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Laser-Raman spectroscopy (Communication arising): Images of the Earth's earliest fossils?
Abstract
Fossil remains of the most ancient, minute forms of life on Earth and
other planets are hard to recognize. Schopf et al. claim to have
identified the biological remnant material known as kerogen in
microscopic entities in rock by using Raman spectroscopic analysis. On
the basis of a substantial body of published evidence, however, we
contend that the Raman spectra of Schopf et al. indicate that these are
disordered carbonaceous materials of indeterminate origin. We maintain
that Raman spectroscopy cannot be used to identify microfossils
unambiguously, although it is a useful technique for pinpointing
promising microscopic entities for further investigation.
... However, in 2002, the biogenicity of these cellular microfossils became the subject of intense debate (Brasier et al., 2002;Schopf et al., 2002), based on Martin Braiser's contention that the putative microfossils have an inorganic origin. More recently, detailed studies utilizing laser Raman methods, support the view that the Pilbara remains are composed of graphitized kerogen, consistent with a biological origin DeGregorio et al., 2011; but see Pasteris and Wopenka, 2002). Sugitani et al. (2007Sugitani et al. ( , 2010 reported a diverse microfossil assemblage from slightly younger stratiform chert sequences of the Strelley Pool Formation, dated at ca. 3.43 Ga, and Allwood et al. (2007) described diverse, environmentally distinct assemblages of stromatolites from the same stratiform chert sequences of the Pilbara (Fig. 3D). ...
Discoveries in geobiology have dramatically shaped our understanding of the nature, distribution, and evolutionary potential of terrestrial life, paving the way for new exploration strategies to search for life elsewhere in the Solar System. Genomic studies, applied over a broad range of geological environments, have revealed that the vast proportion of species on Earth are microbial. Studies of the fossil record indicate that this has been the case for >75% of our planet's history. Microbial life has been shown to occupy a stunning array of environmental extremes, seemingly only limited by the distribution of liquid water and its chemical activity, nutrient availability, suitable energy sources, radiation, etc. Advances in geomicrobiology have revealed important contributions of microbial processes to many global biogeochemical cycles, and in the evolution of Earth's atmospheric and surface composition. The discovery of a subsurface biosphere, fueled by inorganic chemical energy and able to tolerate extremes in temperature and salinity, has been especially important in opening up new horizons for the astrobiological exploration of Mars, as well as icy satellites of the outer Solar System. Although the environment of life's origin remains uncertain, molecular studies suggest that the last common ancestor of life probably lived in hydrothermal environments where it utilized simple compounds of carbon, hydrogen, and sulfur as sources of chemical energy. This general view is consistent with what we know about late Hadean to early Archean environments on the Earth, as well as model-based interpretations of late, giant impacts that could have exterminated early mesophilic (and possibly photosynthetic) surface life forms, leaving behind only deep subsurface chemotrophic thermophilic microbial communities to re-populate the biosphere. These and related discoveries have contributed extensively to the view that life could be much more broadly distributed, within the Solar System and beyond, than once thought. We now believe it possible that life may have become established in surface environments on Mars during the first half billion years of the planet's history, when liquid water was widespread there. Furthermore, a subsurface hydrosphere on Mars (suggested by both models and geomorphic evidence) may have provided a continuously habitable zone for life over most of Martian history and could still support an active, deep biosphere on Mars today. Exploration of the outer Solar System supports the presence of saline brines (perhaps oceans) beneath the icy crusts of Europa, Callisto, and possibly Ganymede, along with plausible energy sources for life based on chemical disequilibria between oxidized and reduced compounds. It also appears that interior zones of liquid water may also exist on Enceladus, a moon of Saturn, while hydrocarbon oceans of liquid methane discovered on Titan may provide alternative solvents for novel life forms completely unlike anything found on Earth. Ongoing efforts to systematically explore potentially habitable environments elsewhere in our Solar System have helped catalyze the development of astrobiology, an emerging interdisciplinary science that seeks to understand the origin, evolution, distribution, and future of life in the cosmos. Geobiology, which studies interactions of biological and physical-chemical systems and how they have evolved over the history of Earth, is a central focus of astrobiology, providing fertile ground for the growth of conceptual models and new technological tools needed to implement the search for extraterrestrial life elsewhere in the Solar System.
... Color images available online at www.liebertonline.com/ast Wopenka, 2002Wopenka, , 2003Marshall et al., 2010); however, in the case of a planetary mission, a positive detection would indicate samples that should either be investigated further by using additional techniques capable of assessing biogenicity, or cached for future return to Earth for more detailed analysis (Mustard et al., 2013). ...
Knowledge and understanding of the martian environment has advanced greatly over the past two decades, beginning with NASA's return to the surface of Mars with the Pathfinder mission and its rover Sojourner in 1997 and continuing today with data being returned by the Curiosity rover. Reduced carbon, however, is yet to be detected on the martian surface, despite its abundance in meteorites originating from the planet. If carbon is detected on Mars, it could be a remnant of extinct life, although an abiotic source is much more likely. If the latter is the case, environmental carbonaceous material would still provide a source of carbon that could be utilized by microbial life for biochemical synthesis and could therefore act as a marker for potential habitats, indicating regions that should be investigated further. For this reason, the detection and characterization of reduced or organic carbon is a top priority for both the ESA/Roscosmos ExoMars rover, currently due for launch in 2018, and for NASA's Mars 2020 mission. Here, we present a Raman spectroscopic study of Archean chert Mars analog samples from the Pilbara Craton, Western Australia. Raman spectra were acquired with a flight-representative 532 nm instrument and a 785 nm instrument with similar operating parameters. Reduced carbon was successfully detected with both instruments; however, its Raman bands were detected more readily with 785 nm excitation, and the corresponding spectra exhibited superior signal-to-noise ratios and reduced background levels. Key Words: Raman spectroscopy-Archean-Organic matter-Planetary science-Mars. Astrobiology 15, 420-429.
... [25] However, similar spectral shapes to those described were observed in carbonaceous matter with abiotic origin, thus leading to a debate about the origin of carbonaceous structures observed in old rocks. [25][26][27][28][29][30][31][32] Finally, it is concluded that, although the high sensitivity of Raman spectroscopy to carbonaceous matter makes it a very powerful tool for detecting potential microfossils in geological samples, the shape of the Raman spectrum alone cannot be used as a proof of biogenicity. In low grade metamorphosed silicified rocks, fossilized microorganisms can be relatively well preserved. ...
Demonstrating the biogenicity of carbonaceous microfossils can be relatively difficult because of their small size and simple shape, and to the degradation of the associated organic molecules with time. For Precambrian fossils, it generally requires the use of several techniques to study the shape and the composition of the structure itself, as well as its mineral environment. The ability to identify both organic matter and minerals using Raman spectroscopy makes it a key technique in the field of micropaleontology. Raman instruments are also being developed for the upcoming missions to Mars, ExoMars and Mars 2020, both dedicated to the search for past or present traces of life. However, demonstrating the biotic origin of carbonaceous matter in geological materials using this technique is controversial. Here, we show that Raman mapping instead of single spot analysis can detect variations in the composition of carbonaceous matter associated with fossilized microbes in the 800-Ma-old microfossils from the Draken Formation, Svalbard. This discovery is of great interest because it permits assessment of the biotic origin of a fossilized carbonaceous structure. Raman mapping could thus be of crucial importance in the near future for detecting potential fossilized microbial remains in Martian rocks. Copyright © 2015 John Wiley & Sons, Ltd.
... There are however a number of abiotic sources of reduced carbon, including meteoritic infall [61] and volcanic activity [62]. The suitability of Raman spectroscopy for the detection and characterization of such deposited carbon has already been discussed, however, it is also clear that the biogenicity of a particular population of carbon cannot be determined by Raman spectroscopy alone [55,63,64]. Nevertheless, the detection of abiotic reduced carbon on an extra-terrestrial planetary surface, such as that of Mars, would be a valuable result in itself. ...
The first Raman spectrometers to be used for in situ analysis of planetary material will be launched as part of powerful, rover-based analytical laboratories within the next 6 years. There are a number of significant challenges associated with building spectrometers for space applications, including limited volume, power and mass budgets, the need to operate in harsh environments and the need to operate independently and intelligently for long periods of time (due to communication limitations). Here, we give an overview of the technical capabilities of the Raman instruments planned for future planetary missions and give a review of the preparatory work being pursued to ensure that such instruments are operated successfully and optimally. This includes analysis of extremophile samples containing pigments associated with biological processes, synthetic materials which incorporate biological material within a mineral matrix, planetary analogues containing low levels of reduced carbon and samples coated with desert varnish that incorporate both geo-markers and biomarkers. We discuss the scientific importance of each sample type and the challenges using portable/flight-prototype instrumentation. We also report on technical development work undertaken to enable the next generation of Raman instruments to reach higher levels of sensitivity and operational efficiency.
... It does not need extensive sample preparation and can give comparatively quick information. However, when applying this method in fossil studies, the interpretation of the data is not always clear and often critically discussed (e.g in case of putative microfossils in Archaean rocks; Kudryavtsev et al. 2001;Brasier et al. 2002;Pasteris & Wopenka 2002;Schopf et al. 2002;Schopf et al. 2005). Already Marshall et al. (2012) made an approach for investigating BST type preservation on a fossil from the Cambrian Spence Shale with Raman spectroscopy. ...
Understanding the taphonomy of organic matter of sponges will be helpful in reconstructing a more exhaustive picture of the evolutionary history of these organisms from fossil records. The so-called ‘round sponge fossils’ (RSF) from the Burgess Shale-type (BST) Chengjiang Lagerstätte predominantly yield explicit organic remains, which seem more durable than the carbonaceous components of other fossils in the same Lagerstätte. In order to characterize these carbonaceous remains with Raman spectroscopy, a quick and non-destructive technique with the ability of analyzing the molecular composition and crystal structure in high resolution, 5 RSF specimens were examined in this study. Another Cambrian sponge fossil from the Xiaoyanxi Formation and a few algal remains from the Ediacaran Wenghui Biota were
also measured for comparison. The resulting Raman spectra of the macroscopic fossils confirmed previous observations on microfossils by Bower et al. (2013) that carbonaceous material with compositionally complex precursor material and low diagenetic thermal affection will plot in a certain region in a ΓD over R1 diagram. The results also successfully differentiated the sponge material from the algal material, as well as the fossil-derived signal from the background. However, it is still uncertain whether the different clustering of the RSF data and the algal data reflects the variance of precursor material or only the
diagenetic and geological history. The variance within the RSF data appears to be larger than that within the algal data. Considering the similar diagenetic history of the RSF, this is possibly reflecting the difference in precursor material. Nonetheless, further measurements on other fossil algal and poriferan material must be involved in the future, in order to improve and testify the current interpretation. Despite the properties revealed by Raman spectroscopy, the taphonomy of carbonaceous material in RSF has not been investigated. According to our observation, as well as the phenomenon described in previous studies, the preservation of the carbonaceous material in RSF does not show obvious taxonomical preferences. Because the RSF are polyphylogenetic and currently lack evidence to indicate that they represent any special development stage of sponges, we infer that this unusual carbonaceous preservation is due to diagenetic bias relating to their specific morphology, which in turn is possibly controlled by similar living environments. Again, to test these inferences, more detailed taxonomical and paleoecological studies are necessary.
... However, detecting ancient life on Earth has also been a challenge. This is because of the difficulties encountered in distinguishing ancient microfossils from pseudo-fossils [3][4][5] . Also, what has been interpreted as chemical (e.g. ...
The earliest evidence for amino acids on Earth is in Precambrian sedimentary rocks with varied metamorphic histories. Igneous rocks rarely contain such compounds, exceptions being those introduced via the migration of fluids into fractures subsequent to crystallization. Martian meteorites are excellent examples of ancient igneous rocks that apparently contain amino acids associated with minerals precipitated in rock fractures. The challenge has been to determine whether the organic compounds present in ancient terrestrial and extraterrestrial materials are indigenous and, if so, are representative of past life or pre-biotic synthesis. A summary of what is known to date about amino acids in ancient terrestrial and extraterrestrial materials is presented. Alternative approaches for distinguishing their origin(s) are discussed.
... Further, metamorphic processes have the capability of transforming complex organic hydrocarbons and disordered carbonaceous materials into ordered graphitic materials (Buseck & Huang, 1985;Wopenka & Pasteris, 1993;Beyssac et al., 2002b)albeit intrinsically difficult for some types of organic matter to graphitize, such as carbonaceous material exhibiting high microporosity and ⁄ or low oxygen fugacity (Bény-Bassez & Rouzaud, 1985;Large et al., 1994). To make things even more complicated, graphite generated from biological precursors can be microchemically indistinguishable from well-ordered abiotic graphite-derived de novo from metamorphism, both in terms of carbon isotopic composition (Van Zuilen et al., 2002) and Raman characterization (Pasteris & Wopenka, 2002), and thus, a combination of independent but mutually reinforcing morphological and biogeochemical data should be used to verify biogenicity (Schopf et al., 2002). Thus, the summation of taphonomic and diagenetic modifications to microfossil morphology and biochemistry-or 'Archeanization' (termed as such by Knoll et al., 1988)-presents serious challenges to early life exploration and raises the question: can we institute a means for distinguishing veritable biological remains (even when they are metamorphically modified) from abiotically generated imposters? ...
The identification and confirmation of bona fide Archean-Paleoproterozoic microfossils can prove to be a challenging task, further compounded by diagenetic and metamorphic histories. While structures of likely biological origin are not uncommon in Precambrian rocks, the search for early fossil life has been disproportionately focused on lesser thermally altered rocks, typically greenschist or lower-grade metamorphism. Recently, however, an increasing number of inferred micro- and macrofossils have been reported from higher-grade metasediments, prompting us to experimentally test and quantify the preservability of organic-walled microfossils over varying durations of controlled heating and under two differing redox conditions. Because of their relatively low-intensity natural thermal alteration, acritarchs from the Mesoproterozoic Ruyang Group were chosen as subjects for experimental heating at approximately 500°C, with durations ranging from 1 to 250 days and in both oxic (normal present day conditions) and anoxic conditions. Upon extraction, the opacity, reflectivity, color, microchemistry, and microstructures of the heated acritarchs were characterized using optic microscopy, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results differ for acritarchs prepared under oxic vs. anoxic conditions, with the anoxic replicates surviving experimental heating longer and retaining biological morphologies better, despite an increasing degree of carbonization with continuous heating. Conversely, the oxic replicates show aggressive degradation. In conjunction with fossils from high-grade metasediments, our data illustrate the preservational potential of organic-walled microfossils subjected to metamorphism in reducing conditions, offer insights into the search for microfossils in metasediments, and help to elucidate the influence of time on the carbonization/graphitization processes during thermal alteration.
... Such analyses are important for, as an example, verifying the biological or abiological origin of putative Archean microfossil-like structures. [19,22,23,24,25] . Two-dimensional images of microfossil-like objects and their embedding minerals can also provide evidence for the syngenicity of the studied objects with the surrounding rock. ...
Organic-walled microfossils of uncertain origin, classified to an informal group named acritarchs, are most commonly interpreted as the resting cysts of marine eukaryotic phytoplankton. Some acritarchs have recently been interpreted as vegetative cells of chlorococcalean green algae, based on internal bodies that have been interpreted as their asexual reproductive structures (spores). To verify this interpretation, we applied confocal Raman spectroscopy and atomic force microscopy (AFM) to study the ultrastructure and nanostructure of exceptionally preserved acritarchs with internal bodies from the early Silurian cherts (ca 430 Ma-old) of Frankenwald (Germany). Three-dimensional Raman mapping showed the spatial distribution of carbonaceous material and other minerals in the walls of the analysed internal bodies and confirmed that these structures are comparable with spores of chlorococcalean microalgae. Our findings document therefore the oldest thus far known vegetative cells of sporulating green algae. The combination of confocal Raman and AFM techniques yielded detailed information about the nanostructure and fossilisation mode of the mineralised organic walls of both the central vesicles and the enclosed spore-like bodies. Copyright (c) 2011 John Wiley & Sons, Ltd.
The utility of fluorescence lifetime imaging microscopy (FLIM) for identifying bacteria in complex mineral matrices was investigated. Baseline signals from unlabelled Bacillus subtilis and Euglena gracilis , and Bacillus subtilis labelled with SYTO 9 were obtained using two‐photon excitation at 730, 750 and 800 nm, identifying characteristic lifetimes of photosynthetic pigments, unpigmented cellular autofluorescence, and SYTO 9. Labelled and unlabelled B. subtilis were seeded onto marble and gypsum samples containing endolithic photosynthetic cyanobacteria and the ability to distinguish cells from mineral autofluorescence and nonspecific dye staining was examined in parallel with ordinary multichannel confocal imaging. It was found that FLIM enabled discrimination of SYTO 9 labelled cells from background, but that the lifetime of SYTO 9 was shorter in cells on minerals than in pure culture under our conditions. Photosynthetic microorganisms were easily observed using both FLIM and confocal. Unlabelled, nonpigmented bacteria showed weak signals that were difficult to distinguish from background when minerals were present, though cellular autofluorescence consistent with NAD(P)H could be seen in pure cultures, and phasor analysis permitted detection on rocks. Gypsum and marble samples showed similar autofluorescence profiles, with little autofluorescence in the yellow‐to‐red range. Lifetime or time‐gated imaging may prove a useful tool for environmental microbiology.
LAY DESCRIPTION : The standard method of bacterial enumeration is to label the cells with a fluorescent dye and count them under high‐power fluorescence microscopy. However, this can be difficult when the cells are embedded in soil and rock due to fluorescence from the surrounding minerals and dye binding to ambiguous features of the substrate. The use of fluorescence lifetime imaging (FLIM) can disambiguate these signals and allow for improved detection of bacteria in environmental samples.
The Pilbara craton of northwestern Australia is known for what were, when reported, the oldest known microfossils and paleosols on Earth. Both interpretations are mired in controversy, and neither remain the oldest known. Both the microfossils and the paleosols have been considered hydrothermal artefacts: carbon films of vents and a large hydrothermal cupola, respectively. This study resampled and analyzed putative paleosols within and below the Strelley Pool Formation (3.3 Ga), at four classic locations: Strelley Pool, Steer Ridge, Trendall Ridge, and Streckfuss, and also at newly discovered outcrops near Marble Bar. The same sequence of sedimentary facies and paleosols was newly recognized unconformably above the locality for microfossils in chert of the Apex Basalt (3.5 Ga) near Marble Bar. The fossiliferous Apex chert was not a hydrothermal vein but a thick (15 m) sedimentary interbed within a sequence of pillow basalts, which form an angular unconformity capped by the same pre-Strelley paleosol and Strelley Pool Formation facies found elsewhere in the Pilbara region. Baritic alluvial paleosols within the Strelley Pool Formation include common microfossil spindles (cf. Eopoikilofusa ) distinct from marine microfossil communities with septate filaments ( Primaevifilum ) of cherts in the Apex and Mt Ada Basalts. Phosphorus and iron depletion in paleosols within and below the Strelley Pool Formation are evidence of soil communities of stable landscapes living under an atmosphere of high CO 2 (2473 ± 134 ppmv or 8.8 ± 0.5 times preindustrial atmospheric level of 280 ppm) and low O 2 (2181 ± 3018 ppmv or 0.01 ± 0.014 times modern).
The oldest carbonaceous matter in the solar system, aged at 4.5 billion years old, can be found trapped in meteorites. On Earth, the oldest carbonaceous matter of biological origin is fossilised in cherts dated at 3.5 billion years old. The EPR study of samples of this primitive carbonaceous matter provides information on the nature, the environment and the means by which carbon-based radicals were formed. This information is precious to develop scenarios for the formation of organic matter in the solar system and the emergence of life on Earth. The same methods could be used to analyse samples collected on Mars.
Confocal Raman Spectroscopy and Microscopy have attained a significant increase in recognition over the past two decades and the method is now well established as another instrument in the geoscientists’ toolbox. Here we present and discuss the use and benefit of the method, considering aspects related to sample preparation, effects of the interaction of lasers on specific sample surfaces, as well as instrumental consideration to address these aspects. Further we present examples how confocal Raman microscopy can be applied in mineral phase and phase composition imaging as well as crystallographic orientation imaging in a variety of geological materials, including shocked minerals, carbonaceous materials and fluid inclusions.
The inability to unambiguously distinguish the biogenicity of microfossil-like structures in the ancient rock record is a fundamental predicament facing Archean paleobiologists and astrobiologists. Therefore, novel methods for discriminating biological from nonbiological chemistries of microfossil-like structures are of the utmost importance in the search for evidence of early life on Earth. This, too, is important for the search for life on Mars by in situ analyses via rovers or sample return missions for future analysis here on Earth. Here, we report the application of synchrotron X-ray fluorescence imaging of vanadium, within thermally altered organic-walled microfossils of bona fide biological origin. From our data, we demonstrate that vanadium is present within microfossils of undisputable biological origin. It is well known in the organic geochemistry literature that elements such as vanadium are enriched and contained within crude oils, asphalts, and black shales that have been formed by diagenesis of biological organic material. It has been demonstrated that the origin of vanadium is due to the diagenetic alteration of precursor chlorophyll and heme porphyrin pigment compounds from living organisms. We propose that, taken together, microfossil-like morphology, carbonaceous composition, and the presence of vanadium could be used in tandem as a biosignature to ascertain the biogenicity of putative microfossil-like structures. Key Words: Microfossils-Synchrotron micro-X-ray fluorescence-Vanadium-Tetrapyrrole-Biosignature. Astrobiology 17, xxx-xxx.
Filamentous microstructures from the 3.46. billion. year (Ga)-old Apex chert of Western Australia have been interpreted as remnants of Earth's oldest cellular life, but their purported biological nature has been robustly questioned on numerous occasions. Despite recent claims to the contrary, the controversy surrounding these famous microstructures remains unresolved.Here we interrogate new material from the original 'microfossil site' using high spatial resolution electron microscopy to decode the detailed morphology and chemistry of the Apex filaments. Light microscopy shows that our newly discovered filaments are identical to the previously described 'microfossil' holotypes and paratypes. Scanning and transmission electron microscopy data show that the filaments comprise chains of potassium- and barium-rich phyllosilicates, interleaved with carbon, minor quartz and iron oxides. Morphological features previously cited as evidence for cell compartments and dividing cells are shown to be carbon-coated stacks of phyllosilicate crystals. Three-dimensional filament reconstructions reveal non-rounded cross sections and examples of branching incompatible with a filamentous prokaryotic origin for these structures.When examined at the nano-scale, the Apex filaments exhibit no biological morphology nor bear any resemblance to younger . bona fide carbonaceous microfossils. Instead, available evidence indicates that the microstructures formed during fluid-flow events that facilitated the hydration, heating and exfoliation of potassium mica flakes, plus the redistribution and adsorption of barium, iron and carbon within an active hydrothermal system.
Among the most plausible environments for the origin of life are marine hydrothermal systems, where geochemical energy sources and refugia from sterilizing meteorite impacts would have been plentiful. Here, values of Gibbs energy were calculated for the formation of individual cellular building blocks from inorganic reactants. In our model, corresponding redox reactions occurred at the interface between two end-member fluids—low temperature (25 °C), mildly acidic (pH 6.5), relatively oxidized (Eh -0.30 mV) seawater and moderately hot (140 °C), alkaline (pH 9), reduced (Eh -0.71 mV) hydrothermal vent fluid. The thermodynamic calculations demonstrate that biomass synthesis is most favorable at moderate temperatures, where the energy contributions from HCO3- and H+ in seawater coupled to the reducing power in hydrothermal fluid are optimized. The models further show that the net synthesis of cellular building blocks may yield small amounts of energy over the ranges of temperature and chemical composition investigated. This is counter to conventional wisdom for anabolic processes and lends further support to marine hydrothermal systems as particularly favorable sites for the emergence of life.
Desert varnishes are thin, dark mineral coatings found on some rocks in arid or semi-arid environments on Earth. Microorganisms may play an active role in their formation, which takes many hundreds of years. Their mineral matrix may facilitate the preservation of organic matter and is therefore of great relevance to martian exploration. Miniaturized Raman spectrometers (which allow nondestructive analysis of the molecular composition of a specimen) will equip rovers in forthcoming planetary exploration missions. In that context, and for the first time, portable Raman spectrometers operating in the green visible (532 nm as currently baselined for flight) and in the near-infrared (785 nm) were used in this study to investigate the composition (and substrate) of several samples of desert varnish. Rock samples that were suspected (and later confirmed) to be coated with desert varnish were recovered from two sites in the Mojave Desert, USA. The portable spectrometers were operated in flight-representative acquisition modes to identify the key molecular components of the varnish. The results demonstrate that the coatings typically comprise silicate minerals such as quartz, plagioclase feldspars, clays, ferric oxides, and hydroxides and that successful characterization of the samples can be achieved by using flightlike portable spectrometers for both the 532 and 785 nm excitation sources. In the context of searching for spectral signatures and identifying molecules that indicate the presence of extant and/or extinct life, we also report the detection of β-carotene in some of the samples. Analysis complications caused by the presence of rare earth element photoluminescence (which overlaps with and overwhelms the organic Raman signal when a 785 nm laser is employed) are also discussed. Key Words: Desert varnish-Raman spectroscopy-ExoMars-Portable spectrometers-Planetary science. Astrobiology 15, 442-452.
Over the last two decades, there has an been increasing interest in applying vibrational spectroscopy in palaeontological research. For example, this chemical analytical technique has been used to elucidate the chemical composition of a wide variety of fossils, including Archaean putative microfossils, stromatolites, chitinozoans, acritarchs, fossil algae, fossil plant cuticles, putative fossil arthropods, conodonts, scolecodonts and dinosaur bones. The insights provided by these data have been equally far ranging: to taxonomically identify a fossil, to determine biogenicity of a putative fossil, to identify preserved biologically synthesized compounds and to elucidate the preservational mechanisms of fossil material. Vibrational spectroscopy has clearly been a useful tool for investigating various palaeontological problems. However, it is also a tool that has been misapplied and misinterpreted, and thus, this review is dedicated to providing a palaeontologist who is new to vibrational spectroscopy with a basic understanding of these techniques, and the types of chemical information that can be obtained. Two example applications of these techniques are discussed in detail, one looking into fossil palynomorph taxonomy and other into the enigmatic Burgess Shale-type preservation.
The search for sp(2)-bonded carbonaceous material is one of the major life detection strategies of the astrobiological exploration programmes of National Aeronautics and Space Administration and European Space Agency (ESA). The ESA ExoMars rover scheduled for launch in 2018 will include a Raman spectrometer with the goal of detecting sp(2)-bonded carbonaceous material as potential evidence of ancient life. However, sp(2)-bonded carbonaceous material will yield the same Raman spectra of well-developed G and D bands whether they are synthesized biologically or non-biologically. Therefore, the origin and source of sp(2)-bonded carbonaceous material cannot be elucidated by Raman spectroscopy alone. Here, we report the combined approach of Raman spectroscopy and gas chromatography-mass spectrometry biomarker analysis to Precambrian sedimentary rocks, which taken together, provides a promising new methodology for readily detecting and rapidly screening samples for immature organic material amenable to successful biomarker analysis.
Subsurface environments are known to support and preserve diverse microbial communities. Giant pool fingers from Hidden Cave, New Mexico consist of mm-scale dark micritic calcite layers alternating with clear dogtooth spar crystals and contain morphological and geochemical evidence of past microbial communities. We used Fourier Transform infrared spectroscopy to identify fatty acids, proteins, PO2-carrying compounds, and polysaccharides spatially related to morphological fossil filaments throughout the surface micritic laminations and central pool finger regions. These biomolecular signatures are important components that contribute to the biosignature suite under development that identify microbial involvement in carbonate precipitation on Earth and remotely.
The ESA/Roscosmos ExoMars rover will be launched in 2018. The primary aim of the mission will be to find evidence of extinct or extant life by extracting samples from the sub-surface of Mars. The rover will incorporate a drill that is capable of extracting cores from depths of up to 2 m, a Sample Preparation and Distribution System (SPDS) that will crush the core into small grains and a suite of analytical instruments. A key component of the analytical suite will be the Raman Laser Spectrometer (RLS) that will be used to probe the molecular and mineralogical composition of the samples. In this work we consider the capability of the proposed Raman spectrometer to detect reduced carbon (possibly associated with evidence for extinct life) and to identify the level of thermal alteration/maturity. The Raman analysis of 21 natural samples of shale (originating from regions exhibiting different levels of thermal maturity) is described and it is shown that reduced carbon levels as low as 0.08% can be readily detected. It is also demonstrated that the Raman spectra obtained with the instrument can be used to distinguish between samples exhibiting high and low levels of thermal maturity and that reduced carbon can be detected in samples exposed to significant levels of oxidation (as expected on the surface of Mars).
Once life appeared, it evolved and diversified. From primitive living
entities, an evolutionary path of unknown duration, likely paralleled by
the extinction of unsuccessful attempts, led to a last common ancestor
that was endowed with the basic properties of all cells. From it,
cellular organisms derived in a relative order, chronology and manner
that are not yet completely settled. Early life evolution was
accompanied by metabolic diversification, i.e. by the development of
carbon and energy metabolic pathways that differed from the first, not
yet clearly identified, metabolic strategies used. When did the
different evolutionary transitions take place? The answer is difficult,
since hot controversies have been raised in recent years concerning the
reliability of the oldest life traces, regardless of their
morphological, isotopic or organic nature, and there are also many
competing hypotheses for the evolution of the eukaryotic cell. As a
result, there is a need to delimit hypotheses from solid facts and to
apply a critical analysis of contrasting data. Hopefully, methodological
improvement and the increase of data, including fossil signatures and
genomic information, will help reconstructing a better picture of life
evolution in early times as well as to, perhaps, date some of the major
evolutionary transitions. There are already some certitudes. Modern
eukaryotes evolved after bacteria, since their mitochondria derived from
ancient bacterial endosymbionts. Once prokaryotes and unicellular
eukaryotes had colonized terrestrial ecosystems for millions of years,
the first pluricellular animals appeared and radiated, thus inaugurating
the Cambrian. The following sections constitute a collection of
independent articles providing a general overview of these aspects.
Phosphatized microfossils from ca. 580Ma from the Doushantuo Formation in the Weng'an region of South China were analyzed by Fourier transform infrared (FTIR) microspectroscopy for their chemical characterization. Two morpho-types of phosphatized embryo-like fossils (Megasphaera and Megaclonophycus) were analyzed, together with algal fossils. Transmission IR spectra of the microfossils have absorption bands of around 2960cm-1 and 2925cm-1, indicating the presence of aliphatic hydrocarbon (anti-symmetric aliphatic CH3 and aliphatic CH2), and have an additional band of around 1595cm-1, probably derived from aromatic moieties (aromatic CC). In addition, IR microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo-like fossils. The embryo-like fossils appear to show three types of CH3/CH2 peak height ratios (R3/2) and aromatic CC/CH2 peak height ratios (RCC/2 values): (1) high-R3/2/low-RCC/2 type (R3/2=~0.2-1.0 and RCC/2~0-2), (2) low-R3/2/medium-RCC/2 type (R3/2=~0.2-0.6 and RCC/2=~1-4); and (3) low-R3/2/high-RCC/2 type (R3/2=~0.2-0.6 and RCC/2~1-8). All three types are contained in both Megasphaera and Megaclonophycus. Raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration, indicating that the organics were embedded in situ prior to thermal maturation. The IR spectroscopic differences might reflect differences in original organic compositions of microorganisms, and/or immediate post-mortem alteration. This suggests that the origins of phosphatized embryo-like fossils are more diverse than was previously recognized on the basis of their morphology. A comparison of R3/2 and RCC/2 values in the embryo-like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo-like fossils belong to animal embryo, others are algae, but none of them originate from bacteria.
The oldest part of the Pilbara Craton is 3.80–3.55 Ga crust. Between 3.53 and 3.22 Ga, mantle plume activity resulted in eight successive volcanic cycles forming the Pilbara Supergroup. Large volumes of granitic magma were intruded during the same period. By 3.22 Ga, a thick continental crust, the East Pilbara Terrane, had been established. Between 3.22 and 3.16 Ga, rifting of the East Pilbara Terrane separated off two additional terranes (Karratha and Kurrana), with intervening basins of oceanic crust. After 3.16 Ga, the three terranes began to converge, resulting in both obduction of oceanic crust (Regal Terrane) and, in another area, subduction to form a 3.13 Ga island arc (Sholl Terrane). At 3.07 Ga, the Karratha, Regal, and Sholl Terranes collided to form the West Pilbara Superterrane, and this collided with the East Pilbara Terrane. The 3.05–2.93 Ga De Grey Superbasin was deposited as a succession of basins: Gorge Creek, Whim Creek, Mallina, and Mosquito Creek. Eventual closure of the basins, between 2.94 and 2.93 Ga, formed two separate orogenic belts on either side of the East Pilbara Terrane. Post-orogenic granites were intruded between 2.89 and 2.83 Ga. The 2.78–2.63 Ga Fortescue Basin developed in four stages: (i) rifting of the Pilbara Craton; (ii) folding and erosion; (iii) large igneous province (LIP) volcanism; and (iv) marine sedimentation on a passive margin. A review of all known evidence for early life in the Pilbara Craton is provided. In hydrothermal settings, most of the evidence occurs as filamentous and spheroidal microfossils, organic carbon, microbial mats, and rare stromatolites. By contrast, shallow-water marine sedimentary rocks contain a diverse range of stromatolites, and microbial mats. Lacustrine and shallow-water marine carbonate rocks in the Fortescue Basin contain abundant and morphologically diverse stromatolites, widespread microbial mats, and organic carbon.
The Pilbara Craton in Western Australia contains the best-preserved and most complete record of Archean rocks in the world. As such, they are some of the most studied rocks in the world; paleontologists, isotopic geochemists, geologists and geobiologists have all investigated these rocks for clues about the early biosphere and atmosphere. Here we show using high-resolution transmission electron microscopy that the carbonaceous material found in the Apex chert, and potentially in other associated units, was formed by multiple processes such as abiotic catalytic synthesis and/or biological synthesis. We use these data as well as the geological history of the craton to demonstrate that when the rocks of the Pilbara Craton experienced a high degree of post-depositional hydrothermal alteration, carbonaceous material could have been remobilized and redeposited. As the carbonaceous material within the Apex chert samples was formed over nearly a billion years, bulk chemistry, even at the micron level, will be unable to unambiguously delineate the presence of life in these ancient rocks, although nanoscale observations may provide a way forward in the search for ancient life.
The organic matter was isolated from a chert of the Warrawoona deposit and its chemical structure analysed using high resolution transmission electron microscopy, solid state nuclear magnetic resonance, infrared and electron paramagnetic resonance.
Surface-enhanced Raman spectroscopy will increase sensitivity by several orders of magnitude over conventional Raman, and should be considered for future missions. We demonstrate detection of organic pigments from ice containing snow algae.
The combination of spectroscopic and analytical techniques applied to the kerogen of the Warrawoona chert (3.5 Byr) leads to new information on the controversial question of the origin of life especially the syngeneity of the archean organic matter.
Micro-Fourier transform infrared (FTIR) spectroscopic imaging analyses nondestructively revealed micrometer to millimeter-scale
distributions of organic and inorganic functional groups in Proterozoic stromatolitic chert containing prokaryotic fossils
from ∼1,900 Ma Gunflint Formation. CH3/CH2 absorbance ratios (R
3/2) indicate a bacterial origin, but not Archaea, of most carbonaceous matter in the chert as well as in the microfossils themselves.
However, the characterizations of the stromatolitic chert also show that R
3/2 value of carbonaceous matter existing with carbonates could be overestimated or underestimated. This technique is useful
for searching and characterizing rapidly the organic matter in terrestrial and extraterrestrial samples at the micrometer
to millimeter scale, and may provide useful information on the affinities of microfossils in the chert.
The oldest putative microfossils on Earth occur in the 3.5 Ga Apex chert of the Warrawoona Group, Western Australia. We have
analyzed disseminated interstitial carbon found within Apex chert using transmission electron microscopy (TEM) and electron
energy loss spectroscopy (EELS) to address the controversy regarding its state of structural disorder. We found that the carbonaceous
material is structurally amorphous, with no evidence of graphitization, and contains aromatic domains, most likely as polyaromatic
ring structures, similar to preserved kerogen in bona fide microfossils. In addition, amorphous carbonaceous material occurs
as a grain boundary phase between quartz crystals and within fluid inclusions in quartz crystals, indicating that hydrocarbons
moved through the chert during crystallization and hydrothermal alteration. The results suggest that the carbonaceous material
is similar in structure to microfossil kerogen, implying the microbe-like features within Apex chert are also microfossils.
However, this kerogen-like material may also be produced abiotically via Fischer-Tropsch-type (FTT) synthesis reactions in
an ancient hydrothermal vent.
The problems involved with the interpretation of carbon isotopes as indicators for early life in highly metamorphosed early Archean rocks have prompted the search for additional chemical and isotopic biomarkers. Here we report an attempt to identify the origin of carbonaceous matter in the 3.8 Ga old Isua Supracrustal Belt in southern West Greenland by measuring the concentration and isotopic composition of a trapped nitrogen component. Stepped-combustion/pyrolysis-mass spectrometry of carbonaceous matter in several rock samples revealed three different reservoirs of trapped nitrogen: (1) nitrogen associated with a very small amount of reactive carbonaceous material, (2) nitrogen intercalated in graphite, correlated with intercalated radiogenic argon, (3) nitrogen strongly retained at defects or chemically bound in the graphite structure. The δ15N of nitrogen associated with reactive carbonaceous matter (ca. +6‰) overlaps with that of average Phanerozoic sedimentary organic matter, and is believed to be part of nonindigenous postmetamorphic biologic material. In situ Raman spectroscopy confirmed the high degree of crystallinity of the metamorphosed indigenous carbonaceous material, and this material is further referred to as graphite. Graphite interpreted as epigenetic (associated with Mg,Mn-siderite in metacarbonates) contains a very small strongly retained nitrogen component with a low δ15N ratio (−3 to −1‰). This range overlaps with values that are typically found in Archean kerogens, but also those of a metamorphically emplaced inorganic basaltic source. Geological constraints suggest that this graphite incorporated nitrogen from surrounding metabasaltic rocks. Graphite interpreted as syngenetic and biogenic found in a turbidite deposit is relatively similar to this Mg,Mn-siderite-derived graphite, based on degree of graphite crystallinity, amount of trapped radiogenic argon, low nitrogen concentration and δ15N signature. We conclude that nitrogen concentration and its isotope ratio in graphite cannot be used conclusively as a biomarker in these rocks from the highly metamorphosed Isua Supracrustal Belt.
The Drake Equation was originally composed as an attempt to quantify the potential number of extraterrestrial civilizations in our Galaxy which we might be able to detect using a radio telescope. Since this equation was first formulated, nearly 50 years ago, we have discovered that life on Earth arose very early in its history, and has filled virtually every habitable, potentially extreme, niche available. This suggests that simple forms of life might be plentiful where possible, and can be observed remotely by atmospheric biosignatures in the host planet. We consider modifications to the Drake Equation to reflect this new understanding.
In recent decades, the documented fossil record has been extended to some 3500 Ma (million years) ago. Hundreds of fossiliferous units have been discovered that contain thousands of microbial fossils, and the rules for accepting ancient microfossil-like objects as bona fide have come to be well established. Of these, criteria for establishing biogenicity have proven the most difficult to satisfy. Three new techniques have now been devised to help answer this need: (1) ion microprobe mass spectrometry has been used to measure the carbon isotopic compositions of individual Precambrian microfossils. (2) Laser-Raman imagery has been used to analyze the molecular structure of the carbonaceous matter comprising such cellular fossils and associated particulate organic matter. (3) Atomic force microscopy has been used to reveal the nanometer-scale structure of the kerogenous components of individual Precambrian microscopic fossils. These new techniques not only provide means for elucidation of the isotopic composition, molecular structure, and submicron-scale fine structure of individual microscopic fossils, but hold promise for understanding the geochemical maturation of ancient organic matter and clarifying the nature of minute fossil-like objects of putative but uncertain biogenicity, whether Precambrian or extraterrestrial.
High Resolution Transmission Electron Microscopy (HRTEM) makes possible the imaging of the profile of the polyaromatic layers, allowing a knowledge of carbons, such as disordered natural carbons from meteorites and from Precambrian metasediments
We question the biogenicity of putative bacterial andcyanobacterial `microfossils' from3465 Ma Apex cherts of the Warrawoona Group in WesternAustralia. They arechallenged on the basis of integrated multidisciplinary evidenceobtained from field andfabric mapping plus new high-resolution research into theircontext, sedimentology,filament morphology, `septation' and arrangement. They cannotbe distinguished from(and are reinterpreted as) secondary artefacts of amorphouscarbon that formed duringdevitrification of successive generations of carbonaceoushydrothermal dyke vein quartz.Similar structures occur within associated carbonaceous volcanicglass. The nullhypothesis of an abiotic or prebiotic origin for such ancientcarbonaceous matter issustained until mutually supporting contextural, morphologicaland geochemicalevidence for a bacterial rather than abiotic origin is forthcoming.
Until recently, the deep-branching relationships in the bacterial domain have been unresolved. A new phylogenetic approach (termed compartmentalization) was able to resolve these deep-branching relationships successfully by using a large number of genes from whole genome sequences and by reducing long branch attraction artefacts. This new, well-resolved phylogenetic tree reveals the evolutionary relationships between diverse bacterial groups that leave important traces in the geological record. It shows that mesophilic sulphate reducers originated before the Cyanobacteria, followed by the origination of sulphur- and pyrite-oxidizing bacteria after oxygen became available in the biosphere. This evolutionary pattern mirrors a similar pattern in the Palaeoproterozoic geological record. Sulphur isotopic fractionation records indicate that large-scale bacterial sulphate reduction began in marine environments around 2.45 billion years ago (Ga), followed by rapid oxygenation of the atmosphere about 2.3 or 2.2 Ga. Oxygenation was then followed by increasing oceanic sulphate concentrations (probably owing to pyrite oxidation and continental weathering), which then resulted in the disappearance of banded iron formations by 1.8 Ga. The similarity between the phylogenetic and geological records suggests that the geochemical changes observed on the Palaeoproterozoic Earth were caused by major origination events in the mesophilic bacteria, and that these geochemical changes then caused additional origination events, such as aerobic respiration. If so, then constraints on divergence dates can be established for many microbial groups, including the Cyanobacteria, mesophilic bacteria, mesophilic sulphate reducers, methanotrophs, several anoxygenic phototrophs, as well as for mitochondrial endosymbiosis. These dates may also help to explain a large number of other changes in the geological record of the Neoarchean and Palaeoproterozoic Earth. This hypothesis, however, does not agree with the finding of cyanobacterial and eukaryote lipids at 2.7 Ga, and suggests that further work needs to be done to elucidate the discrepancies in both these areas.
Precambrian microbial fossils show carbonaceous cellular structure, which often resemble in shape and size cyanobacteria and other prokaryotes. Morphological taxonomy of these minute, simple, and more or less degraded fossils is, however, often not enough to determine their precise phylogenetic positions. Here we report the results of micro-FTIR spectroscopic analyses of well-preserved microfossils in ∼850 Ma and ∼1900 Ma stromatolites, together with those of 8 species of extant prokaryotes and 5 of eukaryotes for comparison. These Proterozoic fossils have low CH3/CH2 absorbance ratios (R3/2 < 0.5) of aliphatic CH moieties, suggesting selective preservation of long, straight, aliphatic carbon chains probably derived from bacterial membrane lipids. All the observed R3/2 values of coccoids, filaments and amorphous organic matter resemble lipid fractions of extant Bacteria including cyanobacteria, but not Archaea. The results indicate that Proterozoic microfossils belong to Bacteria, which is consistent with the cyanobacterial origin inferred from morphology. Moreover, the R3/2 value of fossilized cell would reflect chemical composition of its precursor membrane lipid, thus could be a useful new tracer for distinguishing Archaea, Bacteria and possibly Eucarya for fossilized and extant microorganisms.
Raman spectroscopy has long been used in geosciences and a wealth of data and publications are available. The majority of
this information originates from point measurements using micro-Raman setups. With the application of confocal Raman imaging,
additional analytical possibilities arise with respect to analyzing the three-dimensional spatial distribution of inorganic
as well as organic phases on the centimeter to sub-micrometer scale. This chapter will highlight some of the key aspects experimenters
should take into consideration when performing confocal Raman measurements as well as experimental results showing the insight
gained into geological samples by the use of confocal Raman imaging.
In this paper we report on electron paramagnetic resonance (EPR) of paramagnetic centers in the carbonaceous matter of primitive
siliceous rocks, known to contain the most ancient traces of life. The EPR lines observed are attributed to π-radicals stabilized
by the aromatic structure of this matter. It is found that the line widths and the line shapes vary continuously during geological
periods and show a progressive pattern evolution from Gaussian–Lorentzian (recent) to purely Lorentzian (ca. 2000 million
years [Myr]), and to supra-Lorentzian (ca. 3500 Myr). Artificial ageing experiments allowed us to define four stages of maturation
of the organic matter according to the evolution of g-factors, EPR intensity, peak-to-peak line width and line shape. We suggest that the peculiar supra-Lorentzian line shape
observed for organic matter older than 2000 Myr should be related to a low-dimensional (two- or one-dimensional) spatial distribution
of electron spins. From these results, we derive a relationship between the line shape and the age of the organic matter,
valid for ages ranging from about 600 Myr to about 3500 Myr.
Once life appeared, it evolved and diversified. From primitive living entities, an evolutionary path of unknown duration, likely paralleled by the extinction of unsuccessful attempts, led to a last common ancestor that was endowed with the basic properties of all cells. From it, cellular organisms derived in a relative order, chronology and manner that are not yet completely settled. Early life evolution was accompanied by metabolic diversification, i.e. by the development of carbon and energy metabolic pathways that differed from the first, not yet clearly identified, metabolic strategies used. When did the different evolutionary transitions take place? The answer is difficult, since hot controversies have been raised in recent years concerning the reliability of the oldest life traces, regardless of their morphological, isotopic or organic nature, and there are also many competing hypotheses for the evolution of the eukaryotic cell. As a result, there is a need to delimit hypotheses from solid facts and to apply a critical analysis of contrasting data. Hopefully, methodological improvement and the increase of data, including fossil signatures and genomic information, will help reconstructing a better picture of life evolution in early times as well as to, perhaps, date some of the major evolutionary transitions. There are already some certitudes. Modern eukaryotes evolved after bacteria, since their mitochondria derived from ancient bacterial endosymbionts. Once prokaryotes and unicellular eukaryotes had colonized terrestrial ecosystems for millions of years, the first pluricellular animals appeared and radiated, thus inaugurating the Cambrian. The following sections constitute a collection of independent articles providing a general overview of these aspects.
There is an apparent preservational paradox in the early rock record. Cellularly preserved and ensheathed microfossils which are remarkably preserved from the late Archaean (c.2700 Ma) onward, have rarely been found in the earlier rock record and when they are their biogenicity is debated. Likewise, the abundance and morphological complexity of stromatolites appears much reduced in the early Archaean and even these lack compelling associations with organic remains of microbial mats. This ‘preservational dark age’ may have arisen because microfossils and microbial mats were absent, because conditions for their preservation were rare or, as we suggest here, because scientists have largely been looking in the wrong places.
To illustrate the potential of looking far beyond ‘chertified Bahamian lagoons’, we make a traverse across the key potential habitats for early life on Earth and identify some exciting and new taphonomic windows, in the search for Earth’s earliest microfossils, trace fossils and stromatolites. Such habitats include hitherto little explored pillow lavas, hydrothermal vents and beach sandstones. These new windows are already starting to provide surprising insights into the nature of the earliest vital processes.
In the prospect of the search for traces of primitive life on Earth and Mars, we investigated the possibility to detect and visualize the spatial distribution of minute amounts of organic matter in ancient rocks, in a non-destructive way, by Electron Paramagnetic Resonance Imaging (EPRI). We studied a series of non- or moderately metamorphosed siliceous rocks (cherts) of different ages ranging from 45 Ma to 3490 Ma and embedding fossile organic matter. In the case of the oldest cherts containing only mature insoluble organic matter (IOM), with IOM• radicals characterized by a single Electron Paramagnetic Resonance (EPR) line, we could obtain three-dimensional images with sub-millimetric resolution of the organic matter distribution inside samples containing as low as 1014–1015 radicals per gram. In the case of younger cherts, containing less mature organic matter, and thus several types of organic radicals, we showed that selective imaging of each type of radical was possible provided that the EPR spectra did not overlap. Selective imaging of either the organic radicals or of the oxygen vacancy (E' centres) of the mineral matrix, which are ubiquitous in siliceous rocks, was possible, selecting either one or the other paramagnetic species with high power in-phase, 1st harmonic detection or with 90°-out-of-phase, 2nd harmonic detection of the EPR. The influence of ferromagnetic inclusions in the mineral matrix on the EPRI of the organic matter was also addressed. Image artifacts due to the ferromagnetic resonance signal of these inclusions could be easily removed by background substraction from the EPR spectra before image reconstruction. We thus showed that selective imaging by EPR of minute amounts of fossile organic matter in ancient rocks could be possible despite the magnetic complexity of such materials.
Structures resembling cyanobacterial microfossils from the ca. 3465 Ma old Apex chert of the Warrawoona Group in Western Australia have until recently been accepted as providing the oldest morphological evidence for life on Earth, and have been taken to support an early beginning for oxygen-releasing photosynthesis. Eleven species of filamentous prokaryote, principally distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils. They were contrasted with other microfossils that were dismissed as either unreliable or irreproducible. The aim of this paper is to provide a detailed account of research recently reported by us on the type and recollected material, involving optical and electron microscopy, digital image analysis and other techniques. All previously figured holotype materials are illustrated here, and the context for all the published materials is re-evaluated.The Apex chert ‘microfossils’ occur near the top of a 1.5-km long chert dyke complex associated with major synsedimentary growth faults. Highly localised, glassy felsic tuffs erupted explosively from this and other fissures during the early stages of volcanism, and were followed by the deposition of essentially hydrothermal black and white BaSO4 rich cherts that infiltrated the feeder dykes, underplating and dilating adjacent stratiform cherts before the start of the next volcanic cycle. The Apex chert ‘microfossils’ occur within multiple generations of these metalliferous hydrothermal vein cherts some 100 m down the dyke system. Comparable structures occur in associated volcanic vent glass and in hydrothermal cherts at least 1 km deep. We find no supporting evidence for a primary biological origin. We reinterpret the purported microfossil-like structures as pseudofossils that formed from the reorganization of carbonaceous matter, mainly during recrystallization from amorphous to spherulitic silica.
The biological origin of organic matter in the oldest siliceous sediments (cherts) is still debated. To address this issue, the insoluble organic matter (kerogen) was isolated from a chert of the Warrawoona group. The chemical structure of the kerogen was investigated through a combination of analytical techniques including solid-state 13C nuclear magnetic resonance and pyrolysis. Although dominated by aromatic hydrocarbons, the pyrolysate comprises a homologous series of long chain aliphatic hydrocarbons characterized by odd-over-even carbon number predominance. This distribution is only consistent with a biological origin. As kerogen must be contemporaneous of the solidification of the chert, this observation should be regarded as an evidence for the presence of life on Earth, 3.5 By ago.
Carbonaceous matter occurring in chert deposits of the 3.4–3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ13C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ13C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ13C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer–Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material.
Hydrogen-lean kerogen (atomic H/C < 0.46) isolated from the 3.4 Ga Strelley Pool Chert in the North Pole area, Pilbara Craton, Western Australia, were studied by vibrational spectroscopy (Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy), nuclear magnetic resonance spectroscopy (solid state 13C NMR spectroscopy), catalytic hydropyrolysis followed by gas chromatography mass spectrometry (HyPy–GC–MS), and isotope ratio mass spectrometry (IRMS). The kerogen occurs in sedimentary rocks as clasts and clots deposited together with other detrital materials that are finely disseminated throughout a chert matrix. The bulk kerogen δ13C values range from −28.3 to −35.8‰. Solid-state 13C NMR spectroscopy and FTIR spectroscopy reveals that the kerogen is highly aromatic (fa varying from 0.90 to 0.92) and contains only minor aliphatic carbon or carbon-oxygenated (C–O) functionalities. The Raman carbon first-order spectra for the isolated kerogens are typical of spectra obtained from disordered sp2 carbons with low 2-D ordering (biperiodic structure). The implications of the Raman results show low 2-D ordering throughout the carbonaceous network indicate the incorrect usage of the term graphite in the literature to describe the kerogen or carbonaceous material in the Warrawoona cherts. Hydropyrolysates contain aromatic compounds consisting of 1-ring to 7-ring polycyclic aromatic hydrocarbons which were covalently bound into the kerogen as well as alkanes (linear, branched and cyclic) which were most probably trapped in the microporous network of the kerogen. These PAHs have mainly C1- and C2-alkylation while C3+-substitued aromatics are low in abundance and do not show a high degree of branched alkylation. For the first time we have shown a correlation between elemental analysis (H/C atomic ratios), Raman spectroscopic parameters (ID1/IG, ID1/(ID1 + IG), and La), and the degree of alkylation of bound polyaromatic molecular constituents generated from HyPy for Archaean kerogens. Similarities in molecular profiles exist between HyPy products of Strelley Pool Chert kerogens and an oil-window-mature Mesoproterozoic kerogen from Roper Group (ca. 1.45 Ga), which is biogenic in origin, suggesting that the Strelley Pool Chert kerogens may also be derived from diagenesis and thermal processing of biogenic organic matter. A combination of Raman spectroscopy, for identifying the least metamorphosed kerogens, used together with HyPy for liberating trapped and bound molecular components of these kerogens, offers a powerful strategy for assessing the origins of Earth's oldest preserved organic matter.
Investigating carbonaceous microstructures and material in Earth's oldest sedimentary rocks is an essential part of tracing the origins of life on our planet; furthermore, it is important for developing techniques to search for traces of life on other planets, for example, Mars. NASA and ESA are considering the adoption of miniaturized Raman spectrometers for inclusion in suites of analytical instrumentation to be placed on robotic landers on Mars in the near future to search for fossil or extant biomolecules. Recently, Raman spectroscopy has been used to infer a biological origin of putative carbonaceous microfossils in Early Archean rocks. However, it has been demonstrated that the spectral signature obtained from kerogen (of known biological origin) is similar to spectra obtained from many poorly ordered carbonaceous materials that arise through abiotic processes. Yet there is still confusion in the literature as to whether the Raman spectroscopy of carbonaceous materials can indeed delineate a signature of ancient life. Despite the similar nature in spectra, rigorous structural interrogation between the thermal alteration products of biological and nonbiological organic materials has not been undertaken. Therefore, we propose a new way forward by investigating the second derivative, deconvolution, and chemometrics of the carbon first-order spectra to build a database of structural parameters that may yield distinguishable characteristics between biogenic and abiogenic carbonaceous material. To place Raman spectroscopy as a technique to delineate a biological origin for samples in context, we will discuss what is currently accepted as a spectral signature for life; review Raman spectroscopy of carbonaceous material; and provide a historical overview of Raman spectroscopy applied to Archean carbonaceous materials, interpretations of the origin of the ancient carbonaceous material, and a future way forward for Raman spectroscopy.
Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from about 3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer--Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.
Raman spectra of carbons derived from copper-containing polyfurfuryl alcohol are studied as a function of heattreatment temperature. The structure-sensitive nature of the bands in the 2600–3300 cm−1 region as compared with that of the bands at 1580 and 1360 cm−1 is discussed. A band at 2950 cm−1, observed only on disordered carbons, is suggested to be a defect-induced mode.
Light scattering in graphite intercalation compounds gives key insights into the physics of these layered structures. In this chapter a review is presented of experimental Raman scattering studies and their interpretation based on models of the lattice dynamics of pristine and intercalated graphite. The periodic layer structure of intercalation compounds makes it possible to model the dynamical matrix by a Brillouin zone folding of the pristine graphite matrix. The stage dependence of the Raman-active modes is reported which correlates with a stage dependent strain. Resonant enhancement of the scattering cross-section permits observation of modes related to the intercalate layer. Explicit results are obtained for the internal modes of Br2 molecules in the graphite-Br2 system. Stage I alkali metal compounds show a lineshape of the Breit-Wigner-Fano form which implies a coupling between a sharp vibrational mode and a Ramanactive continuum. Second-order Raman scattering results for intercalated graphite are reported. A brief summary is also given on Raman scattering studies of intercalated graphite fibers, adsorbed molecules on graphite surfaces and ion-implanted graphite.
Laser Raman spectroscopy has been applied to the study of various carbons ranging from highly graphitized to amorphous materials. A calculation is presented for such materials based on the assumption of short correlation lengths for normal modes that break the wave-vector selection rules. This calculation leads to expressions for the first-order Raman scattering intensity in terms of the vibrational density of states of a single graphite layer weighted by the electron-phonon couping of the modes to the electromagnetic radiation.
Ion implantation of graphite is characterized with respect to lattice damage and the distribution of implanted ions. Both the depth profile of the damage and of the implanted ions are shown to follow the models previously developed for ion-implanted semiconductors. Raman spectroscopy is used in a variety of ways to monitor different aspects of the lattice damage while Auger spectroscopy is used to monitor the implantation profile. Both first- and second-order Raman spectra are reported as a function of ionic mass and ion energy. The surface damage is examined by scanning electron microscopy while the microcrystalline regions in an amorphous background are observed by scanning transmission electron microscopy.
An extensive survey of the Raman spectral activity of a wide variety of carbon materials has produced experimental evidence for at least five structure-sensitive lines or bands. In addition to the well-known, always present 1580 cm−1 graphite line and the 1360 cm−1 disordered carbon line, there is a disorder line at ∼1620 cm−1 that is responsible for the apparent blue-shift of the graphite line in very disordered carbons; and there are lines at ∼2700 and ∼2735 cm−1 that are strong in graphite and annealed carbons but absent in very disordered carbons. These additional lines increase the capability of Raman spectroscopy to characterize carbon materials.
Ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 Ma Bitter Springs Formation, Australia, and the approximately 2100 Ma Gunflint Formation, Canada. The delta 13C(PDB) values from individual microfossils of the Bitter Springs Formation ranged from -21.3 +/- 1.7% to -31.9 +/- 1.2% and the delta 13C(PDB) values from microfossils of the Gunflint Formation ranged from -32.4 +/- 0.7% to -45.4 +/- 1.2%. With the exception of two highly 13C-depleted Gunflint microfossils, the results generally yield values consistent with carbon fixation via either the Calvin cycle or the acetyl-CoA pathway. However, the isotopic results are not consistent with the degree of fractionation expected from either the 3-hydroxypropionate cycle or the reductive tricarboxylic acid cycle, suggesting that the microfossils studied did not use either of these pathways for carbon fixation. The morphologies of the microfossils suggest an affinity to the cyanobacteria, and our carbon isotopic data are consistent with this assignment.
Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil 'lookalikes' can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of life's history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser--Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.
- B Wopenka
- J D Pasteris
Wopenka, B. & Pasteris, J. D. Am. Mineral. 78, 533-557 (1993).
- C Beny-Bassez
- Rouzaud
Beny-Bassez, C. & Rouzaud, J. N. Scan. Electr. Microsc.
119-132 (1985).
- R Vidano
- D B Fischbach
Vidano, R. & Fischbach, D. B. J. Am. Ceram. Soc. 61,
13-17 (1978).
- P Lespade
- R Al-Jishi
- M S Dresselhaus
Lespade, P., Al-Jishi, R. & Dresselhaus, M. S. Carbon 20,
427-431 (1982).
- J Pasteris
Pasteris, J. D. in Applications of Microanalytical Techniques to
Understanding Mineralizing Processes (eds McKibben, M. A.,
Shanks, W. C. & Ridley, W. I.) 233-250 (Soc. Econ. Geol.,
Littleton, Colorado, 1998).
- J W Schopf
Schopf, J. W. Science 260, 640-646 (1993).
- A B Kudryavtsev
- J W Schopf
- D G Agresti
- T Wdowiak
Kudryavtsev, A. B., Schopf, J. W., Agresti, D. G. & Wdowiak, T. J.
Proc. Natl Acad. Sci. USA 416, 73-76 (2002).