Hydrothermal Systems: Doorways to Early Biosphere Evolution
Jack D. Farmer, Department
of Geology, Arizona State University, P.O. Box 871404, Tempe, AZ 85287, USA
ABSTRACT
Hydrothermal systems may have provided favorable environments for the prebiotic synthesis of organic compounds necessary for life and may also have been a site for life's origin. They could also have provided a refuge for thermophilic (heat-loving) microorganisms during late, giant-impact events. Phylogenetic information encoded in the genomes of extant thermophiles provides important clues about this early period of biosphere development that are broadly consistent with geological evidence for Archean environments. Hydrothermal environments often exhibit high rates of mineralization, which favors microbial fossilization. Thus, hydrothermal deposits are often rich storehouses of paleobiologic information. This is illustrated by studies of the microbial biosedimentology of hot springs in Yellowstone National Park that provide important constraints for interpreting the fossil record of thermophilic ecosystems. Hydrothermal processes appear to be inextricably linked to planetary formation and evolution and are likely to have existed on other bodies in the solar system. Such environments may have sustained an independent, extraterrestrial origin of life. Thus, hydrothermal systems and their deposits are considered primary targets in the search for fossil evidence of life elsewhere in the solar system.
INTRODUCTION
Hydrothermal systems develop anywhere in the crust where water coexists with a heat source. Hydrothermal systems were important in the differentiation and early evolution of Earth because they linked the global lithospheric, hydrologic, and atmospheric cycles of the elements (Des Marais, 1996). Over geologic time, volatile chemicals released by hydrothermal systems have contributed significantly to the evolution of the oceans and the atmosphere.
Most terrestrial hydrothermal systems are sustained by magmatic heat sources. Variations in the temperature (and density) of fluids drive convective circulation in the crust, producing large-scale transfers of energy and materials. As hot fluids move through the crust, they interact chemically with their host rocks, leaving behind distinctive geochemical, mineralogical, and biological signatures. The chemical precipitates of hydrothermal systems, called sinters, typically consist of simple mineral assemblages dominated by silica, carbonate, metallic sulfides and oxides, and clays. The mineralogy of hydrothermal deposits depends both on host rock composition and on the temperature, pH, and Eh of hydrothermal fluids.
This paper covers: (1) the importance of hydrothermal systems in the history of the biosphere, (2) the nature of biogeological information contained in hydrothermal deposits (e.g., travertine spring systems at Mammoth Hot Springs, Yellowstone National Park, Wyoming [Fig. 1]), and (3) hydrothermal systems as potential environments for prebiotic synthesis and biological evolution on other bodies in our solar system.
HYDROTHERMAL SYSTEMS AND EARLY BIOSPHERE EVOLUTION
Molecular phylogenies derived from comparisons of genetic sequences of living species have radically altered our view of the biosphere and of the contribution of microbial life to planetary biodiversity. By comparing genetic sequences in highly conserved molecules like ribosomal RNA, living species have been shown to cluster into three domains — the Archaea, the Bacteria, and the Eukarya (Fig. 2). When we look at the distribution of organisms within this "universal tree," exciting patterns emerge. The deepest branches (those nearest the common ancestor of all life) and the shortest branches (i.e., the most slowly evolving) of the domains Bacteria and Archaea are all populated by heat-loving species (hyperthermophiles) that only grow at temperatures >80 °C (Figure 2, red branches; see Woese, 1987; Stetter, 1996). The distribution of thermophiles suggests that hydrothermal systems may have been a "cradle" for early biosphere evolution. The most deeply branching thermophiles (i.e., most primitive) are all chemosynthetic organisms that use hydrogen and sulfur in their metabolism. Thus, both chemotrophy and thermophily are generally regarded as the most likely characteristics of the common ancestor of all life on our planet.
To date, only a small fraction (perhaps 1%–2%) of the total biodiversity on Earth has been sampled (Pace, 1997). However, the sampling of environments covered by the RNA tree is very broad and the three-domain structure of the RNA tree has only been strengthened with the addition of new taxa. The thermophilic character of the deep branches has also been widely embraced, although it has become clear that precise branching orders have been complicated by the transfer of genetic information between domains (Doolittle, 1998). Therefore, the properties of the common ancestor may undergo some revision as additional sequences are obtained from a broader range of environments.
Despite the uncertainties, the high-temperature nature and other properties of the common ancestor implied by the RNA tree are consistent with a wide variety of independent geological evidence, which indicates that hydrothermal, reducing environments were widespread on early Archean Earth. Theoretical calculations (Turcotte, 1980) indicate that crustal heat flows were much higher during the Archean and volcanism was more widespread. The restriction of komatiitic lavas (surface eruptions of high-temperature peridotite magmas) to primarily Archean terranes indicates that average crustal temperatures must have been much higher at that time. This view is also supported by trends in oxygen isotope abundances for well-preserved siliceous sediments (cherts) that suggest a steady decline in average surface (climatic) temperature from early Archean highs of 50–70 °C, to present values (Knauth and Lowe, 1978).
On early Earth, hydrothermal systems could also have been created by large asteroid or comet impacts. The lunar cratering record suggests that following an initial period of heavy bombardment, which lasted from ~4.6 to 3.8 Ga, both impactor size and flux declined dramatically (Maher and Stevenson, 1988). Theoretical considerations suggest that the stable atmosphere and oceans necessary for life's origin would have been possible only after ~4.4 Ga and that life could have been established on Earth by ~4.2 Ga (Chang, 1994; Zahnle et al., 1988).
Hydrothermal systems may also have been a primary site for life's origin. Thermodynamic calculations for hydrothermal environments suggest that a variety of complex organic compounds (potential precursor molecules for living systems) are synthesized at high temperatures (Shulte and Shock, 1995). Recent experimental work (Voglesonger et al., 1999) has demonstrated the synthesis of alcohols under simulated black smoker conditions, lending support to thermodynamic arguments for the role of hydrothermal systems in prebiotic organic synthesis.
As mentioned above, the origin of life appears to have overlapped with the end of heavy bombardment. Early biosphere development could have been frustrated by one or more late, giant impacts which would have disrupted early habitats, possibly extinguishing all surface life. For example, models indicate that a giant impact of ~500 km diameter would create a transient atmosphere of molten rock vapor that would evaporate the oceans over a period of months (see Chang, 1994). This would produce a steam atmosphere that would rain out over a period of a few thousand years, eventually restoring the oceans (Zahnle et al., 1988). Such high-temperature events could eliminate most, if not all, surface life, allowing only hyperthermophilic (primarily subsurface chemotrophic) species to persist.
If this scenario is true, then the thermophilic nature implied for the common ancestor of life could be simply a legacy of one or more late giant impacts that occurred during late bombardment (Gogarten-Boekel et al., 1995). Impact flux models suggest the last such events could have occurred as late as ~3.9 Ga, the age of the Imbrium Basin on the Moon and the approximate age of the oldest preserved metasedimentary sequences on Earth (Isua Supergroup, Akilia Island, Greenland), dated at >3.85 Ga (Nutman et al., 1997). These same sequences are purported to contain the oldest fossil biosignatures on Earth, chemofossils characterized by light (biologically fractionated?) carbon isotope signatures preserved in iron-rich metasedimentary rocks (Mojzsis et al., 1996; Mojzsis and Harrison, 2000). The geological events outlined above are broadly consistent with basic branching patterns observed in the RNA tree.
The root of the RNA tree is generally placed at the midpoint of the long branch that separates the Bacteria from the Archaea (Fig. 2). As noted previously, the deep basal branches are occupied by hyperthermophilic species that exhibit chemotrophic strategies based on hydrogen and sulfur. In contrast, photosynthesis, the surface metabolic strategy that supports most of the productivity on Earth today, apparently originated within the sulfur bacteria (anoxygenic photoautotrophs; Fig. 2). Oxygenic photosynthesis, an event of singular importance in the history of the biosphere, first appeared in the cyanobacteria and was later transferred to plants (domain Eukarya) through the development of endosymbiotic associations with that group (Fig. 2; Margulis and Chapman, 1998). Deposition of the banded iron formations, a proxy for the buildup of oxygen in the oceans, peaked around 2.5 Ga, at which time Earth's surface environment began to undergo a dramatic changeover to the highly oxidizing conditions that prevail today (Des Marais, 1997).
PALEOBIOLOGY OF THERMAL SPRINGS
Studies of ancient hydrothermal systems can provide important constraints for reconstructing the evolutionary history of thermophilic ecosystems on Earth (Walter, 1996). Comparative studies of the geochemistry, microbial biosedimentology, and fossilization processes in modern thermal spring systems (see Cady and Farmer, 1996) have provided a basis for constructing facies frameworks (e.g., Farmer and Des Marais, 1992), which have utility for interpreting the paleobiology of ancient deposits (e.g., Walter et al., 1996, 1998). Such studies also hold importance for refining our strategies to explore for signatures of life or prebiotic chemistry on other bodies in the solar system, as well as providing more robust criteria for recognizing biogenic features in ancient terrestrial and extraterrestrial materials (Farmer, 1995).
We have studied active travertine (carbonate-precipitating) thermal springs located at Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, Wyoming (Bargar and Muffler, 1975; Farmer and Des Marais, 1994; Fouke et al., 2000). The following highlights three microfacies within this system that represent major differences in mineralogy, in community composition and style of microbial mat development, in stromatolite morphogenesis, and in sedimentary fabrics. Trends in the composition of microbiotas along thermal gradients (from high to low temperature), broadly mirror the inferred sequence of evolutionary events implied by the RNA tree for the global biosphere (cf. Figs. 2 and 4).
VENT MICROFACIES
Angel Terrace is a sulfide spring system (Castenholz, 1977) with vent temperatures
and pH values of ~74 °C and ~6.7, respectively (Farmer and Des
Marais, 1994). On Angel Terrace, vents exhibit both a high rate of discharge (~75–100
cm/s) and a rapid rate of carbonate (aragonite) precipitation (~35 cm/yr;
Farmer and Des Marais, 1994). Vents and proximal channels (Figs. 3A and 3B) are
dominated by filamentous sulfide and hydrogen-oxidizing species (Fig. 3B) that
comprise the group Aquificales (Anna Louise Reysenbach, 1999, personal commun.).
The Aquificales group (represented by Aquifex in Fig. 2) presently represents
the deepest branch in the RNA tree. In shallow channels where flow rates are high,
Aquifex mats form bacterial "streamers" (Fig. 3B) that become encrusted,
forming characteristic sinter fabrics that preserve original flow orientations.
These fabrics are retained during the recystallization of aragonite to calcite.
Remarkably similar bacterial streamers have been described from ancient siliceous
thermal spring sinters in the Carboniferous Drummond Basin of northeast Queensland,
Australia (Walter et al., 1996, 1998).
MID-TEMPERATURE POOLS
The floors of mid-temperature (45–60 °C) ponds on Angel Terrace (Fig.
3C), are covered by centimeter-thick photosynthetic mats (Fig. 3D). These mats
are dominated by a number of cyanobacterial form taxa (Farmer and Des Marais,
1994), including species of Spirulina, Oscillatoria, Synechococcus, and
the green sulfur bacterium Chloroflexus, an anoxygenic photoautotroph (see
Fig. 2). In this part of the system, flow rates are much lower (~10 cm/s),
as are rates of aragonite precipitation (~5 cm/yr). Light-induced gliding
of the filamentous cyanobacteria produces a variety of tufted (coniform) and ridged
mat structures (Fig. 3D). Collectively, mat species produce a dense, gelatinous
slime (exopolymer) matrix that entraps bubbles of photosynthetic oxygen and other
gases, forming "lift-off" structures (Fig. 3D). These features eventually become
mineralized to form distinctive associations of fabrics and microtextures that
survive recrystallization to calcite. Much of the aragonite precipitation at mid-temperatures
occurs just beneath the mat surface where pH is elevated during photosynthesis,
favoring carbonate precipitation (Farmer and Des Marais, 1994).
DISTAL SLOPE ENVIRONMENTS
On the distal slopes of Angel Terrace (Fig. 3E), lying below the mid-terrace
ponds discussed above are shallow (centimeter deep) terracette ponds (Fig. 3F)
that are characterized by slow flow rates (<50 cm/s) and low rates of carbonate
precipitation (<2 cm/yr). Temperatures are also low, falling in the range,
~40–15 °C. The microbial communities of these distal slope environments
comprise a diverse association of microorganisms, including many species of diatoms
and other representatives of the Eukarya, as well as grazing insects and protozoans.
At the lowest temperatures, small enclaves of higher plants also survive on distal
slopes (Fig. 3E). The filamentous cyanobacteria present include species of Spirulina
and Calothrix, the latter characterized by heavy exopolymer sheaths that
are relatively resistant to degradation. The primary precipitate in this part
of the system is calcite, which tends to nucleate on organic surfaces, eventually
forming small spherulites of calcite that accrete as they are rolled downslope
along the floors of shallow channels (Fig. 3F). As the spherulites grow, they
entomb cyanobacterial sheaths, epiphytic diatoms, and other attached organisms,
which are often seen in thin sections of spherulites.
FACIES MODEL FOR TRAVERTINE THERMAL SPRINGS
Observations of modern springs at Angel Terrace were used to construct the
integrated biofacies and lithofacies model shown in Figure 4. The model is organized
around systematic variations in temperature, pH, and flow rates typically observed
along spring outflows in the Mammoth Hot Springs area. Such facies frameworks
are useful tools for reconstructing the paleobiology of ancient sinter deposits.
We have recently extended this model to include important aspects of aqueous and
solid-phase geochemistry, documenting systematic trends in oxygen and carbon isotopes,
and elemental abundances along spring outflows (Fouke et al., 2000). Systematic
variations in carbon and oxygen isotope abundances with declining temperature
were entirely accounted for by CO2 outgassing and evaporation, with
no evidence of significant biological fractionation, even where mats were well
developed (Fouke et al., 2000). This result emphasizes the importance of textural
information in reconstructing the paleobiology of ancient subaerial sinters.
HYDROTHERMAL PROSPECTING ELSEWHERE IN THE SOLAR SYSTEM
Because subsurface fluids and crustal heat sources could have also coexisted on other planetary bodies in the solar system, hydrothermal deposits are also important targets for planetary exploration and the search for extraterrestrial life. Potential targets for hydrothermal systems include active vents on the floor of a putative subsurface ocean on Europa (Reynolds et al., 1983) and possibly other icy satellites in the outer solar system (Pendleton and Farmer, 1997). Results from the Galileo mission have significantly advanced our understanding of Europa, providing support for a substantial subcrustal ocean maintained by tidal frictional heating of the Moon's interior. Given the potential for abundant water, a sustained heat source, and reduced compounds, hydrothermal systems on Europa could provide long-term habitats for chemotrophic microbial ecosystems similar to those found in deep-sea vent environments on Earth (Pappalardo et al., 1999).
Hydrothermal systems may also have been important during the early history of the dark asteroids (Cronin et al., 1988), which are considered the most likely parent bodies for carbonaceous (C-1) chondrites. These meteorites show evidence of extensive aqueous alteration of minerals over a temperature range of 50–100 °C. The Murchison meteorite, perhaps our best studied carbonaceous meteorite, contains a diverse assemblage of biologically important amino acids (Cronin, 1989) that were apparently synthesized on the meteorite parent body during an early, transient hydrothermal phase (Oro and Mills, 1989). Along with comets, carbonaceous meteorites are believed to have contributed significantly to the early inventory of prebiotic organic compounds needed for the origin of life (Chyba and Sagan, 1992).
Hydrothermal environments also appear to have been widespread on Mars early in the planet's history (Farmer, 1996, 1998). Siliceous thermal spring deposits have been cited as important targets in the search for evidence of an ancient biosphere on Mars (Walter and Des Marais, 1993; Cady and Farmer, 1996). Based on our studies of terrestrial analogs, the discovery of ancient hydrothermal systems on Mars would provide access to: (1) localized environments capable of sustaining high rates of microbial productivity, and (2) high rates of mineralization (chemical precipitation), favorable for capturing and preserving microbial biosignatures. Thus, hydrothermal deposits are considered high-priority targets in the exploration for a Martian fossil record (Farmer and Des Marais, 1999).
Although active surface hydrological systems appear to have largely disappeared on Mars after ~3.5 Ga, models suggest that a global groundwater system could still be present on Mars today (Clifford, 1993; Carr, 1996). This view is supported by the presence of large outflow channels in some younger Martian terranes formed during catastrophic releases of groundwater (Baker, 1982). Many Martian outflow channels originate within chaos terranes, collapse features thought to have formed by melting of the shallow cryosphere. Some chaos features show clear associations with potential magmatic heat sources (e.g., chaos at the base of Apollonaris Patera in Figure 5) that could have sustained hydrothermal systems for prolonged periods (Farmer, 1996).
The thermal emission spectrometer is a mid-infrared mapping spectrometer presently in orbit around Mars. The spectrometer recently detected a large deposit of coarse-grained (specular) hematite at Sinus Meridiani (Christensen et al., 2000). On Earth such deposits normally form by aqueous precipitation at elevated temperatures. This discovery lends credibility to the idea that hydrothermal systems were once active in shallow crustal environments on Mars.
CONCLUSIONS
Hydrothermal systems appear to have played a fundamental role in the early evolution of Earth and in the endogenous synthesis of prebiotic organic compounds that were the basic building blocks for life. The phylogenetic information encoded in the genomes of extant thermophilic species appears to provide important clues about early biosphere evolution and the processes that shaped its history. In addition to providing a potential site for life's origin, hydrothermal environments may also have been a refuge for thermophilic organisms during the late, giant-impact events that overlapped with early biosphere evolution. Hydrothermal environments typically sustain high rates of inorganic mineral precipitation favorable for capturing and preserving a microbial fossil record and integrated studies of the microbial biosedimentology, paleontology, and geochemistry of modern and ancient hydrothermal deposits provide important constraints for interpreting the fossils of thermophilic ecosystems. Hydrothermal systems are considered primary targets in the exploration for prebiotic chemistry and life on other bodies in the solar system (e.g., Mars, Europa, and dark asteroids) and could have provided cradles for the emergence of life in other planetary systems within our galaxy, and beyond.
ACKNOWLEDGMENTS
Work on hydrothermal systems was supported by grants from the NASA Astrobiology Institute and the NASA Exobiology Program. I thank Dave Des Marais (NASA Ames), Malcolm Walter (Macquarie University, Sydney), Bruce Fouke (University of Illinois), Laurie Leshin (Arizona State University), and Norman Pace (University of Colorado) for informative discussions, and Steve Mojzsis (University of California, Los Angeles) and Horton Newsome (University of New Mexico) for valuable reviews of the manuscript. I also thank Sue Selkirk and Maria Farmer (both from Arizona State University) for technical assistance in preparing the figures.
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Manuscript received April 21, 2000;
accepted May 10, 2000.
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