Since Earth lacks a direct record of its own formation history, means for extending our knowledge into the Hadean Eon (pre-3900 Ma) of such qualities as surface temperature, atmospheric composition, volume and timing of crustal differentiation, as well as the sophistication of life forms that could have lived (survived?) then, will rely on geophysical models for these processes, and on geochemical searches for older traces found within younger rocks.
Current understanding places the window for life's appearance on Earth in the approximately 600 million years between the formation of the planet at ca. 4550 Ma and the first manifestation of sediments at ~3900 Ma; the oldest known sediments also contain chemical evidence consistent with life (Mojzsis et al., 1998). The first significant watery veneer (that could have hosted developing life forms on the planet) is assumed to have occurred within the context of meteoritic bombardments and the formation of the primordial crust. Surface states on the early Earth needed support the emergence of life require the presence and stability of liquid water; this condition has been used as the principal constraint for an origin of life.
The known Archean (pre-2500 Ma) geologic record disappears altogether at 4000 Ma, which has been interpreted to be the age of the oldest components of the North American craton (Bowring et al., 1989). On Earth, the only available direct source of information about Hadean crusts or anything else is provided by a small population of zircon grains contained in much younger detrital sediments; these zircons have been recognized to be as old as 4270 Ma (Froude et al., 1983; Compston and Pidgeon, 1986) or about 95% of the age of Earth. It is plausible that life emerged during the Hadean in aqueous environments where concentration and polymerization of biologically important molecules could occur, possibly at deep hydrothermal systems away from thermal shocks from impacts during episodes of bombardment. The nature of the earliest environments for the emergence of life remains poorly understood, but research efforts currently in progress aim to: (i) search for older remnants of crust potentially hosting sediments than are currently known, (ii) determine past mantle oxygen fugacities from ancient mineral assemblages and their compositions to place constraints on the composition of the primordial atmosphere, (iii) outline the role of liquid water in early crustal differentiation, (iv) use isotopic paleontology of remnantc carbon in ancient rocks to make inferences about early metabolic styles, and (v) look for evidence of the influence of extraterrestrial input to the sedimentary system in the early Archean.
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Compston WA, & Pidgeon, RT, Nature, 321, 766-769, (1986).
Froude, DO et al, Nature, 304, 616-618, (1983).
Mojzsis, SJ, Krishnamurthy, R & Arrhenius, G, The RNA World, Cold Spring Harbor Press, 2, 1-47, (1998).
Large micrometeorites with sizes of 50-500 µm were recovered from the Antarctica and Greenland ice sheets. Their flux measurements in the ice indicate that they represent nowadays the dominant extraterrestrial matter that survives the hypervelocity impact with the Earth's atmosphere. Mineralogical, chemical and isotopic compositions of micrometeorites show that they are mainly related to the relatively rare group of the CM carbonaceous chondrites (about 2% of the meteorite falls) and not to the most abundant meteorites (see Engrand and Maurette, 1998, for a review of earlier results).
But marked differences between these two classes of carbonaceous extraterrestrial objects indicate that micrometeorites represent a new population of solar system objects, strongly depleted in both differentiated objects and chondrules, and suggest that they could have originated in the outer solar system. This conclusion is supported by a recent comparison of the silicate mineralogy of micrometeorites with that of the coma of comet Hale-Bopp, both showing pyroxene to olivine ratios larger than those measured in carbonaceous chondrites (with the exception of the CR chondrites).
Micrometeorites contain complex organic molecules in contact with potential catalysts for hydrolysis reactions of their organic matter. They can be considered as microscopic chemical chondritic reactors, which could initiate a complex "shooting-star" chemistry with gases and waters in favorable planetary environments such as oceans on Earth, Mars and Titan. If the composition of micrometeorites and meteorites is somewhat invariant with time, micrometeorites could have brought a significant amount of reduced carbon to the surface of these planetary bodies about 4 Ga ago, when their flux was enhanced by a factor of about 1000. In addition, such microscopic chemical reactors could have contributed to the synthesis of prebiotic organic molecules on the early Earth and Mars. Moreover, the distribution of their D/H ratios measured in the constituent water of their hydrous silicates agrees with the distribution measured for the terrestrial oceans. This could also imply that such a CM-type carbonaceous material depleted in chondrules may have contributed to the formation of the oceans at an even earlier time.
Engrand C & Maurette M, Meteoritics Planet. Sci., 33, 565-580, (1998).
The major share (> 90%) of rare gases of the present-day atmosphere were settled very early, possibly at the end of the accretional period. If CO2 and molecular nitrogen followed a similar history, then the partial pressure of these volatiles might have been more then sufficient to provide adequate conditions for the development of life. Critical to this assumption is the ratio between rare gases and major volatiles that is assumed for the parent carriers or reservoirs, as well as the fractionation between these classes of elements during atmosphere formation. The present amount of carbon at the Earth's surface is sufficient to provide a CO2 partial pressure at the Earth's surface equivalent to approx. 40 bars. This constitutes a lower limit since carbon has been efficiently recycled in the mantle. Assuming a chondritic C/Ar ratio, one obtains a CO2 pressure of the order of 103 bars using the present-day 36Ar content of the atmosphere, clearly inadequate for providing ad hoc environmental conditions early in the Earth's history. A similar conclusion is reached in the case of nitrogen. To solve this apparent discrepancy, three types of possibilities may be advocated. (i) carbon and nitrogen were trapped in silicates or metal more efficiently than rare gases. The possibility that the core may contain ~ 1% C (B.J. Wood, 1993) provides a reasonable terrestrial reservoir for carbon since it may account largely for the "missing" C at the Earth's surface. (ii) the Earth's surface was contributed by a component having volatiles in non-chondritic proportions. Candidates include a solar-type end-member, but this possibility is at odds with isotopic signatures of surface volatiles; or comets, although mass balance considerations suggest limited contribution of such end-member (this conclusion may however evolve when precise cometary compositions become available, N. Dauphas et al., submitted) (iii) surface volatiles were depleted and fractionated during atmospheric escape, in which case carbon should not have been present as CO2 in order to be largely depleted relative to argon.
In all cases the volatile composition of the mantle provides critical constraints on early environmental conditions. Except neon (and, possibly, helium), volatiles in the mantle are not solar, but chondritic. Evidences for that includes a non-solar 38Ar/36Ar ratio (B. Marty et al., EPSL, in press), and near-chondritic elemental ratios (N/Ar, Ne/Ar). The C/N and C/Ar ratios are higher than chondritic, and this C excess reflects in fact efficient carbon recycling. These observations strongly suggest that volatiles were contributed to our planet after core formation, and that the surface inventory evolved following specific contributions different from those of the mantle, and later on through surface-mantle interactions. Discounting carbon recycling allows to estimate a CO2 partial pressure of ~102 bar at the Earth's surface, and poses the problem of carbon precipitation during the Hadean.
Wood BJ EPSL 117, 593, 1997
For the kind of life we know on Earth to emerge from inanimate components, several specific geochemical requirements apply. An obvious condition is the generation of organic source molecules. A second is the requirement for decrease of entropy by selective concentration, ordering and catalytic bond formation between source molecules to form components of bioactive oligomers.
Experimental exploration of these processes suggests positively charged intercalating double layer metal hydroxide minerals such as hydrotalcite and green rust as effective concentrators and reactors, producing sugar phosphates such as ribose-2,4-diphosphate, the backbone component of p-RNA. They could thus assume the role of primitive compartments, where growth of biomolecular precursor species of increasing complexity could have taken place.
The structures and observed molecular interactions in such host minerals are discussed.
The basal position that extant hyperthermophilic organisms occupy inrooted phylogenetic trees has been interpreted by some authors as evidence of a heat-loving last common ancestor of all living beings, and extrapolated to assume a high-temperature origin of life. In order to gain insights on the nature of early biological systems, we have used an alternative approach based on the comparison of completecellular genomes. The resulting set of sequences, which is likely to be an essential and highly conserved pool of proteins common to all organisms, is dominated by molecules involved in gene expression and RNA metabolism. These findings support the hypothesis that during early cellular evolution RNA molecules played a more prominent role. Given the chemical liability of RNA, our findings place strong environmental constraints on the geochemical conditions in which early biological evolution may have taken place during early Archaean times.
Carbon isotope ratios in the oldest known sedimentary rocks in West Greenland indicate biological activity on Earth as early as 3.8 Ga (1). This places the emergence of life in the Hadean Eon (~4.6-3.8 Ga), contemporaneous with the heaviest bombardment of Earth by comets and asteroids. In part because of the protection afforded against such impacts (2), but also because of favorable energetics for organic synthesis reactions (3, 4), it has been argued that life may have emerged in submarine hydrothermal systems where entrained seawater and hydrothermal fluids mix. We calculate that the chemical disequilibrium states in these systems due to mixing provide sufficient energy to synthesize amino acids from CO2 or CO, H2, H2S, and NH4+. Constraints on Hadean ocean compositions, derived in part from atmospheric models of Kasting (5) and reaction-path models of hydrothermal fluids (6), were combined with values of standard Gibbs free energies as functions of temperature and pressure to evaluate overall Gibbs free energies of amino acid synthesis reactions. Reaction of hot (400°C), reduced, alkaline hydrothermal fluids with warm (85°C), CO2-rich, mildly acidic seawater are extremely conducive to amino acid synthesis. A 20 to 1 mass ratio of seawater to hydrothermal fluid produces a 100°C solution in which 17 of the 20 amino acid synthesis reactions using CO2 as the carbon source are exergonic even at millimolal concentrations for each amino acid. As an example, the reaction to form phenylalanine would release ~375 kJ/mol! Millimolal concentrations of amino acids are approximately six orders of magnitude greater than the concentrations in present-day seawater, but are likely to be similar to intracellular concentrations in living microorganisms. We postulate that the emergence of a metabolic system capable of amino acid synthesis on the Hadean Earth was an inescapable consequence of the disequilibrium states established in hydrothermal systems.
Mojzsis et al, Nature, 384, 55-59, (1996).
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Shock and Schulte, Journal of Geophysical Research, in press, (1998).
Amend and Shock, Science, 281, 1659-1662, (1998).
Kasting, Science, 259, 920-926, (1993).
McCollom and Shock, Journal of Geophysical Research, 103, 547-575, (1998).
Detailed stratigraphic documentation of meter to submillimeter-scale banding and lamination of c. 3.8 Ma Isua banded iron formation (BIF) has been undertaken using hand samples and up to 300 m long drill cores. The fine-scale lamination in the BIF is likely to result from temporally cyclic chemical changes of initially unknown periodicity in the sedimentary environment. Investigation of lamination over long sequence intervals may provide suggestive evidence of the nature of the cyclicity. An attempt is also made to identify stratigraphically widely separated volcanic ash layers in the sediment column. This would possibly allow determination of corresponding time intervals using radioisotopic methods. A reliable chronology, enabling estimates of sediment accumulation rates is also a prerequisite for constraining the early Archean flux of extraterrestrial debris using for example Ir in the metasediments as a tracer (cf. Mojzsis, 1997).
The accumulation of Isua BIF partly overlapped with the period of the lunar late heavy bombardment and its decay between 3.9 and 3.7 Ma. Related events have not left any visible traces in the comparatively short metasediment sections so far investigated. However, access to the sedimentary record in the drill cores allows the study of relatively long time intervals of the BIF history, enabling the establishment of any markedly enhanced supply of extraterrestrial material.
Since these metasediments have been subjected to amphibolite grade metamorphism, much of the initial textural and mineral evidence of extraterrestrial material may have been lost. A resistant cosmic tracer is however provided by the platinum group metal concentrates (nuggets; Brownlee et al., 1984) that may form at atmospheric melting of extraterrestrial debris. Sudden enhancement of aluminosilicates in the BIF microstrata could possibly also serve as an indirect impact indicator, reflecting the effects of tsunami waves in the basin in the form of coastal erosion, and the resulting increased supply and accumulation of detrital material.
Brownlee DE, Bates BA & Wheelock MM, Nature, 309, 693-695, (1984).
Mojzsis SJ, PhD Thesis, University of California, San Diego, 300 pp., (1997).
Searching for traces of life on Earth in the early Archaean is fraught with difficulty. Due to geological activity, most rocks older than 3.5 Ga have been removed from the surface. The very few formations that are still exposed, have all been subjected to high or intermediate grade metamorphism. High temperature and pressure during these metamorphic episodes have deformed any morphological fossils beyond recognition, turned organic matter into graphite, and may have affected unprotected stable carbon isotope signatures that could be indicative for a biogenic origin. Hence there is a need for identification of properties that can be relied upon for deciding between a biogenic or abiogenic origin of solids in highly altered rocks.
A chemofossil has been defined as a bioorganic information repository resistant to physical chemical changes since formation (Mojzsis and Arrhenius, 1998). Here a new type of chemosfossil indication is proposed; specific trace element abundances in graphite. A group of transition elements (including V, Ni, Co, Cu, and Mo) is commonly found in excess in biologically derived sedimentary material; in part due to endemic concentrations in living matter, but principally due to transition element complexation by specific organic molecules like porphyrins during sedimentation and burial. The metallated compounds are incorporated into the kerogen and it is speculated that the metals are retained during graphitization in the form of carbides or sulfides. Abundances of such trace elements, are expected to be lower in graphite formed from a non-biogenic source, e.g. at reduction of carbon dioxide in metamorphic fluids. Since the graphite itself is chemically unaffected by high grade metamorphism, the excess trace metals in graphite are expected to remain as resistant potential biomarkers.
In order to evaluate this hypothesis, trace metal concentrations of biogenically and abiogenically derived graphite in metamorphic rocks and synthetic preparations are compared with corresponding concentrations in graphite, isolated from >3.7 Ga old metasediments from the Isua Supracrustal Belt in southern West Greenland. The high melting points of graphite, metal carbides and sulfides suggest that this test may be applied to even partly melted rocks. Tracing life in ancient rocks then would not be restricted to moderately metamorphosed sediments, but could be applied to highly altered and partly melted early Archaean gneisses. Analyses of graphite ash are presented and discussed in terms of applicability to the proposed interpretation.
Mojzsis S & Arrhenius G, Orig. Life. Evol. Biosphere, (in press).
The NERCMAR drillcore through the Belingwe greenstone belt revealed sequences containing organic-rich horizons and sulphide facies ironstones (the Manjeri Formation) between basement granite gneiss and overlying Reliance Formation komatiitic volcanics (Hunter et al., 1998), and which were poorly preserved in outcrop. Stable isotopic analysis of these horizons and additional analyses from shallow drill and outcrop samples in both Manjeri and Cheshire Formations (Abell et al., 1985), allows tentative reconstructions of the bacterial processes taking place.
Comparison of stromatolitic and non-stromatolitic outcrop data and core samples reveal significant differences between the lower Manjeri Formation and the upper Cheshire Formation, the two sequences being separated by the thick volcanic pile comprising the Reliance and Zeederberg Formations. Cheshire formation stromatolites represent a cyanobacterial mat community with a narrow range of 13C of carbonates close to 0‰. The associated Cheshire shales contain kerogen with most values within the range -37‰ to -44‰, suggesting the activity of methanogenic bacteria throughout the sequence. The older Manjeri stromatolites also have carbonate 13C close to 0‰, though extracted kerogen is consistently around -22‰. Greater heterogeneity of carbon isotopic values is recorded in sulphide-rich horizons of Manjeri Formation drillcore. Some organic horizons intercalated with sulphide ironstones give evidence for methanogens (-37‰), but most kerogen values are around -30‰. Significant sulphur isotopic heterogeneities exist in the ironstone horizons (34S of -19‰ to +17‰, see Grassineau et al., 1997). Disseminated sulphides away from these horizons show more restricted values. The general trend is from +3‰ at the base of the NERCMAR core, to 0‰ in the middle, and -5‰ throughout one of the Cheshire shale horizons.
Worldwide evidence suggests that methanotrophy is present in many sedimentary successions at the Archaean-Proterozoic transition, but earlier evidence of methane cycling is more sporadic (Hayes, 1994). The isotopic data from the 2.7 Ga Ngesi Group record a snapshot of oxygenic photosynthesis and methanotrophic activity, and record a simultaneous increase in the activity of sulphur-reducing bacteria.
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Grassineau et al., J. Conf. Abs.,4, (1997)
Hayes JM, Early Life on Earth - Nobel Symposium, Columbia University Press, 84, 220-236, (1994).
Hunter MA, Bickle MJ, Nisbet EG, Martin A & Chapman HJ, Geology, 26, 883-886, (1998).
Recently the representation about structure-organismic constraint of the abiogenesis, generation of live organisms from their nonbiologic predcessors, from the mineral individuals of hydrocarbon structure receive more and more wide development and confirmation in the decision of the problem of the origin and early development of life. It proves by new experimental data, recent discoveries of natural structure-ordered hydrocarbon systems, including complex polymer crystals, establishment of the phenomenon of abiogenic synthesis biologically of the important molecules and protein amino acids during high-temperature formation and metamorphysm of solid bitumens. One prominent example of such prebiologic mineral organisms are fibrous kerite crystals from pegmatites.
Kerite crystals show fibrous and cylindrical habits, frequently with spheres at the ends and internal axial channel. Spiral-like individuals twisted in one direction (left or right; chiral selection is carried out according to the epitaxial mechanism) as well as complex regeneration aggregates are frequently observed. Fibrous kerite crystals have elemental composition nearly identical to that of protein (H=5.02-7.06%; O=9.00-23.00%; C=60.38-76.51%; N=8.79-8.92%). They contain all chemical elements typical of the living matter and all elements-catalysts. Heating the crystals in the range from 293 to 973 K resulted in release of a variety of hydrocarbon gases to the inner channels and environment. The crystals are distinguished by anomalously high contents of all "protein" amino acids, which are synthesized from abiogenic components during crystallization. Protein self-assembly and evolution of some organismic functions described as biological ones are possible.
We relied on fibrous kerite crystals to develop a model of a protobiologocal organism, genetic predecessor of biological life forms and propose a concept of hydrocarbon crystallization of life (Yushkin, 1996; Yushkin, 1998).
Life formed and evolved as one single whole, an integral seguence of crystallization processes occuring in complex hydrocarbon systems, not as a result of random events and combinations of genetically different components. Both minerals and bioorganisms evolve governed by common ontogenetic laws.
In evolution of traditional conceptual currents during development of problems of an abiogenesis, holobiosis and the genobiosis, organismobiosis as the most realistic, i. e. evolution of structures and functions in order molecular hydrocarbon systems, protoorganisms which gave rise to byological systems is put forward by us.
Yushkin N, Journ. of Crist. Growth., 167, 237-246, (1996).
Yushkin N, Instruments, Methods and Missions for Astrobiology. SPIE, 344, 234-246, (1998).
The unambiguous detection of ancient life is a crucial necessity in assessing the timing of life on our planet. To-day, the strategy to reveal features of past life forms is also of utmost importance towards an approach to seek out living beings on other planets. Among other very few biomarkers used today (stromatolite structures, autigenic minerals, biological degradation compounds and isotopic analysis), morphological recognition of living forms still plays a critical role in Precambrian micropaleontological studies. The underlying principle supporting life detection using morphological and textural tools derived from the old idea that there is a substantial morphological difference between the inanimate and the animate worlds: it was thought that certain complex shapes with non-crystallographic symmetry were characteristic of life. The most recent and conspicuous application of this "law" is the fossil-like microstructures found in ALH84001 meteorite. However, complex inorganic shapes with symmetry properties characteristic of primitive life form under laboratory conditions emulating geological scenario, thus becoming of geological interest as they can be used as geochemical markers. This is the case of carbonate precipitation in silica-rich alkaline brines forming what I called induced morphology crystal aggregates [Garcia-Ruiz, 1985]. Morphological emulation is particularly dramatic when barium or strontium carbonate are precipitated while textural emulation is clear in the case of calcite [Garcia-Ruiz, 1994]. These biomimetic carbonates (of barium, strontium or calcium) patterns are obtained in the laboratory into alkaline silica-rich brines. Extreme as these conditions can be (pH > 10; silica concentration > 500 ppm), there are today few lakes, particularly in the African Rift Valley, where these precipitation environment can be found. Therefore, beyond a chemical curiosity or materials science innovation, this biomimetic carbonate precipitation is a plausible phenomenon under natural conditions [Garcia-Ruiz, 1998]. I claim in this communication the importance of considering this bizarre precipitation phenomenon when detecting primitive life here on Earth or on terrestrial planets. Silica biomorphs form into environments which are likely to be widely spread during earlier stages of the Earth and Mars, planets which are though to share a similar geological history during the Archean (Earth) and Noachian (Mars) periods [Chyba & McDonald, 1995]. In particular, some terrestrial Archean (baritic) cherts are though to be the result of inorganic silica-precipitation under alkaline conditions. Silica leached from alkaline volcanic rocks increases the pH and, since there were no silica absorbing organisms during the Archean times, the pooling waters becomes enriched in silica. This draws a extremely favorable geochemical scenario for the precipitation of silica biomorphs. Similarly, the process leading to precipitate carbonate particles with properties of induce morphology crystal aggregates is much likely to have been active in the geochemical scenario proposed for the carbonate formation in ALF84001 meteorite [McKay et al., 1996; Shearer et al., 1996].
García-Ruiz JM, J. Crystal Growth, 85, 251-262, (1985).
García-Ruiz JM, Origin of Life and Evolution of the biosphere, 24, 451-467, (1994).
García-Ruiz JM, Geology, 26, 843-846, (1998).
Chyba CF & McDonald GD, Annual Review of Earth and Planetary Sciences, 23, 215-249, (1995).
McKay D et al, Science, 273, 924-930, (1996).
Shearer CK , Layne GD , Papike JJ & Spilde MN, Geochim et Cosmochim. Acta, 60, 2921-2926, (1996).
Early Archean continental crust subsoil and/or uranium deposits of the same age have been proposed as the most suitable environments for the prebiotic synthesis to occur. Two types of energy sources were available: the radioactive from the U-Th-40K nuclides decay in both environments, and the uranium nuclear fission. Coming from the mantle the radioactive isotopes were concentrated preferably in pegmatites, from which uranium placer type deposits developed. Large concentrations (and masses) could have been widely and/or locally reached. The result was an unusual body: a nuclear reactor (Nagy et al., 1993; Draganic,1983; Garzon,1996). The radioactive sources were: The U-Th-40K mineral grains and the radioactive radon gases acumulated in the voids (Miller et al., 1976).
Within the crustal voids the ionizing particles emitted by the radioactive source (mainly from 222Rn decay) along with the presence of water, clay minerals and Fe(II) cations, allowed the synthesis of small molecules. The inventory of the ubiquitous continental radioactive source, up to about 1 km deep, some 4 Ga ago, was about 1018 J/a , that is similar to that of electric discharges. The role of radioactivity, in uranium-deposit environments of the same age, must have been even more relevant in favoring the chemical evolution.
On the other hand, nuclear fission could have provided an oscillatory thermal source for the biopolymers synthesis to occur. By using one thermal neutrons group we have obtained the most relevant parameters, for example, critical mass, Mc, as a function of U-concentration, water content and age. Typical values of Mc are about 10-30 kg.
Several empirical and/or observational facts are coherent with the environments and mechanisms proposed:
1.-The presence of carbonaceous matter in certain uraninites (Schidlowski, 1981). 2.-The content of some carbonaceous matter in the large pitchblende lenses in the Oklo deposit (Nagy et al., 1993). 3.-The existence of some carbonaceous radioactive minerals, being THUCHOLITE the most relevant of them. 4.-The presence of organic compounds in certain meteorites. 5.-There is also an acceptable correlation between the geographical distribution of the Archean microfossils and that of the early uranium deposits (Schopf, 1983).
Schidlwoski, M, US Geological Survey, Prof Paper 1161-N, 29 (1981).
Schopf, W J, Earth´s Earliest Biosphere, Princenton University Press, N-J (1983).
Draganic I G. and Draganic Z D, Precambrian Res., 20,283, (1983).
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Nagy et al., Geology, 21,7, (1993).
Since the pioneering work by Nier and Gulbransen (1939) it is known that the incorporation of inorganic carbon into living systems entails sizeable fractionations of the stable carbon isotopes. Consequently, it became firmly established that the observed bias in favour of the light isotope (12C) characteristic of biogenic substances derives, for the most part, from the isotope discriminating properties of the principal carbon-fixing enzyme (ribulose-1,5-bisphosphate carboxylase) that is operative in the main chemosynthetic and specifically photosynthetic pathways, channeling most of the carbon transfer from the nonliving to the living world. With most biochemical processes enzymatically controlled, and all living entities representing stationary states undergoing rapid cycles of anabolism and catabolism, it is generally accepted that the 13C/12C fractionations observed are mostly due to kinetic rather than equilibrium effects.
Most importantly, biological carbon isotope fractionations are basically retained when organic carbon is incorporated in sediments, the enzymatic isotope effect thus being propagated into the rock section of the carbon cycle. The carbon isotope archives preserved in sedimentary rocks have been shown to constitute important sources of information regarding the global state of the terrestrial carbon cycle and the biosphere through geologic time. This holds particularly for the extension of the isotope record to the early Precambrian, testifying that biologically mediated carbon isotope fractionations have persisted over 3.5, if not 3.8 Ga, of recorded Earth history. Since changes in 13Corg and 13Ccarb observed in the oldest terrestrial sediments can be referred to a high-t isotopic reequilibration between the two carbon species in response to amphibolite-grade metamorphism, there is little doubt that the biological signature of the record had originally extended to the very beginning of the presently known record (Schidlowski et al., 1979, 1983), a conclusion recently confirmed by a novel approach utilizing advanced microanalytical (ion microprobe) techniques (Mojzsis et al., 1996).
Postulating a universality of biological principles in analogy to the proven universality of the laws of physics and chemistry, we may expect enzymatic reactions in exobiological systems to be beset with similar kinetic fractionation effects. Hence, the retrieval from the oldest Martian sediments of isotopic fractionations between reduced and oxidized (carbonate) carbon may substantially constrain current conjectures on the possible existence of former life on Mars.
Mojzsis SV, Arrhenius G, McKeegan KD et al, Nature, 384, 55-59, (1996).
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All known rock complexes > 3600 Ma old have been strongly modified by deformation, metamorphism and metasomatism. The earliest organisms on Earth left little or no morphological information about their mode of living, that could sustain these intense geologic overprints. We must therefore rely on geochemical and isotopic evidence for their presence, and couple this evidence with geologic evidence for the depositional environments of the rocks in which the isotopic evidence for life are bestowed. This, however, requires that we understand the geochemical behavior of carbon and define the carbon reservoirs that interacted during high grade metamorphism.
In the > 3700 Ma Isua supracustals the carbon isotopic composition of graphite derived from sedimentary biogenic reduced carbon in metamorphosed pelagic shales were variably altered by metamorphic recrystallisation and reaction with vein carbonate. This alteration is seen as an increase in: 13C from primary values of -20 to ~ -15 which correlate with the extend of metamorphic recrystallization and with the degree of disturbance of the U-Pb isotopic system during a late Archaean event. Oxidation of graphite was associated with uranium enrichment. In general, redox reactions involved in changes in carbon speciation are likely to strongly affect the U-Pb systematics of the rocks that host the carbon. Thus whole rock Pb-Pb isotopic studies provide a powerful tool for assessing the extend of metasomatic alteration, for identifying the contributions form different carbon reservoirs, and for selecting samples that preserve primary carbon isotopic values.
The biological sulfur cycle is believed to be rooted in the beginning of earth history with sedimentary iron sulfide as possible base for the initiation of life. Therefore, scientists have studied ancient sedimentary rocks for clear evidence of biological activities linked to the sulfur cycle, notably the process of bacterial sulfate reduction and subsequent formation of sedimentary iron sulfide. Research has been guided by the observation that this process is associated with a strong displacement of the sulfur isotopic composition during transfer from seawater sulfate to sedimentary sulfide. As a consequence, variable but frequently strongly negative sulfur isotope values characterize the ultimate reaction product, i.e. sedimentary pyrite. The isotopic composition of this reduced form of sulfur is clearly separated from the composition of seawater sulfate. In modern marine settings, this kinetic isotope effect can be as much as several tens of permil.
Observations from early Archean sedimentary rocks are quite different. Previous studies of the Isua Metasediments and more recent accounts for the South African Archean have shown that the isotopic composition of sedimentary pyrite from early Archean successions displays a narrow range with an average close to 0‰. In addition, seawater sulfate is believed to have an isotopic composition around +3‰, as evident from early Archean barite deposits. Thus, reduced and oxidized forms of sulfur in sedimentary rocks display very similar isotopic compositions which are close to average values observable for magmatic sulfur. Alternatively, it has been suggested that the reduced fractionation is a biological response to different environmental conditions.
Results from an ongoing study of the sulfur isotopic composition of sulfides associated with the Isua metasedimentary succession yielded an equally narrow range of values between -1.5 and +4.5‰. Data have been derived from iron sulfides associated with the banded iron formation located in the northeastern part of the Isua Belt. These results are similar to previously published values for the Isua sequence and in good agreement with datasets from other early Archean successions. In comparison to recent marine sediments, both the absolute values for iron sulfides as well as the small isotopic difference between reduced and oxidized sulfur provide no clear evidence for a biological origin of the iron sulfides.
Important insights into the nature of early life are emerging from molecular biology, paleontology, and geochemistry. In particular, the sequencing of DNA, including entire genomes from diverse microorganisms, has shed light on the evolutionary history of prokaryotes and on how living prokaryotes function. In paleontology, the known record of cellularly preserved bacterial fossils has been extended to nearly 3.5 billion years ago (Schopf, 1993). And in geochemistry, analyses of the carbon isotopic composition of sediments suggests life existed as early as 3.8 billion years ago (Mojzsis, et al. 1996) and documents the more or less continuous presence of biologic activity from about 3.6 billion years ago to the present (Strauss and Moore, 1992). A merging of these diverse lines of evidence may help unravel the biochemical characteristics of Earth's earliest biosphere and help constrain the timing of molecular evolutionary events.
As shown by ion microprobe analyses of individual Precambrian microfossils, measurements of the carbon isotopic composition of ancient microscopic specimens provide promising means to decipher the biochemistry of early life. The 13CPDB values determined for four 6 to 10 µm-diameter spheroidal microfossils (Glenobotrydion) from the 850 Ma-old Bitter Springs Formation of central Australia (exposed at the upper surface of a polished petrographic thin section and analyzed using the CAMECA 1270 ion microprobe) are -25.1±0.9‰ (n = 6, MSWD = 2.5), -26.8 ± 1.4‰ (n = 8, MSWD = 1.4), -26.8.0 ± 0.8‰ (n = 6, MSWD = 1.2), and -29.1 ± 0.7‰ (n = 6, MSWD = 1.1). A larger spheroidal microfossil (Myxococcoides) has a value of -28.8±1.4‰ (n = 7, MSWD = 0.4), and two microbial sheaths (Eomycetopsis) have values of -23.6±1.2‰ (n = 6, MSWD = 3.8) and -28.3±1.4‰ (n = 7, MSWD = 0.3).
This preliminary work indicates that reliable carbon isotopic data can be obtained from ancient microfossils with a precision and accuracy of about 1‰ and suggests the presence of isotopic heterogeneity among specimens in the same deposit. Analysis of the Bitter Springs microfossils support their morphologic based assignment to the crown group cyanobacteria whereas preliminary studies of microfossils in the ~2,000 Ma-old Gunflint Formation of southern Canada suggest the presence of other microbial groups as well. In summary, work to date demonstrates the feasibility of ion microprobe analyses of individual microfossils and suggests that this technique holds promise for constraining, and perhaps deciphering, the physiology and phylogenetic relations of ancient microorganisms.
Mojzsis, SJ, Arrhenius, G, McKeegan, KD, Harrison, TM, Nutman, AP, & Friend, RL, Nature, 384, 55-59, (1996).
Schopf, JW, Science, 260, 640-646, (1993).
Strauss, H & Moore, T, The Proterozoic Biosphere: A Multidisciplinary Study, Cambridge University Press, 709-796, (1992).
The narrowing of the window for emergence of life on Earth combined with new observations on Mars is placing doubt on some of the popular speculations about the accretion and early evolution of the terrestrial planets and the Moon. Indications of enhanced extraterrestrial material influx or impacts at the time of late heavy bombardment of the Moon and its expected decay appear to be absent in the sedimentary rock record on Earth, suggesting that the record on the Moon reflects events in lunar orbit, or that the terrestrial sedimentary sequences investigated derive from calmer time intervals, between spikes in impact rate.
Underlying observations are reviewed and possible explanatory scenarios for lunar evolution discussed.
A considerable amount of excitement has arisen over the possibility that a liquid water layer lies beneath the surface of Europa. Liquid water is one prerequisite for life as we know it, and the other is a source of energy, which at Europa's distance from the sun would most likely be internal heat which can drive hydrothermal systems akin to those at Earth's mid-ocean ridges. The lifetime of a Europan ocean and the amount of heat being exchanged into it can both be assessed through models of the thermalhistory of Europa's interior. The lifetime of a Europan liquid-water layer depends on the balance between heat loss at the surface and heat supplied by tidal heating and radioactive decay in the rocky interior. Parameterized convection models utilizing recent results in heat transfer in temperature- and stress-dependent viscosity fluids indicate that even with large amounts of tidal heating (10% Io's value) a pure water ocean on Europa will onlylast about 100~Myr before freezing solid. The presence of ammonia in the ocean leads to an interesting situation: if the heat supply is great enough, a few tens of km of liquid ammonia-water may exist beneath the ice. Whether or not the temperatures in the silicate interior of Europa are sufficient to drive the volcanism necessary for hydrothermal circulation is less clear. Although it is possible to exceed the melting point of silicates due to radiogenic heating, it is most likely that this happened early in Europa's history, and that today the silicate mantle is entirely sub-solidus.
We have investigated either biogenic structures (Hofmann and Farmer, 1997) or biogenic etching (Fehrenbach et al., 1984; Callot et al., 1987), in minerals and rocks, produced by filamentary microorganisms, in non-sedimentary environments. Biofabrics in minerals resulting from the mineralization of microbes in subsurface environments have been identified in oxidized ore bodies and low-T hydrothermal minerals in volcanic rocks from a wide range of localities. Spectacular microscopic "biolithographs" of filamentous siderobacteria are produced in minerals and rocks through powerful processes of etching that are exacerbated during drastic changes in climatic condition. Both subsurface biofabrics and biolithographs are expected to be both much more durable than other types of fossil evidence for ancient life and quite characteristics of the families of organisms. For example, they are not destroyed during subsequent heating at high temperatures, oxidation and flushing by waters. The spores of these microorganisms act as very powerful "hydraulic jacks" as to open their host microfracture network in rocks, thus changing its fractal dimension. Environments of formation for subsurface biofabrics are any water bearing rocks, and for lithographs both surface and subsurface environments. Suitable environments for the development of both types of structures are likely to have existed on the surface and in the subsurface of Mars and Earth. If primitive forms of life did evolve at an earlier time on Mars, they have probably produced biogenic structures of this type in some of the Martian samples to be hopefully returned to the Earth soon in the twenty first century. A preliminary attempt to find micro-lithographs either on polished sections and/or fracture surfaces of the meteorite ALH84001 from Antarctica, supposed to originate from Mars, has been negative as yet.
Hofmann BA & Farmer JD, Conf. on Early Mars, Houston, LPI contrib, 916, 40-41, (1997).
Fehrenbach L, Maurette M, Guichard F, Havette H & Monaco A, Journ. Non Cryst. Solids, 67, 287-501, (1984).
Callot G, Maurette M, Pottier L, Dubois A, Nature, 328, 147-150, (1987).
In origin of life studies, the search for the oldest microfossil evidence is challenging because of the high degree of post-depositional alteration of Early Archean sedimentary rocks that has occurred over the aeons. We present here a study of a partially unaltered Dolomite Unit, located in the Coongan Belt of the eastern Pilbara Craton granite-greenstone terrain of NW Australia, which has a well-constrained deposition age between 3.470 and 3.454 Ga. Our research on microbial mediation during dolomite formation under anaerobic conditions led us to postulate that Early Archean dolomite might contain evidence for early life. Although rare, the Early Archean dolomite potentially is an excellent candidate in which to search for evidence of early microbial life because its surface can easily be etched to reveal microfossils using high resolution scanning electron imaging.
The Coongan Belt is deformed, but the deformation is localised in shear zones, subparallel to the steepened bedding, leaving in between internally undeformed thrust slices. The studied Dolomite Unit is preserved in one of these slices. It forms part of the dominantly mafic volcanic Warrawoona Group directly overlying the felsic volcanic Duffer Formation. It is, in turn, overlain by pillow basalt of the Salgash Subgroup, which mark the termination of an exceptional, approximately 200-m-thick sedimentary sequence revealed in a deep creek incision where the Dolomite Unit has been eroded to a deeper level. Observed facies changes and sedimentary structures indicate a shallowing upward within the sequence. Silica deposition in a deeper water environment is followed by alternating beds of dolomitized breccia and cross-bedded sandstone deposited on an outer shelf below wave base. The sequence culminates in micritic dolomite deposition in an intertidal/supratidal mud flat environment, a setting probably covered by extensive microbial mats.
The micritic dolomite comprises thin, approximately 1 mm or less, alternating darker and lighter grey, undulatory laminations criss-crossed by silica veins. The remnant laminated structure is possibly indicative of a former microbial mat. Distinctive three-dimensional views of filamentous microfossils can be observed in detail in the SEM photomicrographs of the EDTA-etched dolomite surface and permit an analysis of the ancient microbe's motility. These microfossils, which we interpret to be fossil gliding microbes, may represent the earliest constructing organisms, which were capable of binding sediment to form laminae. This microbial activity probably occurred in a near-shore to subaerial environment under strictly anoxic conditions. Indeed, if correctly interpreted, the Pilbara dolomite evidence for the existence of a complex microbial community exhibiting two-dimensional movement indicates a high degree of microbial evolution at 3.45 Ga.
A wealth of new fossil data has been obtained from the well-preserved, Early Archean terrains of the Barberton greenstone belt, South Africa and the Pilbara Craton, Australia, by a new technique making use of high-resolution SEM imaging and electron dispersive analysis of thin sections and rock chips delicately etched in the fumes of HF. These studies provide valuable information about microbial diversity as well as microbial environments. Given the hypothesised similarity in the early environments of the Earth and Mars, these studies serve as a useful database in the search for life on Mars.
The samples studied include finely laminated cherts from the Onverwacht Group, South Africa (3.334-3.474 Ga), and the North Pole chert from the Warrawoona Group, Australia (3.49 Ga). These silicified sediments represent shallow water deposits which are intercolated in volcanic sequences (Walsh & Lowe, in press). The Onverwacht Group sediments were strongly influenced by local hydrothermal activity (ubiquitous tourmaline crystals, penecontemporaneous silicification). The biota lived in thick microbial mats on the sediment surface. Cracked mats containing the evaporite minerals halite, calcite and gypsum on some bedding planes indicate periodic subaerial exposure (Westall et al., in press). The North Pole Chert consists of stromatolitic layers, originally carbonates but now silicified, overlying cross-bedded sediments. This configuration suggests a shallow water environment.
The microfossils include coccoid and bacillar forms, ranging from 0.6 to 3.8 microns in size, and were observed with light and scanning electron microscopy (SEM). They have been identified on the basis of typical bacterial characteristics (size, shape, cell complexity, cell wall texture, pliant behaviour, and occurrence in colonies and consortia) and association with biofilms (or extracellular polymers). They differ demonstrably from abiogenic spheres or rod-shaped crystals. The fossils have been preserved by mineral replacement (no organic carbon measurable with an electron dispersive system) or as moulds in a mineral deposit, whereas the biofilm or polymers have been permineralised (still some measurable, residual organic carbon).
Unetched Onverwacht samples observed both in light and SEM contain 0.5-2 micron wide filaments with textures, such as tapering and rounded ends, suggestive of a biological origin. The intimate relationship of the filaments and the mineral matrix emphasises the co-formation of the structures and the enclosing sediments and, therefore, their originality.
The bacterial biofilms or polymers occur as fine, wavy laminations which are brown in light microscopy. They appear as a veil on bedding planes with an interwoven fibrous, smooth or granular texture. Strands of polymer permeate the quartz matrix. Fossil bacteria are sometimes embedded in their surfaces. Fenestrae are common between the laminations.
These observations suggest that similar Martian environments (submerged volcanic flanks, lagoons, shallow seas, all with associated hydrothermal activity) may produce evidence of past Martian life which would be visible both macroscopically (stromatolitic layering) and microscopically. Mineral casts of microorganisms would not be affected by the presumed oxidising conditions at the Martian surface.
Walsh MM & Lowe DR, Geol. Soc. Am. Special Paper, 329, (in press).
Westall Fde Wit M, Dann J, van der Gaast S, de Ronde C, & Gerneke D, Precambrian Res, (in press).
In the 2.7 Ga Belingwe belt, the isotopic and textural evidence suggest: 1) sulphate-reduction and probably photosynthetic sulphide oxidation; 2) operation of the Calvin cycle both in photosynthetic cyanobacterial stromatolites and possibly in deeper water non-photosynthetic sulphur-bacterial mats; 3) methanogenesis and methane oxidation. If so, the shallow-water cyanobacterial community would have been based on oxygenic photosynthesis, as mats and possibly also as picoplankton. Other microbial mats existed by processing sulphur oxidation states on the boundary between a reduced mud and more oxidising water, both in shallow-water (probably associated with anoxygenic and oxygenic photosynthesis), and in non-photosynthetic deeper water consortia. Locally in the Belingwe basin, coastal (and planktonic?) cyanobacteria would have provided a supply of oxidised water (and fixed nitrogen?), as well as organic detritus, especially from picoplankton, if present. In shallow water, consortia of bacteria would have exploited the supply of sulphate, nitrate and organic debris. Photosynthetic green sulphur bacteria probably oxidised H2S to S0, while sulphate and sulphur reducers operated in the reverse direction. In the Jimmy member of the Manjeri Fm., carbon-rich sulphides occur in a strata-bound setting between sediment and volcanics: in these rocks isotopic and textural evidence and C abundance imply a productive microbial mat, possibly non-photosynthetic. Light carbon in hydrothermal-associated (REE) sulphides implies though does not prove that the Calvin cycle operated in sulphur-based microbial mat, as in modern Beggiatoa.
The Belingwe communities, at 2.7 Ga, support the view that major steps in bacterial phylogeny, as revealed by rRNA studies, had taken place long previously, probably well before 3.5 Ga ago. Possibly photosynthesis evolved in a cell in a shallow-water sulphide-bacterial mat consortium, piggy-backed upon the Calvin cycle. Cyanobacteria are probably a chimaera: they include green sulphur bacteria-like PSI and purple sulphur bacteria-like PSII as well as being capable of nitrogen fixation. On them and their descendants is built the modern atmosphere.
The lowest part of the 2.7 Ga Ngezi group in the Belingwe Greenstone belt, the Manjeri Formation, is composed of sulphur-rich facies associated with organic deposits. Two main units are distinguished: 1] the basal Spring Valley Member, characterised by very dark carbon-rich sediments traversed by thin sulphide layers, and 2] the higher level Jimmy Member characterised by a massive sulphide deposit and localised organic-rich deposits.
This very well-preserved Archean formation warrants a high precision stable isotopic study in order to constrain the signature of the early life. The two units can be separated also by the stable isotopic compositions. Close to 150 sulphides were analysed, the 34S values give a wide range from -18.3 to 16.7‰ (±5.3‰), with a mean peak for 60% of the sulphides at -0.4±1.4‰. The compositions of the Spring Valley samples (from a shallow water sequence) are from -18.3 to 5.4‰ with some sulphides locally very depleted in 34S. In the Jimmy Member where the facies appears to be deeper, the 34S have a larger range (-14.9 and 16.7‰), with one 34S-enriched sample. In the same way, the wide range of the carbon isotopic compositions, from -38 to -17‰, with a major peak at -30.0‰ is slightly shifted between the two units. Spring Valley Member shows 13C between -30.8 and -18.8‰, and Jimmy Member from -37.8 to -27.9‰. In the Manjeri stromatolitic sequence, contemporaneous with Spring Valley, two pristine 13C "end-members" are distinguished: a carbon-rich bleb (? remnant microbial matter) at -35.0‰ (36% carbon) and the carbonates at 0.2±0.6‰ (12% carbon).
The wide range in 34S contrasts with the values obtained from the thick volcanoclastic sequence (-0.7±0.5‰) intercalated between the two carbon-sulphide-rich units. These values and their homogeneity indicate a volcanic signature, whereas the highly heterogeneous sequences on both sides, emphasised by carbon-rich rocks, imply biological activity, illustrated by depleted compositions (<-5‰). The shift in the range between Spring Valley and Jimmy can be explained by isotopic exchange with the slightly 34S-enriched sulphate of the seawater.
In the same way, the wide range of heterogeneous 13C values also show an organic signature, with high activity in the Jimmy Member where the light values indicate high isotopic fractionation during the biological cycle involving anaerobic and methanogenic process. The Spring Valley signature suggests the action of Rubisco in oxygenic photosynthesis explainable by the shallow water environment.
Primitive terrestrial life - defined as a chemical system able to transfer its molecular information via self-replication and also to evolve - probably originated from the evolution of reduced organic molecules in liquid water. Several sources have been proposed for the prebiotic organic molecules: terrestrial primitive atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems and extraterrestrial meteoritic and cometary dust grains. A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. They might have brought more carbon than that involved in the present surficial biomass.
The early histories of Mars and Earth clearly show similarities. Geological observations collected from Martian orbiters suggest that liquid water was once stable on the Mars surface, attesting to the presence of an atmosphere capable of decelerating carbonaceous micrometeorites. The Viking 1 and 2 lander missions did not find organic carbon in the martian soil. It was concluded that the most plausible explanation for these results was the presence, at the Martian surface, of highly reactive oxidants. The Viking lander could not sample soils below 6 cm and therefore the depth of this apparently organic-free and oxidizing layer is unknown. The two SNC meteorites EETA79001 and ALH84001, very likely from martian origin, show the presence of organic molecules suggesting that organic matter required for the emergence of a primitive life may have been present on the surface of Mars. Therefore, microorganisms may have developed on Mars until liquid water disappeared. Since Mars probably had no plate tectonics, the Martian subsurface perhaps keeps a frozen record of the very early forms of a terrestrial-like life.
A multi-user integrated suite of instruments designed to optimize the search for evidence of life on Mars is described. The package includes: Surface inspection and surface environment analysis to inspect the surface geology and mineralogy, to search for visible surficial microbial macrofossils, to study the surface radiation budget and surface oxidation processes, to search for niches for extant life. Subsurface sample acquisition by core drilling. Analysis of surface and subsurface minerals (stratigraphy) and organics to characterize the surface mineralogy, to analyse the surface and subsurface oxidants, to analyse the mineralogy of subsurface aliquots, to analyse the organics present in the subsurface aliquots (elemental and molecular composition, isotopes, chirality). Macroscopic and microscopic inspection of subsurface aliquots to search for life's indicators (paleontological, biological, mineralogical) and to characterize the mineralogy of the subsurface aliquots. The study is led by ESA Manned Spaceflight and Microgravity Directorate.
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