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Molecular evidence for life in the 3.5 billion year old Warrawoona chert
Abstract
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.
... Large saturated hydrocarbon molecules (i.e. with >25 carbons, including diagnostic biomarkers), however, appeared absent from Neoarchean kerogen (Brocks et al., 2003) and from more mature ~3.4 Ga and 3.48 Ga (Derenne et al., 2008;Duda et al., 2018) kerogens. Gas-chromatographic massspectrometry (GC-MS) analyses, performed along catalytic hydropyrolysis of kerogen, have documented an essentially (poly)aromatic carbon structure in kerogens of the Neoarchean (Brocks et al., 2003) and of the ~3.4 Ga Strelley Pool Formation . ...
... Aliphatic hydrocarbons have been detected by pyrolysis of Archean kerogens (Hayes et al., 1983). Pyrolysis (Derenne et al., 2008) and catalytic hydropyrolysis yielded alkanes with a strong decrease in abundances at carbon numbers >18. This alkane size distribution is consistent with that observed in a Mesoproterozoic kerogen and in solvent-extracted cyanobacterial biomass , but is different from some abiotic alkane series generated using hydrothermal ...
... However, it can be noted that higher relative abundances of alkanes with n>18 were observed in other hydrothermal FTT syntheses (McCollom et al., 1999) and that alkane size distributions can be finely tuned using various catalysts in gas-phase (not hydrothermal) FTT syntheses (Subramanian et al., 2016). Moreover, hydrocarbons with an odd-over-even carbon number predominance, consistent with a biogenic origin, have been detected with pyrolysis GC-MS in a 3.48 Ga kerogen of the Dresser Formation (Derenne et al., 2008), but their origin has been questioned as their high abundance (inferred with Nuclear Magnetic ...
The Archean era (4 to 2.5 billion years ago, Ga) yielded rocks that include the oldest conclusive traces of life as well as many controversial occurrences. Carbonaceous matter is found in rocks as old as 3.95 Ga, but the oldest (graphitic) forms may be abiogenic. Due to the metamorphism that altered the molecular composition of all Archean organic matter, non-biological carbonaceous compounds such as those that could have formed in seafloor hydrothermal systems are difficult to rule out. Benthic microbial mats as old as 3.47 Ga are supported by the record of organic laminae in stromatolitic (layered) carbonates, in some stromatolitic siliceous sinters, and in some siliciclastic sediments. In these deposits, organic matter rarely preserved fossil cellular structures (e.g. cell walls) or ultrastructures (e.g. external sheaths) and its simple textures are difficult to attribute to either microfossils or coatings of cell-mimicking mineral templates. This distinction will require future nanoscale studies. Filamentous-sheath microfossils occur in 2.52 Ga rocks, and may have altered counterparts as old as 3.47 Ga. Surprisingly large spheres and complex organic lenses occur in rocks as old as 3.22 Ga and ~ 3.4 Ga, respectively, and represent the best candidates for the oldest microfossils. Titaniferous microtubes in volcanic or volcanoclastic rocks inferred as microbial trace fossils have been reevaluated as metamorphic or magmatic textures. Microbially-induced mineralization is supported by CaCO3 nanostructures in 2.72 Ga stromatolites. Sulfides 3.48 Ga and younger bear S-isotope ratios indicative of microbial sulfate reduction. Ferruginous conditions may have fueled primary production via anoxygenic photosynthesis–as suggested by Fe-isotope ratios–possibly as early as 3.77 Ga. Microbial methanogenesis and (likely anaerobic) methane oxidation are indicated by C-isotope ratios as early as 3.0 Ga and ~ 2.72 Ga, respectively. Photosynthetic production of O2 most likely started between 3.2 and 2.8 Ga, i.e. well before the Great Oxidation Event (2.45–2.31 Ga), as indicated by various inorganic tracers of oxidation reactions and consistent with morphology of benthic deposits and evidence for aerobic N metabolism in N-isotope ratios at ~ 2.7 Ga.
This picture of a wide diversification of the microbial biosphere during the Archean has largely been derived of bulk-rock geochemistry and petrography, supported by a recent increase in studied sample numbers and in constraints on their environments of deposition. Use of high-resolution microscopy and micro- to nanoscale analyses opens avenues to (re)assess and decipher the most ancient traces of life.
... The 13 C-depletion of organic matter indicates the predominance of autotrophic carbon fixation (Ueno et al. 2001(Ueno et al. , 2004. The organic matter has been characterized by Raman microspectroscopy, FTIR spectroscopy, solid-state 13 C nuclear magnetic resonance, and pyrolysis analysis (e.g., Ueno et al. 2001;Derenne et al. 2008;Duda et al. 2018). The results of these organic geochemical analyses of isolated insoluble organic matter reveal that the organic matter is dominated by aromatic structures with aliphatic hydrocarbon moieties. ...
... The results of these organic geochemical analyses of isolated insoluble organic matter reveal that the organic matter is dominated by aromatic structures with aliphatic hydrocarbon moieties. In addition, the pyrolysis fragments of the organic matter comprise long-chain aliphatic hydrocarbons and thiophenes, which are likely related to the existence of microbes including sulfate-reducing bacteria (Derenne et al. 2008;Duda et al. 2018). ...
... The chemical analysis of carbonaceous matter is necessary for understanding its origin and characteristics. However, the bulk analysis of isolated organic matter generally involves the potential risk of post-depositional and experimental contamination of the organic matter (e.g., Brocks et al. 1999;Derenne et al. 2008;Rasmussen et al. 2008;Bourbin et al. 2012). In situ analysis is a more reliable technique that avoids such contamination problems and extracts chemical signatures specific to individual microstructures in petrographic thin sections (e.g., House et al. 2000House et al. , 2013Kudryavtsev et al. 2001;Ueno et al. 2001;Schopf et al. 2002Schopf et al. , 2005Igisu et al. 2006Igisu et al. , 2009van Zuilen et al. 2007;Wacey et al. 2011;Lepot et al. 2013; Williford et al. 2013). ...
Carbonaceous matter in ~ 3.5 Ga hydrothermal vein deposits from the Dresser Formation, Western Australia, was analyzed using Fourier transform infrared (FTIR) microspectroscopy. Based on the spectroscopy, the carbonaceous matter was mainly composed of disordered aromatic structures, with minor aliphatic C–H functional groups. Spatially resolved analysis supports that the aliphatic C–H signatures are derived from kerogenous macro-molecule and not from free bitumen or other artificial sources. The intensity ratios of the asymmetric aliphatic CH3 to the asymmetric aliphatic CH2 (R3/2 value) in the carbonaceous clots range from 0.22 to 0.51. Thermal alteration may increase or not change the R3/2 value of organic matter, as suggested by previous experiments, but it is unlikely to be the cause of the substantially lower R3/2 values when compared with those of primary organic matter. In particular, the low R3/2 values (< ~ 0.4) suggest that the carbonaceous matter mainly contains aliphatic C–H bonds derived from bacterial cells. The carbonaceous clots may have been possibly produced by abiotic reaction such as Fischer-Tropsch-type (FTT) synthesis. However, the organic matter source only produced by the FTT synthesis is inconsistent with the R3/2 values for the analyzed carbonaceous clots. The results obtained by combining these spectroscopic features of the carbonaceous clots together with the previously reported isotopic features may possibly suggest that both bacteria and archaea were colonized in the ~ 3.5 Ga Dresser hydrothermal system. [Figure not available: see fulltext.].
... The origin of the oldest traces of life on Earth has been investigated through chemical and thermal degradation of organic matter (OM) from Archean silicified sediments (Brocks et al., 1999(Brocks et al., , 2003aMarshall et al., 2007;Ventura et al., 2007;Derenne et al., 2008). In addition to the numerous debates about the syngeneity of putative molecular biomarkers Rasmussen et al., 2008;French et al., 2015), the preservation of aliphatic moieties in OM from Archean silicified sediments remains an open issue (Bourbin et al., 2012a). ...
... In contrast to soluble hydrocarbons thought to be highly sensitive to contamination, hydrocarbons released through the thermal cracking of covalent bonds of the insoluble OM, the so-called kerogen macromolecular structure, are considered syngenetic (Brocks et al., 2003b;Marshall et al., 2007;Derenne et al., 2008). Because of thermal alteration extending to greenschist/amphibolite facies metamorphism in Archean metasediment (Westall et al., 2006;Sugitani et al., 2007Sugitani et al., , 2015Delarue et al., 2016), the macromolecular network of Archean kerogens is often depleted in aliphatic moieties (Bourbin et al., 2012a) as reflected by Hydrogen-to-Carbon (H/C) atomic ratios mostly below 0.3 (Hayes et al., 1983;Marshall et al., 2007). ...
... Because of thermal alteration extending to greenschist/amphibolite facies metamorphism in Archean metasediment (Westall et al., 2006;Sugitani et al., 2007Sugitani et al., , 2015Delarue et al., 2016), the macromolecular network of Archean kerogens is often depleted in aliphatic moieties (Bourbin et al., 2012a) as reflected by Hydrogen-to-Carbon (H/C) atomic ratios mostly below 0.3 (Hayes et al., 1983;Marshall et al., 2007). Nevertheless, several studies indicated that some Archean kerogens can also be characterized by H/C atomic ratios higher than 0.3, reaching values up to 0.6 (Hayes et al., 1983;Marshall et al., 2007;Derenne et al., 2008;French et al., 2015;Ferralis et al., 2016). A H/C atomic ratio up to 0.6 in kerogen from the 3.45 Gyr-old Warrawoona Formation (Derenne et al., 2008) has been considered inconsistent with the prehnitepumpellyite and lower greenschist facies metamorphism undergone by the host rock . ...
The search for hydrocarbon molecular biomarkers in Archean metasediments is of prime importance for deciphering the early evolution of life. Suitable criteria are required to identify promising targets for further molecular biomarkers. Possible criteria include the Hydrogen-to-Carbon (H/C) atomic ratio used as a proxy of the aliphatic content of the kerogen matrix. However, H/C ratio values exhibit large variation in Archean kerogens and their significance remains poorly understood. In this study, we thus investigate the significance of the H/C ratios of Archean kerogens by combining elemental analyses, Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), Rock-Eval pyrolysis and Raman spectroscopy. First, NanoSIMS investigations show the H/C ratio of kerogen can be compromised by residual minerals. In addition, Rock-Eval pyrolysis underlines the fact that thermal cracking of Archean kerogens does not just release hydrocarbon covalently linked to the macromolecular network but also a complex mixture of organic pools distinguished by their thermal maturity. Therefore, the H/C ratio alone cannot be used to probe the preservation of aliphatic compounds bound to kerogen since it can be biased by the presence of (i) residual bitumen, as well as (ii) refractory organic matter in secondary hydrothermal veins whose syngenecity is debatable. Rock-Eval pyrolysis then provides a useful and complementary method to check the significance of H/C atomic ratio as a proxy for hydrocarbon preservation in Archean kerogens.
... Ga-old Barberton (South Africa) and Pilbara (Australia) greenstone belts are so degraded by age and low-grade metamorphism (prehenite-pumpellyite to lowermost greenschist facies) that they no longer contain recognizable compositional biomarkers. The kerogens do, however, retain structural characteristics that are indicative of a biological origin (Westall et al., 2006a;Derenne et al., 2008). Evidence of syngenicity of the kerogens analyzed is provided by in situ Raman spectra showing D and G peaks indicative of a maturity consistent with the metamorphic grade of the rocks (Westall et al., 2006a), as well as covalent linking of the molecular structures in the kerogen matrix (Derenne et al., 2008). ...
... The kerogens do, however, retain structural characteristics that are indicative of a biological origin (Westall et al., 2006a;Derenne et al., 2008). Evidence of syngenicity of the kerogens analyzed is provided by in situ Raman spectra showing D and G peaks indicative of a maturity consistent with the metamorphic grade of the rocks (Westall et al., 2006a), as well as covalent linking of the molecular structures in the kerogen matrix (Derenne et al., 2008). ...
... Such contamination led Pflug (1979) to identify eukaryotic yeast cells in a 3.8 Ga-old rock from Isua as in situ microfossils. This type of problem can be overcome by verifying the maturity of the kerogen by high-resolution TEM and Raman spectroscopy and/or by demonstrating chemical bonding between the organic molecules and the mineral matrix that indicates the syngenicity of the kerogen (Marshall et al., 2005;Derenne et al., 2008). Mineralized microbial structures: The most common form of preservation of microbial structures is mineral fossilization. ...
... Ga ago (tail end of the Paleoarchaean all through to the very end of the Mesoarchaean era) [6], even though there was practically very little oxygen [7]. Some of these early life cyanobacteria, through their activities in the formation of stromatolites, can still be found in the Pilbara Craton's Strelley pool chert [8] and Warrawoona group of fossils [9], Cervantes' Lake Thetis [10] and Shark Bay [11], all in Western Australia. The great oxidation event (GOE), which occurred around the generally accepted $2.4-2.1 Ga ago (middle Siderian through all the Rhyacian period) is hypothesised to have marked the enduring presence and spread of atmospheric oxygen from the oceans through cyanobacterial activity. ...
... For simplicity and economic reasons, wellhead injection temperatures and pressures are usually aligned with that of the compressed gas's transportation conditions [90] although temperatures can range between 30°C all through to 200°C and pressures from 1 to 30 MPa, with both temperature and pressure ceiling usually determined by the reservoir's state [91], and the acid gas injection strategy delineated to 9 Hydrogen Sulphide DOI: http://dx.doi.org/10.5772/intechopen.1003662 take advantage of system thermodynamics, including importantly, use of the phase envelope [92]. ...
Hydrogen sulphide (H₂S), a highly toxic and corrosive molecule, is typically found in hydrocarbon reservoirs, sewers and in the waste industry. It can be extremely problematic during drilling, production and processing. This chapter offers a synopsis of H₂S, which is sulphur in its most reduced form of all its numerous oxidation states. It delves briefly into H₂S's history on planet earth before there was life all through to its diminishment during the latter Proterozoic era to present day. It also investigates its various forms of generation and production, and its effect and impact especially as an occupation-based hazard. Its utilisation in enhanced oil recovery (EOR) as a standalone or together with carbon dioxide (CO₂) and its role in geosequestration together with CO₂ is explored.
... Compare this with the fact that sediments in the northwest of Western Australia that are 3.5 × 10 9 years old contain very convincing cellularly preserved filamentous microfossils (Schopf, 1993;Derenne et al., 2008). These are among the oldest fossils known but their morphometrics are consistent with modern filamentous cyanobacteria (Boal & Ng, 2010). ...
... Prokaryotes have dominated the Earth for the bulk of its history (I have put that statement in a tense that suggests they do not dominate the Earth now, but the truth might be other than this). LUCA must have emerged close to the start of the Archaean Eon, about 3.8 billion years ago, because, as noted above, some of the oldest microbial fossils are fully differentiated, photosynthetic bacteria (cyanobacteria) found in Western Australian sediments that are 3.5 × 10 9 years old (Schopf, 1993;Derenne et al., 2008;Boal & Ng, 2010). On the other hand, eukaryotes are generally thought to have appeared no earlier than about 1.5 billion years ago (and some people put their emergence somewhat later than that). ...
This is the complete version of the original manuscript for this book. It includes all illustrations and references and appears here under my original, and preferred, title. My publisher insisted on changing the title!
... Indeed, although most non-Archean CM is usually considered to be of biological origin, abiotic processes such as Fischer-Tropsch-type synthesis and siderite decomposition are often thought to account for Archean CM formations (van Zuilen et al., 2002(van Zuilen et al., , 2003McCollom and Seewald, 2006;see van Zuilen et al., 2007, and references therein for more details about abiotic formation of CM). Molecular characterization of Archean CM has recently been recognized as a promising tool with which to discriminate between biotic and abiotic CM (Brocks et al., 2003;Marshall et al., 2007;Derenne et al., 2008). Unfortunately, in most samples, multiple sources of postdepositional CM and the extensive impact of metamorphism have resulted in the masking or elimination of molecular structures and a lack of univocal molecular biosignatures (Bourbin et al., 2012a;French et al., 2015). ...
... The Dresser sample, on the other hand, presents a surprisingly low aromaticity (57%) for an Archean Sample. However, this result is consistent with the release of significant amounts of aliphatic moieties by pyrolysis (Derenne et al., 2008). Such aliphaticity may reflect the input of poorly ordered CM through hydrothermal circulation as recently demonstrated in the Apex chert (Marshall et al., 2012;Sforna et al., 2014) or late pyrobitumen generation formed through carbonization (Bernard et al., 2012). ...
Unlabelled:
The search for indisputable traces of life in Archean cherts is of prime importance. However, their great age and metamorphic history pose constraints on the study of molecular biomarkers. We propose a quantitative criterion to document the thermal maturity of organic matter in rocks in general, and Archean rocks in particular. This is definitively required to select the best candidates for seeking non-altered sample remnants of life. Analysis of chemical (Raman spectroscopy, (13)C NMR, elemental analysis) and structural (HRTEM) features of Archean and non-Archean carbonaceous matter (CM) that was submitted to metamorphic grades lower than, or equal to, that of greenschist facies showed that these features had all undergone carbonization but not graphitization. Raman-derived quantitative parameters from the present study and from literature spectra, namely, R1 ratio and FWHM-D1, were used to draw a carbonization continuum diagram showing two carbonization stages. While non-Archean samples can be seen to dominate the first stage, the second stage mostly consists of the Archean samples. In this diagram, some Archean samples fall at the boundary with non-Archean samples, which thus demonstrates a low degree of carbonization when compared to most Archean CM. As a result, these samples constitute candidates that may contain preserved molecular signatures of Archean CM. Therefore, with regard to the search for the oldest molecular traces of life on Earth, we propose the use of this carbonization continuum diagram to select the Archean CM samples.
Key words:
Archean-Early life-Kerogen-Raman spectroscopy-Carbonization. Astrobiology 16, 407-417.
... The uniqueness of Archean microfossils constrains the use of destructive methods for determining the environmental context of deposition of this chert unit and the reality of these microfossils. New nondestructive techniques such as Nano-SIMS imagery of microfossils (e.g., Oehler et al. 2009;Schopf et al. 2018) or a combination of analytical techniques could be useful for determining whether the chemical structure of such putative microfossils is consistent with a biological origin (e.g., Derenne et al. 2008). Detailed investigation of the varying generations of cherts by, for example, fluid inclusion micro-geothermobarometry and decrepitation geochemistry of such fluid inclusions would certainly add tremendously to understanding of the history of the Apex Chert. ...
... Millions or billions of years ago, the diagenesis of organic rich sediments led to the formation of rocks consisting of a mixture of minerals and solid carbonaceous matter called kerogen. These more or less hydrogenated carbonaceous materials carry information about ancient life forms and their environments and are key for understanding the various 30 attempts made during life evolution (Derenne, et al., 2008). Metabolism of most living systems, even the most primitive ones, are based on various metalloporphyrin complexes. ...
How the transport of fluids in a confined and complex mixed organic/inorganic matrix can be far below the expected value from topological aspect? A good example of this situation is oil shales. Oil and gas shales are source rocks in which organic matter has matured to form hydrocarbons. They exhibit a dual porous network formed by the intertwining of mineral and organic pores that leads to very low permeability. Still, the exact origin of this extremely low permeability remains somehow unclear. The present communication addresses this important question and provides novel insights on the mechanisms that strongly hinder fluid diffusion in such materials. By combining nuclear and electronic magnetic resonance techniques combined with SEM imaging, we evidence that magnetic locking occurs in kerogen. This locking results from a magnetic coupling between vanadyl present in porphyrins and the organic matrix. We demonstrate that such coupling retards fluid diffusion and is reversible. This key dynamical feature explains the extremely low mobility of oil in shale rocks. This phenomenon may be a more general feature occurring in several systems where fluids are confined in a complex hierarchical matrix that embeds both organic and inorganic radicals resulting from ageing process.
... are frequently used to remove modern organic contaminants from experimental apparatus and rock samples, assuming that the modern organic contaminants did not percolate into the porous space of the sample (Beaumont and Robert, 1999;Derenne et al., 2008;WRIGHT et al., 1997). Moreover, organic solvents, along with trace impurities, can contaminate samples, contributing to errors in the total organic carbon (TOC) values (Arif et al., 2017c;Pan et al., 2020;WRIGHT et al., 1997). ...
The oxidation of fossil fuels produces billions of tons of anthropogenic carbon dioxide (CO2) emissions from stationary and nonstationary sources per annum, contributing to global warming. The natural carbon cycle consumes a portion of CO2 emissions from the atmosphere. In contrast, substantial CO2 emissions accumulate, making it the largest contributor to greenhouse gas emissions and causing a rise in the planet’s temperature. The Earth’s temperature was estimated to be 1°C higher in 2017 compared to the mid-twentieth century. A solution to this problem is CO2 storage in underground formations, abundant throughout the world. Millions of tons of CO2 are stored underground into geological formations annually, including deep saline aquifers. However, these geological formations have minute concentrations of organic material, significantly influencing the CO2 containment security, fluid dynamics, and storage potential. Examining the wetting characteristics and influencing parameters of geological formations is pertinent to understanding the supercritical CO2 behavior in rock/brine systems. Wettability is an important parameter governing the ability of injected CO2 to displace formation water and determine the containment security and storage capacity. Previously, many studies have provided comprehensive overviews of CO2-wettability depending on various factors, such as pressure, temperature, salinity, formation type, surfactants, and chemicals. However, mineral surfaces in these wettability studies are chemically cleaned, and natural geological storage conditions are anoxic (containing organic molecules) where reductive conditions ensue. A severe gap exists in the literature to comprehend the effects of organic material for determining the CO2 storage capacities and how this effect can be reversed using nanomaterial for increased CO2 storage potential. Therefore, we conducted a thorough literature review to comprehend the recent advances in rock/CO2/brine and rock/oil/brine systems containing organic material in different geo-storage formations. We also present recent advances in anoxic rock/CO2/brine and rock/oil/brine systems that have employed nanomaterial for wettability reversal to be more water-wet. This comprehensive review is divided into four parts: 1) reviewing CO2 emissions and geological systems, 2) recent advances in direct quantitative experimental procedures in anoxic rock/CO2/brine systems and effects of organic contaminations on experimental methodology and their controls, 3) effects of organics and nanomaterial in rock/CO2/brine and rock/oil/brine systems, and 4) the future outlook of this study.
... The uniqueness of Archean microfossils constrains the use of destructive methods for determining the environmental context of deposition of this chert unit and the reality of these microfossils. New nondestructive techniques such as Nano-SIMS imagery of microfossils (e.g., Oehler et al. 2009;Schopf et al. 2018) or a combination of analytical techniques could be useful for determining whether the chemical structure of such putative microfossils is consistent with a biological origin (e.g., Derenne et al. 2008). Detailed investigation of the varying generations of cherts by, for example, fluid inclusion micro-geothermobarometry and decrepitation geochemistry of such fluid inclusions would certainly add tremendously to understanding of the history of the Apex Chert. ...
Apex Chert, Microfossils
Wladyslaw Altermann and Daniele L. Pinti
Keywords
Apex Chert · Apex Basalt · Biomarkers ·
Cyanobacteria · Microfossils, World’s oldest
fossils, Pseudofossils
Synonyms
Apex Chert, Apex Basalt Formation, Schopf
locality, Earth’s oldest microfossils
Definition
The Apex Chert is a lenticular and bedded, laminated,
microcrystalline silica (SiO2) deposit
interlayered with and crosscutting submarine
lavas of the Apex Basalt Formation, Pilbara Craton,
Western Australia. The Apex Basalt Formation,
Salgash Subgroup of theWarrawoona Group
dates at 3465–3458 Ma. The origin of the chert is
disputed. The rivalling interpretations: diagenetic
silicification (chertification) of clastic or
carbonate sedimentary and volcano-sedimentary
rocks versus primary silica deposition on the
ocean floor or hydrothermal chert intrusion and
replacement, do not necessarily contradict each
other. Varying chert generations may coexist
where black chert dikes and lenses crosscut dark
gray and whitish stratiform cherts and interlayered
volcanics of the Apex Formation (Marshall et al.
2012). Carbonaceous filaments found in the Apex
Chert beds, Chinaman Creek near Marble Bar,
were interpreted as world’s oldest fossils and as
evidence for the antiquity of life on Earth (Schopf
1993). The name “Schopf locality” was given to
this outcrop after J. William (Bill) Schopf, an
American paleontologist and paleobiologist of
the University of California, Los Angeles, who
found and described these microfossils......
... Cette chimie prébiotique menant à la formation de molécules non oxydées à fait l'objet de nombreuses recherches puisqu'il s'agit en définitive d'établir l'origine de la vie (Knoll, 2003(Knoll, , 2015. Cette dernière était déjà présente il y a 3,5 Ga sous forme de structures microfossiles mises en évidence à Pilbara, en Australie (dans les cherts du Groupe de Warrawoona, Derenne et al., 2008 ;Wacey et al., 2011) et à Barbeton, il y a 3,2 Ga, en Afrique du Sud (dans les cherts du Groupe d'Onverwacht, Brooks & Shaw, 1971 ;Knoll, 2003), dans des stromatolithes. Cette vie a probablement débuté plus tôt, il y a 3,8 à 3,9 Ga (Mojzsis et al., 1996 ;Nutman et al., 2010Nutman et al., , 2016. ...
Résumé : L’oxygène n’est pas apparu aussi brutalement qu’on le pensait sur
notre planète.
Malgré un apport en oxygène lié aux cyanobactéries dès l’Archéen, ce ne se sont
pas ces micro-organismes qui sont à la base de la première grande ‘révolution’ de
l’oxygène qui a eu lieu à la limite Archéen/Paléoprotérozoïque (il y a 2,5 milliards
d’années) dans l’atmosphère, lors du Grand Evénement d’Oxydation. Ce sont les
processus liés au cycle de la tectonique des plaques (activité mantellique et
périodes intenses d’érosion/altération) qui ont contribué de manière
déterminante à l’augmentation de la concentration de l’oxygène atmosphérique
vers 2,5 milliards d’années. Les deux principaux processus responsables de
cette augmentation sont liés à l’enfouissement de la matière organique et de la
pyrite (= FeS2). L’altération des séries riches en ces deux composants conditionnera
ensuite pendant près d’un milliard d’années la composition chimique des océans en
oxygène, soufre et fer. Au cours du temps, l’oxygène proviendra de l’activité des
cyanobactéries et l’atmosphère réductrice du début de l’Archéen sera remplacée par
une atmosphère oxydante à la fin du Précambrien.
Abstract : Oxygen did not appear as abruptly as we thought on our planet.
Despite an oxygen supply related to cyanobacteria, since the Archean, it is not these
microorganisms that are at the base of the first great oxygen revolution that took
place at the Archean/Paleoproterozoic boundary (2.5 billion years) in the atmosphere
during the Great Oxidation Event. Two processes related to the cycle of plate
tectonics (mantle activity and intense periods of erosion/weathering) were
mostly involved in the increase of the of atmospheric oxygen concentration 2.5
billion years ago. These two main processes are related to the burial of organic
matter and those of pyrite (= FeS2). The alteration of series with high contents of
the two elements will then condition for nearly a billion of years the oxygen, sulfur
and iron chemical composition of the oceans. The oxygen will finally come from the
activity of cyanobacteria and the early Archean reducing atmosphere will be replaced
by an oxidizing atmosphere at the end of the Precambrian.
... The vast majority of Paleoarchean cherts in the Pilbara Craton were exposed to these metamorphic conditions, and thus have the potential for preserving molecular information relevant to assessing the source of organic matter. Bulk scale molecular techniques, such as py-GC-MS and NMR, have been used to evaluate the origin of organic matter by identifying molecular features attributable to paleobiological activity 32,47 . Yet the application of bulk analytical techniques to ancient organic materials can be problematic because of contamination introduced during post depositional geological processes, exposure in outcrop, archival and/or during the preparation of samples [48][49][50][51] . ...
Hydrothermal and metamorphic processes could have abiotically produced organo-mineral associations displaying morphological and isotopic characteristics similar to those of fossilized microorganisms in ancient rocks, thereby leaving false-positive evidence for early life in the geological record. Recent studies revealed that geologically-induced alteration processes do not always completely obliterate all molecular information about the original organic precursors of ancient microfossils. Here, we report the molecular, geochemical, and mineralogical composition of organo-mineral associations in a chert sample from the ca. 3.47 billion-year-old (Ga) Mount Ada Basalt, in the Pilbara Craton, Western Australia. Our observations indicate that the molecular characteristics of carbonaceous matter are consistent with hydrothermally altered biological organics, although significantly distinct from that of organic microfossils discovered in a chert sample from the ca. 3.43 Ga Strelley Pool Formation in the same area. Alternatively, the presence of native metal alloys in the chert, previously believed to be unstable in such hydrothermally influenced environments, indicates strongly reducing conditions that were favorable for the abiotic formation of organic matter. Drawing definitive conclusions about the origin of most Paleoarchean organo-mineral associations therefore requires further characterization of a range of natural samples together with experimental simulations to constrain the molecular composition and geological fate of hydrothermally-generated condensed organics.
... The low porosity of cherts is commonly advocated for making the syngenetic organic matter less prone to postdepositional contamination (Derenne et al., 2008). However, Raman spectroscopic data collected on the organic material present within the matrix of the Apex chert, one of the most well studied Archean deposits (e.g., Schopf, 1993;De Gregorio and Sharp, 2006), indicated the presence of two different phases of carbonaceous materials deposited as separate phases in the quartz matrix (Marshall et al., 2012). ...
Insoluble organic materials (kerogens) isolated from ancient sedimentary rocks provide unique insights into the evolution of early life. However, establishing whether these kerogens are indeed syngenetic with the deposition of associated sedimentary host rocks, or contain contribution from episodes of secondary deposition, is not straightforward. Novel geochemical criterions are therefore required to test the syngenetic origin of Archean organic materials. On one hand, the occurrence of mass-independent fractionation of sulphur isotopes (MIF-S) provides a tool to test the Archean origin of ancient sedimentary rocks. Determining the isotope composition of sulphur within kerogens whilst limiting the contribution from associated minerals (e.g., nano-pyrites) is however challenging. On the other end, the Xe isotope composition of the Archean atmosphere has been shown to present enrichments in the light isotopes relative to its modern composition, together with a mono-isotopic deficit in ¹²⁹Xe. Given that the isotopic composition of atmospheric Xe evolved through time by mass dependent fractionation (MDF) until ∼2.5-2.0 Ga, the degree of MDF of Xe isotopes trapped in kerogens could provide a time stamp for the last chemical equilibration between organic matter and the atmosphere. However, the extent to which geological processes could affect the signature of Xe trapped in ancient kerogen remains unclear. In this contribution, we present new Ar, Kr and Xe isotopic data for four kerogens isolated from 3.4 to 1.8 Gy-old cherts and confirm that Xe isotopes from the Archean atmosphere can be retained within kerogens. However, new Xe-derived model ages are lower than expected from the ages of host rocks, indicating that initially trapped Xe components were at least partially lost and/or mixed together with some Xe carried out by younger generations of organic materials, therefore complicating the Xe-based dating method. Whilst non-null Δ³³S values and ¹²⁹Xe deficits relative to modern atmosphere constitute reliable imprints from the Archean atmosphere, using Xe isotopes to provide information on the syngenetic origin of ancient organic matter appears to be a promising - but not unequivocal - tool that calls for further analytical development.
... The kerogen consists of highly aromatic molecules and its bulk δ 13 C values range from À28.3 to À35.8‰. Derenne et al. (2008) noted an odd-over-even carbon number pattern associated with this kerogen, generally considered consistent with a biological origin, although this pattern is also found in meteoritic carbon (R Summons personal communication 2013). The Strelley Pool organic matter also contains sulphur and analysis of the δ 33 S and δ 34 S signatures indicated (1) that sulphur has been incorporated into the Fig. 7.1 (continued) by the mat. ...
... The high stability of Si-O bound under the oxidizing conditions of the Earth's atmosphere and seas means that solid mineral SiO 2 is the end product of most chemical reactions involving Si. The discovery of the oldest putative evidence for life in SiO 2 -rich chert deposits, stimulated a debate about the role of silica (SiO 2 ) in the emergence of life (e.g., Derenne et al., 2008). Nowadays, silica is present in most lineages of living organisms, often conferring some defensive advantage (Hodson et al., 2005;Exley, 2015). ...
... In one study of a kerogen isolated from a black chert vein in the 3.5 Ga Dresser Formation, Pilbara Craton, catalytic pyrolysis in a stream of high-pressure hydrogen gas yielded abundant hydrocarbons with aliphatic chains up to 18 carbons in length with an abrupt step to only trace levels of longer chains [57]. In another study of a similarly aged kerogen from a different locality in the Pilbara Craton, Curie-point pyrolysis afforded alkene-alkane doublets with up to 25 carbon atoms and an odd over even carbon number preference [58]. In both cases, these distributions were claimed to be evidence for Paleoarchean biological activity because of molecular characteristics similar to unambiguous biogenic organic matter. ...
Here we discuss the early geological record of preserved organic carbon and the criteria that must be applied to distinguish biological from non-biological origins. Sedimentary graphite, irrespective of its isotopic composition, does not constitute a reliable biosignature because the rocks in which it is found are generally metamorphosed to the point where convincing signs of life have been erased. Rather, multiple lines of evidence, including sedimentary textures, microfossils, large accumulations of organic matter and isotopic data for co-existing carbon, nitrogen and sulfur are required before biological origin can be convincingly demonstrated.
... The illogically enhanced cellular requirement for such an unavailable element in the modern ocean stems from the early evolution of cells. Around 3.5-3.8 billion years ago, when the first signs of life were beginning to emerge [83,84] , the reducing atmosphere and oceans of the Archean Earth meant that iron was abundant in its soluble ferrous form and thus readily available for cellular incorporation [85,86] . The unique properties of iron (i.e. ...
The effects on bacterioplankton of very low concentrations of nitrogen and phosphorus/iron in the Atlantic were investigated using shipboard enrichment experiments. In the North Atlantic gyre, bacterioplankton abundance and amino acid uptake increased upon combined addition of ammonium and phosphate (9-42% and 120-880% increase, respectively). Outside the gyre, the requirement for phosphate additions was reduced. In the South Atlantic, ammonium additions generally caused an increase in bacterioplankton abundance (10-32% after 48 h) and amino acid uptake (20-300%), particularly in the gyre centre. Conversely, iron additions showed a negligible response. These results contribute towards understanding the effect of low nutrient concentrations on bacterioplankton. Reduced abundance or metabolic activity of bacterioplankton due to low nutrient concentrations may impact marine primary production and carbon fluxes, which is a particular concern due to the expansion of oligotrophic gyres as a result of climate change. Iron and cobalt are essential micronutrients that are susceptible to forming particulate/insoluble species (e.g. via adsorption, oxidation, precipitation). These species can be lost from the surface ocean, thus reducing nutrient bioavailability. Therefore, mechanisms affecting their formation were also investigated. Cell surface iron adsorption was measured in the South Atlantic, with highest levels occurring in the gyre (~180-300 zmol Fe/cell, verses ~3-155 zmol Fe/cell in productive waters). This was hypothesised to be due to low ambient iron in the gyre, resulting in a larger free cell surface area for iron binding. Particulate cobalt formation was enhanced (>150%) in the presence of Aurantimonas (a manganese-oxidising bacteria), was generally elevated under higher manganese concentrations (e.g. ~13-60% increase upon adding MnCl2 to manganese-poor cultures), but was slightly reduced in the presence of nickel (6.2±7.4% decrease) or copper (7.4±12.2% decrease). These results further understanding of factors influencing micronutrient speciation, which is especially important considering potential changes to ocean biogeochemistry as a result of anthropogenic activity.
... The kerogen consists of highly aromatic molecules and its bulk δ 13 C values range from À28.3 to À35.8‰. Derenne et al. (2008) noted an odd-over-even carbon number pattern associated with this kerogen, generally considered consistent with a biological origin, although this pattern is also found in meteoritic carbon (R Summons personal communication 2013). The Strelley Pool organic matter also contains sulphur and analysis of the δ 33 S and δ 34 S signatures indicated (1) that sulphur has been incorporated into the Fig. 7.1 (continued) by the mat. ...
Life on the early Earth inhabited a planet whose environment was vastly different from the Earth of today. An anaerobic and hot early Earth was the birthplace of the first living cells but wide-spread small-scale physico-chemical diversity provided opportunities for a variety of specialists: alkalophiles, acidophiles, halophiles etc. The earliest record of life has been lost due to plate tectonic recycling and the oldest preserved terranes (~3.9–3.7 Ga) are heavily altered by metamorphism, although they may contain traces of fossil life. As of ~3.5 Ga, ancient sediments are so well-preserved that a broad diversity of micro-environments and fossil traces of life can be studied, providing a surprising window into communities of microbes that had already reached the evolutionary stage of photosynthesis. From the wide variety of traces of ancient life that have been reported from the Archaean geological record in Greenland, Canada, South Africa and Western Australia, we examine a few particularly pertinent examples. Biosignatures in the rock record include microfossils, microbial mats, stromatolites, microbially induced sedimentary structures, biominerals, biologically indicative isotopic ratios and fractionations, elemental distributions, organochemical patterns and other geochemical peculiarities best explained by biological mediation. Due to dynamic geological reprocessing over the billions of years since these fossils entered the rock record, identifications of very ancient traces of life have been subject to criticism, hence the often complex arguments regarding their biogenicity. We here highlight a range of unambiguously bona fide and widely supported examples of fossil biosignatures. Fossil biosignatures have great promise as analogues of life that might be detected on other planets. In this respect, the study of the early Earth is particularly pertinent to the search for life on Mars, given the planetary- and microbial-scale similarities that prevailed on both planets during their early histories, together with the lack of subsequent geological reprocessing on Mars, which may make it an ideal repository for a near-pristine fossil record.
... Bulk nitrogen isotopic composition (δ 15 NBulk, with respect to the 15 N/ 14 N ratio of the atmospheric N2) was determined on 5 to 10 mg of organic matter (OM) previously isolated through successive demineralisation of ca. 200 g of rocks using hydrofluoric (HF) and hydrochloric (HCl) treatment (Derenne et al., 2008). Powdered cherts were first stirred at room temperature in dichloromethane/methanol (2/1, v/v; High Performance Liquid Chromatography grade) in order to remove soluble organic compounds. ...
There is compelling evidence for early oxygenation of mid-Archean oceans. However, the biological use of molecular oxygen is still not ascertained. Here we report the nitrogen isotope composition measured in isolated microfossils (δ15Nµm) from the 3.0 billion years old Farrel Quartzite metasediments. We show that the quasi-null bulk δ15N values of Farrel Quartzite organic matter encompass a large 15N isotopic heterogeneity at the scale of isolated microfossils (-21.6 ‰ < δ15Nµm < +30.7 ‰). Rayleigh fractionation is required to yield such large δ15N variations. Based on these data, we propose a model in which negative δ15Nµm values determined on film-like and on spheroidal microfossils are explained by ammonia assimilation in the anoxic deeper levels of the water column, whereas positive δ15Nµm values determined on lenticular microfossils were driven by both ammonia assimilation and aerobic oxidation close to the sea surface. Since ammonium aerobic oxidation requires the presence of free molecular O2 within the water column, we further suggest that positive δ15Nµm values reflect an ocean redox stratification tightly related to O2 production by oxygenic photosynthesisers in a mid-Archean ocean 3.0 Gyr ago.
... In most cases, solvents are applied to small pieces of rock sample devoid of fractures and assuming that the modern contamination did not penetrate into the rocks' porosity (e.g. Wright et al., 1997;Beaumont and Robert, 1999;Derenne et al., 2008). Others prefer to wash rock powders with organic solvents, in order to ensure the complete decontamination of the sample (e.g. ...
Interpreting the organic carbon content (TOC) and stable carbon isotopic composition (δ¹³C) of organic matter in the sedimentary rock record depends on our capability to accurately measure them, while excluding sources of contamination. This however becomes increasingly problematic as we analyze samples with ever-lower organic carbon content. Accordingly, organic solvents are sometimes used to remove contaminating traces of modern organic matter from ancient rock samples. However, especially for very low TOC samples, traces of solvents or their impurities remaining in the sample may contribute a significant organic contamination that can impact the bulk measurements of both TOC and δ¹³C values. This study, including three independent investigations performed in different laboratories, is the first detailed examination of the effect of cleaning treatments on the reliability of TOC and δ¹³C values in a range of natural rock samples and synthetic materials with low TOC content from below detection limit to 3330 ppm. We investigated the four most common organic solvents used to remove modern organic matter: dichloromethane (DCM), n-hexane, methanol and ethanol, and evaluated the effect of grain size and mineralogy. We find that (i) cleaning treatments with methanol, n-hexane and dichloromethane contaminate rock samples when used directly on sample powder, regardless of the grain size; (ii) this pollution buffers the natural variability and homogenizes the δ¹³C values of samples around the isotopic composition of the solvent, i.e. between −27 and −29‰; (iii) the extent of contamination depends on the solvent used, DCM being the most contaminating (up to 6000 ppm) and ethanol the only solvent that does not seem to contaminate rock samples above our detection limit; (iv) sample mineralogy also exerts an influence on the extent of contamination, clay minerals being more prone to adsorb contaminants. We conclude that the response of carbon concentrations and the stable carbon isotopic composition of organic matter in geological samples to cleaning treatments is neither negligible nor systematic when investigating samples with low carbon content.
... Finely laminated Archean cherts containing hydrothermally silicified biofilms also preserve some organic matter, but the high metamorphic grades of these rocks (Westall, Campbell, et al., 2015) ensure that any molecular biosignatures have been erased. Curie point pyrolysis of an Archean chert from the Warrawoona Formation of the Pilbara Craton in Western Australia yielded alkane/alkene doublets with a slight odd/even preference (Derenne et al., 2008), an indisputable biosignature. However, given the metamorphic grade of rocks from the locality (Flannery et al., 2018) and the possibility of contamination from contemporary surface-dwelling microbes, results such as this should be viewed with caution. ...
The Martian surface is cold, dry, exposed to biologically harmful radiation and apparently barren today. Nevertheless, there is clear geological evidence for warmer, wetter intervals in the past that could have supported life at or near the surface. This evidence has motivated NASA and ESA to prioritize the search for any remains or traces of organisms from early Mars in forthcoming missions. Informed by (1) stratigraphic, mineralogical and geochemical data collected by previous and current missions, (2) Earth's fossil record, and (3) experimental studies of organic decay and preservation, we here consider whether, how, and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early Mars. We conclude that Noachian–Hesperian Fe‐bearing clay‐rich fluvio‐lacustrine siliciclastic deposits, especially where enriched in silica, currently represent the most promising and best understood astropaleontological targets. Siliceous sinters would also be an excellent target, but their presence on Mars awaits confirmation. More work is needed to improve our understanding of fossil preservation in the context of other environments specific to Mars, particularly within evaporative salts and pore/fracture‐filling subsurface minerals.
... 50 mm) and isolated kerogen, whereas scanning electron microscopy (SEM) observations were only performed on the kerogen. Kerogen isolation was performed on about 200 g of rock through successive demineralization using HF-HCl (Derenne et al., 2008). Then, a few milligrams of kerogen was deposited on a microscope glass slide for TLM, SEM, and NanoSIMS investigations. ...
The search for life and habitable environments on other Solar System bodies is a major motivator for planetary exploration. Due to the difficulty and significance of detecting extant or extinct extraterrestrial life in situ, several independent measurements from multiple instrument techniques will bolster the community's confidence in making any such claim. We demonstrate the detection of subsurface biosignatures using a suite of instrument techniques including IR reflectance spectroscopy, laser-induced breakdown spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy. We focus our measurements on subterranean calcium carbonate field samples, whose biosignatures are analogous to those that might be expected on some high-interest astrobiology targets. In this work, we discuss the feasibility and advantages of using each of the aforementioned instrument techniques for the in situ search for biosignatures and present results on the autonomous characterization of biosignatures using multivariate statistical analysis techniques. Key Words: Biosignature suites-Caves-Mars-Life detection. Astrobiology 17, 1203-1218.
... 50 mm) and isolated kerogen, whereas scanning electron microscopy (SEM) observations were only performed on the kerogen. Kerogen isolation was performed on about 200 g of rock through successive demineralization using HF-HCl (Derenne et al., 2008). Then, a few milligrams of kerogen was deposited on a microscope glass slide for TLM, SEM, and NanoSIMS investigations. ...
Observations of Archean organic-walled microfossils suggest that their fossilization took place through both encapsulation and permineralization. In this study, we investigated microfossils from the ca. 3.0 Ga Farrel Quartzite (Pilbara, Western Australia) using transmitted light microscopy, scanning electron microscopy, Raman microspectrometry, and nanoscale secondary ion mass spectrometry (NanoSIMS) ion microprobe analyses. In contrast to previous studies, we demonstrated that permineralized microfossils were not characterized by the micrometric spatial relationships between Si and C-N as observed in thin sections. Permineralized microfossils are composed of carbonaceous globules that did not survive the acid treatment, whereas encapsulated microfossils were characterized due to their resistance to the acid maceration procedure. We also investigated the microscale relationship between the (12)C(14)N(-) and (12)C2(-) ion emission as a proxy of the N/C atomic ratio in both permineralized and encapsulated microfossils. After considering any potential matrix and microtopography effects, we demonstrate that the encapsulated microfossils exhibit the highest level of geochemical preservation. This finding shows that the chemical heterogeneity of the microfossils, observed at a spatial resolution of a few hundreds of micrometers, can be related to fossilization processes. Key Words: Carbonaceous matter-Farrel Quartzite-Fossilization-NanoSIMS-Nitrogen-Permineralization. Astrobiology 17, xxx-xxx.
... For example, the odd-over-even predominance found in many sedimentary n-alkanes indicates an origin from biomolecules, namely fatty acids with even numbers of C atoms. It is routinely regarded as diagnostic of a biological origin (for example, of a 3.5 Ga old kerogen; Derenne et al., 2008). Rare cases of even-over-odd predominance in n-alkanes have been reported (Nishimura and Baker, 1986). ...
The “Rare Earth” hypothesis—put forward by Ward and Brownlee in their 2000 book of the same title—states that prokaryote-type organisms may be common in the universe but animals and higher plants are exceedingly rare. If this idea is correct, the search for extraterrestrial life is essentially the search for microorganisms. Various indicators may be used to detect extant or extinct microbial life beyond Earth. Among them are chemical biosignatures, such as biomolecules and stable isotope ratios. The present minireview focuses on the major problems associated with the identification of chemical biosignatures. Two main types of misinterpretation are distinguished, namely false positive and false negative results. The former can be caused by terrestrial biogenic contaminants or by abiotic products. Terrestrial contamination is a common problem in space missions that search for biosignatures on other planets and moons. Abiotic organics can lead to false positive results if erroneously interpreted as biomolecules, but also to false negatives, for example when an abiotic source obscures a less productive biological one. In principle, all types of putative chemical biosignatures are prone to misinterpretation. Some, however, are more reliable (“stronger”) than others. These include: (i) homochiral polymers of defined length and sequence, comparable to proteins and polynucleotides; (ii) enantiopure compounds; (iii) the existence of only a subset of molecules when abiotic syntheses would produce a continuous range of molecules; the proteinogenic amino acids constitute such a subset. These considerations are particularly important for life detection missions to solar system bodies such as Mars, Europa, and Enceladus.
... The uniqueness of these microfossils constrains the use of destructive methods for determining the environmental context of deposition of this chert unit and the reality of these microfossils. New nondestructive techniques such as NanoSIMS imagery of microfossils (Oehler et al. 2009) or a combination of analytical techniques could be useful for determining whether the chemical structure of such putative microfossils is consistent with a biological origin (Derenne et al. 2008). ...
... The diversity of organic moieties that can be preserved in fossils still seems poorly assessed. Biomarkers, i.e., highly resistant organic molecules comprising cell walls, are obvious examples (e.g., Derenne et al. 2008). Some organic geochemistry studies have however shown that additional molecules such as proteins could be preserved more than a 100 million years (Riboulleau et al. 2002). ...
... The diversity of organic moieties that can be preserved in fossils still seems poorly assessed. Biomarkers, i.e., highly resistant organic molecules comprising cell walls, are obvious examples (e.g., Derenne et al. 2008). Some organic geochemistry studies have however shown that additional molecules such as proteins could be preserved more than a 100 million years (Riboulleau et al. 2002). ...
... Lipid biomarkers, molecular fossils, provide important clues to the composition of the microbial community. Molecular fossils that have been found in geological materials on Earth include hopanes and other geologically preserved lipids derived from bacterial hopanoids Ourisson and Rohmer 1992;Summons et al. 1999;Brocks et al. 1999;Derenne et al. 2008;Love et al. 2009). Grotzinger (1989) has pointed to Paleoproterozoic (~2.5 bya) stromatolite reefs of enormous size to argue for oxygenic photosynthesis at that time (Des Marais 2000). ...
Fossil microbes are generally preserved by authigenic minerals, including silica, apatite, iron minerals, clays, and carbonates. An alternative mode of preservation by entombment in calcite, without replacement, has been identified in carbonate cave pool microbialites that were etched and examined in the scanning electron microscope (SEM). Features identified include filaments, threads, and films that show excess carbon in energy dispersive X-ray (EDX) analyses, suggesting preservation of organic matter. Filaments are single smooth or reticulated strands with curving string-like morphology, often hollow, and with a uniform diameter of 0.5 to 1.0 lm. Threads, in contrast, are variable thickness, from several microns down to 0.1 lm, always solid, and commonly branch. Films are thin (, 1 lm) drapes associated with threads. Filaments are interpreted as microbial filaments, while threads and films are interpreted as preserved extracellular polymeric substance (EPS). In addition, microbial filaments and EPS are only revealed via acid etching, suggesting preservation of organic material by entombment, not by replacement with calcite. To determine whether entombed microbes are a common feature of carbonate microbialites that form in different environmental settings, samples of hot spring travertine, caliche soil, and reef microbialite were examined. Whereas the travertine samples were barren, entombed EPS was found in the caliche soil and the reef microbialite; the latter also contained a few entombed filaments. In addition, entombed microbial material has been reported from carbonate cold seep deposits. Such findings indicate that entombment of microbes and EPS in carbonates is not restricted to cave settings, but is more widespread than previously reported. Possible causes for the lack of preservation in travertines include rapid degradation of microbial material either by sunlight due to photolytic degradation, aerobic microbial degradation, detritivore consumption, or elevated temperatures. Rapid carbonate precipitation is ruled out as, somewhat surprisingly, preservation is better in slower growing cave carbonates than in rapidly growing travertines. Potential long-term preservation of organic material entombed in carbonate has implications for the characterization of fossil microbial communities using molecular biomarkers and the search for life on other planets.
... According to Hartgers et al. (1995) the occurrence of alkene/alkane doublets could be explained by the pyrolysis which induced a decarboxylation of esterified fatty acids bound to the macromolecular matrix, leading to a loss of the carbonyl function and the formation of alkanes and alkenes in the same proportion. Derenne et al. 2008 showed that the alkanes and alkenes doublet up to C25 identified during pyrolysis originate from the homolytic cleavage of long alkyl chains through capture and elimination of H°radicals, respectively. Quénéa et al. (2005) show that the n-alkene/n-alkane doublets commonly occur in the pyrolysates of natural macromolecular organic materials where they originate chiefly from the homolytic cleavage of C-C bonds in polymethylene chains. ...
The behavior of aliphatic hydrocarbons during co-composting of sewage sludge activated with palm tree waste was studied for 6 months using Py-GC/MS. The main aliphatic compounds represented as doublet alkenes/alkanes can be classified into three groups. The first group consists of 11 alkenes (undecene, tridecene, pentadecene, hexadecene, heptadecene, octadecene, nonadecene, eicosene, uncosene, docosene, tricosene) and 15 alkanes (heptane, octane, nonane, decane, undecane, dodecane, tetradecane, pentadecane, heptadecane, octadecane, nonadecane, eicosane, uncosane, docosane, and tricosane), which remain stable during the co-composting process. The stability of these compounds is related to their recalcitrance behavior. The second group consists of five alkenes (heptene, octene, nonene, decene, dodecene) and tridecane as a single alkane that decreases during co-composting. The decrease in these compounds is the combined result of their metabolism and their conversion into other compounds. The third group is constituted with tetradecene and hexadecane that increase during composting, which could be explained by accumulation of these compounds, which are released by the partial breakdown of the substrate. As a result, these molecules are incorporated or adsorbed in the structure of humic substances.
The NASA Mars 2020 Perseverance rover is actively exploring Jezero crater to conduct analyses on igneous and sedimentary rock targets from outcrops located on the crater floor (Máaz and Séítah formations) and from the delta deposits, respectively. The rock samples collected during this mission will be recovered during the Mars Sample Return mission, which plans to bring samples back to Earth in the 2030s to conduct in-depth studies using sophisticated laboratory instrumentation. Some of these samples may contain traces of ancient martian life that may be particularly difficult to detect and characterize because of their morphological simplicity and subtle biogeochemical expressions. Using the volcanic sediments of the 3.45 Ga Kitty's Gap Chert (Pilbara, Australia), containing putative early life forms (chemolithotrophs) and considered as astrobiological analogues for potential early Mars organisms, we document the steps required to demonstrate the syngenicity and biogenicity of such biosignatures using multiple complementary analytical techniques to provide information at different scales of observation. These include sedimentological, petrological, mineralogical, and geochemical analyses to demonstrate macro- to microscale habitability. New approaches, some unavailable at the time of the original description of these features, are used to verify the syngenicity and biogenicity of the purported fossil chemolithotrophs. The combination of elemental (proton-induced X-ray emission spectrometry) and molecular (deep-ultraviolet and Fourier transform infrared) analyses of rock slabs, thin sections, and focused ion beam sections reveals that the carbonaceous matter present in the samples is enriched in trace metals (e.g., V, Cr, Fe, Co) and is associated with aromatic and aliphatic molecules, which strongly support its biological origin. Transmission electron microscopy observations of the carbonaceous matter documented an amorphous nanostructure interpreted to correspond to the degraded remains of microorganisms and their by-products (extracellular polymeric substances, filaments…). Nevertheless, a small fraction of carbonaceous particles has signatures that are more metamorphosed. They probably represent either reworked detrital biological or abiotic fragments of mantle origin. This study serves as an example of the analytical protocol that would be needed to optimize the detection of fossil traces of life in martian rocks.
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).
How can the transport of fluids in a confined and complex
mixed organic/inorganic matrix be far below the expected value from a
topological aspect? A good example of this situation is oil shales. Oil and
gas shales are source rocks in which organic matter has matured to form
hydrocarbons. They exhibit a dual porous network formed by the intertwining
of mineral and organic pores that leads to very low permeability. Still, the exact origin of this extremely low permeability remains somehow unclear. The present communication addresses this important question and provides novel insights on the mechanisms that strongly hinder fluid diffusion in such materials. By combining nuclear and electronic magnetic resonance techniques with SEM imaging, we show evidence that magnetic interaction occurs in kerogen. This results from a magnetic coupling between vanadyl present in porphyrins and the organic matrix. We demonstrate that such coupling retards
fluid diffusion and is reversible. This key dynamical feature explains the
extremely low mobility of oil in shale rocks. This phenomenon may be a more
general feature occurring in several systems where fluids are confined in a
complex hierarchical matrix that embeds both organic and inorganic radicals
resulting from the aging process.
The ancient rocks of the Pilbara region of Western Australia have been an important analog site for the study of possible inhabited environments in the search for life on early Mars for over four decades. Here, we review the evidence for Paleo- to Neoarchean life and the habitats that it occupied in the Pilbara Craton and unconformably overlying Fortescue Group of the Mount Bruce Supergroup. Nine major inhabited environments are described, which range from land to sea, and into the subsurface, showing that life had diversified into, and flourished within, a range of different environments early in Earth history.
An important additional component in the search for life on Mars involves the manner in which evidence for early life is preserved. From the examples studied here, early mineralization of organic matter is key to the preservation of reliable biosignatures, in either silica, carbonate, or pyrite, but burial by volcanic ash can also provide excellent preservation.
The effect of standard acid maceration on organic matter (OM) from ancient silicified sediments remains undocumented. Early silicification favours preservation of organic moieties against thermal alteration over time. In this study, we investigated the effects of acid maceration on the structure of OM isolated from the Lower Devonian Rhynie chert. The structure of OM was investigated by combining Rock-Eval pyrolysis and Raman spectroscopy. Besides a loss of thermolabile organic matter owing to solvent extraction, Rock-Eval pyrolysis showed that standard acid maceration also causes a loss of C-H emissions at high pyrolysis temperature (> 500 °C). The standard acid maceration procedure was also associated with the disappearance of the D4 and D5 Raman spectrum shoulders assigned to C–H bonds in aliphatics and bitumens, respectively, entrapped in the macromolecular network. Taken together, Rock-Eval pyrolysis and Raman spectroscopy indicate that standard acid maceration can lead to the chemical degradation of syngenetic hydrocarbonaceous moieties of OM isolated from ancient silicified and thermally altered sediments. In sediments having experienced early silicification, which hampers bitumen migration and favours pyrobitumen formation, we suggest that novel in situ molecular analytical techniques are required to provide a thorough examination of the syngenetic molecular content independent of the soluble/insoluble operational definition.
Some of the oldest traces for planktonic lifestyle have been reported in ca. 3.4 billion years old silicified sediments from the Strelley Pool Formation in Western Australia. Observation of flange appendages suggests that Archean life motility was passive and driven by drifting of microorganisms in their surrounding environment. Until now, the oldest traces for active motility are ca. 2.1 billion years old. Whether or not active motility already existed during the Archean eon remains an open question. In this study, we report the discovery of new 3.4 billion years old microfossils exhibiting a tail-like structure isolated from the Strelley Pool Formation. Exhibiting Raman spectra typically observed in organic-walled microfossils from the Strelley Pool Formation, these microfossils exhibiting a tail-like structure are syngenetic with their host rock. Composed of carbon, nitrogen, and, for one specimen, phosphorus, some of these organic-walled microfossils also exhibit significant level of aliphatic and amide moieties supporting their biogenicity. In addition, these microfossils exhibit a tail-like appendage sharing similar morphological features with locomotory organelles in modern microorganisms such as archaella, flagella, and cilia. This suggests that this observed appendage likely provided them with movement capabilities. If correct, with the ability to move, these microorganisms were capable of escaping from harsh environments and/or colonizing new ecological niches as early as 3.4 billion years ago.
Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining early ecosystems. Applying Fourier transform infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific functional groups, we characterized fossilized biopolymers in 3.5-3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite high aromaticity. This reflects precursor biological heterogeneity since all horizons show equally exceptional preservation and underwent similar grades of metamorphism. Low methylene to end-methyl (CH 2 /CH 3) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n-alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH 2 /CH 3 ratios suggesting mostly longer, unbranched fatty acids from bacterial lipid precursors. Absorbance ratios of end-methyl to methylene (CH 3 /CH 2) in Hooggenoeg, Josefsdal and Footbridge mats yield a range of values (0.20-0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Higher (0.78-1.25) CH 3 / CH 2 ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from biomass. Since silicification commenced during the lifetime of the microbial mat, FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Together, these results show that Bacteria and Archaea flourished together in Earth's earliest ecosystems.
Résumé : L’oxygène n’est pas apparu aussi brutalement qu’on le pensait sur
notre planète (nb: première partie 1/2, ici).
Malgré un apport en oxygène lié aux cyanobactéries dès l’Archéen, ce ne se sont
pas ces microorganismes qui sont à la base de la première grande ‘révolution’ de
l’oxygène qui a eu lieu à la limite Archéen/Paléoprotérozoïque (il y a 2,5 milliards
d’années) dans l’atmosphère, lors du Grand Evénement d’Oxydation. Ce sont les
processus liés au cycle de la tectonique des plaques (activité mantellique et
périodes intenses d’érosion/altération) qui ont contribué de manière
déterminante à l’augmentation de la concentration de l’oxygène atmosphérique
vers 2,5 milliards d’années. Les deux principaux processus responsables de
cette augmentation sont liés à l’enfouissement de la matière organique et de la
pyrite (= FeS2). L’altération des séries riches en ces deux composants conditionnera
ensuite pendant près d’un milliard d’années la composition chimique des océans en
oxygène, soufre et fer. Au cours du temps, l’oxygène proviendra de l’activité des
cyanobactéries et l’atmosphère réductrice du début de l’Archéen sera remplacée par
une atmosphère oxydante à la fin du Précambrien.
Abstract : Oxygen did not appear as abruptly as we thought on our planet.
Despite an oxygen supply related to cyanobacteria, since the Archean, it is not these
microorganisms that are at the base of the first great oxygen revolution that took
place at the Archean/Paleoproterozoic boundary (2.5 billion years) in the atmosphere
during the Great Oxidation Event. Two processes related to the cycle of plate
tectonics (mantle activity and intense periods of erosion/weathering) were
mostly involved in the increase of the of atmospheric oxygen concentration 2.5
billion years ago. These two main processes are related to the burial of organic
matter and those of pyrite(= FeS2) The alteration of series with high contents of the
two elements will then condition for nearly a billion of years the oxygen, sulfur and
iron chemical composition of the oceans. The oxygen will finally come from the
activity of cyanobacteria and the early Archean reducing atmosphere will be replaced
by an oxidizing atmosphere at the end of the Precambrian.
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.
During the Late Neoproterozoic to early Cambrian period, the transition from mainly microbial ecosystems to eukaryotic marine primary productivity was one of the most profound ecological revolutions in Earth’s history. Abundant biomarkers have been reported from high maturity Late Neoproterozoic sedimentary rocks in South China, including the Cryogenian Datangpo and Ediacaran Doushantuo formations. These biomarkers were suggested to be organic molecular evidence of the survival of photosynthetic eukaryotes in palaeo-oceanic environments during the Snowball Earth era. To advance the understanding of Late Neoproterozoic ecosystems and to re-evaluate the provenance and validity of the biomarkers, fresh drill core was collected from black shales of the Datangpo and Doushantuo formations in South China. In order to ensure the indigeneity of the solvent extracted hydrocarbons, their composition was determined by conducting experiments under strict laboratory procedures, including using slice experiments on a precision saw that removed external surfaces. The distribution of the recovered aliphatic and aromatic hydrocarbons indicates very a high thermal maturity for the analysed black shales, which is consistent with the equivalent vitrinite reflectance (~2.5%) and the lack of specific biomarkers. Both prokaryotic (bacterial) and eukaryotic (algal) organisms are likely producers during the Neoproterozoic. However, no long-chain steranes (≥C26 steranes) were detected in the interior of any of the samples, and only trace amounts of hopanes and pregnanes in just one sample from the Doushantuo Formation (xs-199-DST). On the contrary, solvent extraction of millimetre-thick slices cut from the external surfaces of the black shales reveals strikingly high level of contamination from organic residues on the sample surfaces. These contaminants include n-alkanes (C10 ~ C33), isoprenoids, monomethylalkanes, aromatic hydrocarbons (naphthalene, phenanthrene and their alkyl isomers) and abundant biomarkers such as tricyclic terpanes, hopanes, steranes and diasteranes. The concentrations of these biomarkers in the exterior sub-samples exceed blank concentrations by more than three orders of magnitude due to surficial contamination. These results indicate that previously studied Late Neoproterozoic samples from South China likely contain mixtures of abundant biomarker contaminants and some indigenous over-mature hydrocarbons. Therefore, special care need to be taken with regard the existing biomarker evidence from South China that has been used to illustrate the contribution of eukaryotic photosynthesis to marine primary productivity during the Snowball Earth period. No Late Neoproterozoic rocks from South China within the appropriate thermal maturity window for survival of indigenous biomarkers are currently known, but these may be found during future exploration.
Organic matter trapped within Archean sedimentary rocks provides a unique insight into the emergence of early life. However, the potential for postdepositional contributions of carbonaceous matter may question the syngeneticity of organic materials and their ancient host rocks, hence limiting the potential for dating the emergence of life on Earth. Novel criterion is therefore required to guarantee accurate dating of organic materials recovered from Archean rocks. Organic matter was recently identified as a novel archive of the ancient atmosphere. Because the isotopic composition of atmospheric Xe evolved
through time by mass dependent fractionation, the degree of Xe isotopic fractionation in kerogens relative to modern atmosphere can provide a time stamp for the last chemical equilibration between the organic matter and the atmosphere. Here, we present new Xe
isotopic data for kerogens isolated from 3.4 - 1.8 Gyold rocks, some of which are abundant in microfossils. Comparing model ages derived from Xe isotopes with
the geological age of host rocks allows the syngeneticity of ancient organic materials to be tested. Implications regarding our understanding of the composition and evolution of the Archean atmosphere, as well as the time window for the emergence and diversification of life during the Archean eon, will be discussed.
Understanding the composition of the Archean atmosphere is vital for unraveling the origin of volatiles and the environmental conditions that led to the development of life. The isotopic composition of xenon in the Archean atmosphere has evolved through time by mass-dependent fractionation from a precursor comprising cometary and solar/chondritic contributions (referred to as U-Xe). Evaluating the composition of the Archean atmosphere is challenging because limited amounts of atmospheric gas are trapped within minerals during their formation. We show that organic matter, known to be efficient at preserving large quantities of noble gases, can be used as a new archive of atmospheric noble gases. Xe isotopes in a kerogen isolated from the 3.0–billion-year–old Farrel Quartzite (Pilbara Craton, Western Australia) are mass fractionated by 9.8 ± 2.1 per mil (‰) (2σ) per atomic mass unit, in line with a progressive evolution toward modern atmospheric values. Archean atmospheric Xe signatures in kerogens open a new avenue for following the evolution of atmospheric composition through time. The degree of mass fractionation of Xe isotopes relative to the modern atmosphere can provide a time stamp for dating Archean kerogens and therefore narrowing the time window for the diversification of early life during the Archean eon.
Palaeontology is an essential tool for tracing the history of life in the geological record. However, access to the origin of life is blocked because of the lack of preservation of suitable rocks dating from the fi rst billion years of Earth’s history. Nevertheless, study of Early Archaean rocks (~4-3.3 Ga) indicates that the environmental conditions of the early Earth, upon which life emerged, were very different to those of today and provides essential information for guiding investigations into the origin of life in terms of realistic environmental scenarios and possible timing of the appearance of life. Microbial palaeontology investigations of well-preserved, Early Archaean rocks ~3.5 to 3.3 Ga show that the earliest preserved life was diverse and widespread and suggest that it probably appeared in the Hadean, as soon as the Earth’s surface was habitable. The extreme, anaerobic conditions characterising the early Earth, together with the ingredients of life, i.e. carbon molecules, liquid water and energy, were common on other planets and satellites in the early Solar System. Considering carbon and water-based life forms to be a cosmically frequent phenomenon, it is hypothesised that life could have emerged on some of these bodies and that traces of its appearance may still be preserved, for instance on Mars, Europa or Enceladus. Microbial palaeontology as well as information gleaned from extant extremophiles and experimental data provides us with essential information about what kinds of extant or fossilised life forms to look for on another planet or satellite. Moreover, the methods evolved to study and understand the remains of fossil traces of primitive microbial life will aid the search for life and its origins on Mars or other satellites. The perspective of returning to Earth rocks from Mars (or other samples from Europa or Enceladus?) containing potential traces of extraterrestrial life, most likely primitive anaerobic chemotrophs, will be a challenge for microbial palaeontology that we need to start addressing now. Most importantly, it will open up the possibility of establishing the universality of life.
Coastal-plain paleosols in the 3.0 Ga Farrel Quartzite of Western Australia have organic surface (A horizon) and sulfate-rich subsurface (By) horizons, like soils of the Atacama Desert of Chile, Dry Valleys of Antarctica, and 3.7 Ga paleosols of Mars. Farrel Quartzite paleosols include previously described microfossils, permineralized by silica in a way comparable with the Devonian Rhynie Chert, a well known permineralized Histosol. Five microfossil morphotypes in the Farrel Quartzite include a variety of spheroidal cells (Archaeosphaeroides) as well as distinctive large spindles (new genus provisionally assigned to cf. Eopoikilofusa). Previously published cell-specific carbon isotopic analyses of the Farrel Quartzite microfossils, and unusually abundant sulfate considering a likely anoxic atmosphere, allow interpretation of these morphotypes as a terrestrial community of actinobacteria, purple sulfur bacteria, and methanogenic Archaea.
The abundant and diverse assemblage of filamentous microbial fossils and associated organic matter permineralized in the ~ 3465 Ma Apex chert of northwestern Australia—widely regarded as among the oldest records of life—have been investigated intensively. First reported in 1987 and formally described in 1992 and 1993, the biogenicity of the Apex fossils was questioned in 2002 and in three subsequent reports. However, as is shown here by use of analytical techniques unavailable twenty years ago, the Apex filaments are now established to be bona fide fossil microbes composed of three-dimensionally cylindrical organic-walled (kerogenous) cells. Backed by a large body of supporting evidence of similar age—other microfossils, stromatolites, and carbon isotopic data—it seems clear that microbial life was present and flourishing on the early Earth ~ 3500 Ma ago.
Habitability can be defined on many scales. The early Earth was globally habitable because of its global ocean, but early Mars was not. Relatively dry conditions appear to have reigned on Mars throughout its history, but, from a microbial point of view, the necessary conditions for the appearance of life were still theoretically possible. The lack of connectivity between potential habitats in time and space may have resulted in life appearing and disappearing simultaneously in different geographical locations. The absence of habitable environments on geologically long timescales of 100s My together with the likelihood that lakes and seas were covered by ice are inhibiting factors for the evolution of photosynthesis. Martian life thus probably remained in a primitive chemotrophic form. Nevertheless, established life could have colonized newly formed habitats, even on an ephemeral basis, providing that viable cells could be transported into the habitats. For in situ missions and the search for Martian life, its heterogeneous distribution implies that the search for past traces of life will be challenging, but such environments do exist.
Traditional history, beginning around 3000 bce, depends on dateable written records. The expanded scope of big history (13.8 byr) requires new understandings of time. Following the work of J. T. Fraser, I explore the underlying temporalities of big history. To Fraser's atemporality and eotemporality, which govern the history of galaxy, star, and planet formation, I add petrotemporaliity which governs three processes - in the freezing of time in mineralized fossils, the chronological structuring of geohistory from stratified rock, and the slow unwinding of time through the decay of radioactive isotopes - and genotemporality which bypasses the vagaries of species transition and extinction, utilizing instead the incorporation of viral dna within the human genome and its continuity of umwelt to construct a big history of life that connects the earliest life forms with humans. I conclude that the possibility of constructing a big history depends on these underlying temporalities.
Astrobiology is an exciting interdisciplinary field that seeks to answer one of the most important and profound questions: are we alone? In this volume, leading international experts explore the frontiers of astrobiology, investigating the latest research questions that will fascinate a wide interdisciplinary audience at all levels. What is the earliest evidence for life on Earth? Where are the most likely sites for life in the Solar System? Could life have evolved elsewhere in the Galaxy? What are the best strategies for detecting intelligent extraterrestrial life? How many habitable or Earth-like exoplanets are there? Progress in astrobiology over the past decade has been rapid and, with evidence accumulating that Mars once hosted standing bodies of liquid water, the discovery of over 500 exoplanets and new insights into how life began on Earth, the scientific search for our origins and place in the cosmos continues.
Results from a multidisciplinary geoscience program since 1994 are summarized for the North Pilbara terrain of the Pilbara Craton. Major findings include the recognition of three separate terranes with unique stratigraphy, geochronological, and structural histories; the ca. 3.72 to 2.85 Ga East Pilbara granite-greenstone terrane, the ca. 3.27 to 2.92 Ga West Pilbara granite-greenstone terrane, and the ≤3.29 Ga Kuranna terrane in the southeast. These are separated by two late, dominantly elastic sedimentary basins deposited within tectonically active zones; the ca. 3.01 to 2.93 Ga Mallina basin in the west and the undated Mosquito Creek basin in the east. The oldest supracrustal rocks are the ca. 3.51 to 3.50 Ga Coonterunah and ca. 3.49 to 3.31 Ga Warrawoona Groups in the East Pilbara granite-greenstone terrane, deposited on fragments of older sialic crust to 3.72 Ga. The Warrawoona Group is subdivided into three main (ultra)mafic-felsic volcanic cycles including from base to top, the Talga (3.49-3.46 Ga), Salgasli (3.46-3.43 Ga), and newly defined Kelly (3.43-3.31 Ga) Sub-groups. These dominantly basaltic rocks include chert beds containing Earth's oldest stromatolites and are interbedded with significant felsic volcanics erupted intermittently from 3.49 to 3.43 Ga during emplacement of sheeted sodic granitoid sills. Estimates of autochthonous stratigraphic thickness range from 9 to 18 km. Deformation involved extensional growth faulting, local folding, and tilting of greenstones away from synvolcanic granitoid domes. Rapid partial convective overturn of upper and middle crust occurred at 3.32 Ga during voluminous potassic felsic magmatism, followed by deposition of the Budjan Creek Formation at 3.31 Ga. Granitoid plutonism at ca. 3.29 Ga in the Kuranna terrane preceded deposition of ultramafic through felsic volcanics and chert in the West Pilbara granite-greenstone terrane (3.27-3.25 Ga Roebourne Group) and western margin of the East Pilbara granite-greenstone terrane (3.26-3.24 Ga Sulphur Springs Group). Geochemical and isotopic data suggest that volcanism resulted from plume-related rifting of the East Pilbara granite-greenstone terrane, which was accompanied by granitoid plutonism and deformation. Following this was ca. 100 m.y. of relative quiescence during which locally economic concentrations of banded iron-formation and siliciclastics of the Gorge Creek Group were deposited in the East Pilbara granite-greenstone terrane. Thereafter, geologic events are more consistent with microplate tectonics, commencing with deformation at 3.15 Ga followed by deposition of 3.13 to 3.11 Ga bimodal volcanics in the West Pilbara granite-greenstone terrane (Whundo Group), which have juvenile Nd isotope signatures and thus may represent either a rift or island- are succession. Basaltic rocks and minor felsic tuff were deposited in the East Pilbara granite-greenstone terrane at 3.06 Ga and possibly in the West Pilbara granite-greenstone terrane (Regal Formation). At 3.02 Ga. the Whundo and Roebourne Groups share a common history of deposition of banded iron-formation and granitoid plutonism across the Sholl shear zone, suggesting accretion at, or immediately preceding, this time. This was followed by deposition in the Mallina basin of the volcanic Whim Creek Group at 3.01 Ga, possibly as an arc, and then the 2.97 to 2.93 Ga volcanic Bookingarra (west) and clastic De Grey (east) Groups during periods of intracontinental rifting interspersed with compression and granitoid intrusion. The geochemistry of 2.95 Ga high Mg diorites (sanukitoids) indicates a previous episode of subduction during either the Whundo or Whim Creek Groups or both. Final events include emplacement of ultramafic-mafic layered intrusions (2.925 Ga in the West Pilbara granite-greenstone terrane), local shearing and lode Au mineralization (2.92 Ga in the West Pilbara granite-greenstone terrane, 2.90 Ga in the Mosquito Creek basin, 2.89 Ga in the East Pilbara granite-greenstone terrane), and intrusion of fractionated, Sn-Ta-Li-bearing granites to 2.85 Ga (East Pilbara granite-greenstone terrane).
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
Molecular fossils of biological lipids are preserved in 2700-million-year-old shales from the Pilbara Craton, Australia. Sequential
extraction of adjacent samples shows that these hydrocarbon biomarkers are indigenous and syngenetic to the Archean shales,
greatly extending the known geological range of such molecules. The presence of abundant 2α-methylhopanes, which are characteristic
of cyanobacteria, indicates that oxygenic photosynthesis evolved well before the atmosphere became oxidizing. The presence
of steranes, particularly cholestane and its 28- to 30-carbon analogs, provides persuasive evidence for the existence of eukaryotes
500 million to 1 billion years before the extant fossil record indicates that the lineage arose.
Carbon materials usually exhibit a multiscale organization from the subnanometric to the millimetric scales. Such an organization is the fingerprint of the conditions of formation and is responsible for numerous properties. High-resolution transmission electron microscopy (HRTEM) is a relevant tool to image directly the profile of the polyaromatic layers forming the skeleton of these carbons. Quantitative data are required to accurately describe such multiscale organization, to decipher the formation conditions and to foresee industrial properties. In this respect, an in-house image analysis procedure was developed based on the skeletonization of the HRTEM images, followed by the extraction of structural and microtextural data. Thanks to these quantitative data, the organization of disordered carbons can be relevantly revisited. Some applications of this approach are presently tested in the field of carbon characterization, in relation with environment problems, earth or universe sciences, or industrial carbon materials.
Nitrogen concentrations and isotopic compositions of kerogens from Precambrian cherts have been determined for a series of samples from Western and Northern Australia, South and Central Africa and Northern America. The nitrogen concentrations range from 2 to 106 ppm with C/N elemental ratios between 31 and 590. Sample ages range from 3.5 to 0.7 Gyr. They display δ15N values ranging from −6 to +13‰, whereas δ15N values of Phanerozoic marine samples usually range from 0 to +10‰. The oldest samples (3.5 to 3.4 Gyr) appear to be isotopically lighter (down to −6.2‰) whereas the Early Proterozoic samples (2.0 Gyr) display δ15N values (from 0.3 to 10.1‰) similar to the Phanerozoic samples. Changes through geological time of the atmospheric nitrogen isotopic composition or a selective diagenetic preservation of nitrogen-bearing organic compounds cannot account for this isotopic shift. Today in the sea marine organic matter exhibits positive 15N values reflecting the 15N enrichment of the dissolved nitrate (NO−3). Therefore it seems likely that, in the absence of atmospheric oxygen, NO−3 was absent in Archean seas. An Archean nitrogen cycle is proposed for which the negative 15N values reflect a metabolic isotopic fractionation in anoxic conditions with microorganisms using the reduced forms of nitrogen (N2, NH+4). The increase of atmospheric oxygen after the Archean time would have encouraged the biological production of NO−3 and its use as a source for organic nitrogen. In this respect, nitrogen isotopes in kerogens have recorded the evolution of redox-conditions on the Earth's surface.
It is unknown when life first appeared on Earth. The earliest known microfossils (approximately 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences--a approximately 3,800-Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present.
Molecular fossils of biological lipids are preserved in 2700-million-year-old shales from the Pilbara Craton, Australia. Sequential extraction of adjacent samples shows that these hydrocarbon biomarkers are indigenous and syngenetic to the Archean shales, greatly extending the known geological range of such molecules. The presence of abundant 2alpha-methylhopanes, which are characteristic of cyanobacteria, indicates that oxygenic photosynthesis evolved well before the atmosphere became oxidizing. The presence of steranes, particularly cholestane and its 28- to 30-carbon analogs, provides persuasive evidence for the existence of eukaryotes 500 million to 1 billion years before the extant fossil record indicates that the lineage arose.
The formation of lipid compounds during an aqueous Fischer-Tropsch-type reaction was studied with solutions of oxalic acid as the carbon and hydrogen source. The reactions were conducted in stainless steel vessels by heating the oxalic acid solution at discrete temperatures from 100 to 400 degrees C, at intervals of 50 degrees C for two days each. The maximum lipid yield, especially for oxygenated compounds, is in the window of 150-250 degrees C. At a temperature of 100 degrees C only a trace amount of lipids was detected. At temperatures above 150 degrees C the lipid components ranged from C12 to > C33 and included n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanals, n-alkanones, n-alkanes, and n-alkenes, all with essentially no carbon number preference. The n-alkanes increased in concentration over the oxygenated compounds at temperatures of 200 degrees C and above, with a slight reduction in their carbon number ranges due to cracking. It was also noted that the n-alkanoic acids increased while n-alkanols decreased with increasing temperature above 200 degrees C. At temperatures above 300 degrees C synthesis competes with cracking and reforming reactions. At 400 degrees C significant cracking was observed and polynuclear aromatic hydrocarbons and their alkylated homologs were detected. The results of this work suggest that the formation of lipid compounds by aqueous FTT reactions proceeds by insertion of a CO group at the terminal end of a carboxylic acid functionality to form n-oxoalkanoic acids, followed by reduction to n-alkanoic acids, to n-alkanals, then to n-alkanols. The n-alkenes are intermediate homologs for n-alkan-2-ones and n-alkanes. This proposed mechanism for aqueous FTT synthesis differs from the surface-catalyzed stepwise FT process (i.e., gaseous) of polymerization of methylene reported in the literature.
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.
The isotopic composition of graphite is commonly used as a biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed terrestrial rocks. Earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3.8-Gyr-old Isua supracrustal belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean. This interpretation, however, has to be approached with extreme care. Here we show that graphite occurs abundantly in secondary carbonate veins in the ISB that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks (metasomatism). During these reactions, graphite forms from the disproportionation of Fe(II)-bearing carbonates at high temperature. These metasomatic rocks, which clearly lack biological relevance, were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life. The new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed Early Archaean rock record.
We have synthesized inorganic micron-sized filaments, whose microstucture consists of silica-coated nanometer-sized carbonate
crystals, arranged with strong orientational order. They exhibit noncrystallographic, curved, helical morphologies, reminiscent
of biological forms. The filaments are similar to supposed cyanobacterial microfossils from the Precambrian Warrawoona chert
formation in Western Australia, reputed to be the oldest terrestrial microfossils. Simple organic hydrocarbons, whose sources
may also be abiotic and indeed inorganic, readily condense onto these filaments and subsequently polymerize under gentle heating
to yield kerogenous products. Our results demonstrate that abiotic and morphologically complex microstructures that are identical
to currently accepted biogenic materials can be synthesized inorganically.
The 3,430-million-year-old Strelley Pool Chert (SPC) (Pilbara Craton, Australia) is a sedimentary rock formation containing laminated structures of probable biological origin (stromatolites). Determining the biogenicity of such ancient fossils is the subject of ongoing debate. However, many obstacles to interpretation of the fossils are overcome in the SPC because of the broad extent, excellent preservation and morphological variety of its stromatolitic outcrops--which provide comprehensive palaeontological information on a scale exceeding other rocks of such age. Here we present a multi-kilometre-scale palaeontological and palaeoenvironmental study of the SPC, in which we identify seven stromatolite morphotypes--many previously undiscovered--in different parts of a peritidal carbonate platform. We undertake the first morphotype-specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation. Finally, we argue that the diversity, complexity and environmental associations of the stromatolites describe patterns that--in similar settings throughout Earth's history--reflect the presence of organisms.
The terrestrial sediment record indicates that the Earth's climate varied drastically in the Precambrian era (before 550 million years ago), ranging from surface temperatures similar to or higher than today's to global glaciation events. The most continuous record of sea surface temperatures of that time has been derived from variations in oxygen isotope ratios of cherts (siliceous sediments), but the long-term cooling of the oceans inferred from those data has been questioned because the oxygen isotope signature could have been reset through the exchange with hydrothermal fluids after deposition of the sediments. Here we show that the silicon isotopic composition of cherts more than 550 million years old shows systematic variations with age that support the earlier conclusion of long-term ocean cooling and exclude post-depositional exchange as the main source of the isotopic variations. In agreement with other lines of evidence, a model of the silicon cycle in the Precambrian era shows that the observed silicon isotope variations imply seawater temperature changes from about 70 degrees C 3,500 million years ago to about 20 degrees C 800 million years ago.
Microscopic sulfides with low 34S/32S ratios in marine sulfate deposits from the 3490-million-year old Dresser Formation, Australia, have been interpreted as evidence for the presence of early sulfate-reducing organisms on Earth. We show that these microscopic sulfides have a mass-independently fractionated sulfur isotopic anomaly (Delta33S) that differs from that of their host sulfate (barite). These microscopic sulfides could not have been produced by sulfate-reducing microbes, nor by abiologic processes that involve reduction of sulfate. Instead, we interpret the combined negative delta34S and positive Delta33S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.
A direct nondestructive method for source-rock evaluation based on solid-state nuclear magnetic resonance (NMR) techniques is demonstrated. Direct measurement of the aliphatic and aromatic carbon structures of the total organic matter in whole rocks is provided by NMR techniques known as cross polarization with magic-angle spinning. The proportion of organic carbon present in aliphatic structures correlates with the oil-generating potential of a source rock. In addition, whether the organic matter in the rock has been altered during catagenesis can be evaluated. These evaluations are demonstrated for carbonaceous shales from the Retort and Meade Peak Phosphatic Shale Members of the Permian Phosphoria Formation. The NMR results show that the Retort shales have a high poten ial to generate oil, but the Meade Peak shales, having already undergone thermal alteration, have a low potential to generate additional oil.
At the present time procedures for kerogen isolation must be adapted either for physico-chemical analysis or for petrographic analysis. Physico-chemical analysis require good recovery with little chemical alteration of the organic matter (OM), while petrographers need satisfactory preservation of the morphology of macerals and microfossils. In each case the sediment must be ground differently. The former require a fine grain size which results in the breaking of the microfossils, while the latter need a coarse particle size with the result that recovery of OM is not satisfactory. Both use physical and chemical methods. In general, physical methods are better suited to petrographic analysis, while chemical methods are more appropriate for physico-chemical analysis.-from Authors
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.
A direct nondestructive method for source-rock evaluation based on solid-state nuclear magnetic resonance (NMR) techniques is demonstrated. Direct measurement of the aliphatic and aromatic carbon structures of the total organic matter in whole rocks is provided by NMR techniques known as cross polarization with magic-angle spinning. The proportion of organic carbon present in aliphatic structures correlates with the oil-generation potential of a source rock. In addition, whether the organic matter in the rock has been altered during catagenesis can be evaluated. These evaluations are demonstrated for carbonaceous shales from the Retort and Meade Peak Phosphatic Shale Members of the Permian Phosphoria Formation. The NMR results show that the Retort shales have a high potential to generate oil, but the Meade Peak shales, having already undergone thermal alteration, have a low potential to generate additional oil.
The major organic component of carbonaceous chondrites is a solvent-insoluble, high molecular weight macromolecular material that constitutes at least 70% of the total organic content in these meteorites. Analytical pyrolysis is often used to thermally decompose macromolecular organic matter in an inert atmosphere into lower molecular weight fragments that are more amenable to conventional organic analytical techniques. Hydropyrolysis refers to pyrolysis assisted by high hydrogen gas pressures and a dispersed catalytically-active molybdenum sulfide phase. Hydropyrolysis of meteorites has not been attempted previ- ously although it is ideally suited to such studies due to its relatively high yields. Hydropyrolysis of the Murchison macromolecular material successfully releases significant amounts of high molecular weight PAH including phenanthrene, carbazole, fluoranthene, pyrene, chrysene, perylene, benzoperylene and coronene units with varying degrees of alklyation. Analysis of both the products and residue from hydropyrolysis reveals that the meteoritic organic network contains both labile (pyrolysable) and refractory (nonpyrolysable) fractions. Comparisons of hydropyrolysis yields of Murchison macromolecular materials with those from terrestrial coals indicate that the refractory component probably consists of a network dominated by at least five- or six-ring PAH units cross-linked together. Copyright © 2004 Elsevier Ltd
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.
Some of the subjects discussed are related to the early biogeologic history, the nature of the earth prior to the oldest known rock record, the early earth and the Archean rock record, the prebiotic organic syntheses and the origin of life, Precambrian organic geochemistry, the biochemical evolution of anaerobic energy conversion, the isotopic inferences of ancient biochemistries, Archean stromatolites providing evidence of the earth's earliest benthos, Archean microfossils, the geologic evolution of the Archean-Early Proterozoic earth, and the environmental evolution of the Archean-Early Proterozoic earth. Other topics examined are concerned with geochemical evidence bearing on the origin of aerobiosis, biological and biochemical effects of the development of an aerobic environment, Early Proterozoic microfossils, the evolution of earth's earliest ecosystems, and geographic and geologic data for processed rock samples. Attention is given to a processing procedure for abiotic samples and calculation of model atmospheric compositions, and procedures of organic geochemical analysis.
Ruthenium tetroxide oxidation was used to examine the macromolecular insoluble organic matter (IOM) from the Orgueil and Murchison meteorites and especially to characterize the aliphatic linkages. Already applied to various terrestrial samples, ruthenium tetroxide is a selective oxidant which destroys aromatic units, converting them into CO2, and yields aliphatic and aromatic acids. In our experiment on chondritic IOM, it produces mainly short aliphatic diacids and polycarboxylic aromatic acids. Some short hydroxyacids are also detected.Aliphatic diacids are interpreted as aliphatic bridges between aromatic units in the chemical structure, and polycarboxylic aromatic acids are the result of the fusion of polyaromatic units. The product distribution shows that aliphatic links are short with numerous substitutions. No indigenous monocarboxylic acid was detected, showing that free aliphatic chains must be very short (less than three carbon atoms). The hydroxyacids are related to the occurrence of ester and ether functional groups within the aliphatic bridges between the aromatic units. This technique thus allows us to characterize in detail the aliphatic linkages of the IOMs, and the derived conclusions are in agreement with spectroscopic, pyrolytic, and degradative results previously reported.Compared to terrestrial samples, the aliphatic part of chondritic IOM is shorter and highly substituted. Aromatic units are smaller and more cross-linked than in coals, as already proposed from NMR data. Orgueil and Murchison IOM exhibit some tiny differences, especially in the length of aliphatic chains.
Thermal decomposition of siderite has been proposed as a source of magnetite in martian meteorites. Laboratory experiments were conducted to evaluate the possibility that this process might also result in abiotic synthesis of organic compounds. Siderite decomposition in the presence of water vapor at 300°C generated a variety of organic products dominated by alkylated and hydroxylated aromatic compounds. The results suggest that formation of magnetite by thermal decomposition of siderite on the precursor rock of the martian meteorite ALH84001 would have been accompanied by formation of organic compounds and may represent a source of extraterrestrial organic matter in the meteorite and on Mars. The results also suggest that thermal decomposition of siderite during metamorphism could account for some of the reduced carbon observed in metasedimentary rocks from the early Earth.
New data gathered during mapping of c. 3490–3240 Ma rocks of the Pilbara Supergroup in the Pilbara Craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism. For most cherts, fossil hydrothermal fluid pathways are preserved as silica ± barite ± Fe-bearing veins that cut through the footwall and up to the level of individual bedded chert units, but not above, indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events. Silica ± barite ± Fe-bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units, and occurred under alternating high-sulphidation and low-sulphidation conditions. These veins provided pathways to the surface for elements leached from the footwall (e.g., Si, Ba, Fe) and volcanogenic emissions from underlying felsic magma chambers (e.g., CO2, H2S/HS−, SO2).
Pyrolysis with and without tetramethylammonium hydroxide (TMAH), vacuum pyrolysis, and solid state 15N nuclear magnetic resonance (NMR) were used to examine the macromolecular insoluble organic matter (IOM) from the Orgueil and Murchison meteorites. Conventional pyrolysis reveals a set of poorly functionalized aromatic compounds, ranging from one to four rings and with random methyl substitutions. These compounds are in agreement with spectroscopic and pyrolytic results previously reported. For the first time, TMAH thermochemolysis was used to study extraterrestrial material. The detection of aromatics bearing methyl esters and methoxy groups reveals the occurrence of ester and ether bridges between aromatic units in the macromolecular network.No nitrogen-containing compounds were detected with TMAH thermochemolysis, although they are a common feature in terrestrial samples. Along with vacuum pyrolysis results, thermochemolysis shows that nitrogen is probably sequestered in condensed structures like heterocyclic aromatic rings, unlike oxygen, which is mainly located within linkages between aromatic units. This is confirmed by solid state 15N NMR performed on IOM from Orgueil, showing that nitrogen is present in pyrrole, indole, and carbazole moieties.These data show that amino acids are neither derived from the hydrolysis of IOM nor from a common precursor. In order to reconcile the literature isotopic data and the present molecular results, it is proposed that aldehydes and ketones (1) originated during irradiation of ice in space and (2) were then mobilized during the planetesimal hydrothermalism, yielding the formation of amino acids. If correct, prebiotic molecules are the products of the subsurface chemistry of planetesimals and are thus undetectable through astronomical probes.
The distributions of sulphur-containing compounds generated by flash pyrolysis of macromolecular sedimentary organic matter (kerogen, coal, asphaltenes) were studied by gas chromatography in combination with Sselective flame photometric detection or mass spectrometry. The abundance of S-containing pyrolysis products in the pyrolysates relative to other products was highly variable depending on the sample but the types of products were generally similar, being mainly composed of “gaseous” compounds (e.g., hydrogen sulphide) and low molecular weight alkylthiophenes and alkylbenzothiophenes. The distribution patterns of the alkylated thiophenes were dominated by a limited number of all theoretically possible isomers. The alkyl substitution patterns of the dominant isomers bear a strong similarity to those of the organic S compounds present in the GC-amenable fractions of bitumens and immature oils. Therefore, it is suggested that these S-containing pyrolysis products are formed by pyrolysis of related thiophenic and benzothiophenic moieties present in the macromolecular sedimentary substances. Specific examples include those with linear alkyl, iso and anteiso alkyl, isoprenoid alkyl and steroidal carbon skeletons. The presence of higher molecular weight alkylthiophenes and alkylbenzothiophenes with these same carbon skeletons in pyrolysates of S-rich kerogens provided further evidence for the presence of these S-containing moieties. It is likely that these moieties have been formed by abiogenic S incorporation into sedimentary organic matter during early diagenesis.
Five sections of bedded chert in mafic-ultramafic rocks of the Archean Warrawoona Group in the Marble Bar greenstone belt, Pilbara Craton, were analyzed in order to understand their depositional environment and to provide some constraints on Early Archean tectonics. The sections are divisible into two types based on their field occurrence, mineralogy and geochemistry; thicker ones (A and B) that overlie Fe-rich, low-K tholeiites and thinner ones (C1, C2, and C3) overlying komatiitic basalts. The thickest, ferruginous section (A; 45 m thick) is the Marble Bar Chert of the Towers Formation, and is interpreted to have been an in situ precipitate derived from a high-T hydrothermal solution emanating from a mid-oceanic ridge (MOR). The following geochemical features are similar to those of modern hydrothermal iron-rich sediments at a MOR: (i) P, V, Zn, and Y are positively correlated with Fe, (ii) a positive Eu anomaly (normalized to chondrite) decreases from 6.6 to 1.3 and the magnitude of a negative Ce anomaly decreases from 0.6 to 1.0 as ∑REE and LREE/HREE increase. The 13 m thick B-section in the Apex Basalt, dominated by SiO2 and containing significant amounts of Ba (up to 4330 ppm), originated from a low-T MOR hydrothermal solution. This section is characterized by an association with massive black/gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center. Geochemical evidence from the greenstones underlying the B-section indicates that they are of MORB origin.
The phylogenetic positions of sulfate-reducing organisms, as revealed from comparisons of small-subunit ribosomal RNA (SSU rRNA), are spread over both the Archaeal and Bacterial domains, though when they evolved is uncertain. The low-branching positions of some of these groups on the Tree of Life have inspired the hypothesis that the metabolic innovation of microbial sulfate reduction is of great antiquity. Only recently, however, have sulfur isotope data from Precambrian rocks begun to emerge that clearly demonstrate sulfate-reducing microbes had evolved by the early Archean. The large spread of δ34S values of microscopic pyrites aligned along growth faces of former gypsum crystals in the ∼3.47-Ga North Pole barite deposit of northwestern Australia provide the oldest evidence of microbial sulfate reduction and the earliest indication of a specific microbial metabolism. The distinct expression of microbial sulfate reduction in this localized and cool sulfate-rich environment provides the oldest date for calibrating the temporal progress of early evolution on the Tree of Life.
Hydrogen-lean kerogens (atomic H/C<0.4) isolated from the 2.5-billion-year-old (Ga) Mt. McRae Shale, Hamersley Group, at Tom Price, Western Australia, were studied via hydropyrolysis, a continuous-flow technique that degrades organic matter in a stream of high-pressure hydrogen assisted by a dispersed Mo catalyst. The hydropyrolysates yielded predominantly phenanthrene and pyrene, and higher polyaromatic hydrocarbons and alkylated homologues were generated in low relative concentrations. Saturated hydrocarbons were not detected. The molecular and carbon isotopic compositions of the hydropyrolysates are very similar to aromatic hydrocarbons obtained by solvent extraction of the host rocks. Because molecular structures covalently attached to kerogen are unaffected by contamination, this indicates that both the bound and extractable aromatic fractions are syngenetic with the host rocks. Therefore, the results of the hydropyrolysis experiments provide compelling evidence for preserved bitumen of Archean age. The very high proportion of nonalkylated polyaromatic hydrocarbons in the hydropyrolysates is consistent with hydrothermal dehydrogenation of the kerogen, and a marked concentration difference of pyrene in rock extracts and hydropyrolysates might be explained by hydrothermal redistribution of the bitumen. The kerogen and bitumen composition is therefore consistent with models suggesting a hydrothermal origin for the giant iron ore deposits at Mt. Tom Price. Comparison of the Archean samples with hydropyrolysates from immature Mesoproterozoic kerogens from the Roper Group, McArthur Basin, Northern Territory, and with pyrolysis experiments on Proterozoic kerogens in the literature suggests that Precambrian kerogens are frequently highly aromatic and lipid-poor regardless of their degree of thermal preservation.
Although it is widely believed that production of organic compounds by Fischer–Tropsch synthesis and related processes occurs in many geologic environments, unambiguous identification of compounds with an abiotic origin in natural samples has been hampered by a lack of means to discriminate between abiotic compounds and organic matter from biological sources. While isotopic compositions might provide a means to discriminate between biologic and non-biologic sources of organic matter, there are few data presently available to constrain the isotopic composition of compounds produced by abiotic processes in geologic systems. Here, we report results of laboratory experiments conducted to evaluate the isotopic composition of organic compounds synthesized abiotically under hydrothermal conditions. We find the organic products are depleted in 13C to a degree typically ascribed to biological processes, indicating that carbon isotopic composition may not be a particularly effective diagnostic means to differentiate between biologic and non-biologic sources. Furthermore, our results suggest that the isotopic compositions of reduced carbon compounds found in many ancient rocks that have heretofore been attributed to biological sources could be consistent with an abiotic origin in a hydrothermal setting.
The geological record of carbonaceous matter from at least 3.5 Ga to the end of the Precambrian is fundamentally continuous in terms of carbonaceous matter structure, composition, environments of deposition/preservation, and abundance in host rocks. No abiotic processes are currently known to be capable of producing continuity in all four of these properties. Although this broad view of the geological record does not prove that life had arisen by 3.5 Ga, the end of the early Archean, it suggests a working hypothesis: most if not all carbonaceous matter present in rocks older than 3.0 Ga was produced by living organisms. This hypothesis must be tested by studies of specific early geological units designed to explore the form, distribution, and origin of enclosed carbonaceous matter.
A preference for n-parffins with odd numbers of carbon atoms is widespread in Recent sediments. Analyses of numerous Recent sediment samples indicate that molecules with odd numbers of carbon atoms predominate in the heavy n-paraffins for at least the first few thousand years after deposition. The depositional environments studied include a continental shelf, some ocean basins, bays. fresh-water lakes, a saline lake and soils. In contrast, the heavy n-paraffins in a representative selection of crude oils are evenly distributed between molecules of even- and odd-carbon number.Analyses of a limited number of postulated source rocks showed no preference in the heavy n-paraifins for molecules of either odd- or even-carbon number. Some shales and limestones ranging from Eocene to Mississippian in age have been examined for possible application of the n-paraffin distribution to recognition of source beds.
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.
Shales of very low metamorphic grade from the 2.78 to 2.45 billion-year-old (Ga) Mount Bruce Supergroup, Pilbara Craton, Western Australia, were analyzed for solvent extractable hydrocarbons. Samples were collected from ten drill cores and two mines in a sampling area centered in the Hamersley Basin near Wittenoom and ranging 200 km to the southeast, 100 km to the southwest and 70 km to the northwest. Almost all analyzed kerogenous sedimentary rocks yielded solvent extractable organic matter. Concentrations of total saturated hydrocarbons were commonly in the range of 1 to 20 ppm (μg/g rock) but reached maximum values of 1000 ppm. The abundance of aromatic hydrocarbons was ∼1 to 30 ppm. Analysis of the extracts by gas chromatography-mass spectrometry (GC-MS) and GC-MS metastable reaction monitoring (MRM) revealed the presence of n-alkanes, mid- and end-branched monomethylalkanes, ω-cyclohexylalkanes, acyclic isoprenoids, diamondoids, tri- to pentacyclic terpanes, steranes, aromatic steroids and polyaromatic hydrocarbons. Neither plant biomarkers nor hydrocarbon distributions indicative of Phanerozoic contamination were detected. The host kerogens of the hydrocarbons were depleted in 13C by 2 to 21‰ relative to n-alkanes, a pattern typical of, although more extreme than, other Precambrian samples. Acyclic isoprenoids showed carbon isotopic depletion relative to n-alkanes and concentrations of 2α-methylhopanes were relatively high, features rarely observed in the Phanerozoic but characteristic of many other Precambrian bitumens. Molecular parameters, including sterane and hopane ratios at their apparent thermal maxima, condensate-like alkane profiles, high mono- and triaromatic steroid maturity parameters, high methyladamantane and methyldiamantane indices and high methylphenanthrene maturity ratios, indicate thermal maturities in the wet-gas generation zone. Additionally, extracts from shales associated with iron ore deposits at Tom Price and Newman have unusual polyaromatic hydrocarbon patterns indicative of pyrolytic dealkylation.The saturated hydrocarbons and biomarkers in bitumens from the Fortescue and Hamersley Groups are characterized as ‘probably syngenetic with their Archean host rock’ based on their typical Precambrian molecular and isotopic composition, extreme maturities that appear consistent with the thermal history of the host sediments, the absence of biomarkers diagnostic of Phanerozoic age, the absence of younger petroleum source rocks in the basin and the wide geographic distribution of the samples. Aromatic hydrocarbons detected in shales associated with iron ore deposits at Mt Tom Price and Mt Whaleback are characterized as ‘clearly Archean’ based on their hypermature composition and covalent bonding to kerogen.
Rock-Eval pyrolysis of a large set of Cenomanian samples, collected from the black levels (clayey, cherty and mixed) in three sections of the Umbria-Marche basin, showed large differences in organic matter (OM) quantity and quality. The chert samples systematically exhibit much lower TOC contents, markedly lower HI and higher OI. This reflects the extensive oxidative destruction of the initial kerogen that occurred upon the chertification of some clayey sediments. A comparative study, by a combination of microscopic, spectroscopic and pyrolytic methods, was performed on kerogens of the chert and clay layers of a representative mixed level. The various fractions of the initial kerogen underwent differential destruction or alteration during chertification, resulting in (i) relative enrichments of microfossils and woody debris although lignin was altered by demethoxylation and (ii) extensive destruction of the amorphous fraction while it remained predominant. The amorphous fraction retained in the chert kerogen showed large changes in composition related to oxygen incorporation and probably escaped complete destruction owing to oxidative reticulation. The above features account for the pronounced systematic differences in OM abundance and oil potential between the chert and clay layers in the black levels.
Ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes, biologically mediated 13C/12C fractionations have come to be recognized as a common corollary of biochemical reactions. Meanwhile, it is firmly established that the universal bias in favour of 12C characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon-fixing enzymatic carboxylation reaction in the primary metabolism of CO2-fixing (autotrophic) organisms. This preference for 12C has turned out to be one of the most enduring relics of the ‘ordered state’ of the biological precursor substances that may be preserved in fossil organics over billions of years. With the currently known sedimentary record at hand, it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3.8 Ga of recorded Earth history, indicating that microbial (prokaryotic and archaeoprokaryotic) ecosystems had been prolific already on the Archean Earth. While for the time span<3.5 Ga the isotopic evidence is unequivocal, the information encoded in the preceding record is commonly blurred by a metamorphic overprint. This holds particularly for the metasediments of the 3.8 Ga old Isua Supracrustal Belt of West Greenland which, apart from widespread metasomatism, have suffered amphibolite-grade metamorphism. It is known that 13C/12C exchange occurs in organic (kerogenous) rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available (as either fluids or carbonate), with isotopic re-equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction. Thermodynamic equilibria predict, however, that 13C/12C ratios in kerogen and graphite increase during this process. Hence, high-T exchange equilibria are always bound to drive δ13C values in positive direction, the lowermost values encountered being consequently the least exchanged and most pristine. With the lowest values of reduced (graphitic) carbon obtained in early Isua studies falling into the range −22 to −28‰ [PDB], we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre-metamorphic Isua suite. It was, therefore, by no means surprising that the results of recent isotope work performed on apatite-hosted carbonaceous microdomains in Isua banded iron-formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions. Hence, the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope-discriminating properties of the principal CO2-fixing reactions(s) in biological carbon assimilation, suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation. As a consequence, biological modulation of the geochemical carbon cycle had been established at least 3.8 Ga ago, having been fully operative by the time of formation of the Earth's oldest sediments.
Recently, methane (CH(4)) of possible abiogenic origin has been reported from many localities within Earth's crust. However, little is known about the mechanisms of abiogenic methane formation, or about isotopic fractionation during such processes. Here, a hydrothermally formed nickel-iron alloy was shown to catalyze the otherwise prohibitively slow formation of abiogenic CH(4) from dissolved bicarbonate (HCO(3)-) under hydrothermal conditions. Isotopic fractionation by the catalyst resulted in delta(13)C values of the CH(4) formed that are as low as those typically observed for microbial methane, with similarly high CH(4)/(C(2)H(6) + C(3)H(8)) ratios. These results, combined with the increasing recognition of nickel-iron alloy occurrence in oceanic crusts, suggest that abiogenic methane may be more widespread than previously thought.
Eleven taxa (including eight heretofore undescribed species) of cellularly preserved filamentous microbes, among the oldest fossils known, have been discovered in a bedded chert unit of the Early Archean Apex Basalt of northwestern Western Australia. This prokaryotic assemblage establishes that trichomic cyanobacterium-like microorganisms were extant and morphologically diverse at least as early as approximately 3465 million years ago and suggests that oxygen-producing photoautotrophy may have already evolved by this early stage in biotic history.
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.
Reassessing the evidence for the earliest traces of life Earth's Earliest Biosphere. Its Origin and Evolution
- Van
- M Zuilen
- A Lepland
- G Arrhenius
- M R Walter
- H J Hofmann
- J W Schopf
Econ. Geol. 97, 695–732. Van Zuilen, M., Lepland, A., Arrhenius,G., 2002. Reassessing the evidence for the earliest traces of life. Nature 418, 627–630. Walter, M.R., Hofmann, H.J., Schopf, J.W., 1983. In: Schopf, J.W. (Ed.), Earth's Earliest Biosphere. Its Origin and Evolution. Princeton University Press, Princeton, p. 397. Wedeking,K.W.,Hayes,J.M.,1983.CarbonizationofPrecambriankerogens.In:Bjoroy,M. (Ed.), Advances in Geochemistry, pp. 546–553. 480 S. Derenne et al. / Earth and Planetary Science Letters 272 (2008) 476–480
Geology and tectonic of the North Pilbara Terrain
- Van Kranendonk
- M J Hickman
- A H Smithies
- R H Nelson
- D R Pike
Van Kranendonk, M.J., Hickman, A.H., Smithies, R.H., Nelson, D.R., Pike, G., 2002. Geology and tectonic of the North Pilbara Terrain, Pilbara Craton, Western Australia. Econ. Geol. 97, 695–732.
Carbonization of Precambrian kerogens
- Wedeking
Wedeking, K.W., Hayes, J.M., 1983. Carbonization of Precambrian kerogens. In: Bjoroy, M. (Ed.), Advances in Geochemistry, pp. 546–553.