N

Uzon Caldera

5 km

Figure 2. A: The Kamchatka peninsula of far eastern Russia (Image Google Earth). Petropavlovsk is the largest city in Kamchatka. B: Satellite image of the Uzon Caldera. The dotted line represents the approximate location of the caldera rim. Note that Valley of the Geysers is located in the bottom right-hand corner of the satellite image, beneath the bar scale. NASA Landsat 7 ETM + Satellite image at 15 m resolution.

Lake Dal'nee Maar was formed through a phreatomagmatic eruption 7,600 to 7,700 years ago (Ponomareva and Braitseva, 1991). The Uzon Caldera is underlain primarily by basalt and dacite flows and tuffs (Belousov et al., 1984; Zolotarev et al., 1999). A large, deep heat source and a deep water-bearing layer feed both the Uzon Caldera and Valley of the Geysers systems, within which occur several hydrothermal areas with many distinct geochemical features (Belousov et al., 1984).

6. Biosedimentological Comparison of Sinters

The main factors that contribute to modern sinter biofabric formation include (1) chemical input via authigenic mineral precipitation, (2) biological input via growth/death of benthic microbial communities, and (3) detrital input via the deposition of clastic sediment grains. Nearly every hydrothermal feature in the world supports some degree of microbial activity. Microbial biofilms and mats thrive in thermal fluids even though the ecosystem may not be receiving authi-genic minerals or detrital input. On the other hand, when the rate of microbial replication is slow compared to the rates of detrital accumulation or mineral deposition, the influence of biology on the fabric of a sinter may be difficult, if not impossible, to recognize. Field- to microscopic-scale observations of the biosedimentological regimes studied are used here to assess the likelihood that such features would be preserved in the geological record.

Observations were made over a period of 3 years of summer field excursions to the Uzon Caldera. The hydrothermal features described here include Thermophile Spring, Burliashiy Pool, Zavarzin Pool, Ochki Pool, and the outflow channel associated with the capped but leaky well head of the K4 Well. In 2003-2006, these hydrothermal features were characterized by relatively similar, undersaturated dissolved silica concentrations (~100 ppm) and circumneutral pH, though some vents at Ochki Pool were found to be more acidic (bulk geochemical analysis was provided by the University of Georgia, Athens Geology Department, personal communication per Doug Crowe).

7. Thermophile Spring

Thermophile Spring is a moderately sized hydrothermal feature located near the rim of the Eastern Thermal Field in Uzon Caldera. The ~74°C vent pool (~1 m in diameter) drains into an outflow channel ~7 m long. The pH of the fluid at the vent increases from 6 to a maximum of just over 7 at the distal portion of the outflow channel. The main effluent of the pool is characterized by small islands composed of white silica sinter. Beyond the sinter islands, the channel floor is coated by black sinter and sediments. The white and black sinters are covered by white microbial streamers. Thick, laminated phototrophic (green-orange pigmented) microbial mats form in the distal portion of the outflow channel (Fig. 3).

It is interesting to note that the identity and distribution of the microbial communities that presently occupy this system are the same as that described by Gorlenko and colleagues (1987) on the basis of their observations made nearly 20 years ago. Permanently submerged white microbial streamers - free floating string-like mats - occupy a considerable portion of the outflow channel. Near the vent effluent they colonize the outer rims of the high-temperature sinter islands.

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