Pondering the Precambrian #27

Proterozoic:

NeoProterozoic:

Did a massive volcanic eruption trigger the Gaskiers Glaciation?

Deposits from British Columbia may be evidence of deep sea conditions just prior to the Gaskiers Glaciation.

Fossil fats from the Cryogenian may actually be from algae instead of sponges.

Amoebozoa diversified earlier than expected during the Tonian 750 million years ago.

How were banded iron formations created in Egypt from the NeoProterozoic?

Was the Cambrian Explosion really that big a deal? Or did it really originate in the Ediacaran?

First record of carbonates with spherulites and cone-in-cone structures from the Ediacaran of Norway has been found.

MesoProterozoic:

The Yili Block was likely located in the NW margin of the supercontinent Rodinia.

PaleoProterozoic:

The apparent cyclical deposition of the Dales Gorge Member banded iron formation appears to be related to sea level rise and fall or tectonic activity.

The Vempalle Formation dolomites of India appear to have more in common with Phanerozoic carbonates than Proterozoic ones.

There's a sedimentation record from 2.27 BYA to 1.96 BYA in West Africa with evidence of volcanic ash and activity in the layers.

The first drillings into the Temagami Anomaly have uncovered potential links to the Sudbury Impact.

The Francevillian Biota of Rhyacian Gabon seems to have had something able to move.  Scientists have found evidence of trackways from shallow, oxygenated waters.

Archean:

Evidence from Mauritania suggests the Archean geophysics were as complicated as today.

Evidence of a breakup of a continent from the Archean.

There appears to be evidence of major crustal growth circa 2.6 BYA and 3.2 BYA.

There is evidence plate tectonics were active, despite prior theory, during the Archean.

MesoArchean:

There is evidence of a continental margin at the tail end of the MesoArchean in North China.

A subduction zone has been found from the MesoArchean/NeoArchean boundary.

Evidence of a continental rift was found in Hainan, China from the MesoArchean.

There is evidence of ephemeral oxygen oases in the Mesoarchean Ocean.

PaleoArchean:

South African barite deposits were not laid down in a marine environment, but from a spring.

Life was thriving during the paleoarchean 3.5 billion years ago.

EoArchean:

How the first continents formed in the EoArchean.

Hadean:

Was the Theia impact the reason for the heterogeneity of Earth's crust?

Origin of Life:

Was the repeated drying and wetting cycle of the margin of water the source of energy for the original polymerization of amino acids et al?

Did hydrogen peroxide play a crucial role in the origin of life?

Pluripotency and the origin of multicellular life.

Could the RNA world hypothesized never have existed?

Shallow pools of water might have been the original location for the origin of life, not the sea.

Evidence of Bacteria Producing Soil at the PaleoArchean/MesoArchean Boundary

Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols

Authors:

Sabhan et al

Abstract:

Regionally traceable paleosols in the lower Moodies Group of the Barberton greenstone belt (ca. 3.22 Ga, northeastern South Africa and Swaziland) contain locally abundant silicified nodules, originally composed of pedogenic carbonates and sulfates, interbedded with heavy-mineral laminae dominated by pyrite. Pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and δ34S ratios clearly different from those of the cores. While cores have low Co and Ni concentrations and high Co/Ni ratios, rims show as much as 5.5 wt% of these elements and low Co/Ni ratios, reflecting the weathering of nearby ultramafic rocks. In-situ sulfur isotope analyses of pyrite cores show δ34SVCDT (Vienna Canyon Diablo troilite) values between +5‰ and –5‰, while the rims show δ34VCDT values between –20‰ and –24.5‰, suggesting biogenic fractionation of sulfur. The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and 34S-depleted sulfur for the growth of authigenic pyrite. This indicates that vadose-zone soil-forming processes in the Archean involved not only physical and chemical modification of moist, unconsolidated sediment in a terrestrial environment but also already included its microbiological modification.

'Whiffs' of Oxygen From Archean Sediments

Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle

Authors:

Aulbach et al

Abstract:

Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt–associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000–550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2σ, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, ΔFMQ, at 1 GPa) of Archean sample suites (ΔFMQ –1.19 ± 0.33, 2σ) is significantly lower than that of post-Archean sample suites, including MORB (ΔFMQ –0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth's mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first "whiffs" of oxygen in sediments of that age.

Traces of Storms From the PaleoArchean Found

Sedimentology of the ∼3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa

Authors:

Trower et al

Abstract:

The Mendon Formation is the uppermost unit of the 3.5–3.26 Ga Onverwacht Group in the Barberton Greenstone Belt, South Africa. It consists of a cyclic stack of komatiitic volcanic units separated by thin cherty sedimentary layers. In most areas, the uppermost Mendon Formation is a sedimentary interval characterized by black chert, banded black-and-white chert, and banded ferruginous chert, although the detailed patterns of lithofacies in different sections are more complex. Previously reported zircon U/Pb ages suggest that Mendon deposition could represent more than 70 Myr of time between ∼3334 Ma and ∼3260 Ma.

This study presents sedimentological and petrographic observations of the upper Mendon Formation from across the central part of the Barberton Greenstone Belt in order to investigate sediment sources, depositional processes, and environments of sedimentation. The dominant mode of sedimentation was quiet settling of carbonaceous grains and, in the deepest sections below storm wave base, fine ferruginous material, resulting in finely laminated black and grey chert. In situ carbonaceous laminations are rare, suggesting that benthic microbial mat growth had little direct influence on deposition. The hemipelagic background deposition was punctuated by occasional inputs of fine pyroclastic debris, formation and deposition of silica granules, and reworking by infrequent storm events. Storm deposits are represented by coarse-grained, poorly-sorted intraclast breccias, some of which include distinctive intraclasts sampling lithofacies that are not observed in situ. Despite considerable lateral variability, correlative temporal trends are resolvable in many Mendon sections: there is an upward-deepening of the overall depositional setting recorded in the oldest upper Mendon sections, consistent with the previous interpretation that Mendon time was characterized by rifting (Lowe, 1994a, 1999a). Younger Mendon cycles include thick, relatively ferruginous basal sections, interpreted to reflect the deepest water deposition. These sections are capped by black chert and silicified ashes with more evidence of disturbance and reworking by storms, reflecting gradual shoaling. This sedimentological analysis is broadly consistent with previous geochemical and tectonic analyses and provides a better picture of depositional patterns during uppermost Onverwacht time, before the distinct change in tectonic regime marked by impact spherule layer S2 and the onset of Fig Tree Group orogenesis and related siliciclastic deposition.

Reworking of atmospheric sulfur in a Paleoarchean hydrothermal system

Reworking of atmospheric sulfur in a Paleoarchean hydrothermal system at Londozi, Barberton Greenstone Belt, Swaziland

Authors:

Roerdink et al

Abstract:

Anomalous fractionation of the minor isotopes of sulfur (Δ33S, Δ36S) in Archean pyrite is thought to reflect photochemical reactions in an anoxic atmosphere, with most samples falling along a reference array with Δ36S/Δ33S ≈ −1. Small deviations from this array record microbial sulfate reduction or changes in atmospheric source reactions. Here, we argue that reworking of atmospheric sulfur with distinct minor sulfur isotope ratios (Δ36S/Δ33S ≠ −1) produced additional variability in sulfide Δ33S and Δ36S-values in a 3.52 Ga hydrothermal barite deposit at Londozi, Barberton Greenstone Belt, Swaziland. In situ measurement of the four stable sulfur isotopes in pyrite revealed Δ36S–Δ33S relationships and a Δ36S/Δ33S trend (−3.2 ± 0.4), which is significantly different from the co-variation between Δ36S and Δ33S in the co-existing barite that reflects ambient Paleoarchean seawater sulfate. This argues against biological or thermochemical sulfate reduction at the time of barite deposition, and requires incorporation of sulfide generated in a chemically distinct atmosphere before 3.52 Ga. We propose a model that combines reworking of this sulfur by hydrothermal leaching, deep mixing with juvenile sulfur and surface mixing with biogenic sulfide to explain the observed variation in δ34S, Δ33S and Δ36S. These interactions between abiotic and biological processes in the Londozi hydrothermal system complicate the interpretation of biosignatures based on deviations in Δ33S and Δ36S from the Archean reference array.

Evidence of an Impact During the PaleoArchean Found in Australia

Scientists have found evidence of a huge asteroid that struck the Earth early in its life with an impact larger than anything humans have experienced.

Tiny glass beads called spherules, found in north-western Australia were formed from vaporised material from the asteroid impact, said Dr Andrew Glikson from The Australian National University (ANU).

"The impact would have triggered earthquakes orders of magnitude greater than terrestrial earthquakes, it would have caused huge tsunamis and would have made cliffs crumble," said Dr Glikson, from the ANU Planetary Institute.

"Material from the impact would have spread worldwide. These spherules were found in sea floor sediments that date from 3.46 billion years ago."

The asteroid is the second oldest known to have hit the Earth and one of the largest.

Dr Glikson said the asteroid would have been 20 to 30 kilometres across and would have created a crater hundreds of kilometres wide.

link.

PaleoArchean Crust in South Africa With Evidence of Subduction

Chronology of the oldest supracrustal sequences in the Palaeoarchaean Barberton Greenstone Belt, South Africa and Swaziland

Authors:

Kröner et al

Abstract:

Zircon age data for felsic metavolcanic rocks of the Sandspruit and Theespruit formations, the two oldest supracrustal components in the Palaeoarchaean Barberton Greenstone Belt, show that these two successions are time-equivalent and constitute one single volcanic event at ca. 3530 Ma. The Sandspruit felsic rocks are ubiquitously metasomatized, intensely deformed and intruded by, and tectonically interlayered with, ca. 3450 Ma granitoid sills that are probably part of the Theespruit Pluton. One metasomatized Sandspruit sample contains abundant metamorphic zircons with a weighted mean 207Pb/206Pb age of 3220.1±1.6 Ma, reflecting a widespread metamorphic event in parts of the eastern Kaapvaal craton in South Africa and Swaziland.

Several samples of felsic metavolcanic rocks of the Theespruit Formation confirm a previously established magmatic emplacement age of ca. 3530 Ma, but slightly older rocks up to 3552 Ma were found in the easternmost exposure of the Theespruit sequence near the South African/Swaziland border and may represent a lower lithostratigraphic level than exposed farther west.

Hf-in-zircon isotopic data for most felsic metavolcanic rocks confirm earlier results suggesting that these rocks predominantly originated from melting of a felsic continental basement, possibly related to the oldest, ca. 3660-3550 Ma components of the Ancient Gneiss Complex in Swaziland. However, several Sandspruit samples also suggest that a juvenile source was involved in their generation, perhaps a mafic underplate. We see no evidence in the geochemistry and isotopic signatures of felsic volcanic rocks of the Sandspruit and Theespruit formations for partial melting of a metabasaltic protolith and for Palaeoarchean oceanic crust that formed in connection with subduction. We rather favour a plateau-type setting on older continental crust.

Evidence of Microbial Activity From PaleoArchean Australia

Carbonaceous microstructures from sedimentary laminated chert within the 3.46 Ga Apex Basalt, Chinaman Creek locality, Pilbara, Western Australia

Authors:

Hickman-Lewis et al

Abstract:

Hydrothermal black chert veins intruding the 3.46 Ga Apex Basalt contain some of Earth’s oldest microfossil-like objects, whose biogenicity has been questioned. Whilst these black chert veins have been studied in great detail, relatively little is known about the stratiform, seafloor, sedimentary cherts that are conformably interbedded with volcanic rocks of the Apex Basalt.

Herein, we document and assess the biogenicity of carbonaceous microstructures present in the lowermost of the stratiform chert units (informally known as the ‘Apex chert’), at the Chinaman Creek locality in the Marble Bar greenstone belt, Pilbara Craton, Western Australia. Carbonaceous material mostly occurs within clotted grey-black cherts and microgranular ‘grainstone-like’ cherts within the stratiform unit, the latter being the major focus of this study. In the clotted cherts, carbon occurs as lobate, fluffy grains, rare compressed flakes, and as a grain boundary phase around spherulitic silica. There is no morphological evidence to support the biogenicity of these microstructures. In contrast, the microgranular chert contains fluffy and flaky carbonaceous grains, plus laminated grains comprising multiple non-isopachous wrinkled carbonaceous laminae, with noted thickening towards some ridge crests, as determined by confocal laser scanning microscopy. Roll-up structures provide evidence of an initial plasticity, interpreted to have formed via the tearing-up and current-induced plastic deformation of microbial mat fragments. Geochemical mapping, using laser Raman micro-spectroscopy and NanoSIMS, respectively demonstrates the antiquity of the carbon, and reveals a close correlation between carbon, nitrogen and sometimes sulphur, concentrated within dark brown to black laminae. Adjacent to microgranular zones are zones of more persistent carbonaceous, undulose, filament-like laminae that entrain relict sediment grains. These microstructures are directly comparable to a sub-type of microbially induced sedimentary structure (MISS), widely reported from younger siliciclastic sediments colonised by microbial biofilms.

The morphology and chemical composition of both the non-isopachous laminated grains and the filament-like laminae are consistent with a biological interpretation, suggesting microscopic MISS were present in the microgranular stratiform ‘Apex chert’. However, the fact that neither macroscopic MISS nor bona fide microfossils have yet been reported from this unit, coupled with the proximity of these structures to active hydrothermal vents, potentially discharging hot carbon-rich fluids, urges caution in such an interpretation. The Chinaman Creek ‘Apex chert’ investigated here is one of at least five sedimentary, laminated cherts within the Apex Basalt. These horizons are promising targets in the search for biological activity within a dominantly volcanic Archaean environment.

Evidence of an Impact From the PaleoArchean in Australia

A new ∼3.46 Ga asteroid impact ejecta unit at Marble Bar, Pilbara Craton, Western Australia: a petrological, microprobe and laser ablation ICPMS study

Authors:

Glikson et al

Abstract:

The Archean record contains seventeen asteroid impact ejecta units interpreted in terms of terrestrial vestiges of an extended late heavy bombardment (LHB) (, and ). Correlated impact ejecta units include the 3.47 Ga in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa, and Pilbara region of Western Australia, with multiple ejecta units in the 3.25–3.22 Ga and 2.63–2.48 Ga intervals. This paper reports the discovery and investigation of a new impact ejecta unit within the Marble Bar Chert Member (MBCM) of the felsic volcanic Duffer Formation, east Pilbara Craton, Western Australia. The age of the MBCM is constrained by a 3459±2 Ma U-Pb zircon date from the uppermost volcanic unit of the Duffer Formation and by a 3449±3 Ma U-Pb zircon date from the overlying felsic volcanic Panorama Formation, stratigraphically above the intervening un-dated Apex Basalt. The ejecta unit, observed in a drill core (ABDP 1) ∼4 km south-southwest of Marble Bar, consists of multiple lenses and bands of almost totally silicified impact spherules 1–2 mm in diameter. All internal primary textures of the spherules have been destroyed. Nonetheless, Fe-rich spherule rims, largely composed of secondary siderite, are well preserved. Chemical analyses of the rims reveal iron-magnesium carbonate displaying high Fe, Mg, Ni, Co and Zn. Whole-rock and in-situ analyses (X-ray fluorescence, Inductively Coupled Plasma Mass Spectrometry (ICPMS), electron-microprobe (EMP) and EMP-calibrated laser ICPMS) reveal that the rims contain high Ni abundances and high Ni/Cr ratios (less than 50). The spherules are separated by an arenite matrix and spherule lenses also occur within bedded chert. The spherules are particularly visible over some ∼14 m of true stratigraphic thickness in which chert breccia is interpreted to represent a tsunami-generated diamictite affected by hydrothermal fragmentation and veining. Despite the almost total silicification of the MBCM whole-rock nickel sulphide (NIS) results indicate high Ir (2 ppb) and a low Pd/Ir ratio (2.0), consistent with geochemical features of impact ejecta units. Dense concentrations of spherules at the 57-58 m level and the 77 m level of the core, separated by banded chert, raise the possibility of two distinct impact events. Stratigraphic and isotopic age data distinguish between the 3459–3449 Ma age of the MBCM ejecta unit and ∼3470.1 ± 1.9 Ma impact ejecta units in the Antarctic Creek Member, Mount Ada Basalt, about 40 km to the west of Marble Bar. In combination with a 3472 ± 2.3 Ma impact unit in the Barberton greenstone belt, these impact ejecta units record large Paleoarchean asteroid impacts, significant for understanding early earth bombardment rates and early crustal evolution.

Where did all the Impactors Come From During the PaleoArchean?

Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors

Authors:

Johnson et al

Abstract:

Ancient layers of impact spherules provide a record of Earth's early bombardment history. Here, we compare different bombardment histories to the spherule layer record and show that 3.2–3.5 Ga the flux of large impactors (10–100 km in diameter) was likely 20–40 times higher than today. The E-belt model of early Solar System dynamics suggests that an increased impactor flux during the Archean is the result of the destabilization of an inward extension of the main asteroid belt (Bottke et al., 2012). Here, we find that the nominal flux predicted by the E-belt model is 7–19 times too low to explain the spherule layer record. Moreover, rather than making most lunar basins younger than 4.1 Gyr old, the nominal E-belt model, coupled with a corrected crater diameter scaling law, only produces two lunar basins larger than 300 km in diameter. We also show that the spherule layer record when coupled with the lunar cratering record and careful consideration of crater scaling laws can constrain the size distribution of ancient terrestrial impactors. The preferred population is main-belt-like up to ∼50 km in diameter transitioning to a steep distribution going to larger sizes.

Pondering the Precambrian #3

Proterozoic:

NeoProterozoic:

The Royal Tyrrell Museum has a lecture on the Ediacaran.

There is stronger evidence for the biomarkers for sponges being present starting in the Cryogenian.

MesoProterozoic:

The shallow seawaters of the Calymmian MesoProterozoic were very hypoxic, showing evidence of having .1% of the current oxygen in seawater.

PaleoProterozoic:

There is evidence of intracontinental rifting from China during the Rhyacian Paleoproterozoic.

Was there a 'Mawson continent' during the Paleoproterozoic?

Archean:

The SETI Institute has a lecture on volcanism during the Archean.

PaleoArchean:

At least part of the trace fossils from Australia are pseudofossils (not real fossils).

EoArchean:

There is evidence of atmospheric oxygen from the EoArchean, 800 million years earlier than ever detected (or suspected) before.

Hadean:

In an almost extraordinary claim, the Siberian Craton may have bits of the crust from the Hadean!

Pondering the Precambrian #2

Precambrian in General:

The magnetic field of the Earth has been surprisingly consistent over the course of Deep Time despite the changes happening internally.

Eukaryotes appear to have acquired mitochondria late in the process of their evolution away from other forms of life.

Studying modern stromatolites gives insight to Precambian life.

What meandering rivers prior to terrestrial plant life were like.

NeoProterozoic:

Was an increase in UV light caused by rapidly flipping magnetic poles the root cause of the Cambrian Explosion?

NeoArchean:

Cerium anomalies in from NeoArchean Brazil suggest the ocean was mildly oxygenated before the Great Oxygenation Event.

MesoArchean:

Evidence subduction (and plate tectonics) were active during the MesoArchean .

PaleoArchean:

Evidence from Swaziland suggests the minerals found there do NOT support subduction (and plate tectonics) as being their source as was apparently thought previously.

Doubt is being cast on PaleoArchean carbon deposits, trace evidence of microbes, being really from the PaleoArchean.

EoArchean:

There is evidence 'proto' crust may have existed 3.8 billion years ago.

Evidence for Cavity-dwelling Microbial Life (coelobionts) From PaleoArchean Tidal Flats

Evidence for cavity-dwelling microbial life in 3.22 Ga tidal deposits

Authors:

Homann et al

Abstract:

Cavities are considered plausible and favorable habitats for life on early Earth. In such microenvironments, organisms may have found an adequate protection against the intense ultraviolet radiation that characterized the Archean ozone-free atmosphere. However, while there is clear evidence that benthic life existed in the Paleoarchean, the oldest traces of cavity-dwelling microbes (coelobionts) have been found in Neoarchean rocks. Here we present the results of a detailed investigation of early silicified cavities occurring in the oldest well-preserved siliciclastic tidal deposits, the 3.22 Ga Moodies Group of the Barberton Greenstone Belt (South Africa). Downward-growing microstromatolitic columns composed of kerogenous laminae are commonly present in planar, bedding-parallel, now silica-filled cavities that formed in sediments of the peritidal zone. In-situ δ13CPDB (PDB—Peedee belemnite) measurements of the kerogen range from –32.3‰ to –21.3‰ and are consistent with a biogenic origin. Scanning electron microscopy analysis of the silicified cavities shows well-preserved chains of cell-sized molds that are interpreted as fossil filamentous microorganisms. The geological context, the morphology of the microstromatolites, the δ13C composition of the kerogen, and the presence of microfossils all suggest that a microbial community inhabited the cavities. These results extend the geological record of coelobionts by ∼500 m.y., supporting the view that cavities were among the first ecological niches to have been occupied by early microorganisms.

Evidence of PaleoArchean Atmospheric Oxygen

A new study shows that iron-bearing rocks that formed at the ocean floor 3.2 billion years ago carry unmistakable evidence of oxygen. The only logical source for that oxygen is the earliest known example of photosynthesis by living organisms, say University of Wisconsin-Madison geoscientists.

"Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen," says Clark Johnson, professor of geoscience at UW-Madison and a member of the NASA Astrobiology Institute. "Recent work has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23 billion years old, and quite well preserved, and we believe they show definite signs for oxygen in the oceans much earlier than previous discoveries."

The most reasonable candidate for liberating the oxygen found in the iron oxide is cyanobacteria, primitive photosynthetic organisms that lived in the ancient ocean. The earliest evidence for life now dates back 3.5 billion years, so oxygenic photosynthesis could have evolved relatively soon after life itself.

Until recently, the conventional wisdom in geology held that oxygen was rare until the "great oxygenation event," 2.4 to 2.2 billion years ago.

The rocks under study, called jasper, made of iron oxide and quartz, show regular striations caused by composition changes in the sediment that formed them. To detect oxygen, the UW-Madison scientists measured iron isotopes with a sophisticated mass spectrometer, hoping to determine how much oxygen was needed to form the iron oxides.

link.

These are NOT the Fossils you Were Looking for: PaleoArchean Microfossils may Only be Mineralogical Artifacts

3.46 Ga Apex chert ‘microfossils’ reinterpreted as mineral artefacts produced during phyllosilicate exfoliation

Authors:

Wacey et al

Abstract:

Filamentous microstructures from the 3.46 billion year (Ga)-old Apex chert of Western Australia have been interpreted as remnants of Earth’s oldest cellular life, but their purported biological nature has been robustly questioned on numerous occasions. Despite recent claims to the contrary, the controversy surrounding these famous microstructures remains unresolved.

Here we interrogate new material from the original ‘microfossil site’ using high spatial resolution electron microscopy to decode the detailed morphology and chemistry of the Apex filaments. Light microscopy shows that our newly discovered filaments are identical to the previously described ‘microfossil’ holotypes and paratypes. Scanning and transmission electron microscopy data show that the filaments comprise chains of potassium- and barium-rich phyllosilicates, interleaved with carbon, minor quartz and iron oxides. Morphological features previously cited as evidence for cell compartments and dividing cells are shown to be carbon-coated stacks of phyllosilicate crystals. Three-dimensional filament reconstructions reveal non-rounded cross sections and examples of branching incompatible with a filamentous prokaryotic origin for these structures.

When examined at the nano-scale, the Apex filaments exhibit no biological morphology nor bear any resemblance to younger bona fide carbonaceous microfossils. Instead, available evidence indicates that the microstructures formed during fluid-flow events that facilitated the hydration, heating and exfoliation of potassium mica flakes, plus the redistribution and adsorption of barium, iron and carbon within an active hydrothermal system.

Evidence of the PaleoArchean Sulfur Cycle

Paleoarchean sulfur cycling: Multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

Authors:

Montinaro et al

Abstract:

Mass-dependent and mass-independent sulfur isotope fractionation archived in volcanic and sedimentary rocks from the Barberton Greenstone Belt (3550–3215 Ma), South Africa, provide constraints for sulfur cycling on the early Earth. Four different sample suites were studied: komatiites and tholeiites, barite, massive and disseminated sulfide ores, and non-mineralized black shales.

Variable but generally slightly positive δ34S values between −0.7 and +5.2‰, negative Δ33S values between −0.50 and −0.09‰, and a negative correlation between δ34S and Δ33S as well as between Δ33S and Δ36S for komatiites and tholeiites from the Komati Formation and from the Weltevreden Formation are outside the expected range of unfractionated juvenile sulfur. Instead, results suggest alteration of oceanic crustal rock sulfur through interactions with fluids that most likely derived their sulfur from seawater.

Barite from the Mapepe Formation displays positive δ34S values between +3.1 and +8.1‰ and negative Δ33S values between −0.77 and −0.34‰. The mass-independent sulfur isotope fractionation indicates an atmospheric sulfur source, notably photolytic sulfate, whereas the positive δ34S values suggest bacterial sulfate reduction of the marine sulfate reservoir.

Non-mineralized black shale samples from the presumed stratigraphic equivalent of the Mapepe Formation show positive δ34S values between 0.0 and +1.3‰ and positive Δ33S values between +0.59 and +2.45‰. These results are interpreted to result from the reduction of photolytic elemental sulfur, carrying a positive Δ33S signature.

Positive δ34S values ranging from +0.7 to +3.5‰ and slightly negative Δ33S values between −0.17 and −0.12‰ characterize massive and disseminated sulfides from the Bien Venue Prospect. Results suggest unfractionated juvenile magmatic sulfur source as the primary sulfur source, but a contribution from recycled seawater sulfate, which would be indicative of submarine hydrothermal activity, cannot be ruled out.

Massive and disseminated sulfides from the M’hlati prospect are distinctly different from massive and disseminated sulfide from the Bien Venue Prospect. They show negative δ34S values between −1.2 and −0.1‰ and positive Δ33S values between +2.66 and +3.17‰, thus, displaying a sizeable mass-independent sulfur isotopic fractionation. Again, these samples clearly exhibit the incorporation of an atmospheric MIF-S signal. The source of sulfur for these samples has positive Δ33S values, suggesting a connection with photolytic elemental sulfur.

In conclusion, the sulfur isotope signatures in Paleoarchean rocks from the Barberton Greenstone Belt are diverse and indicate the incorporation of different sources of sulfur. For komatiites and tholeiites, barite and massive and possibly also disseminated sulfides from Bien Venue, multiple sulfur isotopes are related to ambient seawater sulfate and its photolytic origin, while massive and disseminated sulfides from M’hlati and non-mineralized black shales are related to a second (photolytic elemental sulfur) end member.

A Record of PaleoArchean Sea Water Chemistry?!

Fluid inclusion analysis of silicified Palaeoarchaean oceanic crust – A record of Archaean seawater?

Authors:

Farber et al

Abstract:

In recent years, the role of Archaean seawater and hydrothermal fluid in the extensive silicification of Palaeoarchaean volcano-sedimentary successions has been a matter of considerable debate. In an attempt to constrain the conditions of silica precipitation, and the sources and chemical composition of the interacting fluids, we used fluid inclusion microthermometry, bulk crush-leach and oxygen isotope analyses of chert and quartz veins in silicified komatiites and sediments from the Barberton greenstone belt, South Africa. Chert vein margins consist of microcrystalline quartz and carbonaceous matter, whereas the vein centres are often filled with macrocrystalline quartz that contains abundant fluid inclusions.

Oxygen isotope ratios of vein chert and macrocrystalline quartz vary from 18 to 21‰, with the macrocrystalline quartz having slightly higher δ18O values (0.7 ± 0.3‰). The data are consistent with silica precipitation during low-temperature (≤100 °C) hydrothermal processes on the Archaean seafloor. Macrocrystalline quartz contains homogeneous 2-phase (L+V) inclusions at room temperature with a relatively constant vapour fraction. The inclusions have a salinity of 3–11 wt.% NaCl equiv. and homogenisation temperatures (Th) of 150–200 °C. Whereas some of the inclusions form intragranular fluid inclusion clusters that appear to be primary, other inclusions form transgranular fluid inclusion trails and are clearly secondary. Both types of inclusions share the same microthermometric characteristics, indicating that fluid entrapment occurred during a later metamorphic event and not near the seafloor. The event likely coincided with regional deformation and metamorphism at 3.23 Ga. Chlorite thermometry from vein host rocks reveals peak conditions of ∼257 ± 31 °C. In conjunction with microthermometry, the data correspond to a crustal depth of 3–6 km. A secondary origin of fluid inclusions is also consistent with the Cl/Br, Na/Cl and Na/K ratios of the macrocrystalline veins, which are similar to those found in metamorphic quartz veins. The fluid inclusions thus do not provide information on the conditions and temperatures during chert formation. While the fluids potentially carry the geochemical signature of modified Archaean seawater (i.e. hydrothermal fluid), characterised by low Cl/Br and Na/K ratios, and low Mg-contents, the fluid composition was likely modified during regional metamorphism.

Hydrothermal Microbial Ecosystems Were Flourishing, Diverse During PaleoArchean

Archean (3.33 Ga) microbe-sediment systems were diverse and flourished in a hydrothermal context

Authors:

Westall et al

Abstract:

Interacting, diverse microbe-sediment systems exist in natural environments today but have not yet been recognized in the oldest records of life on Earth (older than 3.3 Ga) because of lack of distinctive biomarker molecules and patchy preservation of microbial paleocommunities. In an in-situ outcrop- to microbial-scale study, we have differentiated probable phototrophic, chemolithotrophic, and chemo-organotrophic fossil microbial signatures in a nearshore volcanogenic sedimentary setting in 3.33 Ga rocks of the Josefsdal Chert, Barberton greenstone belt, South Africa, while demonstrating the importance of contemporaneous hydrothermal activity. Hydrothermal fluids, as a nutrient source, strongly controlled the development and distribution of the microbial communities and, as a silicifying agent, contributed to their rapid fossilization. We thus show that intricate microbe-sediment systems are deep-rooted in time and that at least some early life may indeed have been thermophilic.

Adaptations of PaleoArchean Microbial Mats to Coastal Habitats

Morphological adaptations of 3.22 Ga-old tufted microbial mats to Archean coastal habitats (Moodies Group, Barberton Greenstone Belt, South Africa)

Authors:

Homann et al

Abstract:

Microbial life was well established and widespread by the Paleoarchean; however, the degree of evolutionary advancement such as microbial motility, intra- and inter-species interactions, phototropism, or oxygenic photosynthesis by that time remains highly debated. The 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB, South Africa) are Earth's oldest well-preserved siliciclastic tidal deposits. They exhibit a unique assemblage of microbial mats, providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings. The fossil mats are preserved as kerogenous laminations (0.5–1 mm thick) that can be traced laterally for ∼15 km in a ∼1000 m-thick succession of fine- to coarse-grained tidal sandstones and conglomerates. We here present a detailed stratigraphic and depositional facies analysis, documenting the association of the three principal mat morphotypes with specific environmental settings: (1) planar-type in coastal floodplain, (2) wavy-type in intertidal, and (3) tufted-type in upper inter- to supratidal facies. All mat types indicate a flourishing phototrophic biota; moreover, the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments.

Was There an Oxygen Spike, Crash and Mass Extinction During the PaleoArchean?

Variations in the abundance of photosynthetic oxygen through Precambrian and Paleozoic time in relation to biotic evolution and mass extinctions: evidence from Mn/Fe ratios

Author:

Jackson

Abstract:

This paper reports new information about variations in the abundance of photosynthetic oxygen through Precambrian and Paleozoic time. Non-detrital marine sediments (cherts, limestones, and dolomite) were analysed for NH2OH·HCl/acetic acid-extractable Mn and Fe, and the Mn/Fe ratio (a proxy for the oxidation–reduction potential of the sediment at the time of deposition) was plotted against geologic age. The method has never before been applied to ancient sediments, but previously published data produced independently by other methods confirmed its applicability and underlying assumptions. The Mn/Fe ratio was unexpectedly high ca. 3.416 Ga, implying localised oxidation due to oxygen production by cyanobacteria, but fell dramatically over the interval 3.416–3.298 Ga, suggesting mass mortality or mass extinction of early Archaean cyanobacteria owing to asteroid impacts. However, the ratio increased continuously, though at episodically varying rates, from a minimum at ∼1.8783 Ga to a maximum at ∼0.680 Ga, signifying accumulation of oxygen in the atmosphere and hydrosphere. The rate of increase was relatively high at first but dropped abruptly at some point during the interval 1.8783–1.6 Ga, possibly signalling the appearance of eukaryotic herbivores. The ratio increased exponentially from 1.6 to 0.8 Ga and then rose more rapidly from 0.8 to 0.680 Ga, indicating a late Proterozoic upsurge of oxygen production, whereupon it fell catastrophically to a minimum in the Cambrian, reflecting widespread anoxia due to mass extinction of Ediacaran organisms. The crisis at the Precambrian–Cambrian boundary was followed by a logarithmic increase from the Cambrian to the Permian, indicating a resurgence of photosynthetic activity.