellar region despite there being a visible algal component in the crust.