Figure 5. A: Field photo of Zavarzin Pool. B: Filamentous cyanobacteria (positive for chlorophyll-a autofluorescence). C: Mineral grains (1) bound by biological material (2).

photo-trophic mats have developed along the floor of the outflow channel, and their silicification preserves several distinct sinter fabrics along the length of the outflow channel (Goin, 2007). The outflow channel is ~1 m wide and extends ~3.5 m away from the effluent before grading into standing water at the edge of Chloride Lake (Fig. 6). Fluid that sprays out of the well-head at 97°C cools to ~85°C at the beginning of the outflow channel at the base of the well-head. The hydrothermal fluid cools to ~30°C in pools located outside the main runoff channel and at the end of the channel where it encounters Chloride Lake. The pH varies from 6.6 proximal to the well-head to 7.0 near the end of the main runoff channel.

The high-temperature biofacies at K4 Well is dominated by small diameter (less than 0.5 |m) non-pigmented filaments that form short (1-2 mm) pink-to-reddish colored streamers. Where the water temperature in the main channel drops to <75°C, phototrophic non-oxygenic (green pigmented but negative for

Figure 6. A: Field photo of the K4 Well outflow channel. B: Microfossil casts lie approximately tangential to a horizontally laminated biofabric characteristic of the mid-temperature K4 Well sinter. C: Note the colloidal silica particles (arrow) that coat adjacent filaments.

chlorophyll-a) bacterial filaments ~2|im in diameter dominate the mats. The microbial communities submerged beneath fluids with temperatures <70°C include bacterial morphotypes that exhibit the same characteristics as those of bacterial morphotypes found in the ~75°C fluid and those of filamentous cyano-bacteria (positive for chlorophyll-a autofluorescence). The lower temperature biofacies (40-60°C) consists predominantly of cyanobacteria characterized by rods and several different filamentous morphologies. The pools outside the main outflow channel, with temperatures <40°C, contain floating tufts of green-pigmented phototrophic filaments characterized by narrow diameters (0.8 |im), though the lack of chlorophyll-a autofluorescence indicates that these filaments are not cyanobacteria.

The silicification of biofilms in the outflow channel at K4 Well preserves sinter biofabrics with a demonstrable biogenic input. The biofabrics preserve information indicative of the biofilm architecture as well as the dominant morphologies of the organisms. The outflow channel at K4 Well represents a system where the processes of biologic input, chemical precipitation, and negligible detrital input result in excellent preservation of biofilm features in the sinter biofabrics.

12. Discussion

The ternary diagram shown in Fig. 7 illustrates how the various processes contribute to the formation of a sinter biofabric in surficial hydrothermal systems. The situation at Thermophile Spring demonstrates that the presence of biology alone is not enough to produce biofabrics; hot spring systems dominated almost

Figure 7. Ternary diagram (modified after Hoffman, 1973) illustrates the main processes that contribute to the formation of hydrothermal sinters. The examples discussed here represent hydrothermal ecosystems dominated by near end-member contributions (i.e., Ochki Pool is dominated by chemical contributions; Thermophile Spring is dominated by biological contributions; and Burliashy Pool is dominated by detrital contributions) and approximate mid-point contributions (i.e., biological and chemical processes contribute to K4 Well sinter formation and biological and detrital processes contribute to Zavarzin Pool sinter deposits).

Figure 7. Ternary diagram (modified after Hoffman, 1973) illustrates the main processes that contribute to the formation of hydrothermal sinters. The examples discussed here represent hydrothermal ecosystems dominated by near end-member contributions (i.e., Ochki Pool is dominated by chemical contributions; Thermophile Spring is dominated by biological contributions; and Burliashy Pool is dominated by detrital contributions) and approximate mid-point contributions (i.e., biological and chemical processes contribute to K4 Well sinter formation and biological and detrital processes contribute to Zavarzin Pool sinter deposits).

entirely by biological contributions require a means to convert some aspect(s) of the microbial community into a biofabric. Though hot springs can develop prolific microbial mats and biofilms, the additional contribution of authigenic mineral deposition or the accumulation of detrital debris in association with the microbial communities is necessary to produce a biofabric. Without a biofabric to become lithified or buried and, hence, enter the rock record - one of the main mechanisms of retaining a biological signature of microbially dominated communities (e.g., microfossils, the fabrics of biologically influenced sedimentary structures, chemical fossils, Cady, 2001) - is lost.

Burliashiy Pool and Ochki Pool, the former dominated by detrital input, the latter chemical mineral precipitation, illustrate how difficult it is to form biofab-rics in environments dominated by end-member processes other than biological input. In Burliashiy Pool and the adjacent transient pools, the amount of detrital input and the turbulence of the vent activity prevent the development of micro-bial biofilms and mats on the inner pool surfaces. Biofabrics have not developed in, and adjacent to, Burliashiy Pool because the environment is not physically stable. Biofabric development at the near chemical end-member situation that occurs at Ochki Pool is minimal because the geochemical conditions (low pH, high arsenic concentrations) and relatively calm (i.e., non-splashing, variable, or surging) and low water level in the pool are not conducive to the development of biofilms on sinter substratums. The lack of periodic wetting on the sinter rims around the vents and pool promotes the deposition of opal-A via evaporation, and the lack of well-developed and sustained microbial activity on those surfaces eliminates the potential to preserve distinct biofabrics in the sinter.

Our biosedimentological comparison of the main formation processes involved in biofabric development in Uzon Caldera hot springs underscores the concept that only certain combinations of biological, chemical, and physical processes will lead to the formation and preservation of sinter biofabrics.

Hydrothermal ecosystems like Zavarzin Pool, which receives a significant amount of detrital input in the absence of authigenic mineral precipitation, generates a distinctive binding-and-trapping biofabric on the floor of the hot spring pool. Should the mat/sediment biofabric remain cohesive during post-depositional disruption (e.g., Schieber, 1999; Gerdes et al., 2000; Noffke et al., 2002), the bio-fabric may be preserved in the rock record. It is worth noting that in hot spring ecosystems with only mixed detrital-biological inputs, the biofabrics may lack any obvious sedimentological or morphological characteristics that would indicate the presence, behavior, or metabolic activity of the populations that contributed to their formation. The link back to the paleoecology would be weak without organic chemofossils (e.g., biomarkers and evidence of biological isotopic fractionation) in the carbonaceous remains of such biofabrics, should they be preserved.

The most easily recognized biofabrics, and those most likely to enter the fossil record, are hydrothermal ecosystems characterized by the growth of microbial biofilms and mats that thrive in mineralizing fluids (e.g., Walter and Des Marais,

1993). In the outflow channel of the K4 Well, silica precipitation occurs in the presence of ubiquitous biofilms that colonize the accretionary sinter surfaces. Seasonal variations in evaporation-driven mineral precipitation lead to seasonal variations in the growth rates of the cyanobacterially dominated microbial communities. The interplay of these factors results in the formation of distinctive stromatolitic biofabrics at K4 Well (Goin, 2007).

In summary, the mixed end-member systems of Zavarzin Pool and K4 Well are characterized by well-developed benthic microbial communities, a fluid medium, and the deposition of detrital grains or the accumulation of authigenic precipitates, all of which are attributes required for stromatolite formation. While a trapping and binding fabric is generated at Zavarzin Pool, degradation of the organic relicts of the microbial communities would leave a collection of detrital grains without a demonstrable biogenic contribution to the sediment fabric. The mineralization of microbial mats and biofilms at K4 Well results in the formation of distinctive biofabrics and enhances the potential to preserve evidence of the biogenic contribution to the fabric, even if the carbonaceous remains of micro-bial cells or extracellular remains would be removed from the sinter by post-depositional taphonomic processes.

13. Implications

While it is tempting to overlay a mask on the ternary diagram shown in Fig. 7 for the purpose of defining, in a quantitative way, that portion of the plot where biofabrics will form, there are a number of practical reasons why this would be difficult to accomplish for a particular spring, let alone for the variety of hot springs discussed here. Though hot springs often appear to be highly stable, environmental conditions can change rapidly if flow from the effluent is diminished or diverted from the stream channels and runoff plains. Even slight differences in the rate and volume of fluid flow or changes in fluid temperature and composition can alter the apparent stability of any one microbial community in a hot spring ecosystem. Any changes upstream will also result in a variety of changes in the microbial communities downstream. For example, flow and geochemical changes can alter the nature of the hydrodynamic conditions, the flux of nutrients to organisms, the degree of saturation of the hydrothermal precipitates, and the types of interactions that occur within microbial consortia that consist of many populations of organisms and interlopers like viruses and - at lower fluid temperatures - algae and plankton. Microbial communities become susceptible to predation by grazing eukaryotes when fluid flow decreases and water levels drop below the top of the biofilms or microbial mats. Though it is obvious that the behavior of any one microbial population within a community depends upon many factors, how these populations respond to a variety of environmental clues is not fully understood.

As microbiologists, microbial ecologists, geomicrobiologists and geochem-ists grapple with strategies to categorize, quantify, understand, model, and predict the nature of microbial communities and biogeochemical interactions in natural environments, it is important to remember that we have only begun to study, under laboratory controlled conditions and in natural settings, the various bio-sedimentological processes that interact to produce biosignatures - such as bio-fabrics - in the rock record. It is one thing to search for evidence of life as we know it in ancient rocks here on Earth, yet quite another to search for life elsewhere in the universe. Regardless of the remaining challenges, remarkable opportunities lie ahead for those involved in the continued search for life beyond the confines of the blue planet.

14. Acknowledgements

The authors wish to acknowledge support for this work from the NSF Microbial Observatory Program (MCB-0241001) and the NASA Exobiology Program (NNG04GJ84G).

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Biodata of Hubertus Porada, author (with co-author Patrick G. Eriksson) of "Cyanobacterial Mat Features Preserved in the Siliciclastic Sedimentary Record: Paleodeserts and Modern Supratidal Flats."

Professor Hubertus Porada (retired) is currently a guest researcher in the Department of Applied Geology at the University of Göttingen, Germany. He obtained his Ph.D. from the University of Göttingen in 1964, gained practical experience in the Geological Survey of Namibia and habilitated within a Special Research Programme (SFB 48) at the University of Göttingen in 1983. Professor Porada's scientific interests are in the areas of: geodynamics, sedimentology of metamorphic rocks and, more recently, geobiology of microbial mats.

E-mail: [email protected]

Professor Patrick G. Eriksson is currently the Chair of Geology at the University of Pretoria, South Africa. He obtained his Ph.D. from the University of Natal in 1984 and habilitated from the Ludwig-Maximilians University, Munich, in 1998. Professor Eriksson's scientific interests are in the areas of: Precambrian basin evolution, sedimentary paleoenvironments, continental freeboard and controls on sea level changes during the Precambrian, and, more recently, sedimentology of microbial-mat related clastic sediments.

E-mail: [email protected]

Hubertus Porada Patrick G. Eriksson

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