Jessica C Goin And Sherry L Cady

Department of Geology, Portland State University, Portland, OR 97201, USA

Abstract Though the majority of microorganisms in hot spring ecosystems fail to be preserved as bona fide (carbonaceous) microfossils, the presence of microbial biofilms on accretionary surfaces of hot spring sinters can influence the development of sinter fabrics. The extent of biological influence on a primary sinter fabric depends upon the behavior of the microbial community at the time the sinter accretes as well as on the input of sedimentary processes - chemical and physical - that occur during sinter growth. Our ability to recognize the influence of benthic microbial communities on the fabric of a hydrothermal deposit, whether on Earth or, potentially, another rocky planet like Mars, requires an understanding of the interactions between microbial communities, authigenic mineral deposition, and detrital grain accumulation during sinter formation. Examples of hot springs in Uzon Caldera, Kamchatka, Russia, are discussed to illustrate how changes in the relative input of biological, chemical, and physical processes contribute to sinter biofabric formation and preservation. The conclusions drawn from this comparison are relevant to the search for evidence of life in any type of hydrothermal deposit found on a rocky planet.

1. Biosignatures in Hydrothermal Deposits

Understanding the early history of life on Earth is important not only for the advancement of origin of life hypotheses, but also for the quest to determine whether life may occur on other planetary bodies. Astrobiologists face the same challenge as paleontologists who search for and study life's earliest history, namely, how to recognize definitive evidence for life once a potential paleobio-logical repository is discovered. The types of biosignatures that could be found in hydrothermal deposits on a rocky planet include bona fide microfossils and the carbonaceous remnants of microbial communities, a variety of organic and inorganic chemical fossils, and microbially influenced sedimentary deposits.

Bona fide microfossils, which retain enough morphological fidelity to be recognizable and, by definition (Schopf, 1975), are carbonaceous (composed of complex organic biopolymers), provide definitive evidence for life (Schopf, 1999). To demonstrate that a microfossil-like object is indeed a bona fide microfossil requires confirmation that it contains carbonaceous cell structures, has a recognizable three-dimensional cellular morphology, and is syngenetic with the deposit in which it was found (Schopf and Walter, 1983; Buick, 1990). The latter criteria is required to date microfossils and avoid mistaking younger contaminants as being the same age as the rock in which they were found. An important constraint on syngenicity is the geochemical maturity of the fossilized carbonaceous matter, whether it is from cellular or extracellular remains, which must be consistent with the degree of alteration experienced by its host rock. Though bona fide microbial fossils and evidence of fossilized extracellular substances become increasingly rare in rocks of progressively older age, life can leave a variety of traces in the rock record (Cady et al., 2003; Westall and Southam, 2006).

Chemical fossils include, for example, organic remains and biominerals characterized by metabolically fractionated stable isotopic signatures; biominerals characterized by a degree of chemical purity or structural order unattainable without biomolecular templates and controlled growth; and lipid biomarkers indicative of cellular remains and relict biomolecules from cells and their extracellular matrix. Chemical fossils have been particularly important in the search for ancient life on Earth (Schidlowski et al., 1979; Rosing, 1999).

Microbial communities also leave fossil evidence of their presence, behavior, and metabolic activities by generating or influencing the development of biofab-rics and sedimentary structures such as microbialites and biogenic stromatolites. Microbially influenced sedimentary structures are commonly generated by the interaction of benthic microbial communities and the accumulation of minerals or mineraloids, either through trapping and binding of detrital grains or via authigenic mineral precipitation. Microbialites can occur as thrombolites (characterized by a clotted internal structure) or stromatolites (characterized by a laminated internal structure) (Burne and Moore, 1987). Biogenic stromatolites retain information that has been essential in understanding the paleoecology of early life and its environmental setting (Grotzinger and Knoll, 1999). The various contributions of biological, chemical, and detrital processes involved in the formation of biogenic stomatolites are reflected in the characteristics of their biofab-rics (Hofman, 1973). The emphasis here is how these different biosedimentological processes contribute to the development and preservation of sinter fabrics that form in surficial hot spring environments.

2. Implications for Astrobiology

In March of 2007, the Mars Exploration Rover Spirit traversed a deposit that consists primarily of non-crystalline silica and is hypothesized to be the remains of a hydrothermal deposit (Fig. 1) (NASA/JPL News Release). Hydrothermal

Figure 1. Deposit of amorphous silica uncovered by the Mars Exploration Rover Spirit (Image courtesy of NASA/JPL/Cornell, The tire track marks are approximately 20 cm wide.

Figure 1. Deposit of amorphous silica uncovered by the Mars Exploration Rover Spirit (Image courtesy of NASA/JPL/Cornell, The tire track marks are approximately 20 cm wide.

deposits can form on any rocky planet where subsurface water is present (e.g., Farmer and Des Marais, 1999). The potential for crater impacts to have initiated hydrothermal activity on Mars, as discussed by Newsom et al. (2001), suggests that any evidence observed on Mars that is consistent with hydrothermal activity should be explored for biosignatures. Recent models also indicate that the presence of subsurface water (liquid or solid) during impact events on Mars would lead to hydrothermal activity (Rathbun and Squyres, 2002). These impact hydrothermal systems could be relatively long-lived; an impact with a crater diameter of 30 km could generate a system that would endure for up to 67,000 years (Abramov and Kring, 2005). As discussed by Hode and colleagues (2008, this volume), mineral assemblages associated with an impact crater in Sweden were precipitated from fluids with temperatures that may have supported thermophilic life (Hode et al., 2003).

Though it is not yet known whether life ever occurred or thrived on Mars in the form of microbial mats and biofilms in hydrothermal systems, any evidence of sinter-like fabrics in highly siliceous Mars rocks will lead to extensive interrogation of the deposit over the full range of spatial scales for possible biosignatures.

3. Stromatolites

Though stromatolites have been studied for over 150 years, the meaning of the term "stromatolite" is still debated. Early stromatologists applied Linnean names to stromatolites based upon their morphology. It has since become apparent that stromatolite morphology is strongly dependent upon environmental factors (e.g., Hofmann, 1969; Grotzinger and Knoll, 1999). Historically, there have been two competing definitions of a stromatolite: (1) one descriptive of the structure - a laminated, lithified sedimentary structure - and (2) the other focused on the mechanism of formation - a laminated structure generated through authigenic mineral precipitation onto benthic microbial communities or detrital mineral grain trapping by benthic microbial communities (e.g., Hofmann, 1969, 1973; Walter, 1976; Burne and Moore, 1987). Of course, a biologically influenced stromatolite could be described by either definition, but because of the difficulty in determining the biogenicity of ancient stromatolites (Lowe, 1994; Grotzinger and Rothman, 1996), it should be clear when using this term which definition is implied. Here, we use the descriptive definition of the term stromatolite and preface it with the modifier biogenic when we can be certain that biology influenced the way it formed. Riding (1999) suggested that the term stromatolite be defined as "a laminated, benthic microbial deposit" and the modifiers "possible" or "probable" be used to indicate when biogenicity is uncertain. Regardless of the formation mechanism, the key characteristic of any stromatolite is its internal lamination (Hofmann, 1969).

Stromatolitic deposits are widespread in the Proterozoic rock record, with maximum diversity and abundance from 2.8 to 1 Ga (Riding, 2000). There are fewer exposures of stromatolites that formed during the Archean Eon, yet stromatolitic structures as old as 3.5 Ga have been described (Schopf, 1994; Allwood et al., 2006). The decline of stromatolite abundance after 1 Ga coincides with the rise of metazo-ans, and grazing activity is proposed as a major cause of the decline of stromatolite diversity toward the end of the Proterozoic Eon (Awramik, 1971).

Of relevance to the search for ancient and extraterrestrial life are the different types of stromatolites that form in a wide range of modern environments, such as in Shark Bay, Australia (e.g., Awramik and Riding, 1988); Exuma Sound in the Bahamas (e.g., Reid et al., 2000); and in hot spring ecosystems (Walter et al., 1972). A question commonly asked in ancient stromatolite studies is whether modern stromatolites formed in an analogous way to ancient structures that display similar morphologies? For example, the Shark Bay stromatolites are marine carbonate structures that formed in a sedimentary setting and display shapes similar to the majority of ancient stromatolites. Yet the participation of eukaryotic algae, which entrains much larger detrital grains in the modern structures (Awramik and Riding, 1988), along with dramatic secular changes in the composition of the Earth's atmosphere and oceans, limit the use of this modern locale as a biological and biosedimentological homologue for ancient biogenic-stromatolite forming environments. Regardless, modern biogenic stromatolites provide exceptional opportunities to document how the interaction of biosedimentological processes contribute to the formation of such structures.

4. Sinter Biofabrics

The formation of a stromatolite requires (1) a substratum for the attachment of a microbial community and growth of the stromatolite, (2) a fluid medium, generally fresh or marine water, (3) a benthic microbial community, (4) the deposition of small detrital grains or authigenic mineral precipitates, and (5) a rhythm in the deposition of sediment or growth of the community (Hofmann, 1973). During the growth of a stromatolite, the type of fabric that forms in association with the structure depends on the contributions of four main factors: (1) the accumulation of skeletal remains of organisms (e.g., corals), (2) the input of clastic material (e.g., varves), (3) chemical precipitation (e.g., stalactites), and (4) the input of non-skeletal biological remains (e.g., algal mats) (Hofmann, 1973). As discussed below, the main factors that contribute to hot spring sinter fabrics are chemical, microbial, and detrital processes.

The description of Conophyton-like stromatolitic structures in hot spring sinters in Yellowstone National Park led to the recognition of one of the first modern analogues for ancient siliceous stromatolites (Walter et al., 1972). The study of modern sinter deposition has increased our understanding of how microfossils form, sinter fabrics develop, and chemical biosignatures become preserved in sinters (e.g., Walter and Des Marais, 1993; Cady and Farmer, 1996; Jones et al., 2001; Konhauser et al., 2001; Konhauser et al., 2003; Lalonde et al., 2005). The environment in which most mid-to-high temperature sinter stromatolites form excludes metazoan grazers and allows for the encrustation and entombment of components of microbial mats and biofilms.

The relative importance of biotic and abiotic factors in sinter stromatolite formation has not been firmly established. Modern sinters show evidence for microbial involvement in sinter fabric formation across a broad range of temperatures (Cady and Farmer, 1996). Yet arguments have been made for either abiotic or biotic factors playing a dominant role in sinter stromatolite morphogenesis. For example, a study of high-temperature siliceous sinter at Yellowstone National Park led Lowe and Braunstein (2003) to conclude that, though organisms were present, they did not control the development of stromatolite-like features in the sinter; hence, they consider the sinters abiotic in origin. Detailed study of Icelandic hot springs led Konhauser and colleagues (Konhauser et al., 2001) to propose that laminated moderate temperature sinter fabrics result from the annual growth and die-back of cyanobacteria, a process that favors a biogenic origin such stromatolites.

The examples discussed below illustrate how, in several hot springs in the Uzon Caldera, biosedimentological processes contribute to the formation of sinter fabrics. The Uzon Caldera, in fact, is an ideal location for this comparative biosedimentological study of sinter fabrics, given the presence of multiple thermal basins with a wide variety - geochemically and sedimentologically - of hot springs, the remote location and international preserve status of the area (which limits the impact of human visitors on the site), the presence of natural and a few man-made hydrothermal features (the latter of which constrains their date of origin), and the distribution of numerous and diverse hot springs within a small geographic area. It is worth noting that the conclusions drawn from this comparison are relevant to the search for evidence of life in any type of hydrothermal deposit found on a rocky planet.

5. The Uzon Caldera

The Kamchatka peninsula of far eastern Russia is an active volcanic region characterized by numerous hydrothermal basins. The Uzon Caldera, in the Kronotsky National Biosphere Reserve, is located approximately 200 km north of Petropavlovsk, the only major city in Kamchatka (Fig. 2A). The Uzon Caldera formed during the collapse of Mount Uzon ~40,000 years ago, an age based on carbon-14 dating of the ignimb rites produced during the eruption (Florenskii, 1988). The caldera, a 7 by 10 km oval depression, is associated physically and geochemically with the Valley of the Geysers (Fig. 2B). The large

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