Microorganisms from the Gunflint Chert

INFLUENCIA Y REGISTRO DE MICROORGANISMOS EN FORMACIONES DE HIERRO BANDEADO (BIFS) Y OTROS TIPOS DE MINERALIZACIONES - Revisión y síntesis bibliográfica

Preprint

Full-text available

Dec 2023

Gabriel David Moreno Aguilar

Hace aproximadamente 2,450 millones de años (m.a.), tuvo lugar un evento trascendental conocido como el Gran Evento de Oxidación, o GOE por sus siglas en inglés. Este evento se caracteriza por un notable aumento en la concentración de oxígeno en la atmósfera, probablemente debido a la actividad de cianobacterias que adquirieron relevancia al generar grandes cantidades de oxígeno mediante la fotosíntesis. Diversas evidencias en el registro geológico respaldan este acontecimiento, siendo la más destacada la formación de extensas formaciones de hierro bandeado, conocidas como BIFs (Banded Iron Formations). Estas rocas sedimentarias, precipitadas químicamente, presentan bandas alternantes ricas en óxidos de hierro (hematita o magnetita) junto con bandas de chert. Se atribuye principalmente a la oxidación del hierro por el oxígeno generado por las cianobacterias, al menos para aquellos BIFs posteriores a este período. Existen diferentes evidencias que sugieren la influencia de bacterias en la formación de estos, sin embargo, destacan los microorganismos fósiles descritos para el Gunflint Chert, los cuales comparten características con organismo actuales que realizan fotosíntesis tanto anoxigénica, como oxigénica, adicionalmente se describen microfósiles en un sistema análogo de BIF de edad cuaternaria, lo que evidencia entonces que microorganismos favorecieron la formación de estos depósitos den el pasado. Por otro lado, se muestran diferentes reportes de biomineralización y procesos de oxidación en otro tipo de mineralizaciones, como enriquecimientos auríferos en placeres o enriquecimiento supergéno en los depósitos tipo pórfido.

Microbiota and Organosedimentary structures from Vindhyan supergroup around Chandrehi, Madhya Pradesh

Article

Dec 1983

The present paper records the microbiota and stromatolite from Vindhyan Super-group around Chandrehi Section in Madhya Pradesh. The microbiota is recorded from Semri, Rewa and Bhander groups. It comprises bacteria - Biocatenoides; algae - Huronispora, Sphaerophycus, Aphanocapsiopsis, Corymbococcus; acritarcha - Orygmatosphaeridium, Bavlinella, Polyedryxium and other problematical remains.

Organosedimentary structures and biota from Upper Proterozoic of Equatorial Africa

Article

Dec 1983

P.K. Maithy

Organosedimentary structures and biota from the Upper Proterozoic of Lower Zaire, Kasai, Shaba, North-east Zaire and South-east Burundi are recorded. These structures belong to oncolites, viz., Ambigolamellatus, Asterosphaeroides, Polysphaerula, Babetosphaera and Burundiocyathus f. nov. and catagraphs - Conferta and Vesicularites. The stratigraphical significance of the organosedimentary structures in the Upper Proterozoic of Equatorial Africa has also been given.

Microbiota from the Kushalgarh Formation, Delhi Supergroup, India

Article

Dec 1984

Well-preserved and diverse microfossils are described for the first time from the Kushalgarh Formation of Ajabgarh Group around Baraud Village in Alwar District, Rajasthan. The assemblage comprises 10 genera and 17 species belonging to filamentous and coccoid blue-green algae, of which two genera and 10 species are new. The new forms are Myxococcoides compactus sp. nov., Palaeolyngbya distinctica sp. nov., P. elongata sp. nov., P. baraudensis sp. nov., Palaeoscytonema indica sp. nov., P. intermingle sp. nov., P. misrae sp. nov., Ghoshia bifurcata gen. et sp. nov., Primorivularia robusta sp. nov. and Vesicophycus problematieus gen. et sp. nov. Morphological features and the size range of taxa suggest procaryotic nature of the assemblage. Coccoid forms are aggregated in colonies and dominated by filamentous forms. Baraud assemblage contains hormogonia, heterocysts, and false branching and the most significant is the record of true branching showing Stigonematalean affinity. The composition of the microflora suggests that the rocks of the Delhi Supergroup are equivalent to Vindhyans. This is also supported by the radiometric dates.

Acritarch-like Microorganisms from the 1.9 Ga Gunflint Chert, Canada

Article

Full-text available

Apr 2022
ASTROBIOLOGY

Fossil evidence of eukaryotic life older than 1.8 Ga has long been debated because known fossils of that age usually lack cellular micro- and ultra-structures that bear strong affinities to eukaryotes. These include fossils of the ∼1.9 Ga Gunflint Chert microbiota that, despite being exceptionally well preserved, have suffered from cellular degradation, which poses challenges to studying their delicate cellular structures. In this study, we use an extended-focal-depth imaging technique, in combination with scanning electron microscopy, to document multiple types of large (10-35 μm diameter), cyst-like bodies based on distinctive details such as (1) radially arranged internal strands similar to those in some acritarchs and dinoflagellates; (2) regularly spaced long tubular processes, stubby pustules, and/or robust podia on the cell surface; (3) reticulate cell-wall sculpturing such as scale-like tubercles, pits, and ridges; and (4) internal bodies that may represent membrane-bound organelles. These micro- and ultra-structures provide strong morphological evidence for the presence of protists in the late Paleoproterozoic.

TREATISE ONLINE Number 158

Chapter

Dec 2021

Adriana Heimann Rios

Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils

Article

Mar 2021

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.

An Alternative Approach for Assessing Biogenicity

Article

Full-text available

Oct 2020

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.

Primordial magnetotaxis in putative giant paleoproterozoic magnetofossils

Article

May 2024
P NATL ACAD SCI USA

Magnetotactic bacteria produce chains of nanoscopic iron minerals used for navigation, which can be preserved over geological timescales in the form of magnetofossils. Micrometer-sized magnetite crystals with unusual shapes suggesting a biologically controlled mineralization have been found in the geological record and termed giant magnetofossils. The biological origin and function of giant magnetofossils remains unclear, due to the lack of modern analogues to giant magnetofossils. Using distinctive Ptychographic nanotomography data of Precambrian (1.88 Ga) rocks, we recovered the morphology of micrometric cuboid grains of iron oxides embedded in an organic filamentous fossil to construct synthetic magnetosomes. Their morphology is different from that of previously found giant magnetofossils, but their occurrence in filamentous microfossils and micromagnetic simulations support the hypothesis that they could have functioned as a navigation aid, akin to modern magnetosomes.

Microbiota from stromatolite Stratifera of the Bilara Group (Marwar Supergroup), Western Rajasthan

Article

Dec 1984

P.K. Maithy

Three types of microbiotas referable to Nanococcus vulgaris, Huronispora psilata and Kakabekia umbellata are reported from the stromatolite- Stratifera collected near Bhadora. The biota suggests that the stratified stromatolite may have been formed due to the exogenic activities of the recorded algae.

Greenalite and its role in the genesis of early Precambrian iron formations – A review

Article

Mar 2021
EARTH-SCI REV

Iron formations are marine sedimentary rocks comprising Fe-rich and Si-rich bands that were most widely deposited before ~2.45 Ga, prior to the Great Oxidation Event (GOE). They host most of the world's iron resources but their scientific importance lies in their potential for tracking the evolution of ancient seawater chemistry. Most models of their deposition (see reviews by Bekker et al., 2010, Bekker et al., 2014; Konhauser et al., 2017) assume that iron oxides in iron formations were derived from original Fe(III)-oxides/hydroxides that were precipitated in the photic zone from Fe(II)-rich seawater via biologically mediated oxidation, the Fe(II) ultimately being derived from hydrothermal vent plumes. In this review, we present an alternative model for the origin of iron formations centred on petrographic observations that indicate that the depositional (original) iron phase was an Fe(II)-silicate (greenalite) that precipitated in vent plumes as nanometer-sized particles and silt-sized flocs. The greenalite was deposited throughout the oceans and, in times of increased mafic-ultramafic volcanism, on continental shelves. Paragenetic studies indicate that the bulk of the iron oxides in iron formations formed by replacement of Fe(II)-rich precursors during metamorphism, deformation and exhumation. The origin of early Precambrian iron formations by precipitation of greenalite from vent plumes implies that iron deposition was largely an abiotic chemical process that did not require biologically driven oxidation, consistent with the lack of microfossils and indigenous organic matter.

A late Paleoproterozoic (1.74 Ga) deep-sea, low-temperature, iron-oxidizing microbial hydrothermal vent community from Arizona, USA

Article

Full-text available

Feb 2021
GEOBIOLOGY

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’.

Triple Oxygen Isotope Trend Recorded by Precambrian Cherts: A Perspective from Combined Bulk and in situ Secondary Ion Probe Measurements

Article

Jan 2021

Astrobiological applications of μ-mapping X-ray fluorescence spectrometry

Article

Full-text available

Mar 2024

In situ elemental imaging of planetary surface regolith at a spatial resolution of 100s to 1000s of microns can provide evidence of the provenance of rocks or sediments and their habitability, and can identify post-depositional diagenetic alteration affecting preservation. We use high-resolution elemental maps and XRF spectra from MapX, a flight prototype in situ X-ray imaging instrument, to demonstrate this technology in rock types relevant to astrobiology. Examples are given for various petrologies and depositional/diagenetic environments, including ultramafic/mafic rocks, serpentinites, hydrothermal carbonates, evaporites, stromatolitic cherts and diagenetic concretions.

Organic carbon generation in 3.5-billion-year-old basalt-hosted seafloor hydrothermal vent systems

Article

Full-text available

Feb 2023

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.

Deep-UV Raman Spectroscopy of Carbonaceous Precambrian Microfossils: Insights into the Search for Past Life on Mars

Article

Oct 2022

The current strategy for detecting evidence of ancient life on Mars-a primary goal of NASA's ongoing Mars 2020 mission-is based largely on knowledge of Precambrian life and of its preservation in Earth's early rock record. The fossil record of primitive microorganisms consists mainly of stromatolites and other microbially influenced sedimentary structures, which occasionally preserve microfossils or other geochemical traces of life. Raman spectroscopy is an invaluable tool for identifying such signs of life and is routinely performed on Precambrian microfossils to help establish their organic composition, degree of thermal maturity, and biogenicity. The Mars 2020 rover, Perseverance, is equipped with a deep-ultraviolet (UV) Raman spectrometer as part of the SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) instrument, which will be used in part to characterize the preservation of organic matter in the ancient sedimentary rocks of Jezero crater and therein search for possible biosignatures. To determine the deep-UV Raman spectra characteristic of ancient microbial fossils, this study analyzes individual microfossils from 14 Precambrian cherts using deep-UV (244 nm) Raman spectroscopy. Spectra obtained were measured and calibrated relative to a graphitic standard and categorized according to the morphology and depositional environment of the fossil analyzed and its Raman-indicated thermal maturity. All acquired spectra of the fossil kerogens include a considerably Raman-enhanced and prominent first-order Raman G-band (∼1600 cm-1), whereas its commonly associated D-band (∼1350 cm-1) is restricted to specimens of lower thermal maturity (below greenschist facies) that thus have the less altered biosignature indicative of relatively well-preserved organic matter. If comparably preserved, similar characteristics would be expected to be exhibited by microfossils or ancient organic matter in rock samples collected and cached on Mars in preparation for future sample return to Earth.

Multiwavelength Ablation/Ionization and Mass Spectrometric Analysis of 1.88 Ga Gunflint Chert

Article

Feb 2022

The investigation of chemical composition on planetary bodies without significant sample processing is of importance for nearly every mission aimed at robotic exploration. Moreover, it is a necessary tool to achieve the longstanding goal of finding evidence of life beyond Earth, for example, possibly preserved microbial remains within martian sediments. Our Laser Ablation Ionization Mass Spectrometer (LIMS) is a compact time-of-flight mass spectrometer intended to investigate the elemental, isotope, and molecular composition of a wide range of solid samples, including e.g., low bulk density organic remains in microfossils. Here, we present an overview of the instrument and collected chemical spectrometric data at the micrometer level from a Precambrian chert sample (1.88 Ga Gunflint Formation, Ontario, Canada), which is considered to be a martian analogue. Data were collected from two distinct zones-a silicified host area and a carbon-bearing microfossil assemblage zone. We performed these measurements using an ultrafast pulsed laser system (pulse width of ∼180 fs) with multiple wavelengths (infrared [IR]-775 nm, ultraviolet [UV]-387 nm, UV-258 nm) and using a pulsed high voltage on the mass spectrometer to reveal small organic signals. We investigated (1) the chemical composition of the sample and (2) the different laser wavelengths' performance to provide chemical depth profiles in silicified media. Our key findings are as follows: (1) microfossils from the Gunflint chert reveal a distinct chemical composition compared with the host mineralogy (we report the identification of 24 elements in the microfossils); (2) detection of the pristine composition of microfossils and co-occurring fine chemistry (rare earth elements) requires utilization of the depth profiling measurement protocol; and (3) our results show that, for analysis of heterogeneous material from siliciclastic deposits, siliceous sinters, and cherts, the most suitable wavelength for laser ablation/Ionization is UV-258 nm.

-NC-ND license Correlating trace element compositions, petrology, and Raman spectroscopy data in the ~3.46 Ga Apex chert, Pilbara Craton, Australia

Article

Full-text available

Jan 2021
PRECAMBRIAN RES

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.

Correlating trace element compositions, petrology, and Raman spectroscopy data in the ~3.46 Ga Apex chert, Pilbara Craton, Australia

Article

Full-text available

Oct 2021
PRECAMBRIAN RES

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.

Interpretation of sponge fossil faunas: A neontological approach to a paleontological problem

Thesis

Full-text available

Sep 2021

Pablo Aragonés

The tempo and mode of early animal evolution remains one of the biggest conundrums in biology. Stratigraphy shows that there is a gap, not attributable to poor preservation, of at least ~100 Myr between the oldest animal fossils and the divergence times implied by molecular phylogenies. Sponges, due to their position in the metazoan tree, are a good candidate for the earliest fossil evidence for the diversification of animals. Nevertheless, the search for the earliest animals represents a major challenge due to the lack of criteria by which to recognize such organisms in the fossil record. Here I describe a way to quantify the sponge filtration or ‘sponge pump character’ by analyzing the ratio between the two major components of the aquiferous system: external surface area–SA and osculum cross-sectional area–OSA, which allows comparison between extant sponges and many fossil forms. Second, I addressed the question of whether sponges make use of induced flow, an often-made assumption when interpreting fossils as sponges. The analysis includes the three major classes of Porifera: Demospongiae, Hexactinellida and Calcarea, both are represented by extant and fossil taxa from two extremes of the Phanerozoic, the Cambrian and Eocene. The data show that this ratio is narrow, 0.01 – 0.001 in modern sponges, and the slope of the ratio can be used to distinguish classes of sponges. I then examined the ratio of OSA/SA for the putative Ediacaran sponge Thectardis avalonensis, and found it to be similar to some Cambrian sponge genera. Moreover, the slope of the ratio across different sizes of T. avalonensis is similar to that of demosponges. However, the original argument for the Poriferan affinity of T. avalonensis was based on the idea that the main mechanisms that sponges use to filter water is through current induced flow, meaning that shape alone drives the flow in and out of a sponge at no extra metabolic cost. I tested this hypothesis in the second chapter of this thesis by analyzing data from tank experiments in the demosponge Geodia barretti. The analysis of the filtration to respiration ratio showed increased filtration at ambient currents below 10 cm s-1 but a reduction at higher ambient current speeds, which contrast the traditional view of shape induced flow. Instead, these results support the evidence that sponges have a high degree of control over their filtration, with the consequence that induce flow should not be taken as a criterion for ascribing to the sponges any structure based on this assumption. Overall, my work shows that a more solid understanding of the biology of modern sponges provides a wealth of information with which to examine the Precambrian record of sponges in particular, and the greater picture of early animal evolution in general. This is because having a correct interpretation of the fossil record is essential to properly calibrating molecular phylogenies.

Searching for biosignatures in sedimentary rocks from early Earth and Mars

Article

Jun 2021

The recognition of past habitable environments on Mars has increased the urgency to understand biosignature preservation in and characterize analogues of these environments on Earth. In this Review, we examine the detection and interpretation of potential biosignatures preserved in deposits rich in carbonates, silica and clay. Many of the earliest chemical, textural and morphological evidence of life on Earth are found in carbonates and carbonate-hosted phases. Early diagenetic chert within carbonate deposits can exceptionally preserve microbial body fossils, and clay minerals that form in ultramafic terrains can protect organic matter. On Mars, similar deposits older than 3.5 billion years could contain biosignatures or remnants of prebiotic processes that have long been erased from Earth. Terrestrial analogues for the deposition of magnesium carbonate minerals in Jezero crater, Mars, present patterns that can guide the collection of samples with the highest astrobiological potential by the Perseverance rover. Continued characterization of terrestrial analogue sites and rigorous examination of the processes that impact the preservation of isotopic signals, organic compounds, and microbial textures and fossils will advance the interpretation of Martian deposits.

Precambrian organic compounds from the Ketilidian of South-West Greenland. Parts I and II

Article

Apr 1968

Well preserved Ketilidian sedimentary rocks from South-West Greenland, about 2000 million years old and containing macroscopic and microscopic fossils, have been examined for organic material. The results of the work on the extractions from a coal-graphite layer are presented together with a short description of the local geology around this layer. Straight chain and branched paraffins, mono-, di-, or polycyclic aromatic hydrocarbons, borneol, camphor and other monoterpenoid compounds, fatty acids and methyl esters of fatty acids are present in samples from the coal-graphite layer, together with other organic substances as yet unidentified. Pristane, phytane and other isoprenoid hydrocarbons have been found in small amounts. The presence of these organic compounds indicates a biological origin of the coal-graphite layer. Besides the coal-graphite other sediments (dolomites and quartzites) from the succession have yielded organic material. Further work on these samples and on the coal-graphite is in progress.

Microfossils of the Paleoproterozoic Turee Creek Group: Biological evolution resulting from atmospheric change?

Conference Paper

Full-text available

Nov 2017

Non-pollen palynomorphs in deep time: unravelling the evolution of early eukaryotes

Article

Jan 2021

Most of the Precambrian (> 541 Ma) fossil record, which includes the time before the onset of macroscopic multicellular life, consists of minute organically preserved remains of soft-bodied microorganisms, i.e. non-pollen palynomorphs (NPP). These microfossils include single-celled prokaryotic or eukaryotic organisms, filamentous sheets, and bacterial cellular aggregates, and occur in marine and lacustrine sediments through most of the Earth's history. Ancient NPP have informed our understanding of one of the biggest evolutionary steps in the history of life: the origin of the eukaryotic cell and the subsequent diversification of eukaryotic life before the evolution of macroscopic forms. The oldest widely accepted eukaryotic microfossils are large cells with opening structures and ornamentation from upper Palaeoproterozoic units in China and India. NPP diversify through Meso- and Neoproterozoic Eras, notably recording the oldest multicellular life around 1 billion years ago. In the latest Neoproterozoic, during the advent of animal life, the NPP record allows us to study the cryptic evolution of animals while they were exclusively small and poorly or non-mineralised before the appearance of organisms with hard parts. We review the current understanding of the early eukaryotic evolution and major advances in Precambrian palaeontology through the lens of the NPP record in deep time.

Un point sur... : Lumière sur les débuts de la vie

Article

Full-text available

Jan 2021

Resistance of Bacillus subtilis var. niger Spores Occluded in Water-insoluble Crystals to Three Sterilization Agents

Article

Jul 1967
Appl Microbiol

The resistance to destruction of spores of Bacillus subtilis var. niger occluded in crystals of calcium carbonate and exposed to ethylene oxide and moist and dry heat was determined and compared with the destruction of unoccluded spores. Occluded spores could not be inactivated with ethylene oxide. Resistance to inactivation was approximately 900 and 9 times higher for occluded than for unoccluded spores subjected to moist and dry heat, respectively, at 121 C. The protective effect may be due either to the unavailability of oxygen for destruction by oxidation or to inhibition of the loss of essential cell constituents by vaporization. Evidence also implicates the crystal structure as a thermal conductivity barrier. Occluded spores retained viability over a 3-year period compared with unoccluded spores which decreased over 90% during this period. Occluded spores in insoluble materials are seldom encountered in the technology of sterilization, but could be the most critical factor in the sterilization of interplanetary vehicles. Entrapped spores in insoluble materials are usually difficult to detect, and are very stable as well as extremely resistant to destruction by heat and ethylene oxide.

Ediacaran Doushantuo-type biota discovered in Laurentia

Article

Full-text available

Nov 2020

The Ediacaran period (635–541 Ma) was a time of major environmental change, accompanied by a transition from a microbial world to the animal world we know today. Multicellular, macroscopic organisms preserved as casts and molds in Ediacaran siliciclastic rocks are preserved worldwide and provide snapshots of early organismal, including animal, evolution. Remarkable evolutionary advances are also witnessed by diverse cellular and subcellular phosphatized microfossils described from the Doushantuo Formation in China, the only source showing a diversified assemblage of microfossils. Here, we greatly extend the known distribution of this Doushantuo-type biota in reporting an Ediacaran Lagerstätte from Laurentia (Portfjeld Formation, North Greenland), with phosphatized animal-like eggs, embryos, acritarchs, and cyanobacteria, the age of which is constrained by the Shuram–Wonoka anomaly (c. 570–560 Ma). The discovery of these Ediacaran phosphatized microfossils from outside East Asia extends the distribution of the remarkable biota to a second palaeocontinent in the other hemisphere of the Ediacaran world, considerably expanding our understanding of the temporal and environmental distribution of organisms immediately prior to the Cambrian explosion.

Authigenic anatase within 1 billion‐year‐old cells

Article

Full-text available

Sep 2020

The siliciclastic ~1 Ga‐old strata of the Torridon Group, Scotland, contain some of the most exquisitely preserved three‐dimensional organic‐walled microfossils (OWMs) of the Precambrian. A very diverse microfossil assemblage is hosted in a dominantly phosphatic and clay mineral matrix, within the Diabaig and the Cailleach Head (CH) Formations. In this study, we report on several microfossil taxa within the CH Formation (Leiosphaeridia minutissima, Leiosphaeridia crassa, Synsphaeridium spp. and Myxococcoides spp.) that include populations of cells containing an optically transparent and highly refringent mineral, here identified using electron microscopy as anatase (TiO2). Most anatase crystals occur entirely within individual cells, surrounded by unbroken carbonaceous walls. Rarely, an anatase crystal may protrude outside a cell, interpreted to correspond to zones where the cell wall had broken down prior to anatase precipitation. Where an anatase crystal entombs an organic intracellular inclusion (ICI), the ICI is large and well preserved. These combined observations indicate that the intracellular anatase is an authigenic sedimentary phase, making this the first report of in situ precipitated anatase intimately associated with microfossils. The ability of anatase to preserve relatively large volumes of intracellular and cell wall organic material in these cells suggests that the crystallisation of anatase entombed cellular contents particularly quickly, soon after the death of the cell. This is consistent with the strong affinity of Ti for organic material, the low solubility of TiO2, and reports of Ti occurring in living organisms. With the data currently available, we propose a mineralisation pathway for anatase involving Ti complexation with organic ligands within specific cells, leading to localised post‐mortem anatase nucleation inside these cells as the complexes broke down. Further overgrowth of the anatase crystals was likely fuelled by very early diagenetic mobilisation of Ti that had been bound to more labile organic material nearby in the sediments.

Bacterial Surface-Mediated Mineral Formation

Chapter

Apr 2014

Gordon Southam

Taphonomy of Biosignatures in Microbial Mats on Little Ambergris Cay, Turks and Caicos Islands

Article

Full-text available

Sep 2020

Microbial mats are taxonomically and metabolically diverse microbial ecosystems, with a characteristic layering that reflects vertical gradients in light and oxygen availability. Silicified microbial mats in Proterozoic carbonate successions are generally interpreted in terms of the surficial, mat building community. However, information about biodiversity in the once-surface-layer can be lost through decay as the mats accrete. To better understand how information about surface microbial communities is impacted by processes of decay within the mat, we studied microbial mats from Little Ambergris Cay, Turks and Caicos Islands. We used molecular techniques, microscopy and geochemistry to investigate microbial mat taphonomy – how processes of degradation affect biological signatures in sedimentary rocks, including fossils, molecular fossils and isotopic records. The top < 1 cm of these mats host cyanobacteria-rich communities overlying and admixed with diverse bacterial and eukaryotic taxa. Lower layers contain abundant, often empty, sheaths of large filamentous cyanobacteria, preserving their record as key mat-builders. Morphological remains and free lipid biomarkers of several bacterial groups, as well as diatoms, arthropods, and other eukaryotes also persist in lower mat layers, although at lower abundances than in surface layers. Carbon isotope signatures of organic matter were consistent with the majority of the biomass being sourced from CO2-limited cyanobacteria. Porewater sulfide sulfur isotope values were lower than seawater sulfate sulfur isotope values by ∼45–50‰, consistent with microbial sulfate reduction under sulfate-replete conditions. Our findings provide insight into how processes of degradation and decay bias biosignatures in the geological record of microbial mats, especially mats that formed widely during the Proterozoic (2,500–541 million years ago) Eon. Cyanobacteria were the key mat-builders, their robust and cohesive fabric retained at depth. Additionally, eukaryotic remains and eukaryotic biosignatures were preserved at depth, which suggests that microbial mats are not inherently biased against eukaryote preservation, either today or in the past.

MICROFACIES ANALYSIS, DEPOSITIONAL ENVIRONMENT AND ECOLOGY OF NEOPROTEROZOIC LIMESTONE OF SINCHA FORMATION, KATHUA DISTRICT, JAMMU

Article

Full-text available

Jun 2020

This paper presents the results of microfacies analysis and depositional environment of Sincha Formation exposed at Siara in Kathua District of Jammu region. The Sincha Formation comprises of stromatolite bearing dolomitic limestones with interbedded thin beds of chert and stand out as ribs amidst the phyllites, shales and sandstones. The characteristic feature of these limestones is alternation of white and grey colour bands of limestone giving the rocks a zebra strip appearance. We studied these rocks for sedimentological and palaeontological signatures under the microscope and distinguished five microfacies types and various types of primitive microflora (Cyanobacteria-Archaeoellipsoides). The observed microfacies associations and microflora were used to infer depositional environments and standard facies zones, and environmental and ecological conditions during the deposition of the limestones of Sincha Formation.

First report of Tappania and associated microfossils from the late Paleoproterozoic Chuanlinggou Formation of the Yanliao Basin, North China

Article

Jan 2024
PRECAMBRIAN RES

The first radiation of eukaryotic life in Earth history began at least by the late Paleoproterozoic as evidenced by complex organic-walled microfossils from North China, India and Australia. However, among these oldest eukaryotic fossils, the spinose Tappania plana Yin, 1997, which has been reported from the late Paleoproterozoic to early Mesoproterozoic sediments worldwide, is strangely absent in the well-known microfossil assemblages from the coeval strata in the Yanliao Basin, North China. Here we report the new finding of T. plana and associated microfossils from ∼ 1.64 Ga Chuanlinggou Formation in the eastern Yanliao Basin. T. plana occurs in the assemblage from the upper Chuanlinggou Formation and exhibits typical irregularly distributed tubular processes and neck-like extensions. The assemblage is dominated by a wide variety of filamentous microfossils likely representing cyanobacterial or other bacterial sheaths and trichomes, and includes 21 species (belonging to 10 genera) and 3 unnamed forms. In contrast, the assemblage from the lower Chuanlinggou Formation is composed of only simple spheroidal (Leiosphaeridia) and fusiform (Schizofusa) morphotypes, distinct from those of the upper assemblage. This taxonomic discrepancy likely resulted from different paleoenvironments. Overall, the occurrence of T. plana in the Yanliao Basin expands its paleographic distribution and further reinforces its stratigraphic potential as an index fossil for the late Paleoproterozoic and early Mesoproterozoic strata. Furthermore, the microfossil assemblages reported here document a microbiota overwhelmingly dominated by prokaryotic organisms, which is consistent with the hypothesis that eukaryotes likely constituted as marginal groups in ancient ecosystems during mid-Proterozoic.

Microbially induced precipitation of silica by anaerobic methane-oxidizing consortia and implications for microbial fossil preservation

Article

Dec 2023
P NATL ACAD SCI USA

Authigenic carbonate minerals can preserve biosignatures of microbial anaerobic oxidation of methane (AOM) in the rock record. It is not currently known whether the microorganisms that mediate sulfate-coupled AOM—often occurring as multicelled consortia of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB)—are preserved as microfossils. Electron microscopy of ANME-SRB consortia in methane seep sediments has shown that these microorganisms can be associated with silicate minerals such as clays [Chen et al ., Sci. Rep. 4 , 1–9 (2014)], but the biogenicity of these phases, their geochemical composition, and their potential preservation in the rock record is poorly constrained. Long-term laboratory AOM enrichment cultures in sediment-free artificial seawater [Yu et al ., Appl. Environ. Microbiol. 88 , e02109-21 (2022)] resulted in precipitation of amorphous silicate particles (~200 nm) within clusters of exopolymer-rich AOM consortia from media undersaturated with respect to silica, suggestive of a microbially mediated process. The use of techniques like correlative fluorescence in situ hybridization (FISH), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and nanoscale secondary ion mass spectrometry (nanoSIMS) on AOM consortia from methane seep authigenic carbonates and sediments further revealed that they are enveloped in a silica-rich phase similar to the mineral phase on ANME-SRB consortia in enrichment cultures. Like in cyanobacteria [Moore et al ., Geology 48 , 862–866 (2020)], the Si-rich phases on ANME-SRB consortia identified here may enhance their preservation as microfossils. The morphology of these silica-rich precipitates, consistent with amorphous-type clay-like spheroids formed within organic assemblages, provides an additional mineralogical signature that may assist in the search for structural remnants of microbial consortia in rocks which formed in methane-rich environments from Earth and other planetary bodies.

Late Precambrian microfossils from the Visingsö Beds in southern Sweden

Chapter

Jul 1976

Gonzalo Vidal

A fungal mycelium containing abundant endoconidia from the Lower Devonian Rhynie cherts of Scotland

Article

Apr 2023
REV PALAEOBOT PALYNO

The Paleobiologic Record of Cyanobacterial Evolution

Chapter

Apr 2014

J. William Schopf

Treatise Online no. 160: Part B, Volume 2, Chapter 7: Microfossils of Prokaryotes (Bacteria and Archaea): Research History, Taphonomy, and Paleobiology

Article

Dec 2021

Late Precambrian microfossils from Deoban Limestone Formation, Lesser Himalaya, India

Article

Full-text available

Dec 1986

A well-preserved microbiota consisting of filamentous Cyanobacteria, viz., Oscillatoriopsis, Cyanonema, Siphonophycus, Eomycetopsis, Gunflintia and Animikiea; spheroidal unicells, viz., Glenobotrydion, Globophycus, Sphaerophycus and Myxococcoides; Eubacteria, viz., Archaeotrichion, Biocatenoides; and acritarch (?plankton) Kildinosphaera, is described from petrographic thin sections of cherts from the Deoban Formation, Garhwal Lesser Himalaya. The assemblage has been compared with other authentic Proterozoic records. The palaeomicrobial community is interpreted to have inhabited a protected shallow intertidal environment.

Evolutionary diversification of paleoproterozoic prokaryotes: New microfossil records in 1.88 Ga Gunflint Formation

Article

Sep 2022
PRECAMBRIAN RES

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.

Genesis: early life survived in the Polar Circles by precipitating banded iron formation (~ 3.7–1.85 Ga) followed by stratified ferruginous siliciclasts until ~ 580 Ma, when tectonically shifted to lower latitudes initiating the ‘Cambrian Explosion’

Article

May 2022

Zeev Lewy

The inclination of Planet Earth’s axis of rotation by 23½° resulted in extreme climatic changes. Weak, solar radiation upon the Polar Circles during half-a-year alternated with half-a-year of darkness, turning them into freezing terrains for nearly the whole Precambrian. Exhalant hydrothermal solutions formed huge lakes over Polar Regions, undergoing intensive evaporation and condensation. Chemical interactions incidentally created primitive live forms, surviving as chemoautotrophic bacteria under the weakest UV rays. Daily changing solar radiation emitting UV rays over low latitudes prevented any life form. Some of these polar bacteria developed photosynthesis, improving their nourishment simultaneously releasing oxygen. The high content of ferrous iron in the lakes absorbed toxic oxygen forming iron oxides as banded iron formation (BIF). Excess photosynthetic oxygen molecules escaped into the anoxic atmosphere. At ~ 1.8 Ga oxygenated meteoric water infiltrated the continental subsurface oxidizing hydrothermal fluids, precipitating underground layered iron-oxides followed by silica only during the dry summers. The dilution of the rising solutions terminated biologically induced BIF precipitation. Consequently, intensive evaporation cemented sililiciclasts into stratified ferruginous formations, becoming abundant in the Late Neoproterozoic as NIF. The co-occurrence of Paleoproterozoic BIF and Neoproterozoic NIF sites evidence the tectonic and climatic stability of the Polar Circles. Magmatic convection currents split them ~ 750 Ma ago, but only after 580 Ma shifted the individual plates radially to low latitudes with advance of ‘Plate Tectonics’. The polar bacteria connected with the open sterile sea for the first time had to adapt to daily changing ecosystems by combining into mobile primitive eukaryotes (Ediacaran Biota) and further diversify, erroneously referred to the ‘Cambrian Explosion’. Geological evidence corroborated the activity of convection currents since Earth’s consolidation controlling its inner heat budget and supplying hydrothermal solutions forming lakes on the Polar Circles where life originated and iron ores accumulated.

High Mass Resolution fs-LIMS Imaging and Manifold Learning Reveal Insight Into Chemical Diversity of the 1.88 Ga Gunflint Chert

Article

Full-text available

May 2022

Extraction of useful information from unstructured, large and complex mass spectrometric signals is a challenge in many application fields of mass spectrometry. Therefore, new data analysis approaches are required to help uncover the complexity of such signals. In this contribution, we examined the chemical composition of the 1.88 Ga Gunflint chert using the newly developed high mass resolution laser ionization mass spectrometer (fs-LIMS-GT). We report results on the following: 1) mass-spectrometric multi-element imaging of the Gunflint chert sample; and 2) identification of multiple chemical entities from spatial mass spectrometric data utilizing nonlinear dimensionality reduction and spectral similarity networks. The analysis of 40′000 mass spectra reveals the presence of chemical heterogeneity (seven minor compounds) and two large clusters of spectra registered from the organic material and inorganic host mineral. Our results show the utility of fs-LIMS imaging in combination with manifold learning methods in studying chemically diverse samples.

Geochemistry, depositional environment and tectonic setting of the BIF's of the Late Archaean Chitradurga Schist Belt, India

Article

Apr 1995
CHEM GEOL

Banded iron formations (BIF's) of the Chitradurga Schist Belt (CSB) occur in association with (1) arenite-stromatolitic carbonate-argillite, (2) volcanic-volcaniclastic and (3) greywacke-argillite volcanic suites. Cherty oxide-, carbonate- and sulphide-facies BIF's are exposed within 15 km from west to east. In addition, shales (Sh) and shaly banded iron formation (SBIF) occur with all the three types. Cherty carbonate-, oxide- and sulphide-facies BIF's show close similarities in their majorand trace-element constituents except in their CO2, S and Ce contents. Pure cherty BIF's are depleted in ΣREE, Zr, Hf, Cr, Ni, Co, Nb, Ta, V, Sc and Y, and are generally enriched in La and have strong positive Eu anomalies with respect to SBIF. Many samples of cherty oxide- and carbonate-facies BIF's exhibit negative Ce anomalies. However, sulphide-facies BIF's are devoid of negative Ce anomalies, and indicate that in this case oxidation of Ce³⁺ to Ce⁴⁺ has not taken place. Simultaneous increase of LREE-HREE, HfZr, CrZr, RbSr, VSc, NbTa, NdFe, and several other diagnostic pairs, suggest that the variation in the BIF composition is governed by the type and quantity of the clastic component. Available data indicate that FeO and SiO2 from the BIF's were mainly provided by the hydrothermal waters added at the vent sites of the Archaean Mid-Oceanic Ridges (AMOR) or at some unidentified off-the-shelf volcanically active environment. As a consequence of thermal and chemical potential differences and upwelling, the FeO- and SiO2-enriched ocean water was transported to the shallow-shelf environment, where it reacted with photosynthetic O2 to precipitate Fe-oxides and -hydroxides. Upon burial in shallow shelf regions, the oxides and hydroxides locally reacted with the organic carbon to produce Fe-carbonates. Primary carbonates were also precipitated in the basin along with other types of BIF's. In the deeper part of the ocean, Fe-sulphides were precipitated in a reducing environment near vent sites. Terrigenous and volcaniclastic debris were delivered to the basin during transgression and volcanic activity. It is therefore proposed that the compositional characteristics of the BIF's from the CSB are dependent on the availability of dissolved CO2, O2, particulate carbon and clastic input in the different parts of the basin. Facies changes are related to the compositional layering of the ocean and its transgressioe-regressive cycles. A mini plate model with increased ridge-trench length, shelf area and hydrothermal water flux of higher exit temperature combined with the advanced stages of biological activity explains the genesis of BIF's found in greenstone belts.

Actualistic approaches to interpreting the role of biological decomposition in microbial preservation

Article

Oct 2021

Taphonomic processes, especially post‐mortem biological decomposition, act as crucial controls on the microbial fossil record. Information loss during the fossilization process obscures interpretation of ancient microbial ecology and limits our view of preserved ecosystems. Conversely, taphonomic information can itself provide insight into fossilization pathways and processes. This information‐gain approach requires specific attention to taphonomic patterns in ancient assemblages and robust modern analogue data to serve as points of reference. In this study, we combine experimental taphonomy with decomposition models in order to constrain taphonomic hypotheses regarding Proterozoic microfossil assemblages. Several filamentous and coccoidal prokaryotic and eukaryotic phototrophs were evaluated for taphonomic pattern over the course of a short (~100 days) decomposition experiment. In parallel, simple numerical models were constructed to explain potential taphonomic pathways. These analogue data were then compared to two Mesoproterozoic fossil assemblages, the ~1.5 Ga Kotuikan Formation, Siberia, and the ~1 Ga Angmaat Formation, Canada. Concordant with previous experiments and observations, our results suggest that sheath morphology is more persistent than cell/trichome morphology during early stages of decomposition. These experiments also suggest that taphonomic change in cell morphology may follow one of several trajectories, resulting in distinct taphonomic endpoints. Model output suggests two categories of underlying mechanism and resultant taphonomic trajectory: (1) uniform decomposition, resulting in a low overall taphonomic grade and poor preservation, and (2) faster decomposition of structurally compromised individuals, producing a final population with better overall preservation of very few individuals. In this experiment, cells of coccoidal organisms exhibit the first pattern and trichomes of filamentous organisms and some sheaths exhibit the second. Comparison with preserved microfossil assemblages suggests that differences in taphonomic pattern between parts of an assemblage could be useful in assessing taphonomic processes or degree of taphonomic loss in an entire assemblage.

Precambrian Paleobiology: Precedents, Progress, and Prospects

Article

Full-text available

Aug 2021

J. William Schopf

In 1859, C. R. Darwin highlighted the “inexplicable” absence of evidence of life prior to the beginning of the Cambrian. Given this lack of evidence and the natural rather than theological unfolding of life’s development Darwin espoused, over the following 50 years his newly minted theory was disputed. At the turn of the 19th century, beginning with the discoveries of C. D. Walcott, glimmerings of the previously “unknown and unknowable” early fossil record came to light – but Walcott’s Precambrian finds were also discounted. It was not until the breakthrough advances of the 1950’s and the identification of modern stromatolites (1956), Precambrian phytoplankton in shales (1950’s), stromatolitic microbes in cherts (1953), and terminal-Precambrian soft-bodied animal fossils (1950’s) that the field was placed on firm footing. Over the following half-century, the development and application of new analytical techniques coupled with the groundbreaking contributions of the Precambrian Paleobiology Research Group spurred the field to its international and distinctly interdisciplinary status. Significant progress has been made worldwide. Among these advances, the known fossil record has been extended sevenfold (from ∼0.5 to ∼3.5 Ga); the fossil record has been shown consistent with rRNA phylogenies (adding credence to both); and the timing and evolutionary significance of an increase of environmental oxygen (∼2.3 Ga), of eukaryotic organisms (∼2.0 Ga), and of evolution-speeding and biota-diversifying eukaryotic sexual reproduction (∼1.2 Ga) have been identified. Nevertheless, much remains to be learned. Such major unsolved problems include the absence of definitive evidence of the widely assumed life-generating “primordial soup”; the timing of the origin of oxygenic photosynthesis; the veracity of postulated changes in global photic-zone temperature from 3.5 Ga to the present; the bases of the advent of eukaryotic sexuality-requiring gametogenesis and syngamy; and the timing of origin and affinities of the small soft-bodied precursors of the Ediacaran Fauna.

Carbon‐rich microfossils preserved in the Proterozoic crater‐filling breccias of the Sudbury impact structure, Canada

Article

Dec 2020

Two forms of carbon‐rich microfossils were discovered in the breccias of the Onaping Formation, Sudbury impact structure. The first form is represented by single particles scattered in the matrix of the breccias and included in the vesicles in altered glass. These particles are leaf‐shaped, stem‐shaped, tubular, and spherical objects ranging from 5–10 μm to 200–300 µm in size. It is suggested that algal flora inhabiting the ocean basin before the Sudbury impact was the source of plant material in the Onaping Formation. The second form of carbon‐rich microparticles in the Onaping Formation is represented by plant detritus in carbon‐bearing fragments of mudstones included in the breccia matrix. The microparticles in the mudstones are mainly ribbon‐like shreds from 3–5 µm to 200–300 µm long, while rare particles have more complex shapes. The matrix of the mudstones is a fine‐grained clay‐like substance with a network of micron‐wide open‐joint fissures. Contents of carbon in the mudstone matrix are 12–15 wt%. Muddy bottom sediments of the pre‐impact sea are supposed as a source of mudstone fragments in the breccias. Fragments of mudstones and carbon‐rich microparticles are an important source of organic carbon in the breccias of the Onaping Formation. Discovery of microfossils in the breccias of the Onaping Formation suggests the presence of a previously unknown complex algal flora that inhabited the pre‐impact sea before the impact event 1.85 billion years ago at the very end of the Paleoproterozoic.

Microbial Oxidation of Fe(II) and Mn(II) at Circumneutral pH

Chapter

Apr 2014

David Emerson

The biogeochemistry of ferruginous lakes and past ferruginous oceans

Article

Oct 2020
EARTH-SCI REV

Anoxic and iron-rich (ferruginous) conditions prevailed in the ocean under the low-oxygen atmosphere that occurred through most of the Archean Eon. While euxinic conditions (i.e. anoxic and hydrogen sulfide-rich waters) became more common in the Proterozoic, ferruginous conditions persisted in deep waters. Ferruginous ocean regions would have been a major biosphere and Earth surface reservoir through which elements passed through as part of their global biogeochemical cycles. Understanding key biological events, such as the rise of oxygen in the atmosphere, or even the transitions from ferruginous to euxinic or oxic conditions, requires understanding the biogeochemical processes occurring within ferruginous oceans, and their indicators in the rock record. Important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes; their sediments commonly host siderite and Ca-carbonates, which are important Precambrian records of the carbon cycling. Lakes that were ferruginous in the past, or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid-depth oceanic waters during the Proterozoic. Modern ferruginous meromictic lakes, which host diverse anaerobic microbial communities, are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans. Such lakes are believed to be rare, but regional and geological factors indicate they may be more common than previously thought. While physical mixing processes in lakes and oceans are notably different, many chemical and biological processes are similar. The diversity of sizes, stratifications, and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems: near-shore, off-shore, silled basins, or those dominated by terrestrial or hydrothermal element sources. Ferruginous systems, both extant and extinct, lacustrine and marine, host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of Earth's surface environment.

Diverse communities of Bacteria and Archaea flourished in Palaeoarchaean (3.5-3.3 Ga) microbial mats

Article

Full-text available

Sep 2020
Palaeontology

Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining early ecosystems. Applying Fourier transform infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific functional groups, we characterized fossilized biopolymers in 3.5-3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite high aromaticity. This reflects precursor biological heterogeneity since all horizons show equally exceptional preservation and underwent similar grades of metamorphism. Low methylene to end-methyl (CH 2 /CH 3) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n-alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH 2 /CH 3 ratios suggesting mostly longer, unbranched fatty acids from bacterial lipid precursors. Absorbance ratios of end-methyl to methylene (CH 3 /CH 2) in Hooggenoeg, Josefsdal and Footbridge mats yield a range of values (0.20-0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Higher (0.78-1.25) CH 3 / CH 2 ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from biomass. Since silicification commenced during the lifetime of the microbial mat, FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Together, these results show that Bacteria and Archaea flourished together in Earth's earliest ecosystems.

Triple oxygen isotopes of cherts through time

Article

Aug 2020
CHEM GEOL

It has long been recognized that the stable oxygen isotope ¹⁸O/¹⁶O ratios of chemical sediments show a systematic decrease with increasing age. This continuous, 3.8 Ga long secular trend has been attributed to 80 °C hot early Earth oceans, a ~15‰ lower ¹⁸O/¹⁶O ratio of Archean seawater, or diagenetic obliteration of the original isotope signal. We demonstrate that high-precision triple oxygen isotope measurements including the rare third stable oxygen isotope ¹⁷O on silica rocks (cherts) can be used to shed light on this major problem in geochemistry. Our triple oxygen isotope data do not support 80 °C hot oceans in the Archean. The data also exclude that the ¹⁸O/¹⁶O of seawater was much lower in the Archean. Also, a simple freshwater-origin of cherts is not supported by the new data. We discuss the effects of diagenetic alteration on the triple oxygen isotope compositions of cherts and show that the Archean and Proterozoic cherts could have equilibrated with fluids with higher ¹⁸O/¹⁶O than today's oceans. For the Archean cherts, the high ¹⁸O/¹⁶O fluids required to explain the observations could reflect the composition of ambient seawater or of modified hydrothermal fluids. Oceans with higher ¹⁸O/¹⁶O ratio would be expected for an Archean “water world” without large continents. High ¹⁸O/¹⁶O fluids, however, could also have formed locally by reactions between seawater and the oceanic crust. Our study shows that high-precision triple oxygen isotope data provide new understandings of processes deep in time.

Morpho- and Chemo-Fossil Evidence of Early Life

Chapter

Jun 2020

Mark Harrison

This chapter summarizes what is known about the timing of the emergence of life on Earth from the morpho- and chemo-fossil (chemical and isotopic signals remaining from the decomposition of living organisms) records. The geologic record back to ca. 3.5 billion years includes low grade sedimentary rocks in which organic residues of microbiota present during deposition have remained substantially intact. As different metabolic mechanisms variably fractionate carbon isotopes toward isotopically light values, a longstanding strategy has been to measure δ¹³C in these organic residues, or kerogens, for biologic signatures. When compared to carbon isotopes in inorganic carbonate rocks, a consistent offset is seen throughout the past 3.5 billion years with inorganic carbon averaging δ¹³C close to 0‰ and kerogens yielding δ¹³C of approximately −25‰. As the latter value is broadly characteristic of oxygenating photosynthesis, this relationship has been seen as evidence of past biologic activity. However, as metamorphic grade increases, kerogens are reacted to simpler hydrocarbons, ultimately yielding graphitic residues. The discovery of isotopically light carbon isotopes in microscopic graphite inclusions in rocks as old as ca. 3.83 billion years and in a 4.1 Ga zircon extends the possible emergence of life on this planet back into the Hadean eon. Although inorganic mechanisms exist that could potentially produce light δ¹³C signatures, these isotopic data are consistent with molecular clock calibrations of genomic mutations which suggest a lower bound for the time of life’s origin between 4.1 and 4.4 billion years.

Microorganisms from the Gunflint Chert

Abstract

No full-text available

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