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Figure 3. A: Aerial photograph of the Eastern Thermal Field. The circle encloses Thermophile Spring and its outflow channel, the vent appears white, the orientation of the outflow channel turns just beyond the vent, and the dark area is the thick accumulation of microbial mats (photograph provided by C. Romanek). B: Streamers mat fabrics that lie over thickly layered microbial mats. C: Middle layer of a mat with abundant segmented cyanobacteria (positive for chlorophyll-a autofluorescence).

Figure 3. A: Aerial photograph of the Eastern Thermal Field. The circle encloses Thermophile Spring and its outflow channel, the vent appears white, the orientation of the outflow channel turns just beyond the vent, and the dark area is the thick accumulation of microbial mats (photograph provided by C. Romanek). B: Streamers mat fabrics that lie over thickly layered microbial mats. C: Middle layer of a mat with abundant segmented cyanobacteria (positive for chlorophyll-a autofluorescence).

In the mid-to-high temperature regime they colonize the rocky bottom of the outflow channel for at least 4 m. At the mid-temperature portion of the outflow channel they colonize loose particles of black sediment that have accumulated along the bottom of the channel floor. These white streamers are made up of filamentous Thermothrix spp. and sulfur reducers (Gorlenko et al., 1987). The subaerial portion of the sinter that surrounds the vent and the sinter islands is coated by pink and green biofilms in the subaerial zone above the water line. These subaerial biofilms are dominated by large cyanobacterial cocci (identified on the basis of chlorophyll-a autofluorescence) and small rods. Thick microbial mats that occupy the outflow channel fluids with temperatures <60°C are characterized by long streamers that protrude across the microbial mat surface. These mats are made up of Chloroflexus spp., Oscillatoria spp., and other cyanobacteria (Gorlenko et al., 1987).

Though Thermophile Spring has continued over the years to host diverse and extensive microbial communities that occupy the entire length of the outflow channel, the potential to preserve biogenic sinter fabrics associated with any of the communities is limited. The morphologically distinctive streamers, for example, are not becoming entombed in the sediment on the outflow channel floor. And though minor amounts of subaerial spicular sinter occur around the vent, most likely forming as evaporative-driven silica precipitates, the system lacks enough mineral accumulation in the subaqueous portion of the outflow channel to preserve biologically influenced sinter.

8. Burliashiy Pool

Burliashiy Pool, a nearly circular pool with many vents, is ~5 m diameter. The pool lies in a topographically low area of the basin and is surrounded and underlain by lake sediments. The turbulence from the vents introduces course grained sediments and weathered diatom frustules that deposit on the bottom of the pool as an unconsolidated layer of sediment. Burliashiy Pool is 90°C and has a pH of ~6. A series of adjacent small pools with active turbulent vents and high temperatures is associated with Burliashiy Pool. In these pools, the lack of ben-thic microbial mat development or cementation of the sediment through authi-genic mineral precipitation leads to the accumulation of an unconsolidated layer of clastic grains. The apparent absence of a benthic microbial community due to the turbulent nature of the vent fluids in these pools and in Burliashiy Pool eliminates the possibility that a lithified sedimentary biofabric would develop in these systems. The sediments also lack the type of cohesion that would have been possible had they been colonized by benthic microbial biofilms. The biosedimen-tological regime of these hot springs consists of an accumulation of easily disrupted layers of clastic grains in the submerged regions around the effluents.

9. Ochki Pool

Ochki Pool consists of a large, shallow pool surrounded by a relatively thick and laterally extensive lithified siliceous sinter rim (Fig. 4). A series of small vents that protrude through the sinter rim at various distances from the main pool maintain its relatively constant fluid volume, though the water level lies beneath the surface of the sinter rim. Hence, several small vents around Ochki Pool can be accessed by walking to them on top of the lithified sinter. Ochki Pool measures several meters wide and is greater than 10 m long. This entire system lies in an inlet located along the western-most edge of the Central Thermal Field. The range of fluid temperatures in the vents and main pool (40-75°C) and pH (3-6) indicates that the area is fed by several geochemically distinct subsurface aquifers.

Though an extensive amount of sinter has been deposited in the Ochki Pool area in the past, only the rims around the vents and the main pool show any evidence of recent sinter deposition. Spicular and laminar sinter material along the rim of the pool and around the vents has built up primarily as a result of the evaporation and cooling of episodically splashed fluids that wet the sinter surface

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