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Microfossil Recognition in Archean Rocks: An Appraisal of Spheroids and Filaments from a 3500 M.Y. Old Chert-Barite Unit at North Pole, Western Australia
Abstract
Archean microfossils are notoriously difficult to recognize. Most that have been reported have subsequently proved to be either younger contaminants or abiogenic pseudofossils. To avoid mistaking contaminants for genuine Archean objects, a set of rigorous collecting and preparation procedures should be followed. To avoid mistaking pseudofossils for authentic microfossils, a hierarchical series of recognition criteria can be used. Six classes of microfossil-like objects from a ~3500 m.y. old chert-barite unit in the Warrawoona Group at North Pole, Western Australia, are evaluated using the above technique. -from Author
... Due to their simple shape, various types of Archean spherical and filamentous carbonaceous microstructures have commonly been regarded as dubiofossils or discarded as non-fossils as they could have formed during accretion and/or displacement of carbonaceous matter associated with growth of mineral templates (Brasier et al., 2005;Buick, 1990;Lepot, 2020;McMahon & Cosmidis, 2021). Dubiofossils formed in early Archean volcanoclastic rocks as volcanic vesicles became covered by carbonaceous matter (Hickman-Lewis, Cavalazzi, Foucher, & Westall, 2018;Wacey, Noffke, Saunders, Guagliardo, & Pyle, 2018a;Wacey, Saunders, & Kong, 2018b). ...
... The cellular nature of Paleoarchean microspheres has been debated, competing with the alternative hypotheses of carbonaceous coating of abiotic silica spherulites (Brasier et al., 2005;Buick, 1990) Figure S1). Silica botryoids in another SPF chert displayed hemispherical, coalescent quartz microstructures coated by carbonaceous matter (Sugitani, Mimura, Nagaoka, Lepot, & Takeuchi, 2013). ...
... Carbonaceous halos observed around stellate carbonaceous masses in Paleoarchean chert have been interpreted to result from displacement of carbonaceous matter away from a central core of organic matter during the growth of spherulites of chalcedony (Buick, 1990), based on the experiments of Oehler (1976). Dense carbonaceous masses found at the center of single and coalescing siliceous hemispheres outlined by carbonaceous matter, observed in Paleoproterozoic cherts by Tyler and Barghoorn (1954) have been re-interpreted as pseudofossils of botryoidal chalcedony by Mendelson and Schopf (1992). ...
The morphogenesis of most carbonaceous microstructures that resemble microfossils in Archean (4–2.5 Ga old) rocks remains debated. The associated carbonaceous matter may even—in some cases—derive from abiotic organic molecules. Mineral growths associated with organic matter migration may mimic microbial cells, some anatomical features, and known microfossils—in particular those with simple spheroid shapes. Here, spheroid microstructures from a chert of the ca. 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton (Western Australia) were imaged and analyzed with a combination of high‐resolution in situ techniques. This provides new insights into carbonaceous matter distributions and their relationships with the crystallographic textures of associated quartz. Thus, we describe five new types of spheroids and discuss their morphogenesis. In at least three types of microstructures, wall coalescence argues for migration of carbonaceous matter onto abiotic siliceous spherulites or diffusion in poorly crystalline silica. The nanoparticulate walls of these coalescent structures often cut across multiple quartz crystals, consistent with migration in/on silica prior to quartz recrystallization. Sub‐continuous walls lying at quartz boundaries occur in some coalescent vesicles. This weakens the “continuous carbonaceous wall” criterion proposed to support cellular inferences. In contrast, some clustered spheroids display wrinkled sub‐continuous double walls, and a large sphere shows a thick sub‐continuous wall with pustules and depressions. These features appear consistent with post‐mortem cell alteration, although abiotic morphogenesis remains difficult to rule out. We compared these siliceous and carbonaceous microstructures to coalescent pyritic spheroids from the same sample, which likely formed as “colloidal” structures in hydrothermal context. The pyrites display a smaller size and only limited carbonaceous coatings, arguing that they could not have acted as precursors to siliceous spheroids. This study revealed new textural features arguing for abiotic morphogenesis of some Archean spheroids. The absence of these features in distinct types of spheroids leaves open the microfossil hypothesis in the same rock. Distinction of such characteristics could help addressing further the origin of other candidate microfossils. This study calls for similar investigations of metamorphosed microfossiliferous rocks and of the products of in vitro growth of cell‐mimicking structures in presence of organics and silica.
... Subsequent refinements and restatements of Cloud's criteria for establishing the biogenicity of ancient evidence for life resulted in new criteria for recognizing ancient microfossils and critical appraisals of proposed ancient microfossil assemblages from specific localities. Buick (1990), for example, challenged previous biological interpretations of non-carbonaceous "microbiota" from the~3.5 Ga Dresser Formation at North Pole, Pilbara, and Brasier et al. (2002Brasier et al. ( , 2005 and Wacey et al. (2016) demonstrated an abiogenic origin for the microfossil-like suite of objects that, at the time, was considered the oldest convincing evidence for ancient fossil remains (the~3.46 Ga Apex chert, also Pilbara; Schopf and Packer 1987). ...
... Once the syngenetic context, both regional and local, of ancient filamentous microfossil-like objects has been demonstrated, high-resolution (sub-micrometric) analytical techniques are required to distinguish their biogenic origin (c.f., Oehler and Cady 2014). For example, one of the traditional criteria for recognizing ancient filaments is the presence of microscopically visible septae that regularly subdivide elongate filaments (Buick 1990). However, a high-resolution study that re-examined (Maldanis et al. 2020) shows that septae can be lost during mineral replacement of filamentous cells. ...
... Coccoidal objects exhibit substantial size variation. Although in early work, a statistically normal size distribution was considered a requirement for demonstrating the biogenic origin of a coccoid population (e.g., Schopf and Walter 1983), it is now known that a normal distribution of spherical objects can also arise naturally in inorganic particle populations (Buick 1990). Spheroidal microfossil-like objects also exhibit considerable morphological differentiation: within millimeters, hollow and solid forms, both solitary and grouped, have been identified (Sugitani et al. 2007(Sugitani et al. , 2015. ...
Life is the outcome of a complex network of chemical reactions and molecular interactions that emerged on Earth once primitive chemical automata could self-assemble in such a way that enabled them to self-reproduce and evolve. Yet exactly how, where and when life first appeared on our planet remains unknown. In this chapter, we review the various lines of evidence from fossil and geochemical traces of early life preserved in the geological record, which provide fundamental, albeit often rudimentary, insight into early life. The oldest fossils record the nature of life more than half a billion years after it emerged on Earth and suggest that considerable metabolic diversity had already evolved by this time. Microfossils, microbial mats, stromatolites and organic and inorganic geochemical signatures have been interpreted as compelling evidence for Archean biodiversity. In spite of the chemical enigmas of the earliest life and its limited record, characterization of the various classes of biosignatures indicative of life in its geological context provides guidance as to the earliest co-evolution of the geosphere and biosphere.
... Although a plausible biological morphology alone is not decisive proof of the microbial origin of the filamentous structure, it is a required criterion for identifying microbial fossils (section S4) (3,(23)(24)(25). Several morphological features of the studied filaments are notably similar to other permineralized microfossil examples found throughout the geological record [e.g., (4,(24)(25)(26)]. ...
... Disarticulation ( Fig. 3A and fig. S8, E and F) is a common attribute of fossilized microbial filaments [e.g., (4,(24)(25)(26)] and could reflect either or both structural (i.e., cell division points) (23,26) and taphonomic discontinuities (i.e., due to compressions or less preserved and degraded remains) (2,23,27). High morphological 3D fidelity of the filament preservation is consistent with very early and rapid silicification [e.g., (24,28,29)], which is to be expected of Paleoarchean silica-saturated hydrothermal systems (10,19). ...
... Disarticulation ( Fig. 3A and fig. S8, E and F) is a common attribute of fossilized microbial filaments [e.g., (4,(24)(25)(26)] and could reflect either or both structural (i.e., cell division points) (23,26) and taphonomic discontinuities (i.e., due to compressions or less preserved and degraded remains) (2,23,27). High morphological 3D fidelity of the filament preservation is consistent with very early and rapid silicification [e.g., (24,28,29)], which is to be expected of Paleoarchean silica-saturated hydrothermal systems (10,19). ...
Subsurface habitats on Earth host an extensive extant biosphere and likely provided one of Earth’s earliest microbial habitats. Although the site of life’s emergence continues to be debated, evidence of early life provides insights into its early evolution and metabolic affinity. Here, we present the discovery of exceptionally well-preserved, ~3.42-billion-year-old putative filamentous microfossils that inhabited a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt in South Africa. The filaments colonized the walls of conduits created by low-temperature hydrothermal fluid. Combined with their morphological and chemical characteristics as investigated over a range of scales, they can be considered the oldest methanogens and/or methanotrophs that thrived in an ultramafic volcanic substrate.
... The alternative strategy recommended by, for example, Hofmann (1972) and Buick (1990), is to describe inconclusively fossil-like features as "dubiofossils" (Hofmann, 1972). The use of this neutral term avoids shoehorning ambiguous objects into favored categories on the basis of inconclusive (or cherry-picked) evidence or the falsification of inappropriate "null hypotheses". ...
... But the term "dubiofossil" remains a helpful stopgap, like the paleontological term "problematicum". If desired, the dubiofossil category can be made more precise by scoring the quality of the evidence using biogenicity criteria, even if they do not finally compel either a biotic or an abiotic interpretation (Buick, 1990;McLoughlin & Grosch, 2015;Neveu et al., 2018). Thus, in Buick's (1990) (2015) present a hierarchical scheme for evaluating the biogenicity of metavolcanic-and ultramafic-hosted microalteration textures. ...
... If desired, the dubiofossil category can be made more precise by scoring the quality of the evidence using biogenicity criteria, even if they do not finally compel either a biotic or an abiotic interpretation (Buick, 1990;McLoughlin & Grosch, 2015;Neveu et al., 2018). Thus, in Buick's (1990) (2015) present a hierarchical scheme for evaluating the biogenicity of metavolcanic-and ultramafic-hosted microalteration textures. ...
The search for a fossil record of Earth's deep biosphere, partly motivated by potential analogies with subsurface habitats on Mars, has uncovered numerous assemblages of inorganic microfilaments and tubules inside ancient pores and fractures. Although these enigmatic objects are morphologically similar to mineralized microorganisms (and some contain organic carbon), they also resemble some abiotic structures. Palaeobiologists have responded to this ambiguity by evaluating problematic filaments against checklists of "biogenicity criteria". Here, we describe material that tests the limits of this approach. We sampled Jurassic calcite veins formed through subseafloor serpentinization, a water-rock reaction that can fuel the deep biosphere and is known to have occurred widely on Mars. At two localities ~4 km apart, veins contained curving, branched microfilaments composed of Mg-silicate and Fe-oxide minerals. Using a wide range of analytical techniques including synchrotron X-ray mi-crotomography and scanning transmission electron microscopy, we show that these features meet many published criteria for biogenicity and are comparable to fossilized cryptoendolithic fungi or bacteria. However, we argue that abiotic processes driven by serpentinization could account for the same set of lifelike features, and report a chemical garden experiment that supports this view. These filaments are, therefore, most objectively described as dubiofossils, a designation we here defend from criticism and recommend over alternative approaches, but which nevertheless signifies an impasse. Similar impasses can be anticipated in the future exploration of subsurface palaeo-habitats on Earth and Mars. To avoid them, further studies are required in biomimetic geochemical self-organization, microbial taphonomy and micro-analytical techniques, with a focus on subsurface habitats.
... Hydrothermal activity and metamorphism, by promoting the recrystallization of the mineral matrix and/or the circulation of organic fluids, can greatly modify the chemistry, ultrastructure, and even the general morphology of microfossils. Many abiotic objects, such as self-organized (organo-)mineral aggregates (Hopkinson et al., 1998;García-Ruiz et al., 2003Cosmidis and Templeton, 2016;Muscente et al., 2018;McMahon, 2019), mineral exfoliations (Wacey et al., 2015), tracks of the displacement of individual mineral grains [ambient inclusion trails; (e.g., Knoll and Barghoorn, 1974)], interstitial fillings (Brasier et al., 2005), or bitumen droplets (Buick, 1990), are consequently difficult to distinguish from true but structurally simple and strongly altered microfossils (Fig. 1). ...
... In Table 1, we present the three main lists of criteria put forward by Buick (1990), Schopf et al. (2010), and Brasier and Wacey (2012). By way of illustration, the three lists of criteria are applied in Table 1 to two well-documented potential microfossil assemblages: one from the 1.88 Ga Gunflint formation, whose biogenicity is widely recognized, and one from the 3.46 Ga Apex chert, whose biogenicity is strongly debated. ...
... (2) Evaluation of the biogenicity of the more ancient, hydrothermally affected Apex chert assemblage depends on the individual lists of criteria. When using the list from Buick (1990), the criterion of sedimentary setting is not satisfied-the assemblage originates from a paleohydrothermal vein-leading to a conclusion of nonbiogenicity. The two other lists do not lead to a clear conclusion on biogenicity, due to divergent views in literature. ...
The search for signs of life in the ancient rock record, extreme terrestrial environments, and other planetary bodies requires a well-established, universal, and unambiguous test of biogenicity. This is notably true for cellular remnants of microbial life, since their relatively simple morphologies resemble various abiogenic microstructures that occur in nature. Although lists of qualitative biogenicity criteria have been devised, debates regarding the biogenicity of many ancient microfossils persist to this day. We propose here an alternative quantitative approach for assessing the biogenicity of putative microfossils. In this theoretical approach, different hypotheses-involving biology or not and depending on the geologic setting-are put forward to explain the observed objects. These hypotheses correspond to specific types of microstructures/systems. Using test samples, the morphology and/or chemistry of these systems are then characterized at the scale of populations. Morphologic parameters include, for example, circularity, aspect ratio, and solidity, while chemical parameters could include elementary ratios (e.g., N/C ratio), isotopic enrichments (e.g., d13C), or chirality (e.g., molar proportion of stereoisomers), among others. Statistic trends distinguishing the different systems are then searched for empirically. The trends found are translated into ''decision spaces'' where the different systems are quantitatively discriminated and where the potential microfossil population can be located as a single point. This approach, which is formulated here on a theoretical level, will solve several problems associated with the classical qualitative criteria of biogenicity. Most importantly, it could be applied to reveal the existence of cellular life on other planets, for which characteristics of morphology and chemical composition are difficult to predict.
... Due to their simple shapes, and association with rocks that underwent complex alteration (during diagenesis, metamorphism, and sometimes metasomatism and/or weathering), many microstructures that have been considered as Archean microfossils have been re-evaluated as dubiofossils, pseudofossils, or contaminants (e.g., Brasier et al., 2005;Buick, 1990;Schopf and Walter, 1983). Thus, criteria have been developed to demonstrate the antiquity of microfossils, as summarized by Brasier et al. (2005), including 1) their observation in thin sections where they are enclosed in primary minerals (such as quartz) that would be stable enough to withstand metamorphism, 2) the replicability of their finding, and 3) their association with specific sedimentary structures (e.g. ...
... An important feature that has been proposed to distinguish abiotic spherical structures and cellular microfossils is the distribution of organic matter at the nanoscale. Some clearly abiotic, coalescing (Dunlop et al., 1978) have been reinterpreted as pyrobitumen droplets based on their dense carbonaceous structures observed under the optical microscope (Buick, 1990). Hence, cellularity inferences requires further ultrastructural investigations down to the nanoscale. ...
... A group of tubular microstructures named Ambient Inclusions Trails (AITs) can mimic some features of filamentous microfossils. AITs are commonly found in Archean cherts (Knoll and Barghoorn, 1974;Buick, 1990;Wacey et al., 2008a). These tubes have been inferred to form through displacement of a crystal, enhancing tubular dissolution of its mineral matrix (e.g., quartz) aided by the pressure of ambient fluids generated through biotic and/or thermal decomposition of organic matter (Knoll and Barghoorn, 1974;Wacey et al., 2008a;Luo et al., 2018) and possibly aided by reactions involving clays (Wacey et al., 2016a). ...
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 integrated characterization of the 3D morphology and mineralogical composition of these fossils at the nanoscale leads to complementary means of assessing the biogenicity of microfossils in general. For instance, the presence of kerogen following the morphology of a cell wall or a membrane-like structure that delimits a hollow lumen, such as observed in the Schreiber Beach specimens, is widely considered a key character attesting to the biogenicity of fossilized microorganisms 13,14,25,38 . In poorly-preserved specimens, such as those from Mink Mountain, however, these fine morphological features are frequently not retained. ...
... In this context, the discrimination of thermally mature kerogen from abiotic carbonaceous material -such as crystalline graphite or the condensed Fischer Tropsch-type carbonaceous material formed in hydrothermal settings -is a critical issue 7,14 . In our Mink Mountain species, the homogeneous diameter of these kerogenous features at the micron-scale both in individual fragments of the filaments and throughout their full extent in three dimensions is consistent with a biogenic origin in the cell envelope of the organism 38 . This type of morphometric consistency is one example of a biogenicity criterion that can be important for the evaluation of poorly preserved specimens of disputed biogenicity and can be unequivocally attested using 3D nanoscale imaging over a volume of several cubic micrometers with PXCT. ...
... This work further raises the need for a re-evaluation of controversial structures and a refinement of classical, but potentially outdated, biogenicity criteria (e.g. 38 ) using novel methodological approaches with robust capabilities to discriminate kerogen in 3D and attest that both morphology, mineralogy and local taphonomy at the ultrastructural scale correspond to expectations for biogenic structures. Moreover, the geochemical characterization of the surrounding minerals will enable an accurate interpretation of the palaeoenvironmental setting, petrological context, and taphonomic and diagenetic histories of putative ancient biosignatures that may add confirmation of their singenicity with the host rock. ...
Precambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotron-based ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions.
... While demonstrating the biogenicity of putative prokaryotic microfossils is challenging, criteria have been proposed by several researchers to make such interpretations more consistent: Cloud (1976) proposed that "a given microstructure can be considered demonstrably biogenic only if some of its representatives display a level of cellular, microstructural, or biogeochemical differentiation comparable with that of living organisms and implying a similarity of function and continuity of evolution between them." Further criteria that have been put forward are that the respective microstructures should (1) be similar in size to modern bacteria, (2) occur isolated as well as in groups or colonies, (3) be derived from well-preserved host rocks, (4) exhibit structurally identifiable cell walls, and (5) show variable preservation throughout the sample (Buick, 1990;Schopf et al., 2007;Westall, 1999). The core facies of the Hollard Mound has been identified as a hydrocarbon-seep F I G U R E 6 X-ray diffraction pattern of a Hollard Mound carbonate sample with spheres after decalcification showing peaks for quartz, albite, and pyrite. ...
... While there is little to no internal structure preserved within the cells, their outer structure is preserved by a well-defined, brownish-to-black rim that make them distinct and clearly identifiable from the carbonate cement in which they were preserved (Figures 3 and 4). This pattern of preservation is a criterion for biogenicity (Buick, 1990), while also agreeing with observations derived from experimental taphonomy (Cunningham et al., 2012). This selective preservation is further supported by the identified Raman peaks at ca. 1330 and ca. ...
The giant sulfide‐oxidizing bacteria are particularly prone to preservation in the rock record, and their fossils have been identified in ancient phosphorites, cherts, and carbonates. This study reports putative spherical fossils preserved in the Devonian Hollard Mound hydrocarbon‐seep deposit. Based on petrographical, mineralogical, and geochemical evidence the putative microfossils are interpreted as sulfide‐oxidizing bacteria similar to the present‐day genus Thiomargarita , which is also found at modern hydrocarbon seeps. The morphology, distribution, size, and occurrence of the fossilized cells show a large degree of similarity to their modern counterparts. Some of the spherical fossils adhere to worm tubes analogous to the occurrence of modern Thiomargarita on the tubes of seep‐dwelling siboglinid worms. Fluorapatite crystals were identified within the fossilized cell walls, suggesting the intercellular storage of phosphorus analogous to modern Thiomargarita cells. The preservation of large sulfide‐oxidizing bacteria was probably linked to changing biogeochemical processes at the Hollard Mound seep or, alternatively, may have been favored by the sulfide‐oxidizing bacteria performing nitrate‐dependent sulfide oxidation—a process known to induce carbonate precipitation. The presence of sulfide‐oxidizing bacteria at a Devonian hydrocarbon seep highlights the similarities of past and present chemosynthesis‐based ecosystems and provides valuable insight into the antiquity of biogeochemical processes and element cycling at Phanerozoic seeps.
... Biogenicity. The general appearance and preservation of the microfossils here studied distinguish them from potential abiogenic structure or pseudofossils [27][28][29][30] . Regarding biogenicity 27,28,31,32 , the organic walled microfossils meet all criteria that are commonly used, such as: (i) insertion of the fossil inside the rock, hampering later contamination; (ii) morphologically plausible size and shape; (iii) variation in preservational aspects; and (iv) presence of organic compounds. ...
... The general appearance and preservation of the microfossils here studied distinguish them from potential abiogenic structure or pseudofossils [27][28][29][30] . Regarding biogenicity 27,28,31,32 , the organic walled microfossils meet all criteria that are commonly used, such as: (i) insertion of the fossil inside the rock, hampering later contamination; (ii) morphologically plausible size and shape; (iii) variation in preservational aspects; and (iv) presence of organic compounds. Taxonomic breakdown. ...
Large igneous provinces (LIPs) are major magmatic events that have a significant impact on the global environment and the biosphere, for example as triggers of mass extinctions. LIPs provide an excellent sedimentological and geochemical record of short but intense periods of geological activity in the past, but their contribution towards understanding ancient life is much more restricted due to the destructive nature of their igneous origin. Here, we provide the first paleontological evidence for organic walled microfossils extracted from wet peperites from the Early Cretaceous Paraná-Etendeka intertrappean deposits of the Paraná basin in Brazil. Wet peperites are a volcaniclastic rock formed by the interaction of lava and subaqueous sediments.The Paraná-Etendeka was formed during the Valanginian (ca. 132 Ma) as a continental flood basalt in present day South America and Namibia, and released enormous amounts of carbon dioxide, sulfur dioxide, methane and hydrogen fluoride into the atmosphere. The organic walled microfossils recovered from the Paraná-Etendeka peperites include pollen grains, spores, acritarchs, and other remains of unidentifiable organic matter. In addition to the peperites, organic walled microfossils were also found in heterolithic sandstones and interpillow sandstones. Our findings represent the first insight into the biodiversity of the Paraná Basin during the Early Cretaceous during a period of intense magmatism, and the microfossil assemblages corroborate a regional paleoclimatic transition from arid to more humid conditions that were likely induced by the volcanic activity. We corroborate the potential of wet peperite rocks as a valuable source of paleobiological data and emphasize the importance of sampling volcaniclastic units that have been traditionally considered with lower fossiliferous potential due to their igneous origin.
... As shown herein, fracture-filling carbonate veins in serpentinizing systems can preserve structures resembling microbial biosignatures in the form of filamentous and stromatolitic morphotypes. As with any assessment of biogenicity, multiple lines of evidence must be considered to determine whether such structures represent actual remnants of past life [73][74][75] . Structures resembling filamentous microfossils can be challenging to interpret due to their oftensimple morphology and may instead be attributed to abiogenic processes 76 . ...
... Here we show that Type A filaments are composed of carbon and nitrogen bearing material (Fig. 3), consistent with the presence of biologically derived organic material. Carbonaceous material (e.g., kerogen) associated with biomorphic structures has often been regarded as an indicator of biogenicity 73,87 , although abiotic pathways have been suggested to explain the occurrence of some carbonaceous microfossil-like structures 77 . Nitrogen associated with the microfossils described here provides further evidence of biogenicity as nitrogen is a key element present in a number of essential biomolecules and is known to occur among preserved biologically-derived carbonaceous material [88][89][90] . ...
Serpentinization is a geological process involving the interaction of water and ultramafic rock, the chemical byproducts of which can serve as an energy source for microbial communities. Although serpentinite systems are known to host active microbial life, it is unclear to what extent fossil evidence of these communities may be preserved over time. Here we report the detection of biosignatures preserved in a mineralized fracture within drill cores from the Samail Ophiolite in Oman. Two varieties of filamentous structures were identified in association with iron oxide precipitates. The first type are interpreted as likely microbial remains, while the second type are recognized as potentially microbiological dubiofossils. Additionally, laminated structures composed of carbon and nitrogen rich material were identified and interpreted as having a microbially-associated origin. Our observations affirm the potential to detect subsurface microbial communities within serpentinizing environments and highlight a unique taphonomic window to preserve evidence of rock-hosted life.
... In several ways, this challenge is similar to that of biogenicity tests for putative traces of fossil life in the Earth's rock record. Classical tests of biogenicity in deep time involve the evaluation of multiple biosignature characteristics and context-and contamination-related criteria that need to be satisfied to substantiate a claim (e.g., Buick, 1990;Schopf et al., 2010;Brasier and Wacey, 2012;Neveu et al., 2018). The number and combinations of these characteristics and criteria, however, are subject to debate, since it is easy to include false positives or exclude false negatives. ...
... For instance, convolutional neural network classification models were developed and trained to perform visual palynological identification and taxonomic classification of fossil pollen (Romero et al., 2020). For the purpose of the SSAF, there are three categories to be considered: (Buick, 1990;Neveu et al., 2018;Schopf et al., 2010;Brasier and Wacey, 2012;Rouillard et al., 2021a). There is, however, a growing literature on ''false biosignatures'', i.e., physiochemical processes that lead to the formation of minerals or molecules with life-like features (Cosmidis and Templeton, 2016;Garcia-Ruiz et al., 2003;Jordan et al., 2019;Kotopoulou et al., 2021;Rouillard et al., 2018Rouillard et al., , 2021bMcMahon et al., 2021). ...
The Committee on Space Research (COSPAR) Sample Safety Assessment Framework (SSAF) has been developed by a COSPAR appointed Working Group. The objective of the sample safety assessment would be to evaluate whether samples returned from Mars could be harmful for Earth's systems (e.g., environment, bio-sphere, geochemical cycles). During the Working Group's deliberations, it became clear that a comprehensive assessment to predict the effects of introducing life in new environments or ecologies is difficult and practically impossible, even for terrestrial life and certainly more so for unknown extraterrestrial life. To manage expectations , the scope of the SSAF was adjusted to evaluate only whether the presence of martian life can be excluded in samples returned from Mars. If the presence of martian life cannot be excluded, a Hold & Critical Review must be established to evaluate the risk management measures and decide on the next steps. The SSAF
... The VCJ filaments conform to the majority of established biogenicity criteria for documenting Precambrian microfossils (Buick, 1990;Cady et al., 2003). These criteria include the following: (a) being intrinsic to the rock and pre-dating quartz crystallization (probably from an original silica gel; see Grenne & Slack, 2003) and subsequent quartz veining; (b) having a narrow range of diameters that generally do not vary longitudinally, and that are within the size range of modern micro-organisms; (c) co-existing with other filaments of similar morphology; and (d) occurring in a paleoenvironment analogous to those where bacteria grow today (see below). ...
... These criteria include the following: (a) being intrinsic to the rock and pre-dating quartz crystallization (probably from an original silica gel; see Grenne & Slack, 2003) and subsequent quartz veining; (b) having a narrow range of diameters that generally do not vary longitudinally, and that are within the size range of modern micro-organisms; (c) co-existing with other filaments of similar morphology; and (d) occurring in a paleoenvironment analogous to those where bacteria grow today (see below). The only biogenicity criterion of Buick (1990) not satisfied by the VCJ filaments is their apparent lack of organic matter (e.g., kerogen). The weak EDS carbon signal we found in one of the FIB-TEM sections ( Figure S2) is not spatially associated with a well-defined filament, and its origin could have several explanations, including being contemporaneous to filament formation, or a later contamination related to diagenesis or metamorphism. ...
Modern marine hydrothermal vents occur in a wide variety of tectonic settings and are characterized by seafloor emission of fluids rich in dissolved chemicals and rapid mineral precipitation. Some hydrothermal systems vent only low-temperature Fe-rich fluids, which precipitate deposits dominated by iron oxyhydroxides, in places together with Mn-oxyhydroxides and amorphous silica. Whilst a proportion of this mineralization is abiogenic, most is the result of the metabolic activities of benthic, Fe-oxidizing bacteria (FeOB), principally belonging to the Zetaproteobacteria. These microorganisms secrete micrometer-scale stalks, sheaths, and tubes with a variety of morphologies, composed largely of ferrihydrite that act as sacrificial structures, preventing encrustation of the cells that produce them. Cultivated marine FeOB generally require neutral pH and microaerobic conditions to grow. Here we describe the morphology and mineralogy of filamentous microstructures from a late Paleoproterozoic (1.74 Ga) jasper (Fe-oxide-silica) deposit from the Jerome area of the Verde mining district in central Arizona, USA, that resemble the branching tubes formed by some modern marine FeOB. On the basis of this comparison, we interpret the Jerome area filaments as having formed by FeOB on the deep seafloor, at the interface of weakly oxygenated seawater and low-temperature Fe-rich hydrothermal fluids. We compare the Jerome area filaments with other purported examples of Precambrian FeOB and discuss the implications of their presence for existing redox models of Paleoproterozoic oceans during the ‘Boring Billion’.
... The most common diagenetic changes in stromatolites are silicification and dolomitization, which involve the replacement of original calcium carbonate (CaCO 3 ) by silica (SiO 2 ) during silicification and dolomite [CaMg(CO 3 ) 2 ] in dolomitization. Silicification during early diagenesis leads to the exceptional preservation of original textures, microfossils, and organic chemistry, which are leading indicators in the characterization of biogenicity, indigeneity (i.e., naturally occurring in the environment), and syngenicity (i.e., formed at the same time as the enclosing rock) (Knoll et al., 1988;Buick, 1990;Grotzinger and Knoll, 1999;Van Kranendonk et al., 2003;Sugitani et al., 2007;Schopf and Kudryavtsev, 2012;Braiser et al., 2015). Unlike silicification, dolomitization commonly results in the loss of microbial morphology (Schopf, 1999;Bartley et al., 2000), making the characterization of syngenetic and indigenous biosignatures more difficult (Grotzinger and Rothman, 1996). ...
... Characterizing the effects of secondary dolomitization on microbial fossil preservation is critical for interpreting traces of early life in the geological record. The syngenicity and indigeneity of this type of fossilization in the geologic record can only be comprehensively examined after the degree of alteration of the host rock has been characterized and evaluated against that of the biologic remains (Buick, 1990;Braiser et al., 2004). ...
Stromatolites have been a major focus in the search for ancient microbial life, however, the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized or correlated with their dolomitizing conditions. Although dolomitization rarely preserves microbial morphology, the presence of organic carbon can provide valuable information for characterization of fossils' biogenicity, syngenicity, and indigeneity to their host rock. The Cambrian Allentown Formation in New Jersey, USA, is an excellent example of dolomitized stromatolites and thrombolites containing diagenetically modified microbial biosignatures. Based on XRD and EPMA data, the dolomite composition is typically stoichiometric, with varying degrees of cationic ordering. The outcrop underwent early dolomitization in a marginal-marine setting and later burial diagenesis resulting in multi-generational dolomite formation: (1) microspar dolomite formed by early diagenetic replacement at or near the surface, (2) zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by infilling primary pore spaces within the microspar matrix. The rhombic crystals continued to grow outward in alternating stages of Fe-enriched and -depleted fluids, which were preserved in zoned rims and revealed by cathodoluminescence, and (3) saddle dolomite formed during late stage deep burial with Fe- and Mn-rich fluids, and occurs as a void-filling, high-temperature phase. Organic carbon, characterized using confocal Raman microscopy, has an exclusive distribution within the microspar dolomite, and the D and G bands' characteristics reveal similar thermal alteration to the host rock, indicating that the mapped organic carbon is indigenous and syngenetic with the Cambrian carbonates. The findings presented in this study reveal organic matter found within microspar of various dolomitized facies deriving from different source pools of organic carbon. This study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on Earth and Mars.
... The most common diagenetic changes in stromatolites are silicification and dolomitization, which involve the replacement of original calcium carbonate (CaCO 3 ) by silica (SiO 2 ) during silicification and dolomite [CaMg(CO 3 ) 2 ] in dolomitization. Silicification during early diagenesis leads to the exceptional preservation of original textures, microfossils, and organic chemistry, which are leading indicators in the characterization of biogenicity, indigeneity (i.e., naturally occurring in the environment), and syngenicity (i.e., formed at the same time as the enclosing rock) (Knoll et al., 1988;Buick, 1990;Grotzinger and Knoll, 1999;Van Kranendonk et al., 2003;Sugitani et al., 2007;Schopf and Kudryavtsev, 2012;Braiser et al., 2015). Unlike silicification, dolomitization commonly results in the loss of microbial morphology (Schopf, 1999;Bartley et al., 2000), making the characterization of syngenetic and indigenous biosignatures more difficult (Grotzinger and Rothman, 1996). ...
... Characterizing the effects of secondary dolomitization on microbial fossil preservation is critical for interpreting traces of early life in the geological record. The syngenicity and indigeneity of this type of fossilization in the geologic record can only be comprehensively examined after the degree of alteration of the host rock has been characterized and evaluated against that of the biologic remains (Buick, 1990;Braiser et al., 2004). ...
... In fact, some of the vintage chert beds are considered globally significant for containing the direct evidence of > 2 Ga old life forms (Awramik et al., 1983;Barghoorn & Tyler, 1965;Brasier et al., 2002;Buick, 1990;House et al., 2000;Schopf, 1993;Schopf et al., 2002). Very fine siliceous crystals in chert give high resistance to weathering, recrystallization and metamorphism. ...
The term ‘chert’ ideally refers to fine-grained siliceous (micro/cryptocrystalline) mineral and is also often used for rock with such siliceous mineral aggregate of chemical, biochemical, and organic origin. Petrologically, inorganic non-sedimentary origin or even volcanic derivatives formed by devitrification of metastable felsic volcanic glass can also be included within chert. A new classification scheme for Precambrian cherts is proposed, especially for field workers. Despite several worldwide studies on chert, simple comprehensive classification of chert is not available to date. There are notable differences amongst Archaean, Palaeoproterozoic and Meso-Neoproterozoic cherts. This paper reviews all the Precambrian cherts to divide them into three categories from global context. Archaean and Palaeoproterozoic cherts mostly imply precipitation from silica gel material supplied vide submarine volcanism. This paper also focuses on diagenetic chert concretion, nodules, and geodes in detail. Finally, the Mesoproterozoic Nagari Formation in Cuddapah Basin, India is shown as a case to explain the diagenetic conditions, which could favour chert development by silica supersaturation in the pores. Diagenetic sub-environments are categorized systematically as eogenetic, mesogenetic, and telogenetic types with evidences of each based on photomicrography and outcrop studies. A comprehensive analysis is attempted to understand the development of concretions, nodules and geodes due to diagenesis with respect to the Eastern Ghats Orogeny, which has played a significant role in the prominent development of diagenetic features during mesodiagenetic and telodiagenetic processes.
... Arguments against biogenicity via Fischer-Tropsch synthesis Lollar and McCollom, 2006;McCollom and Seewald, 2006), and arguments against biogenicity of stromatolites presented in Lowe (1994) Apex Basalt ~3.46 Microfossils (Schopf, 1983(Schopf, , 1993Schopf et al., 2007;Schopf and Kudryavtsev, 2012), microbial mats and MISS-like structures (Hickman-Lewis et al., 2016). Arguments against biogenicity of fossils and microstructures (Brasier et al., 2002García-Ruiz et al., 2003Wacey et al., 2016) Kitty's Gap Chert ~3.47 Microfossils (Westall et al., 2006b;Westall et al., 2011) (Awramik et al., 1983) Arguments for abiotic formation of carbonaceous materials (Buick, 1984(Buick, , 1990. Alleon et al. (2019) report the presence of Ni-Fe alloys that could catalyze abiotic formation of carbonaceous materials but prefer a biological interpretation Dresser Formation ~3.48 Biologically fractionated δ 13 C (Ueno et al., 2004), segmented carbonaceous threads (Ueno, 2001), microbial induced sedimentary structures (Noffke et al., 2013), microbial fossils (Ueno, 2001;Ueno et al., 2004) and cell walls (Glikson et al., 2008), microbial palisade fabrics (Djokic et al., 2017), stromatolite morphology ( Van Kranendonk, 2011), stromatolites and syndepositional hydrothermalism (Hickman-Lewis et al., 2023) V. K. Zepeda et al. marine environment featuring the periodic influx of hydrothermal fluids (Van Hickman, 2008;Allwood et al., 2010), however, some have suggested deeper water environments . ...
... Siliceous sediments and chert deposits of soda lakes, particularly Lake Magadi and Nasikie Engida, were proposed as models to understand the deposition of Precambrian cherts (Eugster, 1967(Eugster, , 1969Eugster and Jones, 1968;Hay, 1968;Schubel and Simonson, 1990;Behr and Röhricht, 2000;Behr, 2002;Reinhardt et al., 2019) where primitive biological organisms and putative microfossils were reported (Buick, 1990;Schopf, 1993;García-Ruiz, 1994). Selfassembled silica-carbonate biomorphs were proposed as alternative explanations for these primitive organisms and putative microfossils due to morphological reminiscence and the geochemical plausibility of the chemical cocktails used for the synthesis of biomorphs and chemical gardens (García-Ruiz, 1994, 2000García-Ruiz et al., 2002, 2017, 2020. ...
... Carbonaceous matter in Paleoarchean black chert is generally interpreted to be derived from indigenous cellular carbon. Its presence in simple filaments and spheres is considered to be a key factor in assessing the biogenicity of putative fossil-like objects (12,(66)(67)(68). Our results show that some of the carbonaceous matter in the 3.5-Ga cherts is not indigenous but exotic and, because of the lack of organic-rich source rocks, may include abiotically produced carbon. ...
Carbon is the key element of life, and its origin in ancient sedimentary rocks is central to questions about the emergence and early evolution of life. The oldest well-preserved carbon occurs with fossil-like structures in 3.5-billion-year-old black chert. The carbonaceous matter, which is associated with hydrothermal chert-barite vent systems originating in underlying basaltic-komatiitic lavas, is thought to be derived from microbial life. Here, we show that 3.5-billion-year-old black chert vein systems from the Pilbara Craton, Australia contain abundant residues of migrated organic carbon. Using younger analogs, we argue that the black cherts formed during precipitation from silica-rich, carbon-bearing hydrothermal fluids in vein systems and vent-proximal seafloor sediments. Given the volcanic setting and lack of organic-rich sediments, we speculate that the vent-mound systems contain carbon derived from rock-powered organic synthesis in the underlying mafic-ultramafic lavas, providing a glimpse of a prebiotic world awash in terrestrial organic compounds.
... Despite this potential, recognizing microbial microfossils in the rock record remains difficult. One important reason is their small size and simple morphology (Brasier et al., 2002;Buick, 1990). ...
Deep- sea hydrothermal systems provide ideal conditions for prebiotic reactions and an-cient metabolic pathways and, therefore, might have played a pivotal role in the emer-gence of life. To understand this role better, it is paramount to examine fundamental interactions between hydrothermal processes, non- living matter, and microbial life in deep time. However, the distribution and diversity of microbial communities in ancient deep- sea hydrothermal systems are still poorly constrained, so evolutionary, and ecolog-ical relationships remain unclear. One important reason is an insufficient understanding of the formation of diagnostic microbial biosignatures in such settings and their preser-vation through geological time. This contribution centers around microbial biosignatures in Precambrian deep- sea hydrothermal sulfide deposits. Intending to provide a valuable resource for scientists from across the natural sciences whose research is concerned with the origins of life, we first introduce different types of biosignatures that can be preserved over geological timescales (rock fabrics and textures, microfossils, mineral precipitates, carbonaceous matter, trace metal, and isotope geochemical signatures). We then review selected reports of biosignatures from Precambrian deep- sea hydrothermal sulfide deposits and discuss their geobiological significance. Our survey highlights that Precambrian hydrothermal sulfide deposits potentially encode valuable information on environmental conditions, the presence and nature of microbial life, and the complex in-teractions between fluids, micro- organisms, and minerals. It further emphasizes that the geobiological interpretation of these records is challenging and requires the concerted application of analytical and experimental methods from various fields, including geol-ogy, mineralogy, geochemistry, and microbiology. Well- orchestrated multidisciplinary studies allow us to understand the formation and preservation of microbial biosigna-tures in deep- sea hydrothermal sulfide systems and thus help unravel the fundamental geobiology of ancient settings. This, in turn, is critical for reconstructing life's emergence and early evolution on Earth and the search for life elsewhere in the univers
... To unambiguously demonstrate the syngenicity, biogenicity, and affinity of purported prokaryotic microfossils is a significant challenge, particularly in the study of Precambrian micropaleontology because of the poor age constraints, difficulty in stratigraphic correlation, and simple (and sometimes exotic) morphologies of ancient microorganisms. This challenge is highlighted in the debate on the earliest traces of microbial life on Earth (buick, 1990;braSier & others, 2005;braSier & others, 2006;JavauX, 2019). Below, indigenicity, syngenicity, biogenicity, and affinity are discussed separately for clarity purpose, although these are often intimately related. ...
... However, some aspects can be shared among all categories. According to Buick (1990); Schopf et al. (2005) and Schopf et al. (2012), some criteria for life recognition in the fossil record are: (1) age attributed to the rock; (2) insertion of the fossil within the rock, eliminating the possibility of the structure being a later contamination; (3) time of deposition of the rock equivalent to the burial of the putative microfossil or biosignature; (4) inherent characteristics of organisms (remains or signatures) must be present, such as shape, biologically plausible size ranges, frequency, variable degree of preservation, three-dimensional preservation by permineralization composition; and (5) the presence of organic compounds (e.g. kerogen). ...
... For the entire Archean era, indications for life consist mostly of indirect evidence such as chemical traces of light carbon and isotopic signals consistent with microbial metabolisms, and microbially induced bio-signatures, such as tubes, laments, laminae and stromatolites 8, 9,10,11 . Direct evidence of Archean life consists of putative microfossils, forming structures ranging from a few to tens of micrometers in diameter, that have been interpreted as prokaryotic (e.g., bacterial) in origin 12,13,14,15 . ...
Some of the evidence for the existence of life in the Archean Eon (4.0-2.5 Ga) comes from ‘microfossils’ that are believed to be the mineralized remnants of simple prokaryotic lifeforms. They are spheroidal shaped and a few microns to tens of microns across and most likely represent early prokaryotic organisms. Here we describe spheroidal carbon structures that reach sizes of up to 540 microns from the 2.72 billion years ago (Ga) carbonaceous shales from the Lower Nyanzian Formation in the Geita Greenstone Belt of Tanzania. They are interpreted as microfossils based on their multi-chambered structures with pentagonal to hexagonal sectional geometries or floral forms, and chambers connected by regularly placed, tube-like structures that also connect inner chambers to the outside. The carbonaceous material in the walls is partly composed of disorganised kerogenous carbon with δ13C values averaging -44.6‰ that forms membranes coating a denser core of hematite enriched in P and transition metals (Zn, Cu, Ca). The fossils meet all criteria for syngeneity and biogenicity. Whether these structures are remnants of gigantic bacteria or early eukaryotes such as a testate rhizarian is unclear but they appear to represent a relatively complex organism that lived in a shallow, volcanically-dominated, marine environment where oxygen was locally available.
... Precambrian) rocks is whether they are biogenic relics, abiogenic pseudofossils or contaminants of younger age. Over recent decades, various criteria have been developed to evaluate the potential biogenicity of such microfossil-like objects and a number of firm criteria have come to be well established (Schopf and Walter, 1983;Buick, 1990;Westall, 1999;Sugitani et al., 2007;Rouillard et al., 2021). Three critical criteria, (1) indigenousness and syngenicity, (2) biologically acceptable context, and (3) biologically acceptable chemistry, were followed and are served as guidelines for assessing the biogenicity of CLFs from the Dahongyu granules. ...
After the first great oxygenation event (GOE), microorganism communities flourished in various habitats in shallow marine settings as a response to elevated oxygen contents in atmosphere and seawater. Abundant microorganisms have been reported from microbialites and siliceous dolomites and siliciclastic rocks. Here we report a diverse microfossil assemblage from siliceous granules within chert grainstones of the ca. 1.6 Ga late Paleoproterozoic Dahongyu Formation in the Jixian section, North China. These microfossils are confined to granules and absent in inter-granular chalcedony interpreted as cementing matrix. The microfossil assemblage is made up of the five morphotypes: two types of large spheroidal forms, much smaller unicells, unbranched filaments of cyanobacterial affinity, and chain-like filaments (CLFs) composed of hematitic pseudomorphs after pyritization. The CLF possesses a carbonaceous sheath structure which is mostly obscured by previous pyrite crystals, suggesting a potential bacterial origin. The unique morphology and distinctive chemical composition both distinguish these filamentous objects from filaments known in the Archean and Proterozoic worldwide. However, similar CLFs have been found in a variety of lithofacies worldwide, and coexist with other microfossils or microbial mats, implying that they all may represent a once-widespread group of microbes. The hosting granules are interpreted as depositional products that were transported from shallow marine into a deeper ramp setting, and the microfossils and mat debris therefore likely represent a phototrophic community. The inferred high minus-cement porosity and the predominantly grain-supporting texture of these granules signal the early silicification and cementation. Thus, chert grainstones formed in this way may effectively preserve traces of early life and provide snapshots of early microorganisms after the GOE.
... A number of protocols and strategies have been devised to assess the credibility of candidate biosignatures (e.g. Buick 1990;Brasier and Wacey 2012;McLoughlin and Grosch 2015;Vago et al. 2017;Neveu et al. 2018;Rouillard et al. 2021). Most of these schemes use multiple, nested criteria to assess biogenicity: did the object (or population of objects) form in a demonstrably habitable ( palaeo) environment, with appropriate evidence of endogeneity and syngenicity? ...
It is often acknowledged that the search for life on Mars might produce false positive results, particularly via the detection of objects, patterns or substances that resemble the products of life in some way but are not biogenic. The success of major current and forthcoming rover missions now calls for significant efforts to mitigate this risk. Here, we review known processes that could have generated false biosignatures on early Mars. These examples are known largely from serendipitous discoveries rather than systematic research and remain poorly understood; they probably represent only a small subset of relevant phenomena. These phenomena tend to be driven by kinetic processes far from thermodynamic equilibrium, often in the presence of liquid water and organic matter, conditions similar to those that can actually give rise to, and support, life. We propose that strategies for assessing candidate biosignatures on Mars could be improved by new knowledge on the physics and chemistry of abiotic self-organization in geological systems. We conclude by calling for new interdisciplinary research to determine how false biosignatures may arise, focusing on geological materials, conditions and spatiotemporal scales relevant to the detection of life on Mars, as well as the early Earth and other planetary bodies.
Thematic collection: This article is part of the Astrobiology: Perspectives from the Geology of Earth and the Solar System collection available at: https://www.lyellcollection.org/cc/astrobiology-perspectives-from-geology-of-earth-and-solar-system
... [1,2] The mechanism underlying the growth of the typically resulting hollow tubes is known to be a complex interplay of osmosis, buoyancy, and chemical reaction. [1][2][3] These peculiar structures-once wrongly thought to be remainders of living organisms [4][5][6][7] -have recently attracted growing attention, because of multiple coupled chemical fluxes occurring across the tubular membrane upon ripening. Detailed insights into these processes could be gained through the design of a novel procedure [3,8] allowing the growth of single macroscopic tubes, in which both the inner and outer solutions could readily be sampled for time-dependent concentration measurements, and microelectrodes could be immersed for in situ monitoring of pH and potential differences. ...
Calcium carbonate chemical gardens are tubular mineral structures precipitated upon the addition of sodium carbonate solution to calcium chloride pellets. The tubes are composed of texturally distinct bilayers of aggregates of rhombohedral and cone‐shaped calcite on their exterior and interior surfaces, respectively. These highly crystalline and dense bilayers prohibit the development of significant electrochemical potential differences across the tube walls. More information can be found in the Full Paper by M. Kellermeier, J. M. García‐Ruiz et al. (DOI: 10.1002/chem.202101417).
... However, hydrothermal environments also greatly complicate the search for unaltered remnants of life and their subsequent interpretation (Buick, 1990): ...
The Apex chert unit (~3.46 Ga, Pilbara Craton, Australia) constitutes one of the oldest sedimentary units on Earth in which putative carbonaceous microfossils have been reported. The source of carbonaceous matter (CM) in this unit, however, is hotly debated. Hydrothermal fluids have circulated through the underlying crust and up into the bedded unit; these fluids could have remobilized sedimentary microbial biomass, generated abiological hydrocarbons, or harbored in situ chemolithoautotrophic microbial communities. Are there parts of the unit where microfossils might be best preserved? A potential fossil microbiota-if present-would probably be best preserved in the stratiform portion of the unit where hydrothermal influence seems to have been lowest. In order to shed light on the history of hydrothermal overprinting and the source of carbonaceous fractions in the Apex chert, we correlate here at a high spatial resolution petrographic observations and trace element analyses over a transect from the dyke where putative microfossils were found to the stratiform part where remnants of microbial mats were found. The layered, stratiform part of the unit has positive La anomalies up to 1.7, and Light Rare Earth Element depletions, indicating a seawater source. However, as far as 300 m from the dyke, the stratiform part also shows hydrothermal brecciation, high Eu anomalies (2-12; µ = 4.2) and chondritic Y/Ho ratios (24.3-30.3; µ = 27.0), indicating that hydrothermal fluids have laterally infiltrated over large distances. Overall, the pervasive influence of hydrothermal fluids throughout the entire unit and the presence of carbo-naceous matter both in the sedimentary part and the hydrothermal dyke is consistent with a 'hydrothermal pump' model that was earlier proposed for the nearby Dresser Formation. In this model, organic matter from surface environments is circulated along with hydrothermal fluids and redistributed in the crust and overlying sediments, therefore complicating paleobiological interpretations. Raman measurements show that most of the CM experienced temperatures of ~350 • C, while some samples contain CM with a variable, but markedly lower maturity (temperature ranging from 200 to 350 • C). Correlation to texture points out a potential mixing of pre-metamorphic CM with post-metamorphic CM during late hydrothermal events.
... However, hydrothermal environments also greatly complicate the search for unaltered remnants of life and their subsequent interpretation (Buick, 1990): ...
The Apex chert unit (~3.46 Ga, Pilbara Craton, Australia) constitutes one of the oldest sedimentary units on Earth in which putative carbonaceous microfossils have been reported. The source of carbonaceous matter (CM) in this unit, however, is hotly debated. Hydrothermal fluids have circulated through the underlying crust and up into the bedded unit; these fluids could have remobilized sedimentary microbial biomass, generated abiological hydrocarbons, or harbored in situ chemolithoautotrophic microbial communities. Are there parts of the unit where microfossils might be best preserved? A potential fossil microbiota – if present – would probably be best preserved in the stratiform portion of the unit where hydrothermal influence seems to have been lowest. In order to shed light on the history of hydrothermal overprinting and the source of carbonaceous fractions in the Apex chert, we correlate here at a high spatial resolution petrographic observations and trace element analyses over a transect from the dyke where putative microfossils were found to the stratiform part where remnants of microbial mats were found. The layered, stratiform part of the unit has positive La anomalies up to 1.7, and Light Rare Earth Element depletions, indicating a seawater source. However, as far as 300 meters from the dyke, the stratiform part also shows hydrothermal brecciation, high Eu anomalies (2-12; µ = 4.2) and chondritic Y/Ho ratios (24.3-30.3; µ = 27.0), indicating that hydrothermal fluids have laterally infiltrated over large distances. Overall, the pervasive influence of hydrothermal fluids throughout the entire unit and the presence of carbonaceous matter both in the sedimentary part and the hydrothermal dyke is consistent with a ‘hydrothermal pump’ model that was earlier proposed for the nearby Dresser Formation. In this model, organic matter from surface environments is circulated along with hydrothermal fluids and redistributed in the crust and overlying sediments, therefore complicating paleobiological interpretations. Raman measurements show that most of the CM experienced temperatures of ~350°C, while some samples contain CM with a variable, but markedly lower maturity (temperature ranging from 200-350°C). Correlation to texture points out a potential mixing of pre-metamorphic CM with post-metamorphic CM during late hydrothermal events.
... [1,2] The mechanism underlying the growth of the typically resulting hollow tubes is known to be a complex interplay of osmosis, buoyancy, and chemical reaction. [1][2][3] These peculiar structures-once wrongly thought to be remainders of living organisms [4][5][6][7] -have recently attracted growing attention, because of multiple coupled chemical fluxes occurring across the tubular membrane upon ripening. Detailed insights into these processes could be gained through the design of a novel procedure [3,8] allowing the growth of single macroscopic tubes, in which both the inner and outer solutions could readily be sampled for time-dependent concentration measurements, and microelectrodes could be immersed for in situ monitoring of pH and potential differences. ...
Chemical gardens are self‐assembled tubular precipitates formed by a combination of osmosis, buoyancy, and chemical reaction, and thought to be capable of catalyzing prebiotic condensation reactions. In many cases, the tube wall is a bilayer structure with the properties of a diaphragm and/or a membrane. The interest in silica gardens as microreactors for materials science has increased over the past decade because of their ability to create long‐lasting electrochemical potential. In this study, we have grown single macroscopic tubes based on calcium carbonate and monitored their time‐dependent behavior by in situ measurements of pH, ionic concentrations inside and outside the tubular membranes, and electrochemical potential differences. Furthermore, we have characterized the composition and structure of the tubular membranes by using ex situ X‐ray diffraction, infrared and Raman spectroscopy, as well as scanning electron microscopy. Based on the collected data, we propose a physicochemical mechanism for the formation and ripening of these peculiar CaCO3 structures and compare the results to those of other chemical garden systems. We find that the wall of the macroscopic calcium carbonate tubes is a bilayer of texturally distinct but compositionally similar calcite showing high crystallinity. The resulting high density of the material prevents macroscopic calcium carbonate gardens from developing significant electrochemical potential differences. In the light of these observations, possible implications in materials science and prebiotic (geo)chemistry are discussed.
... The abundance of carbonaceous matter in the Gunflint microfossils has led to the general view that relicts of original cellular carbon are important, if not essential, for putative microfossils to be accepted as bona fide biosignatures (Buick, 1990;Schopf et al., 2002Schopf et al., , 2005McLoughlin and Grosch, 2015). The possibility that carbonaceous matter in ancient rocks and microfossils could be derived from, or contaminated by, organic compounds introduced after deposition has been acknowledged (Brasier et al., 2002;García-Ruiz et al., 2003;Lindsay et al., 2005;Lepot et al., 2009;Oehler et al., 2009;Oehler and Cady, 2014). ...
The 1.88-billion-year-old Gunflint carbonaceous microfossils are renowned for their exceptional morphological and chemical preservation, attributed to early and rapid entombment in amorphous silica. The carbonaceous matter lining and partly filling filamentous and spherical structures is interpreted to be indigenous, representing thermally altered relicts of cellular material (i.e., kerogen). Here we show that stromatolitic black cherts from the Gunflint Formation, Schreiber Beach, Ontario, Canada, were saturated in syn-sedimentary oil. The thermally altered oil (pyrobitumen), which occurs in the stromatolites and intercolumn sediments, fills pores and fractures, and coats detrital and diagenetic grain surfaces. The occurrence of detrital bitumen grains in the stromatolites points to the proximity of shallow seafloor oil seeps and hence the possible existence of chemosynthetic microbes degrading hydrocarbons. We suggest that hydrocarbons that migrated through the silicifying stromatolites infiltrated semi-hollow microbial molds that formed following silica nucleation on the walls or sheaths of decayed cells. Upon heating, the hydrocarbons were transformed to nanoporous pyrobitumen, retarding silica recrystallization and enhancing detailed preservation of the carbon-rich microfossils. Hydrocarbon infiltration of silicified microbes offers a new explanation for the preservation of the Gunflint microfossils and may have played a role in the formation of some of Earth's oldest microfossils.
... 26 taxonomic occurrences of (unquestionable?) microfossils have been documented from six Archaean geological provinces (Alterman and Schopf, 1995). Like Archaean stromatolites, nearly all the recorded Archaean microfossils are challenged (Awramik et al., 1983(Awramik et al., , 1988Buick, 1984Buick, , 1988Buick, , 1990Schopf and Packer, 1987;Schopf, 1993;Brasier et al., 2002Brasier et al., , 2005. A plethora of carbonaceous morphologies have been described from the Archaean cherts and interpreted as microbial fossils (Schopf, 1993). ...
Morphological, petrological and isotopoic studies are conducted on the stromatolitic structures recorded from the Kasia mines of Barbil area, a part of the Mesoarchaean sedimentary succession of the Iron Ore Group (IOG) of the Singhbhum Craton (~3500 Ma). Recognized morphotypes include: stratiform stromatolite, tiny cumulate stromatolite, domical stromatolite, pseudo-columnar stromatolite and columnar stromatolites. The stromatolitic structures were chiefly made up of dolomite which is at places replaced by silica. Microbial influence in building of these stromatolitic structures is strengthened by the occurrence of relatively high TOC concentration (0.30 to 1.50 weight% with mean as) compared to TOC concentration recorded in the Archaean rocks (< 1 weight%). The organic carbon isotope signatures (-39.4 to-28.0 ‰ vs PDB) documented in the spatially distinct layers are unique to anaerobic photosynthetic bacteria and methanogen-methanotrophs. Thus, these stromatolitic structures can be termed as representing one of the Earth's earliest ecosystems.
... In stark contrast to the carbonate matrix of the speleothems, in which only indirect indications of microbial activities have been preserved, the siliceous insoluble residua of the coralloids yielded a number of biomorphic bodies entombed in opal, particularly tubular, elongated, ovoid and spheroidal bodies that are interpreted here as siliceous filamentous and coccoid microbes, respectively. The criteria for the identification of fossil microbes have been exhaustively dealt with by Schopf & Walter (1983), Buick (1990) and Westall (1999). The key points important for establishing whether the potential microfossils are indigenous or merely products of contamination are: (i) syngenicity; i.e. the potential microfossil must belong naturally to the host sediment; (ii) biogenicity; i.e. it must be shown that the potential microfossil is of organic nature; and (iii) occurrence; i.e. it should occur in a depositional environment allowing the presence and preservation of microbes (Westall, 1999). ...
Exotic carbonate – siliceous coralloid speleothems of the Koněprusy Caves, which consist of dominant aggregates of feather‐like, radial fibrous, dogtooth and gothic‐arch calcite crystals, contain a diversity of petrified fossil microbes entombed in siliceous parts of the speleothems. Although a complete diagenetic continuum of SiO2 polymorphs, ranging from opal‐A, through opal‐CT, to moganite and crystalline quartz, was identified to form thin irregular laminae and the infills of secondary pores throughout the speleothems, the microbes have been preserved only in opal aggregates concentrated in younger growth zones of the speleothems. The identified biomorphic bodies included ovoid and spheroidal forms, interpreted as coccoid microbes, and tubular, bent and elongated forms believed to represent putative silicified filamentous microbes of unknown taxonomic affinity. Other biomorphic microstructures resembling biofilms, and plastic deformations and binary fissions of individual coccoid microbes have also been recognized. The silicified microbes, most of which have been heavily encrusted in situ, still reveal the presence of organic carbon and other biogenic elements detectable beneath their opal coats. Micro‐cavities beneath and around coccoid microbes, interpreted as micro‐borings, suggest that the microbes were able to remove the opal substrate by chemical etching. The morphology and mineralization styles of the microbes, the age of which was estimated to be in the range of 10⁰ to 10² ka, exhibit similarities to silicified microbes from present‐day siliceous hot‐spring geysers and travertines. In contrast to the siliceous parts of the coralloids, the calcite crystals forming the matrix of the speleothems, do not contain any calcified microfossils. However, deeply etched calcite crystals, spiky calcite sub‐crystals, needle‐fibre calcite and sparmicrite grains indicate that the processes of calcite dissolution–precipitation mediated by the microbes may have also affected the speleothem exteriors.
... Stromatolites form when photosynthetic microbes trap sand and sediment to create successive layers, which can then be trapped in the rock record (Fig. 2). Stromatolites have been discovered in the Strelley Pool Chert in the Pilbara region of Western Australia, and constitute some of the best evidence of an early terrestrial biome at 3.43 Ga (Walter et al., 1980, Buick 1990, Van Kranendonk et al., 2008. The similarities in age between the Strelley Pool stromatolites and the olivinecarbonate lithology are striking. ...
We discuss the results of a remote sensing study that has revealed new details about an important rock unit dominated by two minerals that can be associated with volcanism (olivine) and life (carbonate). The study, which used a new analysis technique on CRISM data, identified a region where no carbonates or clays are present, only large grain size olivine. This discovery shines new light on the formation and history of the olivine-carbonate rock within Jezero crater that will be explored by the Mars 2020 rover.
... These filaments, ∼2.0 μm wide and ∼30-200 μm long, are typically tubular, non-septate and unbranched. The tubular nature of the carbonaceous filaments, the existence of a "hollow" lumina shown in Video S-1, and the range of widths exhibited by the filaments, are consistent with a biological origin (Buick, 1990). Morphologically, they resemble microbial filaments found in zeolite-carbonate interfaces in subseafloor basalts (Ivarsson et al., 2008). ...
The known deep subsurface biosphere on Earth persists in diversified habitats, including deep within igneous rocks of the oceanic crust. Here, we extend the range of the deep subsurface biosphere to metamorphic ocean crust of a subduction zone. We report fossilised life in zeolite facies rocks, which formed by low grade metamorphism, from the southern Mariana trench. Dense carbonaceous spheroids, filaments, and Frutexites-like structures are preserved in these rocks, which are enriched in organic carbon but depleted in 13C. The distinct difference in the GDGT-0 vs. crenarchaeol and the branched vs. isoprenoid tetraether values between the inner and outer portions of these rocks indicate the in situ production of organic carbon. We
demonstrate that these structures may result from the past activity of potential chemolithoautotrophs within the metamorphic crust, as implied by their morphologies, Raman spectra, carbon isotopes, and biomarker signatures, as well as the Fe oxidation state within whole rocks. We propose that fluid-rock reactions at temperatures within the tolerance of life during low grade metamorphism contributed to microbial subsistence within the biotope. The low grade metamorphic ocean crust of the subduction zone likely represents Earth’s deepest, and one of its largest, microbial ecosystems, which may potentially influence the deep carbon cycle.
... A carbonaceous composition and the thermal maturity of organics are an important criterion for interpreting filaments embedded in silica as biosignatures (Buick, 1990;Schopf et al., 2007). However, the preservation of organic carbon in iron oxides is not thermodynamically favored, and organic carbon can decay quickly in oxidizing environments (Cady and Farmer, 1996;Walter et al., 1996Walter et al., , 1998Farmer, 1999;Sumner, 2004;Hofmann et al., 2008). ...
Iron and silica systems constitute widely recognized preservation agents for mineralized biosignatures in biogeochemical environments spanning from deep-sea hydrothermal vents and metal mineralization to terrestrial sulfide mineralization subjected to supergene oxidation/weathering (gossan) (e.g. Cady and Farmer, 1996; Westall et al., 2006; Peng and Jones, 2012; Williams et al., 2015).
Here we employ mainly imaging using SEM at various resolutions with secondary electron (SE) technique, combined with energy dispersive spectroscopy (EDS) and XRD, in order to describe and evaluate morphological characteristics of mainly Fe-mineralized bio-morphous microstructures, i.e. filamentous, coccoidal and rod-shaped, as putative biosignatures, and related abiotic crystalline solids, in supergene oxidation zones of carbonate-replacement Pb-Ag-Zn mineralization, Lavrion District, Attica, Greece (Skarpelis and Argyraki, 2009).
Underground sampling included solids, streams of gravity-driven material in water flow and acid mine drainage water samples from the old underground mines of Ilarion (Mine No.50), Serpieri, Jean Baptiste (Mine No.1) and Christiana shaft (Mine No. 132) at Kamariza. Surface sampling included solids of the oxidized ore and efflorescent salts, just outside the shaft No. 160 in Plaka, Lavrion. Based on the mineralogy and mineral chemistry, the underground samples (solids and suspended particulate material) have been classified as members of the "Zone of iron oxides containing secondary ore minerals" (mainly secondary minerals of Zn, Cu and As) while surface gossan belongs to the "Zone of leached iron oxides" with prevailing goethite (Blain and Andrew, 1977).
Microscale crystal habits and mineral textures identified include: bladed (devilline, gypsum), tabular (chonichalcite, scorodite), platy (chonichalcite, muscovite, gypsum), prismatic (gypsum), rhombohedral - scalenohedral (smithsonite, hematite), orthorhombic (scorodite), wedge-like prisms (adamite), polygonal (scorodite), reniform - botryoidal (hematite, goethite, malachite), acicular (goethite, chenevixite), "orzo-like" (conichalcite), rosette-arranged (unidentified arsenate), dipyramidal (scorodite, adamite), bundled (halotrichite group minerals), fibrous (halotrichite group minerals), radiating (goethite), stalactitic (goethite), columnar (gypsum), monoclinic (gypsum) and drop/film-like efflorescences.
A number of filamentous, coccoidal and rod-shaped morphological microstructures were identified with obvious differences in structural organization from the surrounding crystal matrix and were interpreted as potential microfossils. Specifically they were interpreted as microbial filaments, coccoids and rods (bacillus), possibly of acidophilic iron-oxidizing bacteria and fungi. Additionally their associations were described as microbial colonies and the multispecies associations as microbial biofilms; and all these features were associated with EPS (extracellular polymeric substances), according to the criteria used by Westall et al (2006). The microbial filaments identified were divided into three categories: 1) Plexi of twisted filaments likely of fungal origin, 2) Multi-branching filaments connected with EPS and 3) Smooth tubular filaments, curvilinear or erect in various sizes, with the largest filament 96 κm long and the smallest just 4 κm . This interpretation resulted from the fulfillment of specific criteria for potential biogenicity (Westall et al., 2006; Williams et al., 2015) which include the geological context (i.e. gossan) that is highly plausible for microbial life and the similarity of the microstructures identified, to biological morphologies in terms of size, shape and morphological characteristics, i.e. cell division, sinuosity, shrinkage, degradation and preserved central filament lumen.
EDS spot analysis made on the different categories of filaments identified, detected only Fe and O. This is interpreted as fossilization possibly associated with Fe3+- (oxy) hydroxide coating as a result of precipitation and nucleation on preexisting bare microbial filaments (Williams et al., 2015). Secondly, the diversity and frequency of biomorphs identified in Plaka surface samples was much wider than in the underground samples in Kamariza mines. Therefore it seems that preservation depends on the level of natural leaching and it is facilitated by rising leaching of iron oxides in gossan. Finally, based on morphological similarities between our putative microfossils and those from Brick Flat Gossan, Iron Mountain, California (Williams et al., 2015), and if the Lavrion microstructures are proven biogenic on the basis of future work involving joint evidence from mineralogy, chemistry, and morphology, the Lavrion gossan may serve as a mineralogical analogue to some ancient martian environments.
... The organic matter is highly subjected to hydrothermal alteration of younger fluid circulation (Buick, 1990), but the crystal morphology surrounding microfossils may be relevant for verifying whether the host rocks have undergone post-depositional processes. ...
La Série sédimentaire du Francevillien âgée de 2,1 milliards d’années du Gabon a fait l’objet de plusieurs investigations notamment à des fins économiques en lien avec les gisements uranifère et manganésifère. Ceci a également promu de nombreuses recherches pour reconstruire les paléoenvironnements et la paléobiodiversité du bassin de Franceville. Les sédiments abritent les plus anciens macrofossiles de tailles et de formes variées, ainsi que les traces laissées par des organismes mobiles, mettant en valeur l’enregistrement grandissant des formes primitives complexes et organisées au Paléoprotérozoïque. Cependant, le style de vie microbien, qui a émergé plus d’un milliard et demi d’années avant la sédimentation du Francevillien, a été peu décrit. Une étude multi-approches et pluridisciplinaire a révélé une grande diversité de structures liées aux voiles (MRS). Ces communautés microbiennes étaient principalement construites par des Cyanobactéries oxyphototrophiques qui ont prospéré dans des environnements peu profonds et dans la zone photique. Etant donné que ces bactéries peuvent avoir produit de grandes quantités d’oxygène très localement, ceci explique à priori la présence rependue de formes de vie avancées à proximité des MRS. Les structures fragiles bactériennes ont ensuite été analysées d’un point de vue minéralogique et géochimique. Les analyses montrent un assemblage minéralogique argileux riche en potassium (K) localisé dans les MRS mais inexistant dans les sédiments encaissants sous-jacents constitués de grès et d’argiles riches en matière organique (black shales). Cela suggère un piégeage des cations K+ par les MRS. Ce K, qui provient de l’eau de mer, a été ensuite relargué dans l’espace interstitiel pendant la dégradation de la matière organique, permettant ainsi la néoformation argileuse riche en K. Ceci confirme l’enrichissement potassique induit par des microbes. En ce qui concerne la teneur en éléments traces (TE) dans les MRS, aucun enrichissement en lien avec les microorganismes a été observé. La concentration de certains TE dans les MRS est plus élevée que celle du sédiment encaissant, mais des facteurs physiques environnementaux et non biologiques pourraient avoir causé ces enrichissements. Les données du redox local de l’eau de mer pour le sédiment encaissant montrent que le milieu de dépôt se traduit par des fluctuations des conditions redox (oxiques/suboxiques). Les signaux isotopiques du carbone organique et de l’azote de la roche totale sont similaires dans les structures bactériennes et le sédiment encaissant. La composition des isotopes du carbone suggère l’occurrence d'un recyclage secondaire d’un matériel carboné dérivé de la photosynthèse. Par ailleurs, les isotopes de l’azote indiquent une limitation azotée où les fixateurs de l’azote n’ont pas efficacement compensé la perte de ce dernier. La fixation de l’azote dans la colonne d’eau aurait été passagère et potentiellement contrôlée par la structure redox de l’océan, tandis que cette voie métabolique associée aux MRS est vraisemblablement commune au royaume des voiles benthiques à travers l’histoire de la Terre. L’ensemble de ces résultats soulignent la manifestation fréquente du mode de vie bactérien dans la série du Francevillien et révèlent si les microbes ont laissé des biosignatures spécifiques. La conservation exceptionnelle des MRS en association avec les macrofossiles du Gabon représente un écosystème marin unique à la suite de la première montée significative de la teneur en oxygène dans l’atmosphère terrestre.
... The organic matter is highly subjected to hydrothermal alteration of younger fluid circulation (Buick, 1990), but the crystal morphology surrounding microfossils may be relevant for verifying whether the host rocks have undergone post-depositional processes. ...
The 2.1-billion-year old (Ga) Francevillian Series in Gabon has been intensively studied because of economic interests in their uranium and manganese ore deposits. This also promoted numerous scientific investigations to reconstruct the palaeoenvironments and palaeobiodiversity of the Francevillian basin. The sediments host the oldest reported macrofossils of various sizes and shapes, and traces left by motile organisms, highlighting the growing record of early complex forms in the Palaeoproterozoic. However, the microbial lifestyle, which emerged more than a billion and a half years before the Francevillian deposition, has been poorly described. Through a combination of analytical technics, a large diversity of mat-related structures (MRS) has been observed. These microbial communities were mainly built by oxyphototrophic Cyanobacteria that thrived in shallow water environments within the photic zone. Considering that these bacteria may have locally produced higher amount of oxygen than in the oxygen-stressed water column, this likely explained the widespread presence of advanced forms of life in the vicinity of MRS. The delicate bacterial structures were then analyzed for their mineralogical and geochemical compositions. A distinct potassium (K)-rich clay assemblage characterizes the MRS, but not the underlying host sandstone and black shale sediments. It suggests that the MRS trapped K+ from the seawater and released it into the pore waters during degradation of organic matter, resulting in K-rich clay neoformation. This confirms the microbially induced K enrichment in the geological rock record. However, the trace element (TE) content in the MRS does not reveal particular microbially mediated enrichments. The concentration of some TE in the MRS is higher relative to that of the host sediments, but physical environmental factors may overwhelm any potential biological signal. The local seawater redox data for the host sediments show that the depositional setting reflects fluctuations in redox conditions (oxic/suboxic). The organic carbon and bulk nitrogen isotopes between the bacterial structures and host sediments are mostly similar. The carbon isotope composition suggests the occurrence of secondary recycling of photosynthetically derived carbonaceous material, while the nitrogen systematic points to a nitrogen limitation by which the N2 fixers did not sufficiently replenish the nitrogen loss. The nitrogen fixation in the water column would have been transient and likely controlled by the ocean redox structure, whereas this metabolic pathway in the MRS is likely common to the realm of benthic mats over Earth’s history. Combined, these results underline the common occurrence of bacterial lifestyle in the Francevillian Series and reveal whether the microorganisms left typical biosignatures. The exceptional conservation of the MRS in association with the Gabonese macrofossils represents a unique marine ecosystem in the aftermath of the first significant rise of oxygen content in Earth’s atmosphere.
Photosynthesis is a very old process on this Earth. Based on fossil discoveries and chemical evidence, cyanobacteria first appeared 2.5–2.6 billion years ago (bya). Their evolution was undoubtedly continued by a number of anaerobic, photosynthetic bacterial life forms. Carbon isotope data revealed that autotrophic carbon fixation may have begun at least a bya. It is unclear, nevertheless, what the earliest photosynthetic organisms were like. The primary elements of the photosynthetic system are the carbon fixation mechanism, electron transport complexes, antenna complexes, and reaction centers. It is most likely true that these components have not all evolved at the same point in time. Consequently, it is better to think of the photosynthetic apparatus as a mosaic made up of numerous structural components, each with its own unique evolutionary background. One early instance of a cyanobacterium’s endosymbiotic absorption by a heterotrophic organism appears to have been the source of the chloroplasts seen in yellow-green algae, glaucophytes, brown algae, cryptophytes, red algae, and other algae in the “red” line of development. The variety of species present in the algae’s “red line” is the outcome of a single secondary endo-symbiotic occurrence in which an organism resembling red algae was ingested by another eukaryote. This “red line” is further expanded by tertiary (third-level) endosymbiotic events. Photosynthetic units are found in reaction centers involving complexes for gathering light. Two of these units are necessary for oxygenic photosynthesis, which currently accounts for the majority of biological transfer of energy in the various trophic levels of the biosphere. The emergence of photosynthesis utilizing oxygen among cyanobacteria, which paved the path for the formation of complex life forms with multicellular levels of organization, had a profound influence on the biology, geology, and environment of Earth. In this review, we have discussed the early evidence of photosynthesis, the origin of reaction centers, antenna, pigments, and how oxygenic photosynthesis came into existence. The origin of the chloroplasts is a necessary event that occurred earlier and was added to the history of photosynthetic origin in this review.
Minerals are the fundamental record of abiotic processes over time, while biominerals are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging due to the superimposi-tion and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, being potential outcomes of both abi-otic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria.
Minerals are the fundamental record of abiotic processes over time, while biominerals, are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging because the record is defined by the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, located on the threshold between abiotic and biotic, can help resolve this issue. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils. While this material has different forms and refers to biological aspects, it is difficult to associate it with any known Fossil Group due to the overlapping geological processes that occur in the Rio do Sul Formation (Pennsylvanian of the Paraná Basin), very close to the contact from a sill of the Serra Geral Group – Lower Cretaceous with a proven thermal effect. The samples were described using a protocol that evaluated: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition and 4) context, assessing indigeneity and syngenicity, and comparing them with abiotic and biotic products. Despite conducting an extensive comparison with abiotic minerals, as well as controlled, induced, and influenced biominerals, no more probable hypothesis was found, excluding the possibility of it being a controlled biomineral due to its patternless diversity of forms and the purely thermometamorphic origin due to the branched elongated form. The occurrence of these structures suggests a complex history: a syndepositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, siderite), which may be linked to the presence of microbial mats, could have served as a template for mineralization and possibly mediated mineral growth. Mesodiagenesis could have also modified the occurrence, but the main agent responsible for its formation was the Cretaceous intrusion, which dissolved and replaced the initial mineral and precipitated calcite, resulting in the dubiofossil. Throughout these steps, physical-chemical and biological reactions, aided by the intrinsic characteristics of the matrix, amount of organic matter, and distance from contact with the intrusive body, may have increased the morphological complexity. This material illustrates how dubiofossils can be the result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.
The evolutionary history of early prokaryotes is recorded in Paleoproterozoic sedimentary rocks. The ca. 1.88 Ga Gunflint Formation is considered key to constrain the course of Paleoproterozoic microbial evolution. However, whether the multicellularity of prokaryote and eukaryote was already present by the Gunflint age remains uncertain. Here, we report novel morphotypes of prokaryotes including colonial, ellipsoidal, spherical, with intracellular inclusions (ICIs), spinous-type, and tail-bearing type, in the Gunflint stromatolitic chert. Biogenicity of such morphotypes was indicated based on their unique microstructures with the parallel C, N, and S distributions and lack of evidence of their post-depositional artifact origin. The new finding of colonial-type microbes in the Gunflint Formation indicates global flourishment of the colonial-type in this age. Moreover, unknown spherical cell-like structures with ICIs were identified, along with microfossils bearing strong similarities to cyanobacterial akinetes. ICIs were more enriched in N-bearing organic compounds than cell wall organic matter. Those ICIs were interpreted as biological contracted protoplasts. These new findings suggest that Paleoproterozoic prokaryotes were more diverse and complex than previously considered and had already acquired adaptability to survive drastic environmental changes. Furthermore, the protruding appendages in the novel spine- and tail-bearing type microfossils likely provided them with advantages in nutrient access and motility respectively, resulting in the promotion of the intercellular interactions. This suggests that functional evolution toward eukaryotes had already started in the Gunflint age.
One of the most compelling objectives in biological sciences is to determine and understand the distribution of life beyond Earth. In this chapter, I discuss the standards of evidence that astrobiologists might use in looking for extraterrestrial life. I use this as an opportunity to address some common misperceptions. They include the idea that astrobiology is only concerned with alien life, the notion that astrobiologists are “hunting” for alien life, and the self-evident but frequently ignored point that all good scientific hypotheses are testable and falsifiable, but not all hypotheses that are testable and falsifiable are good hypotheses. This has particular application to the challenge of advancing credible hypotheses about the search for extraterrestrial life. One purpose of this chapter is to stimulate in the mind of the reader with some thoughts about how we go about claiming evidence for extraterrestrial life, the standards we apply to that endeavor, and how they sit within the more general principles of the scientific method.
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......
An Early Precambrian micaceous metaquartzite subjected to low to moderate metamorphism in the Isua area of Southwestern Greenland was derived from the erosion of preexisting rocks which were probably sialic in composition. This metaquartzite may have been formed before the emergence of life. It contains globular particles of graphite arranged in narrow veins or along foliation or bedding planes. This rock contains no organic compounds besides traces of methane and no biologically significant elements associated with the graphite microstructures. Reaction of primitive methane with ferric oxides appears to have oxidized the methane to the vein graphite and reduced the ferric oxides to ferrous-ferric oxide (magnetite). The graphitic microstructures are likely to be abiotic in origin, although a biological origin is not impossible. Somewhat younger microstructures found in other locations on earth have often been described as microfossils; this origin should be reexamined on the basis of the above mentioned conclusions.
Ambient pyrites of two distinct types were described from middle Precambrian rocks of the Lake Superior area. A new class of this phenomenon is here described from middle Precambrian chert from western Australia. The newly found ambient pyrites are quite minute and characteristically occur in groups forming a "starburst" pattern. All three types of ambient pyrite may be explained in terms of pressure solution initiated by gas evolution from organic material attached to the pyrite. Thermal degradation of the kerogen produces the gases which, due to the impermeability of the encompassing chert, build up the pressures necessary to initiate solution. Pyrite appendages bear a striking resemblance to micro-organisms and, thus, constitute the smallest pseudofossils known.
The beginnings of biospheric evolution had far-reaching biogeochemical consequences for the related evolutions of atmosphere, hydrosphere, and lithosphere. Feedback to the sedimentary record from these several simultaneously interacting aspects of crustal evolution provides the evidence from which historical biogeology is reconstructed. The interpretation of that evidence, however, is beset with pitfalls. Both biogenicity and a primary origin need to be demonstrated, or confidence limits established for each supposed morphological and biochemical fossil. Relevance to biospheric or related evolutions must be critically evaluated for every geochemical and sedimentological anomaly.
Indirect evidence suggests primitive, oxygen-generating autotrophy by ∼ 3.8 × 10 ⁹ years ago (3.8 Gyr or gigayears), while free O 2 first began to accumulate only ∼ 2 Gyr ago. Various reduced substances in the atmosphere and in solution functioned as oxygen sinks, keeping photolytic and biogenic O 2 at levels tolerable by primitive anaerobic and microaerophilic procaryotes.
The oldest demonstrably biogenic and certainly primary microstructures are procaryotes from ∼ or > 2 Gyr old strata around Lake Superior. Improved biologic O 2 mediation, continued carbon segregation, and filling of O 2 sinks initiated atmospheric O 2 buildup, leading to an ozone screen ∼ or < 2 Gyr ago. Consequences were essential termination of banded iron formation, onset of red beds, and O 2 shielding of anaerobic intracellular processes, heralding the eucaryotic cell.
Probable eucaryotes appear in ∼ 1.3 Gyr old rocks in California as large unicells and large-diameter, branched, septate filaments. Likely consequences of eucaryotic evolution were increased atmospheric O 2 , increased carbonate and sulfate ion, and the rise of sexuality. Meiosis had definitely evolved > 0.7 Gyr ago and probably > 1.3 Gyr ago, perhaps simultaneously with the mitosing cell. Whatever the timing, it completed the evolution of the eucaryotic heredity mechanism and foreshadowed (given sufficient free O 2 ) the differentiation of tissues, organs, and advanced forms of life—with all their potential for biogeochemical feedback to sedimentary, diagenetic, and metallogenic processes. The first Metazoa appeared ∼ 0.7 Gyr ago. Being dependent on simple diffusion for O 2 , they lacked exoskeletons. The latter appeared, perhaps 0.6 Gyr ago, when increasing O 2 levels favored the emergence of more advanced respiratory systems.
In view of the continuing controversy about the biogenicity of structurally preserved carbonaceous remains in Archaean rocks, carefully selected samples from the Onverwacht Group, 3.355 X 109 years old, of South Africa, have been subjected to stringent morphological and statistical tests. The same tests have been applied to other samples from rocks of similar lithology that have undergone comparable geological histories from the Late Precambrian part of the Dalradian Supergroup and the Ordovician Ballantrae Group, both from Scotland. The morphological criteria applied are based on those outlined by Cloud and Licari (1968, Abstr. Geol. Soc. Amer. Ann. Meet. Mexico City), and the statistical tests are based on those formulated by Schopf (1975, Origins of Life, in press). 57 criteria used in this paper, assemblages from all three suites of rocks appear to be biogenic, and a consideration of the geological evidence supports this contention.
The supposed worm burrows from the South Stack Group have been interpreted as dewatering structures. The stromatolites from the melange are identified as the Vendian- Cambrian form Georninia, and microfossils from the Llanddwyn Spilitic Group give a Lower Cambrian age for theGwna Group.
In the photic zone of the open sea, procaryotic cells with an ultrastructure typical of chroococcoid cyanobacteria and similar sized populations of autofluorescent bacteria (0.5–1.0 × 1.0 µ m) were observed at concentrations usually between 10 ³ –10 ⁴ cells per ml. The ubiquitous ultrastructural type was an orange‐autofluorescing, phycoerythrin‐containing cyanobacterium amenable to culture. Two other morphological types which have not been cultured occur sporadically in larger concentrations, one apparently favoring deeper water. These phototrophs can account for about 20% of the total bacterioplankton biomass and from 6 to 15% of the total microbial plankton. These cells may play a significant role in oceanic primary productivity.
Experimental data concerning the effects of diagenesis and fossilization on soft-bodied microorganisms (particularly algae preserved in siliceous sediments) have been obtained through systematic monitoring of morphological and organic chemical changes in filamentous blue-green algae (Lyngbya majuscula, Oscillatoriaceae) during simulated fossilization in synthetic chert. With increasing time and temperature, there were tendencies toward reddening, darkening, and fragmentation of algal filaments, reduction in the size of algal cells and sheaths, destruction of micellar order in sheaths (with consequent loss of bire-fringent character), coalescence of trichomes, and destruction of intracellular components with preferential preservation of sheaths and cell walls. In addition, some algal filaments were damaged by growth of quartz spherulites, producing artifactual morphologies that appear to have natural counterparts in Precambrian fossiliferous cherts. Original algal hydrocarbons, and particularly normal alkanes, were relatively stable, but several previously absent hydrocarbons were produced through decomposition of other organic compounds. Isoprenoid compounds, chlorins, and probably porphyrins were derived from the degradation of algal chlorophyll. The δC13PDB value of algal carbon was unaffected in most experiments but decreased by 3‰ under extreme time and temperature conditions. Results of this study suggest trends in the morphological degradation of naturally silicified algae, demonstrate that certain morphologies are artifacts of the silicification process, and suggest possible explanations of observed organic chemical trends in recent and ancient sediments.
The H.Y.C. Pyritic Shale Member of the Barney Creek Formation (ca 1 500 my old; northern Australia) contains several stratiform base metal sulfide deposits of economic significance. Black cherts within these mineral deposits preserve a diverse assemblage of bacterial and algal microfossils. The assemblage differs from most other Precambrian biotas so far described in that it was deposited in deep water, it is not associated with stromatolites or algal mats, and it is dominated by filamentous bacteria, most of which are pyritized. Analysis of the assemblage suggests that the depth of the depositional basin exceeded that of the photic zone, that the bacteria inhabited the basin floor where they maintained anoxic conditions through heterotrophic degradation of detrital organic matter, and that the algae inhabited overlying near surface waters. Most of the algal fossils have been assigned to the Cyanophyta, although two of the described species are potentially referable to the eukaryotic green or red algae. Differences between this assemblage and other biotas described from the McArthur Group suggest that a workable system of biostratigraphic zonation for the Group is feasible.Fossils in the H.Y.C. assemblage are here referred to 21 species and 16 genera, of which 14 species and 6 genera are new. The new taxa are: Bacteria, Biocatenoides incrustata sp. nov., B. pertenuis sp. nov., Ramacia carpentariana gen. et sp. nov., Coleobacter primus gen. et sp. nov., Ferrimonilis variabile gen. et sp. nov.; Chroococcales (Cyanophyta), Nanococcus vulgaris gen. et sp. nov., Bisacculoides tabeoviscus gen. et sp. nov., B. vacua gen. et sp. nov., B. grandis gen. et sp. nov.; Nostocales (Cyanophyta), Oscillatoriopsis schopfii sp. nov., Cyanonema inflatum sp. nov., C. minor sp. nov.; Incertae sedis, Clonophycus elegans gen. et sp. nov., Globophycus minor sp. nov. In addition, the new combination Gunflintia septata (Schopf) is proposed.
Microbial contaminants may be introduced on outcrop as well as en route to or in the laboratory. Micropseudofossils may be natural or man-made. It is possible to recognize such misleading objects and important that they are not allowed to dilute the growing record of authentic pre-Phanerozoic life. Filamentous microbial contaminants from minute cracks in samples of ancient carbonate rocks from Brazil (perhaps 1 Ga old) and South Africa (∼2.3 Ga old) are similar to occurrences previously described as fossils. Published records of supposedly Archean microbial life also include microcontaminants and laboratory artifacts. Although microstructures from sedimentary rocks of the Swaziland system could be fossils, they are not demonstrably so. The oldest structurally preserved fossils yet known seem to be the filaments described by Lois Nagy from stromatolitic limestone in the ∼2.3 Ga old Malmani Dolomite of South Africa. It will be difficult to establish unequivocal older records in the absence of definitive ultrastructural or micro-chemical evidence.
A diverse assemblage of organically preserved unicellular and filamentous microorganisms has been discovered in cherty portions of calcareous, domical, Collenia-like stromatolites from the approximately 1400 Ma-old Vempalle Formation (Papaghni Subgroup, Cuddapah Group) of south-central India. Evidence suggests that this apparently cyanophyte-dominated community, the oldest stromatolitic microbiota now known from the Indian subcontinent, may contain eukaryotic unicells; if so, these fossils would appear to be among the oldest eukaryotes yet reported. Comparison of the Vempalle assemblage with microbiotas preserved in similar stromatolites from other Precambrian units seems to indicate (as recently suggested for stromatolites of the group Conophyton) that Collenia-like stromatolites of differing ages were probably built by differing microbial communities. If such is typical of most stromatolite groups, it seems unlikely that identification of stromatolites at the group-level can provide sufficient grounds for effective subdivision and correlation of Precambrian strata; analyses of such structures at the form-level, however, and of the microorganisms involved in their deposition, could prove to be biostratigraphically useful.
Pseudomicrofossils and authentic microfossils occur in the approximately 1.3 Ga-old Appekunny Argillite, lower Belt Supergroup, Glacier National Park, Montana. These microstructures occur in finely-laminated, dark gray and black mudstones that were deposited in an offshore environment and were subsequently subjected to moderate burial metamorphism. The pseudomicrofossils consist of 0.1–0.3 μm thick organic envelopes that encapsulate 1–20 μm diameter authigenic crystals. These crystals, petrographically identified as apatite (Ca5(PO4)3OH, F, Cl) occur isolated, in clusters, and in filiform aggregates up to 200 μm long. The organic envelopes apparently originated through the accretion, around these crystals, of organic matter which was finely disseminated throughout the sedimentary matrix.The authentic microfossils are poorly preserved sphaeromorphs which, unlike the pseudomicrofossils, are highly compressed parallel to lamination. These microfossils are several tenths of a micrometer to ca. 1 μm thick and range from 22–82 μm long and 17–71 μm wide. They commonly exhibit tears and folds that evidently were produced during compaction of the enclosing argillaceous matrix. Due to poor preservation, these specimens cannot be assigned to known Proterozoic sphaeromorph genera.Inasmuch as the pseudofossils could be misinterpreted as microfossils, they well illustrate the need to exercise caution when interpreting organic microstructures preserved in Precambrian strata. Not being compressed parallel to lamination, the pseudomicrofossils are readily distinguished from the authentic microfossils which are strongly compressed parallel to lamination. The authentic microfossils demonstrate that organic-walled microfossils can be preserved in strata subjected to moderate burial metamorphism, and they provide information useful for recognizing and interpreting microfossils in similarly metamorphosed argillaceous strata.
The Isua supracrustal belt consists of metamorphic rocks, including
metamorphosed sediments, and are the oldest known rocks on Earth. It has
been postulated1,2 that hydrocarbons recently detected in
these rocks are remnants of organisms that lived ~3,800 Myr ago. This
popular interpretation is inconsistent with the high-temperature history
of the Isua rocks. Various types of hydrocarbons were liberated from
these rocks after repeated solvent extraction by matrix dissolution or
pyrolysis. However, the Isua rocks have been metamorphosed to the upper
greenschist or amphibolite facies (400-600 °C) the metamorphic
episodes lasted >106 yr. Consequently, it is very
important that the origins of the key organic constituents be
established. In the study reported here both hydrocarbons and amino
acids were detected in Isua rock samples. In addition to the common
biological amino acids, geologically unstable amino acids and
biologically uncommon ones were detected (such as sarcosine). The extent
of amino acid racemization suggests the apparent continuous diffusion of
biochemicals into the Isua rocks from encrusting lichens since the end
of the last ice age. Also, the cold-temperature history (which retards
racemization) suggests that the amino acids may be modern to a few tens
of thousands of years old. n-Alkanes lacked odd/even carbon chain
preference which may indicate some antiquity. However, the hydrocarbons
also resemble a recent petroleum distillate fraction. Laboratory
experiments, supported by kinetic and thermodynamic calculations, show
that amino acids and aromatic and saturated aliphatic hydrocarbons,
including pristane, could not have survived the known metamorphic
history of the Isua rocks.
Massive yields of discrete groups of highly structured morphological entities are formed by sparking methane and nitrogen over a water surface. These structures were studied by light microscopy and by transmission and scanning electron microscopy. Chemical studies suggest the presence of a cross-linked kerogenous polymer matrix. These findings suggest that organized elements in carbonaceous chrondrites and microstructures in Early Precambrian sedimentary rocks appear to be abiological organic particles which are precursors to the first living cell.
Well preserved filamentous microfossils (Siphonophycus transvaalensis n. sp.) are described here from the carbonate (Campbellrand Subgroup) to iron-formation (Kuruman Iron Formation) transition of the Transvaal Supergroup, South Africa, estimated to be 2.5-2.3 Ga years old. The microfossils occur in petrographic thin-sections of a core sample of carbonate-chert. They are preserved by permineralization in both chert and in sparry ferroan dolomite. Stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence (Campbellrand) to the overlying iron-rich sediments of the Kuruman-Griquatown Iron Formations. The average δ13C value of the kerogen in the sample is about - 36.9%.
The word ‘stromatolite’ should only be applied to organosedimentary structures predominantly accreted by sediment trapping, binding and/or in situ precipitation as a result of the growth and metabolic activities of benthic, principally prokaryotic, micro-organisms. Structures of uncertain origin that resemble stromatolites should be called ‘stromatoloids’. This cautious approach would eliminate the currently common assumption that structures with mesoscopic morphological similarities to microbially accreted sedimentary structures must be biogenic, a misconception that hampers investigations into the antiquity of life.A hierarchical series of meso- and microstructural attributes of stromatolites can be used to assign gradually increasing probabilities of biogenicity to stromatoloids. This method is particularly useful for interpreting ancient noncolumnar stromatoloids with poor microstructural preservation. In a range of Early Archaean pseudocolumnar, nodular and stratiform stromatoloids from North Pole studied using this method, none could be proved to be stromatolites and only a few are probable or possible stromatolites. As these stromatoloids closely resemble previously reported structures from North Pole interpreted as stromatolites, we consider that the evidence for the existence of life c. 3500 my ago at North Pole is less definitive than previously supposed.
THE oldest documented microfossils are from the ~3,300 Myr BP Onverwacht Group of South Africa6–8. Here, we discuss the occurrence of a new assemblage of microfossils from a ~3,500 Myr BP silicified shallow-water to supratidal carbonate sequence of the Warrawoona Group at North Pole, Western Australia. Five morphologies of carbonaceous spheroids are recognised, including some with splits, some with tetragonal tetrad form and others with groups of up to four individuals. Their morphology is very similar to microfossils from the Onverwacht group of South Africa, and statistical tests of size distribution support a biogenic origin. This occurrence suggests that evidence for early Archaean life may be more widespread than generally thought.
An assemblage of microfossils from the mid-Proterozoic Mara Dolomite Member of the Emmerugga Dolomite, McArthur Group, Northern Territory, Australia, has been studied. The assemblage contains 10 species, of which one is new. The classification of Proterozoic microfossils is reviewed and a morphographic scheme adopted. The new assemblage is compared with other McArthur Group assemblages, and the differences between the assemblages are explained in terms of environmental differences. Comparison with other microfossil assemblages world-wide, suggests that these assemblages have stratigraphic potential.
If the information presently available from ancient rocks is representative enough and correctly understood, then it becomes unlikely that a prebiotic evolution took place on the early earth. The origin of life is open to alternative explanations, including extraterrestrial phenomena.
Criteria of biogenicity of microspheroidal objects, which have been interpreted as microfossils, are here reviewed in the light of additional data. Much weight has been placed by some commentators on constrained heterogeneity as a primary criterion of biogenicity. The data from the field and laboratory suggest the need for continuing reservation in the interpretation of these objects. On the basis of these and other data, reasons are given for the alternative explanation that the objects are lithified relics of protobiotic assemblages. The question remains open as to whether the early Archean spheroidal objects are abiotic, protobiotic, or biotic in origin.
A sample of chert from North Pole in the Archaean Pilbara block of Western Australia contains carbonaceous filaments that resemble microfossils. These occur in alternating light and dark laminae that look stromatolitic. However, the filaments are too simple in form for their origin to be determined, so they should be regarded as dubiofossils, perhaps biogenic, perhaps inorganic. Their host laminae were inorganically precipitated in a concordant fissure and thus cannot be stromatolitic. This fissure is younger than the surrounding silicified sediments of the ca. 3500 Ma old Warrawoona Group and possibly formed towards the end of the uplift and associated fracturing of the North Pole Dome, perhaps ca. 2750 Ma ago. The filaments are therefore contaminants in secondary chert.The filament-bearing rock was collected less than a metre from one of the localities (B) from which Awramik et al. reported early Archaean microfossils and possible microfossils. Their filaments from this locality were almost identical to those described here and were found in similar laminae. This suggests that their locality B filaments may also be contaminants in secondary chert. Other filaments found by Awramik et al. at North Pole come from an imprecisely located sample site (locality A) where the rock relationships are unknown. Since the host laminae of these filaments are not demonstrably primary and as cryptic concordant fissures filled with secondary minerals are common in locality A rocks, the filaments from this sample site could be contaminants too. Those that were assigned to Archaeotrichion should be treated as dubiofossils. Thus, the filaments described by Awramik et al. may not be fossil bacteria in ca. 3500 Ma old stromatolites, as they proposed, and are not necessarily the oldest known fossil organisms, as has been claimed.
Two new algal taxa are described from the approximately 2,000 m.y. old Gunflint Iron Formation, Ontario. Leptoteichos golubicii , n. gen. et sp., were relatively large (5-31 mu m) spherical plankters living in the water column above the central portions of the Gunflint basin. Post-mortem degradation processes have acted on this taxon to produce a "spot cell" organization similar to that exhibited by younger putatively eukaryotic cells; however, L. golubicii is here placed among the Algae incertae sedis and considered to be a probable cyanophyte. Megalytrum diacenum , n. gen. et sp., is the preserved sheath of a colonial chroococcalean blue-green alga. It constitutes an allochthonous element of the algal chert facies microbiota of the Gunflint Iron Formation. Earth and Planetary Sciences Organismic and Evolutionary Biology Version of Record
Comparison of partially degraded unialgal cultures of Chroococcus turgidus with coccoid microfossils from the Late Precambrian Bitter Springs formation, Australia, suggests that the Precambrian fossil record has been seriously misinterpreted. Use of degradational features as taxonomic characters has resulted in unrealistically high estimates of Precambrian algal diversity. There is at present no compelling evidence for the presence of eukaryotic microfossils in rocks from the Bitter Springs formation or any older sedimentary sequences. Organismic and Evolutionary Biology Version of Record
The Swaziland Supergroup, Barberton Mountain Land, South Africa, has long been regarded as a promising location for the Earth's oldest fossils because it includes some of the most ancient well-preserved sedimentary rocks, many of which contain carbonaceous matter. Although there have been numerous reports of microfossils from Swaziland Group rocks1-7, the biogenicity of most of the structures has been questioned8-10. Although some of the organic spheroids are probably biogenic10,11, the best early Archaean simple spheroids are generally regarded as `possible microfossils'10 because organic spheroids may form abiotically in several ways12. The discovery of less-simple biological morphologies is therefore important in establishing the existence of early life forms in the early Archaean. Uniformly-sized curving filaments, especially tubular ones, are difficult to explain as anything other than the fossil remains of filamentous organisms. Here we report the discovery of numerous filaments from two different stratigraphical positions in the 3,500-Myr-old Onverwacht Group of the Swaziland Supergroup. Their morphologies and abundance provide convincing evidence for the existence of bacteria- or cyanobacteria-like organisms on the Earth during the early Archaean. This supports recent reports of similar filamentous microfossils from 3,500-Myr-old rocks from Western Australia13.
Two billion year old black chert lenses from the Duck Creek formation, northwestern Western Australia, contain abundant organically preserved microorganisms which are morphologically similar to fossils of approximately the same age from the Gunflint formation, Ontario. Entities include: a relatively small (5-15mum) coccoid taxon morphologically comparable to Huroniospora Barghoorn, a larger coccoid form comparable to an apparently planktonic alga from the Gunflint, Gunflintia Baghoorn, and Eoastrion Barghoorn (Metallogenium Perfil'ev). Gunflint-type assemblages had a wide geographic distribution in middle Precambrian times, and these assemblages may eventually prove useful as biostratigraphic indices.
Improved methods for studying the growth of Mycoplasma hominis (ATCC 14027) have been developed, involving modified growth conditions and preparation of the organisms under minimally distorting conditions. Cells so prepared from batch cultures show relatively uniform exponential growth and appear to be dividing by binary fission; but pleomorphic forms appear upon further incubation. Similar behavior was demonstrated by another laboratory-adapted strain and by three clinical isolates, and therefore seems characteristic of the species. The pleomorphic populations contain small forms having diameters within the 100- to 250-nm size range reported for "elementary bodies." Such forms were isolated from this strain of M. hominis by sequential filtration using gravity alone, after cell aggregates were dispersed by Pronase treatment. Of the small bodies which traversed membranes of 220-nm pore size, a negligible number grew in liquid or on solid media, suggesting that these were not essential reproductive units in a life cycle, but involution forms due to growth in an altered environment.
The stratigraphic importance of fossils is never more apparent than in attempts to unravel the complexities of metamorphic terrains. The age and stratigraphic relationships of the thick metasedimentary and metavolcanic succession of Prins Karls Forland, western Svalbard, have been the subject of investigation and debate since the early part of this century (Hoel 1914; Craig 1916; Tyrrel 1924), and sharply different interpretations have been proposed (e.g. Harland et al. 1979; Hjelle et al. 1979). Until now, such interpretations have been unconstrained by palaeontological data, an understandable consequence of the metamorphic alteration undergone by these rocks. In this paper, we report the discovery of stratigraphically useful microfossils preserved in chert nodules from carbonaceous, dolomitic shales on northern Prins Karls Forland. These fossils have significant implications for the stratigraphic and structural interpretation of Forland metasediments, as well as for the more general problem of palaeontological prospecting in severely deformed and metamorphosed terrains, including those characteristic of the Archean Eon.
Silicified flake conglomerates and in situ stratiform stromatolites of the Upper Proterozoic (c. 700-800 Ma) Limestone-Dolomite 'Series', central East Greenland, contain well preserved microfossils. Five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages, providing a broad palaeontological representation of late Proterozoic peritidal mat communities. Comparison of assemblages demonstrates that the taxonomy and diversity of mat builder, dweller, and allochthonous populations all vary considerably within and among horizons. The primary mat builder in most assemblages is Siphonophycus inornatum, a sheath-forming prokaryote of probable but not unequivocally established cyanobacterial affinities. An unusual low diversity unit in Bed 17 is dominated by a different builder, Tenuofilum septatum, while a thin cryptalgal horizon in Bed 18 is built almost exclusively by Siphonophycus kestron. Although variable taphonomic histories contribute to observed assemblage variation, most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments. Twenty-two taxa are recognized, of which two are described as new: Myxococcoides stragulescens n.sp. and Scissilisphaera gradata n. sp.
Cellularly preserved filamentous and colonial fossil microorganisms have been discovered in bedded carbonaceous cherts from
the Early Archean Apex Basalt and Towers Formation of northwestern Western Australia. The cell types detected suggest that
cyanobacteria, and therefore oxygen-producing photosynthesis, may have been extant as early as 3.3 billion to 3.5 billion
years ago. These fossils are among the oldest now known from the geologic record; their discovery substantiates previous
reports of Early Archean microfossils in Warrawoona Group strata.
A newly discovered population of organic walled microstructures from the Swaziland System, South Africa, is considered to be biological on the following grounds: (i) the structures are carbonaceous and occasionally have internal organic contents; (ii) the population has a narrow unimodal size frequency distribution (average diameter, 2.5 micrometers; range, 1 to 4 micrometers); (iii) the structures are not strictly spherical, but are commonly flattened and folded like younger microfossils; (iv) the sedimentary context is consistent with biogenic origins; and (v) various stages of binary division are clearly preserved.
Endolithic unicellular blue-green algae occur under the surface of orthoquartzite rocks in the dry valleys of southern Victoria Land, Antarctica. This report of primary producers in the Antarctic desert ecosystem suggests that, in future efforts to detect life in extraterrestrial (for example, martian) environments, scientists should consider the possible existence of endolithic life forms.
Blue-green algal cell degradation and its implication for the fossil record Abstracts with Programs
- S M Awramik
- S Golubic
- E S Barghoorn
- S M Awramik
- J W Schopf
AWRAMIK, S.M., GOLUBIC, S., and BARGHOORN, E.S., 1972, Blue-green algal cell degradation and its implication for the fossil record: Geological Society of America, Abstracts with Programs, v. 4, p. 438. AWRAMIK, S.M., SCHOPF, J.W., and WALTER, M.R., 1983, Filamentous fossil bacteria from the Archean of Western Australia: Precam-brian Research, v. 20, p. 357-374.
Evaporitic sediments of early Archean age from a chert-barite bed in the Warrawoona Group
- R Buick
- J S R Dunlop
BUICK, R., and DUNLOP, J.S.R., 1990, Evaporitic sediments of early Archean age from a chert-barite bed in the Warrawoona Group, North Pole, Western Australia: Sedimentology, v.. 237, p. 247-277.
Early biogeologic history: The emergence of a par-adigm: in SCHOPF Earth's Earliest Biosphere: Its Origin and Evolution
- P Cloud
CLOUD, P., 1983, Early biogeologic history: The emergence of a par-adigm: in SCHOPF, J.W., ed., Earth's Earliest Biosphere: Its Origin and Evolution: Princeton University Press, Princeton, p. 14-31.
The Cryptozoic biosphere: Its diversity and geolog-ical significance
- P Cloud
CLOUD, P., 1984, The Cryptozoic biosphere: Its diversity and geolog-ical significance: Proceedings of the 27th International Geological Congress, v. 5, p. 173-198.
Vestiges of a beginning: Geological Society of Amer-ica, Centennial Special Volume
- P Cloud
CLOUD, P., 1985, Vestiges of a beginning: Geological Society of Amer-ica, Centennial Special Volume, v. 1, p. 151-156.
Microbiotas of banded iron for-mations: Proceedings of the National Academy of Science
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CLOUD, P., and LICARI, G.R., 1968, Microbiotas of banded iron for-mations: Proceedings of the National Academy of Science, U.S.A., v. 61, p. 779-786.
Precambrian microflora, Belcher Islands, Can-ada: Significance and systematics
- H J Hofmann
HOFMANN, H.J., 1976, Precambrian microflora, Belcher Islands, Can-ada: Significance and systematics: Journal of Paleontology, v. 50, p. 1040-1073.
Early Proterozoic micro-fossils: in Earth's Earliest Biosphere: Its Origin and Evolution
- H J Hofmann
- J W Schopf
HOFMANN, H.J., and SCHOPF, J.W., 1983, Early Proterozoic micro-fossils: in SCHOPF, J.W., ed., Earth's Earliest Biosphere: Its Origin and Evolution: Princeton University Press, Princeton, p. 321-360.
Interstitial and peloidal microfossils from the
- W P Lanier
LANIER, W.P., 1989, Interstitial and peloidal microfossils from the