Figure 4. A: Ochki Pool consists of several vents that drain into a large, shallow pool surrounded by lithified sinter and many sinter islands. B: The only sinter actively depositing in this area occurs along the rim of an older sinter horizon within which the vents protrude. C: The massive fabric typically has no obvious signs of biological input that would have resulted from the presence of biofilms on the sinter when it accreted.

but are too infrequent to sustain biofilm growth. Phototrophic growth lines the inner rims of the sinter around a few of the vigorously bubbling vents, but in general, the relatively low water level of quiescent vents and the main pool relative to the height of the exposed horizon of sinter causes the sinter to remain dry for the majority of the summer. The dry sinter deposits exhibit very little morphological evidence of biological influence at the time of its formation (alternatively, see Cady and Farmer, 1996 for examples of high-temperature sinter biofabrics constructed via the silicification of subaerial biofilms on rims of perpetually wetted sinters in the splash zone of active silica-depositing hot springs in Yellowstone National Park). A powder X-ray diffraction (XRD) analysis (2-65° 28, step size 0.03° 28, scanning rate of 1°/minute, Phillips X'Pert XRD) of the youngest precipitates deposited on the sinter rims at Ochki Pool reveal that it consists primarily of opal-A with minor amounts of kaolinite, cinnabar, and evaporite minerals (XRD data not shown).

10. Zavarzin Pool

Zavarzin Pool, a 4.5 by 2.25 m pool, is situated in the Eastern Thermal Field, several hundred meters from Thermophile Spring. The broad shallow pool is continuously fed by numerous small vents that keep the pool fluid well-mixed; the temperature (~55°C) and pH (6.3) of the individual vents was the same as those of the mixed pool fluid. The bottom of the pool is colonized by a filamentous mat several millimeters thick that traps detrital material as it settles down on it. The edges of the pool, which are not well lithified, are rimmed with thin greenish mats dominated by cyanobacteria (Fig. 5).

Though the trapping and binding of detrital material into the mat generates a recognizable biofabric that could be preserved, it is not becoming cohesively lithi-fied in this subaqueous environment and is unlikely to be buried and retained in the rock record as a primary sinter biofabric. It is worth noting that, should there be a pulse of detrital input, such as from air-borne ash from a distal volcanic eruption, the preservation of organic-rich horizons in the sediments that accumulate within the pool may occur. However, this type of preservation requires an external episodic influx of sediment, a process not inherent to the active depositional regime of the pool today. XRD analysis has confirmed that the pool floor sediments at Zavarzin Pool consist of detrital grains of opal-A and feldspar.

11. K4 Well

K4 Well is an experimental well that was drilled several decades ago and capped. The well head has been leaking extensively for a number of years (G. Karpov, personal communication, 2004-2007). Several distinct chemotrophic biofilms and

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