Investigating the potential for pseudofossil formation in Earth's ancient sediments.

The earliest evidence for life on Earth comes in two forms; chemical evidence, i.e. compounds believed to have been derived from the activities of living organisms and isotopic signals thought to be indicative of biological activity, and through the presence of structures believed to be microfossils in ancient sedimentary deposits. In both cases, identifying these with confidence requires considerable skill, as there remains a danger that abiotic processes might have achieved the same results. One of these dangers is the potential formation of pseudofossils (structures which resemble fossils but are of non-biological origin). The most ancient structures interpreted as being microfossils take two forms, microbial filamentous and spherical cells, but structures morphologically similar to both of these have been synthesised on the lab by non-biological means, raising the possibility that none of these ancient 'fossils' are actually of biological origin. The researchers were able to recreate these strucutures, also known as organic biomorphs, by oxidising sulphides in the presence of organic materials, under which circumstances they formed spontaneously. This has been shown to be possible under a wide range of conditions likely to have been present on the ancient Earth, and with a wide range of organic compounds serving as precursors.

However, the fact that such biomorphs can be formed in the laboratory does not automatically prove that all, or indeed any, of the various Archaean and Palaeoproterozoic fossil deposits are in fact pseudofossils; indeed some of these have been studied for many years and their status as being of biological origin is not really in doubt. Nevertheless, some caution is clearly needed when establishing the nature of any apparent fossils in these ancient deposits, particularly if their host rocks are sulphide-rich.

In the oceans of the Proterozoic, euxinic (low oxygen, high sulphur) conditions are thought to have been fairly common, and this may have also have sometimes been the case in the Archaean, at least at a local level, although ocean sulphide levels appear to have been low for the most part. Many of the microfossils known from these deposits are associated with pyrite, which indicates the deposits which produced them did indeed have raised sulphur levels. However, there is not currently any clear data on the likelihood of biomorphs being preserved in these deposits.

In a paper published in the journal Geology on 28 January 2021, Christine Nims and Julia LaFond of the Department of Geosciences at Pennsylvania State University, Julien Alleon of the Institut des Sciences de la Terre at the Université de Lausanne, Alexis Templeton of the Department of Geological Sciences at the University of Colorado, Boulder, and Julie Cosmidis, also of the Department of Geosciences at Pennsylvania State University, describe the results of an experiment in which they performed experimental silicification of organic biomorphs along with the Sulphur Bacterium Thiothrix, in order to assess the likelihood of their becoming preserved in the fossil record as pseudofossils.

 

Side-by-side comparison of Precambrian putative organic microfossils and organic biomorphs synthesized in the laboratory. (A) Organic strand from the 3.5 billion-year-old Dresser Formation (Western Australia). OM—organic material; Py—pyrite. (C), (G) Cluster of spheres (C) and 'straw-like' filaments (G) from the 2.4–2.2 billion-year-old Turee Creek Group (Western Australia). Spheres in panel (C) inset are from the 3.4 billion-year-old Strelley Pool Formation (Western Australia). (E), (I) Rosette (E) and cluster of filaments (I) from the 1.9 billion-year-old Gunflint Formation (northeastern North America). (K), (M), (O) Rigid branching filaments (K), 'river' of flexible filaments (M), and cobweb-like network of filaments (O) from the 2.4–2.2 billion-year-old Turee Creek Group. (B), (D), (F), (H), (J), (L), (N), & (P) Organic biomorphs synthesized in the laboratory. Nims et al. (2021).

As the majority of putative Precambrian microfossils are preserved in chert (fine-grained sedimentary rock composed of microcrystalline crystals of quartz), Nims et al. decided to investigate the preservational potential of organic biomorphs through silicification. They also carried out the same experimental procedures on mats of the sulphur-oxidising Bacterium Thiothrix as a control measure; experiments on the silicification of colonial micro-organisms have been undertaken before, but the majority of these have been performed on Cyanobacteria, despite many putative Precambrian microfossils being interpreted as most likely being sulphur-cycling organisms. Nims et al. felt that Thiothrix would be a good analogue for these organisms, as it forms intracellular sulphur globules.

Nims et al. obtained organic biomorphs by reacting dissolved sulphides with yeast extract (which contains a variety of complex organic compounds) in a sterile environment. Both the biomorphs and the Thiothrix mats were then placed into a supersaturated sodium-metasilicate solution, then stored for up to five months at room temperature. Samples were taken from these experiments at regular intervals, and examined using scanning electron microscopy and transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, Raman spectromicroscopy, attenuated total reflectance–Fourier transform infrared spectroscopy, X-ray absorption near-edge structure at the sulphur K-edge, and scanning transmission X-ray microscopy at the carbon K-edge, nitrogen K-edge, and sulphur L-edge.

The biomorphs produced were a mixture of spheres (0.5–3 μm in diameter) and filaments (0.1–1 μm thick). These generally retained their shape during the silicification process; whereas the spherical Thiothrix cells rapidly lost their shape, first becoming elongate, then deforming into flattened ribbons. As non-silicified Thiothrix cells retained their shape. Nims et al. presume this flattening to be a result of the silicification process, rather than the microscopy preparation. In contrast, the silicification process appeared to have little impact upon the shape of the organic biomorphs, other than a small degree of fragmentation in some of the filamentous specimens. In fact, the biomorphs were preserved very rapidly, by the precipitation of nano-colloidal silica on their surfaces, which formed a thin crust around each specimen, preserving it from any further degredation. The Thiothrix cells, in contrast, quickly became covered in a thick silica-gel, possibly due to the presence of extracellular polymeric substances around the Bacterial cells, which may have caused the silica to behave in a different way, or possibly due to the metabolic activity of the cells, which initially reduced the pH of the surrounding media, enabling an increase in the silica saturation, whereas the pH around the biomorphs remained constant at about 7.

 

Scanning electron microscopy images of organic biomorphs (A)–(D) and Sulphur Bacterium Thiothrix cells (E)–(H) prior to and at different times throughout silicification. Note the silica nano-colloids at the surfaces of spherical biomorph in (B) and of Thiothrix filaments in (F). Nims et al. (2021).

During the silification process, the discrete globules of elemental sulphur present within the cells of Thiothrix broke up, with the sulphur becoming diffused out of the cells into the surrounding medium. Sulphur also diffused out of the spherical biomorphs, leaving empty organic vesicles, but here it subsequently re-precipitated along the envelope of the silicified biomorphs, as both sulphur and oxidised sulphur forms (such as sulphate, thiosulphate, and/or sulphones and ester sulphates).

Nims et al. suggest it is likely that the diffusion of sulphur from both the Thiothrix cells and the organic biomorphs was caused by solubilisation as polysulphides, which are highly reactive toward organics, causing rapid organic-matter sulphurisation. This may be the cause of the formation of the sulphur-rich organic envelopes around the organic biomorphs. Attenuated total reflectance–Fourier transform infrared spectroscopy showed that sulphur-bearing groups such as sulphates and sulphones were forming during silification, although it was not possible to confirm the incorporation of intramolecular sulphur into the biomorphs. Early digenetic sulphurisation would favour the preservation of microstructures in the rock record, so establishing whether this actually happens with the organic biomorphs would be a major step in establishing their preservation potential.

 

High-resolution imaging and chemical mapping of organic biomorphs and Sulphur Bacterium Thiothrix cells throughout silicification. (A)–(F) High-angle annular dark field–scanning transmission electron microscopy images and corresponding energy-dispersive X-ray spectroscopy maps of biomorphs prior to silicification (A), (B) and two weeks into silicification (C)–(F). (G)–(L) High-angle annular dark field–scanning transmission electron microscopy images and corresponding energy-dispersive X-ray spectroscopy maps of Thiothrix cells prior to (G), (H) and two months into (I)–(L) silicification. Energy-dispersive X-ray spectroscopy maps show distribution of sulphur in yellow and silica in cyan, except in (H), where carbon is in blue and sulphur in red. Nims et el. (2021).

If sulphur was lost from the organic biomorphs during silicification, then the result would be organic microstructures in the chert which did not contain sulphur-bearing minerals. However, if they were being preserved in an iron- and sulphur-rich environment, we would expect to see the formation of pyrites close to the preserved biomorph structures, something which is commonly observed in cherts which host ancient organic microfossils.

Prior to silicification, the composition of the biomorphs was dominated by carboxylic groups and unsaturated carbon, along with aliphatics, alcohols, and carbon-oxygen groups. During the silicification process, the proportions of aliphatic, aromatic, and unsaturated carbon compounds rose, while those of other organic compounds fell. In Thiothrix cells the composition was dominated by amide groups (the major component of proteins) both prior to and during silicification. It has previously been established that if peptides are present in the synthesis medium, then amides can also be incorporated into organic biomorphs. Nims et al. did not detect any amide groups in the biomorphs, despite these having been formed using yeast extract, which contains peptides. However, nitrogen was found in the envelopes of the biomorphs, in an unidentified inorganic or organic form. Thus, the carbon and nitrogen species present in the biomorphs and Thiothrix were quite different.

The initial carbon/nitrogen ratio of the biomorphs was 0.27, which rose to 0.40 during the silicification process, whereas that of the Thiothrix cells started at 0.16 and rose to 0.42, i.e. a similar value to that of the biomorphs. It is unclear how this proportion would be affected by subsequent high-temperature and pressure diagenesis, although it is possible that this might result in some detectable difference in the carbon/nitrogen ratio of organic biomorphs and Bacterial cells developing.

 

Scanning transmission X-ray microscopy analyses of organic biomorphs and Sulphur Bacterium Thiothrix cells prior to and one week into silicification. (A)–(H) Scanning transmission X-ray microscopyimages and corresponding scanning transmission X-ray microscopy chemical maps of biomorphs (A)–(D) and Thiothrix (E)–(H). Pre-silicification images (A) and (E) show dense sulphur spheres inside spherical biomorphs and Thiothrix cells. Scanning transmission X-ray microscopy maps (B), (D), (F), (H) show distribution of carbon (red), nitrogen (green), and sulphur (blue; in (F) only). (I) X-ray absorption near-edge structure spectra covering carbon (C) and nitrogen (N) K-edges, and calculated N/C ratios. Black rectangle shows the carbon K-edge spectral range (closeup in (J)). Black curves show the fitting functions for N/C ratio calculations. Spectrum of silicified Thiothrix includes a feature at ∼350 eV, corresponding to calcium. (J) Closeup of carbon K-edge X-ray absorption near-edge structure spectra. Energies of the main absorbance features are indicated. Nims et al. (2021).

Nims et al. conclude that organic biomorphs can form via the reaction of sulphides with organic compounds, and are highly likely to be preserved as pseudofossils in chert by the process of rapid silica encrustation, possibly in combination with the sulphurisation of organic matter. Such pseudofossils would not only be extremely similar to fossils produced by Bacteria, or similar Prokaryotes, they would also have similar chemical characteristics. Indeed, such organic biomorphs might actually have better preservational potential than actual microbes.

None of this proves that any Precambrian microfossil assemblage is in fact made up of pseudofossils, but it certainly suggests that there is a possibility for pseudofossil assemblages to exist in these ancient rocks, and indicates that a degree of caution must be used before such assemblages are accepted as being of biological origin. In particular, specimens with an apparently Bacterial morphology and chemical composition will need to be viewed with some caution, particularly if they originate from deposits interpreted as having been laid down in high-sulphur environments. Nims et al. strongly feel that more work must be done to identify possible geochemical signatures which might indicate specimens are of non-biological origin. 

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Identifying the world's oldest impact structure.

Extraterrestrial bombardment flux is speculated to have had major consequences for the development of Earth’s surface environment. However, the terrestrial impact record is fragmentary, principally due to tectonics and erosion, and is progressively erased into the geologic past when, conversely, the bombardment rate was larger than today. The oldest record of impacts on Earth are Archaean to Palaeoproterozoic ejecta deposits found within the Kaapvaal craton of southern Africa and the Pilbara Craton in Western Australia, spanning roughly  3470 to 2460 million years ago; however, no corresponding impact craters have been identified. Currently only two precisely dated Precambrian-age impact structures are known, the 2023-million-year-old, 350 km diameter Vredefort Dome in South Africa, and the 1850 million year old 200 km diameter Sudbury structure in Canada. Other purported Palaeoproterozoic-age impact structures have either poorly constrained ages, or highly contentious impact evidence. A consequence of the incomplete terrestrial impact record is that connections between impact events and punctuated changes to the atmosphere, oceans, lithosphere, and life remain difficult to establish, with the notable exception of the Cretaceous–Paleogene Chicxulub impact. Hitherto, the impact cratering record was absent from 2.5–2.1 billion years ago, when significant changes in the Earth’s hydrosphere and atmosphere occurred.

In a paper published in the journal Nature Communications on 21 January 2020. Timmons Erickson of the Astromaterials Research and Exploration Science Division at NASA's Johnson Space Center, the Institute for Geoscience Research at Curtin University, and the Center for Lunar Science and Exploration of the Universities Space Research Association, Christopher Kirkland, Nicholas Timms, and Aaron Cavosie, also of the Institute for Geoscience Research at Curtin University, and Thomas Davison of the Impacts and Astromaterials Research Centre at Imperial College London, identify the Yarrabubba Impact Structure in Western Australia as the oldest surviving impact structure on Earth, and give their reasons for this assessment and the implications for the history of the Earth's geology and biosphere.

Yarrabubba is a recognised impact structure located within the Murchison Domain of the Archaean granite-greenstone Yilgarn Craton of Western Australia. No circular crater remains at Yarrabubba; however, the structure has an elliptical aeromagnetic anomaly (magnitid anomaly detectable from the air) consisting of an even, low total magnetic intensity domain, measuring approximately 20 km north-south by 11 km east-west. The present day exposure represents a deep erosional level, as neither impact breccias nor topographic expressions of the over-turned rim or central uplift are preserved. Therefore, the 20 km diameter magnetic anomaly has been interpreted to represent the remnant of the deeply buried central uplift of the structure, which is consistent with an original crater diameter of 70 km. Unshocked dolerite dykes formed during either the roughly. 1200 million years ago Muggamurra or roughly 1075 million years ago Warakurna regional volcanism cross-cut the elliptical magnetic anomaly and thus post-date the impact event.

Composite aeromagnetic anomaly map of the Yarrabubba Impact Structure within the Yilgarn Craton, Western Australia, showing the locations of key outcrops and samples used in this study. The image combines the total magnetic intensity (TMI, cool to warm colours) with the second vertical derivative of the total magnetic intensity (2VD, greyscale) data. The demagnetised anomaly centred on the outcrops of the Barlangi granophyre is considered to be the eroded remnant of the central uplift domain, which forms the basis of the crater diameter of 70 km. Prominent, narrow linear anomalies that cross-cut the demagnetised zone with broadly east-west orientations are mafic dykes that post-date the impact structure. Erickson et al (2020).

The main target rocks at the Yarrabubba structure are granitoids collectively known as the Yarrabubba Monzogranite. Identification of shocked quartz and shatter cones in the Yarrabubba Monzogranite confirmed an impact origin for the structure. The structure is centred on a large exposure of granophyre known locally as Barlangi Rock. Barlangi Granophyre is a sodic rhyolite that has been interpreted as an impact-generated melt rock, radiating dykelike apophyses of granophyre outcrop as far as 3 km from the centre of the structure. The Barlangi granophyre has thus been interpreted to have intruded into the Yarrabubba Monzogranite along faults rather than forming a flat-lying, crater-filling melt sheet, similar to metanorite dykes and apophyses interpreted as impact melt that are exposed in the core of the deeply eroded Vredefort impact structure.

The age of the Yarrabubba impact structure was previously constrained only to be younger than the 2650-million-year-old Yarrabubba Monzogranite and older than the 1200-1075-million-year-old cross-cutting dolerite dykes. Zircon crystals from the Barlangi Granophyre have previously yielded a complex age spectrum that span nearly 500 million years, from 2.79 to 2.23 billion years ago. Pseudotachylite veins at Yarrabubba yield a sericite Argon³⁹/Argon⁴⁰ age of about 1.13 billion years, which likely records alteration during younger mafic volcanism.

Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of Argon⁴⁰ to Argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further Argon³⁹ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.

Erickson et al.'s study utilises targeted in situ Uranium-Lead geochronology by secondary ion mass spectrometry to analyse recrystallised domains (neoblasts) in monazite and zircon, which have been shown to yield precise ages for ancient impact events. They present high-resolution orientation mapping and correlated in situ Uranium-Lead analysis to investigate the microstructure and age of shock features in zircon and monazite in target rock and impact melt from the Yarrabubba structure in Western Australia. These results establish Yarrabubba as the oldest preserved impact structure on Earth.

Zircon and monazite are minerals formed by the crystallisation of cooling igneous melts. Whenthey  form they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

Within the Yarrabubba Monzogranite, zircon and monazite grains preserve a range of impact-related microstructures. Zircon displays primary igneous growth zoning that is cross-cut by planar and subplanar shock microstructures, including shock twins and planar deformation bands. Monazite preserves a broader range of impact-related textures including domains with low-angle subgrain boundaries and multiple sets of shock twins along three fracture plains and domains of strainfree neoblasts (grains formed more recently than the surrounding matrix).

Examples of shocked zircon and monazite grains from Yarrabubba monzogranite sample 14YB07 and Barlangi Granophyre impact melt sample 14YB03. (a) Cathodoluminescence (CL) and Inverse Pole Figure (IPF) images of a shocked zircon with deformation twins. The zircon contains primary oscillatory zoning that is cross-cut by shock deformation twins and subplanar low-angle grain boundaries. (b) Backscattered electron (BSE) image and electron backscatter diffraction (EBSD) all Euler map of a shocked monazite with systematic shock twin domains that are overprinted by neoblasts. (c) Cathodoluminescence and Inverse Pole Figure images of a polycrystalline shocked zircon grain from the granophyric impact melt. Note that individual crystallites exhibit concentric Cathodoluminescence zonation patterns while the overall Cathodoluminescence pattern reflects the original zonation of the grain with a Cathodoluminescence brighter core to Cathodoluminescence dark rim. The Inverse Pole Figure orientation map of the zircon is dominantly blue to pink and many of the granules have systematic grain boundaries of either 65° or 90°. (d)  Backscattered electron image and all Euler map of shock-deformed monazite displaying highly deformed and twinned domains that are overprinted by neoblasts, from the Barlangi Granophyre. Location of Uranium-Lead secondary ion mass spectrometry analytical spots are denoted on each grain with the Lead²⁰⁷/Lead²⁰⁶ age and 2σ errors in millions of years. Erickson et al. (2020).

In The Barlangi Granophyre, zircon textures range from unshocked grains preserving primary igneous growth zoning, to grains containing clear evidence of impact metamorphism, such as planar microstructures, polycrystalline aggregates and grains with Zirconium Oxide inclusions. Neoblasts within polycrystalline zircon aggregates contain systematic misorientation relationships with one another, which can only be caused by recrystallisation after formation of twins and the highpressure polymorph reidite, respectively, and are unambiguous indicators of shock metamorphism. While the Zirconium Oxide inclusions index as baddeleyite (monoclinic-Zirconium Oxide), crystallographic orientation relationships among transformation twins demonstrate they originally formed from tetragonal-Zirconium Oxide parent grains. Thermal dissociation of zircon to tetragonal-Zirconium Oxide only occurs in silica-saturated melts above 1673 °C, unequivocally indicating the Barlangi Granophyre was a super-heated impact melt. Monazite grains in the Barlangi Granophyre preserve a similar range of impact-related features to those from Yarrabubba Monzogranite, including crystal-plastic strain, deformation twins diagnostic of shock conditions, and strain-free neoblastic domains.

Yarrabubba Monzogranite (14YB07), zircon. Erickson et al. (2020).

The Uranium-Lead secondary ion mass spectrometry analyses of zircons from the Yarrabubba Monzogranite produced two clusters of dates; the first centred on 2626 million years ago, and the second centred on 1202 million years ago. The older date is interpreted as the primary magmatic crystallisation age of the target rocks, which has previously been constrained to between 2670 and 2630 million years old. The younder date is attributed to partial resetting associated with post-impact dolerite intrusion in the Mesoproterozoic. Monazite Lead²⁰⁷/Lead²⁰⁶ ages from the Yarrabubba Monzogranite also yield a bimodal distribution. Analytical spots from the high-strain shocked host and/or twin domains are variably discordant and record ²⁰⁷Lead/²⁰⁶Lead ages from 2478 to 2323 million years old. These ages may represent either formation during a post-crystallisation metamorphic event or partial radiogenic Lead-loss during the impact event or a subsequent thermal event. In contrast, spots from low-strain, randomly oriented neoblasts cluster around a Lead²⁰⁷/Lead²⁰⁶ age of 2227 million years old (Lead²⁰⁷/Lead²⁰⁶ dating is a varient on Uranium-Lead dating, typically used on bulk rock samples; it relies on the fact that both Lead²⁰⁷ and Lead²⁰⁶ are produced by the decay of uranium in a known ratio, while non-radiogenic Lead²⁰⁴ is not, so that over time the ratio of these isotopes in a sample of rock that originally contained uranium will change at a predictable rate).

Yarrabubba Monzogranite (14YB07), monazite. Erickson et al. (2020).

Barlangi \Granophyre zircon Lead²⁰⁷/Lead²⁰⁶ ages also show a bimodal age distribution. Erickson et al. interpret the oscillatory-zoned cores with apparent ages of 2781 to 2319 million years old to represent inherited (pre-impact) zircon grains that were incorporated into the Barlangi Granophyre as xenocrysts, consistent with zircon ages determined previously. These results indicate the presence of a significant source component in the Barlangi Granophyre that predates the 2.65-billion-year-old Yarrabubba Monzogranite. Individual analyses from polycrystalline zircon domains are variably discordant and yield Lead²⁰⁷/Lead²⁰⁶ ages from 2259 to 2156-million-years-old. Erickson et al. interpret the data to represent near-recent Lead-loss resulting from exposure to surface fluids. The data from recrystallised zircon domains yields an upper age of 2246 million years. Erickson et al. interpret this date to reflect both new zircon growth and near complete resetting of Uraniaum-Lead systematics in pre-existing domains during shock metamorphism. This date falls within the uncertainty of a single previously reported Uraniaum-Lead zircon analysis from the Barlangi Granophyre, which gave a date range of 2262-2206 million years, and which was inferred to indicate a Palaeoproterozoic impact age.

Barlangi Granophyre (14YB03), zircon. Erickson et al. (2020).

Barlangi Granophyre monazite Lead²⁰⁷/Lead²⁰⁶ ages preserve a bimodal distribution similar to monazite from Yarrabubba Monzogranite. Analyses from the highly strained host and twinned domains display variable normal and reverse discordance and record Lead²⁰⁷/Lead²⁰⁶ ages of between 2457 and 2284 million years. In contrast, analyses from low-strain, randomly oriented neoblasts cluster around a  Lead²⁰⁷/Lead²⁰⁶ age of 2231 million years.

Barlangi Granophyre (14YB03), zircon. Erickson et al. (2020).

When combined, all neoblastic monazite domains from both Barlangi Granophyre and Yarrabubba Monzogranite define a cluster on a mean Lead²⁰⁷/Lead²⁰⁶ age of 2229 (±5) million years, which Erickson et al. interpret to record monazite recrystallisation during shock metamorphism and the best estimate of the Yarrabubba impact event. The weighted mean Lead²⁰⁷/Lead²⁰⁶ age for neoblastic zircon of 2246 (±17) million years, overlaps with the Monazite age, but is less precise. The new Yarrabubba impact age of 2229 million years determined by Erickson et al. extends the terrestrial record of impact craters by 200 million years, and demonstrates the potential for discovery of ancient impact structures on Archaean cratons.

Barlangi Granophyre (14YB03), monazite. Erickson et al. (2020).

The age constraints presented by Erickson et al. establish Yarrabubba as the first recognised meteorite impact to have occurred during the Rhyacian Period, a dynamic time in the evolution of Earth following the transition from the Archaean to the Proterozoic eon. At least four glacial diamictite deposits, three of which are found on multiple cratons, are recognised between 2.4 and 2.2 billion years ago. Of these deposits, the over 2.42-billion-year-old Makganyene Diamictite from the Kaapvaal Craton of Southern Africa has been interpreted to represent lowlatitude glaciers that may signify global ice conditions. The youngest Palaeoproterozoic glacial deposit, the Rietfontein Diamictite within the Transvaal Basin of South Africa, has a minimum depositional age of 2225 million years, based on the overlying Hekpoort \Basalt, which is within analytical uncertainty of the Yarrabubba Impact Event. Glacial diamictite deposits do not appear again in the geological record for over 400 million years. What caused the extended absence of glacial conditions after about 2225 million years ago is debatable. The end of the Palaeoproterozoic glaciations at 2225 million years ago occurred within an apparent 50 million year lull in global magmatism from 2266 to 2214 million years ago, making it difficult to appeal to volcanic outgassing as having played a significant role in forcing the glacial termination. Therefore, other mechanisms such as impact cratering need consideration. Radiometric age data presented here demonstrate synchronicity, within uncertainty, between the 2229-million-year-old Yarrabubba Impact Event and the termination of glacial conditions (i.e. the Rietfontein Diamictite) at 2225 million years ago. The geographic extent of the Rietfontein Diamictite is poorly constrained, and it is not yet known if global glacial conditions existed at this time. Nonetheless, Erickson et al. apply numerical simulations to explore the potential effects that a Yarrabubba-sized impact may have had on climactic conditions.

Temporal evolution of the early Palaeoproterozoic Earth. Key features include the Yarrabubba and other impact events, the Great Oxidation Event, 2.06–2.45 billion years ago (Ga) and glaciations, 2.23–2.54 billion years ago. Note the close association of the Yarrabubba Impact Event to the end of the final Palaeoproterozoic glaciation, the Rietfontein, at 2225 million years ago and followed by the large positive carbon isotope excursion known as the Lomagundi Event. Other impacts include the 2.02-billion-years-old Vredefort Dome, and the 2.49-billion-years-old correlated Kuruman Spherule Layer in the Griqualand West basin of South Africa and the Dales Gorge Spherule Layer of the Hamersley Basin in Western Australia. Erickson et al. (2020).

Several factors caused by the Yarrabubba Impact Event could have triggered a change in regional or global climate. Depending on the ambient climate state and palaeogeographic nature of the northern Yilgarn Craton at the time of impact (e.g., ice cover, shallow ocean or carbonate platform overlying silicate basement), which is unknown, significant amounts of carbon dioxide, water vapour or other greenhouse gases could have been released into the relatively oxygen-poor Palaeoproterozoic atmosphere by the impact event. Given that the age of the Yarrabubba impact overlaps with the youngest Paleoproterozoic glacial deposits, Erickson et al. explore scenarios where the Yarrabubba Impact Site could have been covered by a continental ice sheet at the time of impact.

Numerical models using the iSALE shock physics code demonstrate that the formation of a 70-km-diameter impact crater into a granitic target with an overlying ice sheet ranging from 2 to 5 km in thickness results in the almost instantaneous vaporisation of 95–240 km³ of ice and up to 5400 km³ total melting. The vapourised ice corresponds to between 90 and 200 million kilotons of water vapour being jetted into the upper atmosphere within moments of the impact. Impact-generated water vapour in the lower atmosphere would have condensed and rapidly precipitated as rain and snow with no significant long-term climate effects, or could have even triggered widespread glacial conditions via cloud albedo effects during interglacial periods. However, ejection of high-altitude water vapour has potential for greenhouse radiative forcing, depending critically on atmospheric residence time. Uncertainties in the structure and composition of Earth’s Palaeoproterozoic upper atmosphere mean that the precise nature of atmospheric interactions of the collapsing vapour plume is inherently difficult to model. Nevertheless, considering that Earth’s atmosphere at the time of impact contained only a fraction of the current level of oxygen, a possibility remains that the climatic forcing effects of water vapour released instantaneously into the atmosphere through a Yarrabubba-sized impact may have been globally significant. Understanding the residence times of impact-produced water vapour in a cold Palaeoproterozoic atmosphere, and the complex interplay of radiative versus insulative effects of clouds, during glacial conditions requires further investigation. The effects of impact cratering have long been recognised as drivers of climate change. Many studies have described the atmospheric effects of the end-Cretaceous Chicxulub impact structure in Mexico, which resulted in global cooling of oceans and production of widespread acidic rains. While the Yarrabubba Structure, dated at 2229 million years, represents the Earth’s oldest dated impact crater, its coincidence with termination of Palaeoproterozoic glacial conditions prompts further consideration of the ability of meteorite impacts to trigger climate change.

Snapshots of the iSALE model with (a) 2-km-thick ice sheet showing a the initial conditions, (b) the transient crater and (c) the final crater. Superimposed on a is the initial position of tracer particles which were shock-heated to the critical entropy required to begin vaporisation (incipient vaporisation, red) and to completely vapourise ice (complete vaporisation, yellow). The colour scale on the right-hand side of (c) shows the peak shock pressure in the granite basement. (d) The calculated mass of ice shock-heated to the critical entropy for incipient and complete vaporisation, as a function of initial ice thickness. In each impact, the impactor size was 7 km and resulted in a final crater diameter of ~70 km. Erickson et al. (2020).

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Microfossils from the Palaeoproterozoic Hutuo Group of Shanxi Province, China.

Geological and geochemical evidence has revealed that the Neoarchean–Paleoproterozoic period was vitally important for Earth’s evolution. The earliest ‘snowball event’ and major glaciation occurred during this period. This was followed by a great oxidation event, which caused an abnormal positive shift in global carbon isotopes and is referred to as the Lomagundi Event. The emergence of oxygen-producing photosynthetic organisms that led to the sudden increase in atmospheric oxygen has been the focus of several studies. However, until now, convincing fossil records from this key geological time interval (i.e. latest Neoarchean to Palaeoproterozoic) are scarce. During this period, the biosphere experienced multiple geological events, but little is known about it and what is known is dependent on molecule clock dating analyses and estimates. To better understand the biosphere during this time, the metamorphosed Palaeoproterozoic deposits of the Hutuo Group at Wutai Mountain in Shanxi Province, China provide an excellent stratigraphic sequence in which to study well-preserved fossil records of this key period in Earth’s evolution.

In a paper published in the journal Precambrian Research on 5 February 2020, Leiming Yin and Fanwei Meng of the Nanjing Institute of Geology and Palaeontology, Fanfan Kong of the School of Resource and Earth Science at the China University of Mining and Technology, and Changtai Niu, also of the Nanjing Institute of Geology and Palaeontology, describe the results of a study of the microfossils of the Hutuo Group, and describe a number of these.

The Wutai Mountains are located in the Xinzhou area of Shanxi Province, China, between 38°50′ North and 39°05′ North, and between 113°29′ East and 113°44′ East. The Wutai Mountains of the central Trans-North China Orogen are a typical region for the investigation of Precambriansequences. The Precambrian strata in the Wutai Mountains can be divided into the Neoarchean Wutai Group, and the overlying Palaeoproterozoic Hutuo Group, separated by an unconformity.

Geology of Wutai Mountains showing the sample localities. Yin et al. (2020).

The Hutuo Group is distributed in an area of about 1500 km², from northernmost Taihuai-Sijizhuang on the south slope of Wutai Mountain to southernmost Shizui-Dingxiang, and from the upper Taishan River in the east, to Yuanpingqi village in the west. Although the Hutuo Group underwent a major tectonic movement (the ‘Lulianng Movement’) to show strong fold, which has still completely reserved many primary deposited structures, such as cyclothems (alternating stratigraphic sequences of marine and non-marine sediments), wavemarks, cross-bedding, etc. The Hutuo Group is characterised by thick carbonate and silicified rocks and has been divided into three subgroups. At the base is the Doucun Subgroup, which is dominated by terrigenous clastic sediments. This is overlain by the Dongye Subgroup, which is characterised by claret sandstone or slate in the lower part, and transitions upward into interbedded sandstone and carbonate with Stromatolites. It is dominated by dolomitic carbonate in its upper part. This is unconformably overlain by the Goujiazhai Subgroup, which consists of sandstone and local conglomerates. 

The Hutuo Group was previously determined to be more than 2366 million years old, based zircon Uranium-Lead ages (when zircon forms it often contains trace amounts of uranium, which decays into, amongst other things, lead at a known rate; since lead  will not have been present in the original crystal matrix, it is possible to calculate the age of a zircon from the ratio between these elements) from metamorphosed basic volcanic lava. More recent zircon age estimates made with more sensitie techniques are significanty younger, typically between 2150 and 1950 million years old.

Yin et al. investigated the Wenshan, Hebiancun, and Tianpengnao Formations of the Dongye Subgroup of the Hutuo Group for microfossils. The Wenshan Section is located 17.6 km southwest of Wutai County (38°36′ North and 113°7′ East). It comprises the lowermost Wenshan Formation, which is about 500 m thick and composed of metamorphic slate, and in consideration lithology of the single; 7 rock samples of slate were collected from the upper part of this section. The overlying Hebiancun Formation, which is about 1400 m thick and contains well-developed silicified carbonates containing Stromatolites and chert-like concretions. In order to obtain more possible preservation of fossil material from this deposited sequence, especially those of silicified Stromatolites and chert-like concretions, 59 rock samples were collected from middle-upper part of the Hebiancun Formation. In addition, and 76 phyllite rock samples from the uppermost Tianpengnao Formation of the Dongye Subgroup were collected from the Geziling Section, which is about 4 km east of Wutai County (38°44′ North, 113°17′ East).

Stratigraphic column of the Palaeoproterozoic Hutuo Group and sampling horizons of the Dongye Subgroup. Yin et al. (2020).

Standard palynological maceration was mainly used to obtain organic-walled microfossils from 12 slate samples of the Wenshan Formation and 76 phyllite samples from the Tianpengnao Formation. Samples of 50 g were cleaned and macerated with hydrocholric acid (37%) and hydrofluric acid (40%). Organic residues were either concentrated by heavy liquid with a specific gravity of 2.1–2.2, or poured through a 10 μm nylon mesh, and fixed slices were prepared with Canada balsam for the mounting medium of slices and sealed by paraffin. To obtain more fossil material and checking out possible contamination, which was mainly aimed at the phyllite of the Tianpengnao Formation, repeated macerations were performed. In result, total 14 samples including 2 slate samples of the Wenshan Formation and 12 phyllite samples of the Tianpengnao Formations produced organic-walled microfossils.

Additionally, 5 rock thin sections of phyllite of the Tianpengnao Formation (at least 6 mm in thickness) were cleared in distilled water were etched in dilute 8% hydrofluoric acid for 2–3 min, then cleared with distilled water, which were repeated processing in six times. Such etched rock thin sections were observed under scanning electron microscope to show preserved specimens in situ.

Outcrop photographs of the Tianpengnao Formation in the Geziling section and the Hebiancun Formation in the Wenshan section. (A), (B) Black phyllite or greyish-green phyllite interbedded with carbonates in the lower (A) and upper (B) parts of the Tianpengnao Formation. (C) Siliceous concretions in dolostone of the Hebiancun Formation. Yin et al. (2020).

The Tianpengnao Formation (at least 1.95 billion years old), which is the the uppermost formation of the Dongye Subgroup contains a higher diversity of organic-walled microfossils than the underlying formations. Organic-walled microfossils from the Tianpengnao Formation were mostly obtained through palynological preparation of phyllite. They were strongly carbonized and appeared as opaque vesicles. Individual specimen showed wall folds with a few fine spines under the scanning electron microscope. Similar opaque specimens in situ also found in thin sections. of fuchsia phyllite from the lower part of the Tianpengnao Formation. However, greyish-green phyllite from the upper part of the Tianpengnao Formation yielded much better preserved organic-walled microfossils. 

The microfossils found by Yin et al are all considered to be Acritarchs or Cyanobacteria.

Acritarchs are unicellular Eukaryotic organisms (organisms with cells with a discrete nucleus) that appear in the fossil record from about 3200 million years ago until the end of the Permian, and possibly later (depending on what is classified as an Acritarch). They're affinities are unclear, and the group is probably paraphyletic (not all members sharing a common ancestry), though the majority are thought to have been unicellular planktonic Algae or the resting cysts of other unicellular organisms.

Cyanobacteria are filament-forming photosynthetic Bacteria found across the globe and with a fossil record dating back over 3.5 billion years. They are thought to have been the first organisms on Earth to obtain carbon through photosynthesis, and it is also thought that the chloroplasts (photosynthetic organelles) of eukaryotic plants and algae are descended from Cyanobacteria that lived symbiotically within the cells of ancient eukaryotes. Cyanobacteria are often known as Blue-Green Algae, but this is somewhat misleading, as the term Algae is otherwise restricted to photosynthetic eukaryotes (no other group of photosynthetic Bacteria are referred to as Algae), and because not all Cyanobacteria are blue-green in colour; many are dark green or even black.

The first Acritarch described by Yin et al. is assigned to the genus Dictyosphaera, but not to species level. A single specimen was obtained from a siliceous lens in a dolostone containing Stromatolite from the Hebiancun Formation, at the Wenshan Section. This is a spheroidal vesicle, thin-walled, with very fine net-like ornamentation on its surface, forming a polygonal or subrounded mesh with 3–6 μm in diameter; the vesicle diameter is about 52 μm; no excystment structure was observed.

The Achritarch Dictyosphaera sp., in a in thin sections of siliceous material obtained from the Wenshan Section of the Hebiancun Formation. Scale bar is 10 μm. Yin et al. (2020).

The second Acritarch described is placed on a new species and genus, and given the name Dongyesphaera tenuispina, where 'Dongyesphaera' means 'sphere from Dongye' and 'tenuispina' means 'fine-spined'. Six specimens of this Acritarch were obtained by palynological maceration of material from the upper part of the Tianpengnao Formation. They are spheroidal to sub-spheroidal vesicles, the walls of which are psilate (lacking in ornamentation) and prominently wrinkled; with fairly short, conical processes or protrusions of varying length (0.5–4.2 μm, typically 0.8–1.5 μm), their termination showing as round; vesicles are 30–35 μm in diameter; no excystment opening was observed.

Dongyesphaera tenuispina, obtained from the upper greyish-green phyllite of the Tianpengnao Formation in the Geziling Section by palynological maceration. Scale Bars are 10 μm. Yin et al. (2020).

Acritarchs of the genus Leiosphaeridia were found in both palynological maceration and thin section from phyllite of the Tianpengnao Formation, siliceous lenses in dolostone containing Stromatolites of the Hebiancun Formation, and slate of the Wenshan Formation. spp. were obtained by palynological maceration and thin section. Most are strongly carbonized and even opaque. Some specimens obtained from phyllite of the upper part of the Tianpengnao Formation are less carbonized. None of these are assigned to species level by Yin et al. The specimens are spheroid vesicles, with a circular outline in compressed specimens; the wall surface typically is psilate or with inconspicuous ornament; some specimens show irregular folds; the vesicle diameter is 33–65 μm; no excystment structure was observed.

Leiosphaeridia spp., (A) obtained by palynological maceration from the upper greyish-green phyllite; (C) and (F) obtained by palynological maceration from the lower amaranth phyllite; (J) found in a thin section of the lower amaranth phyllite. Yin et al. (2020).

A single specimen is assigned to the genus Satka. This is a spherical colony-like specimen aggregated with many cell-like spheroids that has been compressed and appears to be enveloped by a thin outer membrane. The included spheroids are deformed and show different shapes and sizes. Single spheroids are 3–7 μm in diameter, and the whole vesicle about 40 μm in diameter. This specimen comes from a greyish-green phyllite of upper part of the Tianpengnao Formation, from the Geziling location.

Satka sp., from the upper greyish-green phyllite. Yin et al. (2020).

The first Cyanobacteria described by Yin et al. are assigned to the species Eoentophysalis belcherensis. These are irregular clusters that contain small and large spheroidal to sub-spheroidal cell-like units singly or in pairs; they are in a crowded arrangement, with a common thin envelope. Cell-like units are typically 1.8–2.5 μm in diameter; irregular clusters are 15–25 μm across and are frequently stuck together. These were foiund in thin sections of siliceous lenses in dolostone containing Stromatolites from the Hebiancun Formation.

Eoentophysalis belcherensis, from the Hebiancun Formation. Scale bar is 10 μm. Yin et al. (2020),

Seven specimens found in thin sections of two samples from siliceous concretions in dolostone of the Hebiancun Formation are assigend to a new species of Eoentophysalis; which is given the specific name hutuoensis, meaning 'from Hutuo', in reference to the Hutuo River in the Wutaishan area of Shanxi Province, China. Eoentophysalis hutuoensis comprises cell-like units spheroidal, ellipsoidal or deformed by compression, and 2.5–12 μm in diameter; mostly single, a few in possible pairs and irregular clusters; characteristically crowded in colonies which are enveloped by a thick sheath-like material. Therse are several hundred cell-like units arranged in clusters or extensive colonies, that are enveloped by an opaque sheath-like material (about 5–8 μm thick) and characterized by suborbicular holes (at least 1 μm in diameter); nearly all cell-like units are single, and more distinct in laser scanning confocal microscope images. Some of show ‘lining structure’ (or remains of plasmolysis). Clusters or colonies 78–126 μm across.

Eoentophysalis hutuoensis under laser scanning confocal microscope. Yin et al. (2020).

The third Cyanobacterium described by Yin et al. is assigned to the species Sphaerophycus medium. A single specimen from a siliceous lens in a dolostone containing Stromatolite of the Hebiancun Formation comprises an irregular clump of cells about 135 μm long and 86.5 μm wide; the cells generally not mutually deformed. Individual cells are spheroidal and ellipsoidal and 4.5–12.5 μm in diameter; cell walls are about 0.5 μm thick.

Sphaerophycus medium, from a siliceous lens in a dolostone containing Stromatolite of the Hebiancun Formation. Yin et al. (2020).

The fourth Cyanobacterium described is placed in the genus Pseudodendron, but not assigned to species level. This comprises a single compressed organic carbon specimen that has two short branches connected with a single, main ‘tube-like’ filament along two sides; the surface is nearly psilate and no preserved cellularity or outer enveloped material is observed. The ends of both branches and the main filament are truncated. The main filament 140 μm long, and 5–8 μm wide; the branches 20–48 μm long and 4–4.5 μm wide. Since only one incomplete specimen was found, its placement in Pseudodendron is tentative. The specimen was observed in a thin sections of siliceous lens from a dolostone containing Stromatolite of the Hebiancun Formation from the Wenshan Section.

Pseudodendron sp., from a thin sections of siliceous lens from a dolostone containing Stromatolite of the Hebiancun Formation from the Wenshan Section. Yin et al. (2020).

The final Cyanobacteria described is Siphonophycus kestron. Many specimens of this were found in thin sections of dolostone containing Stromatolites of the Hebiancun Formation. They are single filamentous microfossils, unbranched, nonseptate, surface smooth; only preserved as carbon membrane-like remains due to degradation; and 5–8 μm across.

Specimen of Siphonophycus kestron, from a thin section of dolostone containing Stromatolite of the Hebiancun Formation. Yin et al. (2020).

The Paleoproterozoic (2.5-1.6 billion years ago) was a critical period in  Earth’s evolution. During it, important global events, such as glaciation, atmospheric oxygenation over the early period, and the following Lomagundi-Jatuli isotopic event occurred. 

The Hutuo Group, as a typical Palaeoproterozoic sequence in the North China Craton, is distributed through the Wutai and Luliang mountains in Shanxi Province, China. The lithological character of the group is characterized by metamorphic deposits and volcanic rocks. Based on recent uranum-lead isotopic age dating, the Hutuo Group is constrained to the period between 2.14 and 1.95 billion years ago. Carbon isotope excursions in the Hutuo Group have been documented as a response to the Palaeoproterozoic global glaciation. Glaciogenic diamictite (a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment) has also been discovered in the Shijiazhuang Formation of the Hutuo Group at Wutai Mountain, North China which suggests a locally protracted glacial event could have extended to the Wutaishan area. Based on the uranium-lead isotopic ages and chronological framework of the Hutuo Group, a three-stage evolution in the Carbon¹³ isotope curve has been recognized in carbonates of the Hutuo Formation. The lower to middle part of the Dongye Subgroup shows oscillating positive and negative Carbon¹³ values that range between −5.2 and +2.7 parts per thousand relative to the Pee Dee Belemnite Standard (an increase in the proportion of Carbon¹³ relative to Carbon¹² is often indicative of an increase in photosynthesis, as photosynthetic organisms preferentially extract Carbon¹² from the atmosphere, whereas carbonate forming ones incorporate both in in a proportion reflecting atmosphere composition). 

The microfossils from the Wenshan and Hebiancun Formations would be the fossil records of the geological period manifested by the aftermath of a positive excursion of Carbon¹³ (the Lomagundi-Jatuli isotopic event). The transition from an abnormally high organic carbon burial rate to massive oxidation of organic matter. The microfossils from the uppermost part of the Dongye Subgroup, (i.e. the Tianpengnao Formation), would represent the remains of Microphytoplankton during the geological period characterized by fluctuating Carbon¹³ levels.

The Palaeoproterozoic Hutuo Group was deposited in supra-tidal to sub-tidal environments. Furthermore, the Dongye Subgroup was followed by a remarkable transition of geochemical environments. In such palaeoenvironments, phosphates deposited on the continental margin of North China during late early Palaeoproterozoic. In the middle of the Hebiancun Formation, phosphatic deposits developed in association with dolomitic carbonates and a few terrigenous clasts in the studied Wenshan section. Several phosphatized specimens of spheroidal and filamentous Cyanobacteria and Leiosphaeridia-like forms were found in the phosphatic horizon. Typically, these specimens are poorly preserved, possibly due to late oxidation. Some specimens of Eoentophysalis hutuoensis were preserved as compressed carbon membranes. Such well-preserved Palaeoproterozoic microfossils, especially Eoentophysalis as multicellular colonies, have rarely been reported before. To understand their detailed morphological structure and elemental composition, scanning electron microscope associated with energy spectrum test was used for individual specimen of Eoentophysalis hutuoensis. Many sub-spherical individual cells were embedded within or enveloped by a Carbon membrane. The main elements detected were Carbon, Silicon, Calcium and Magnesium. This could suggest that Cyanobacteria colonies were primitively buried in the dolomitic carbonate and silicification during diagenesis resulted in their preservation. Common early diagenetic silicification was observed in the carbonates as distinct chert layers or concretions intercalated within dolostones of the Hebiancun Formation.

Eoentophysalis belcherensis, from the Hebiancun Formation. Scale bar is 10 μm. Yin et al. (2020),

A few degraded coccoid and filamentous cyanobacteria microfossils have previously been reported from the Hebiancun Formation. Some poorly preserved organic-walled microfossils obtained by palynological maceration have been described from the Doucun Subgroup. Some of those specimens that showed triangular, polygonal and boat-shaped forms that were plausibly interpreted as being like Eukaryotic Protists. In morphological feature discrimination, those specimens probably resulted from taphonomic alteration or were contaminants. Eukaryotic microfossils, except multicellular forms, are normally characterized by a Eukaryotic cytoskeleton and endomembrane system, morphogenetic characters like a multilayered wall, distinct surface ornamentation, and excystment by partial rupture or a circular opening. The oldest fossil evidence for Eukaryotic Protists (e.g. Tappania and other ornamented forms) have been documented from about 1.41 billion years ago elsewhere in the world. The new genus Dongyesphaera described by Yin et al. from the Tianpengnao Formation has a distinct fine spinous ornament on the vesicle wall, which would be recognized as eukaryotic protist. Additionally, a specimen identified as Dictyosphaera sp. was found in the Hebiancun Formation. The morphological genus Dictyosphaera has mostly been described from Late Palaeoproterozoic to Mesoproterozoic sediments in China, Australia and America. It is characterised by a multilayered vesicle wall, polygonal network ornamentation and possible excystment structure and is interpreted as a Eukaryotic Protest. At present, just one specimen identified as Dictyosphaera sp., by displaying network ornamentation on its vesicle surface, was found in the Hebiancun Formation, with an age of approximately 2010 million years. This suggests that the Eukaryotic Protist exercised metabolic activities rarely observed to have occurred in the early Palaeoproterozoic ocean. Up to the middle Palaeoproterozoic sequence, more specimens ornamented with fine conical spines, named as Dongyesphaera tenuispina, occurred in the greyish-green phyllite of the upper part of the Tianpengnao Formation (aged at about 1950 million years). Additionally, in siliceous lenses of dolostone containing stromatolite in the Hebiancun Formation, many Coccoid Cyanobacteria, such as Eoentophysalis and Eogloeocapsa were preserved, and some squashed Leiosphaerids, typically over 50 μm in diameter, have been preserved in situ. No obvious surface ornament was observed. However, the occurrence of the individual specimen of Dictyosphaera in the Hebiancun Formation implies that possible Eukaryotic organisms already existed around 2026 million years ago. Therefore, the microfossil evidence from Yin et al.'s study suggests that Eukaryotic organisms occurred earlier than, at least 2000 million years ago and quite lower morphological diversity of Eukaryotic organisms at the geological epoch. The recent discovery of early Precambrian microfossils, e.g. fungus-like mycelial fossils from 2.4 billion-year-old basalts of the Ongeluk Formation in South Africa, could suggest that Eukaryotic organisms may have occurred earlier than previously thought.

Photomicrographs of microfossils from the Hebiancun and Wenshan Formations in the Wenshan section. (A), (B) Leiosphaeridia sp., (A) broken specimen in thin section of silicified slate; (B) obtained by palynological maceration. (C) Degraded Coccoidal Cyanobacterium-like aggregation in a thin section of flint within crystalline dolostone, (D), (E) Eoentophysalis hutuoensis, in a thin section of a siliceous concretion in dolostone. Yin et al. (2020).

On the basis of microfossils found in samples from the Dongye Subgroup of the Palaeoproterozoic Hutuo Group in the Wutai Mountains of Shanxi Province, China, Yin et al. conclude the following: (1) Based on published geological data, there was an increased influence of oxygen on the carbon cycle during deposition of the Dongye Subgroup. For instance, phosphatised microfossils in phosphatic deposits of the Hebiancun Formation underwent stronger oxidation and show indistinct morphological aspects. (2) Owing to a remarkable increase in oxygen during the early Palaeoproterozoic, Eukaryotic Protists exercised metabolic activities rarely occurred in suitable environment, although Cyanobacteria were dominant in the early Palaeoproterozoic ocean. (3) The morphological diversity of Cyanobacteria appears to increase after about 2.0 billion years ago, especially the occurrence of many colonylike forms, such as the morphological genera Eoentophysalis and Sphaerophycus. Follow on research of microfossil records obtained from terrestrial clastic sediments of lower part of the Hutuo Group and greenschist of Neoarchean ‘Gaofan Subgroup’ is a potential project to reveal evolution of Earth’s life before and after global glaciation event.

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