Alexander T. Brasier

Enhanced accumulation of organic matter: the Shunga event

A number of sedimentary formations deposited globally around 2.0 Ga ago are characterised by high abundances of organic carbon. These formations often contain occurrences of highly concentrated, matured organic material representing metamorphosed oil, now pyrobitumen. Apart from their common names pyrobitumen or anthraxolite, different terminology has been used for these rocks within the pertinent literature, including shungite, thucolite, or Precambrian “coal”. Given their long and frequently complex geologic history, these sedimentary formations exhibit a variable and sometimes substantial degree of metamorphic (thermal) overprint. Consequently, many of them show undisputable signs of thermal mobilisation, migration and likely loss of hydrocarbons/bitumen. This includes the so-called shungite rocks on the Fennoscandian Shield.

Palaeoproterozoic Stromatolites from the Lomagundi-Jatuli interval of the Fennoscandian Shield

When and how life on Earth started is still an open question. Biochemical fingerprints stored in the ancient rock record indicate the presence of traces of life back to some of the oldest sedimentary rocks on the planet. The Earth has thus harboured life throughout most of its geologic history, and biological processes have contributed significantly to shaping the environmental conditions on the surface of the planet. Tracking the nature of ancient life using morphological, mineralogical, chemical and isotopic proxies in the rock record on Earth needs, however, to surmount a number of obstacles. Most important are the effects of post-depositional alteration of the sedimentary host rocks due to exposure to metamorphic temperatures and pressures and metasomatism during the protracted time before their present exposure. Diagenetic and metamorphic overprints may have resulted in recrystallisation of the original mineral assemblages and deformation of the original textural features in the sedimentary rocks, in many cases blurring the biologic signatures and jeopardizing the reliable interpretation of the nature of the lifeform.

Evaluating evidence from the Torridonian Supergroup (Scotland, UK) for eukaryotic life on land in the Proterozoic

Abstract The Stoer, Sleat and Torridon groups lie unconformably on Palaeoproterozoic Lewisian metamorphic rocks. They contain organic carbon microfossils claimed to be non-marine and to include eukaryotes. We consider the evidence for terrestrial interpretations from each formation of the Torridonian Supergroup. The range of sedimentary structures and the boron content of illite led us to the overall conclusion that, based on the currently available evidence, the Torridonian Supergroup was probably entirely non-marine. Evidence for terrestrial life in these rocks comes from microbially induced sedimentary structures, including wrinkle structures with reticulate and elephant skin fabrics. Organic remains and microscopic carbonaceous compressions mostly reported from phosphates in the grey shales of the Stoer, Aultbea and Applecross formations are dominated by sphaeromorph acritarchs. The Diabaig phosphatic lagerstätte includes three-dimensional preservation of eukaryotic and prokaryotic organisms, providing remarkable insights into non-marine life around 1 billion years ago. Supplementary material: Taxonomy of Torridon Group microfossils from thin sections of phosphatic material (adapted from Battison 2012) is available at https://doi.org/10.6084/m9.figshare.c.3522753

The Imandra/Varzuga greenstone belt

The Late Archaean-Early Palaeoproterozoic transition (2500–2000 Ma) represents a hallmark period when the Earth System experienced a series of fundamental upheavals. Among them, the most important was the establishment of an oxygen-rich atmosphere (sometimes referred to as the Great Oxidation Event) and the emergence of an aerobic biosphere. Associated with this, either incidentally or causally, was a cascade of other prominent, global-scale events that considerably modified Earth’s surface environments, either temporarily or permanently; these are reviewed in Parts 1 and 8 in full, and detailed in Part 7. Briefly mentioned here, these include: the severe and global climatic event known as the Huronian glaciation; an unprecedented perturbation of the global carbon cycle, the large-magnitude Lomagundi-Jatuli positive excursion of δ13Ccarb, lasted over 160 Ma; radical changes in the phosphorus and sulphur cycles resulting in accumulation of the first-known massive sulphates and sedimentary phosphates; a radical modification in recycling of organic matter leading to the emergence of a new 13C-depleted carbon reservoir in the form of carbonate concretions; and an unprecedented accumulation of organic-rich sediments and formation of the earliest supergiant petroleum deposits.

Carbon isotopic evidence for organic matter oxidation in soils of the Old Red Sandstone (Silurian to Devonian, South Wales, UK)

Petrographic and calcrete carbon isotope data from seasonally waterlogged Upper Silurian (Přídolí) to Lower Devonian (Pragian) palaeo-Vertisols of the Old Red Sandstone, South Wales, UK, are presented. The δ13C values mostly range from −9 to −12‰ (VPDB), suggesting that the soils were inhabited by abundant vegetation that when oxidized (perhaps with microbial assistance) resulted in CO2-rich soils. Such soils would favour calcrete precipitation through equilibration of soil zone CO2 with the relatively lower atmospheric pCO2. However, reliably estimating palaeoatmospheric pCO2 using these carbon isotope data is a challenge. Supplementary material: Detailed information on stable isotope measurement methods, descriptions of the samples, and carbon and oxygen isotope data are available at www.geolsoc.org.uk/SUP18754.

Earth's Earliest Travertines.

In contrast to a rather extensive marine sedimentary record of the Palaeoproterozoic, the terrestrial record, preserved as palaeosols (ancient soils; Retallack 2001) and caliches or calcretes (carbonate layers in palaeosols; Wright and Tucker 1991), is sparse. Nevertheless, these deposits are important, because they have the potential to provide the least ambiguous information on climatic and atmospheric compositional change during this critical interval of Earth history. In the best of circumstances, marine sediments record oceanic conditions, which can differ markedly from atmospheric conditions, especially in terms of redox. In the modern world, soil environments themselves poorly reflect those at the surface, especially in terms of CO2 and O2 partial pressure, because of respiration by roots and soil microbes. But in the Palaeoproterozoic, with its presumably poorly developed terrestrial biota, soil conditions likely track those of the atmosphere much more closely. (We use “presumably” here, because terrestrial ecosystems may have been extensive (Horodyski and Knauth 1994; Watanabe et al. 2000), and the land surface may have been the incubator for early evolutionary innovation, including the origin of cyanobacteria; e.g. Battistuzzi et al. 2009).

Earliest Cretaceous cocoons or plant seed structures from the Wealden Group, Hastings, UK

Abstract Complete metamorphosis evolved in insects towards the end of the Palaeozoic Era. A wide range of pupation strategies existed and numerous biosedimentary structures associated with these have been described. The fossil record of endogenous materials associated with pupation, e.g. cocoons, is more limited. Here we report six amber-coloured specimens from the earliest Cretaceous of southern England that were tentatively identified on collection as insect cocoons. These were analysed by Fourier transform infrared spectrometry, stereomicroscopy and X-ray microtomography to elucidate their origin. The interpretation of the Fourier transform infrared spectrometry data was inconclusive because the spectra showed some differences from those of amber. A seed pod origin seems likely for at least two of the objects based on their size, shape and the lineations on their surfaces. Three specimens are more cocoon-like based on their overall morphology and a fibrous surface texture. Although plant megaspore membranes have features analogous with these specimens and cannot be ruled out, the similarity to and variability found within insect cocoons, coupled with the range of potential insect architects present at the time of origin, make an insect origin more likely. We review a number of hymenopteran clades whose extant members construct comparable cocoons. The possible cocoons may have been resin-coated to protect the larva inside from predation, or they may have passively come into contact with resin prior to burial. Supplementary material: All TIFF computed tomography slices from the scan, the computed tomography log file, a surface model of the specimen and digital visualizations of both the whole specimen and the perforations are available at https://doi.org/10.6084/m9.figshare.c.3704794

The Palaeoproterozoic global carbon cycle: insights from the Loch Maree Group, NW Scotland

Two Palaeoproterozoic events have particularly interested Earth scientists. These are the global Lomagundi–Jatuli Event, the greatest magnitude positive carbonate carbon isotope excursion in Earth history, and the Shunga Event, the world’s largest organic carbon burial event. Analysis of newly acquired high-resolution C–O isotope data and U–Pb zircon geochronology refine understanding of carbon isotope characteristics and timing of deposition of the Palaeoproterozoic Loch Maree Group of NW Scotland. Petrographic examination reveals a basal unconformity between the Loch Maree Group and Archaean basement, permitting a stratigraphy and younging direction to be assigned. Detrital zircon ages from immediately above the unconformity are dated at c. 2.3 Ga. δ13Ccarbonate data on two temporally discrete carbonate packages range from c. +15 to 2‰ in the older unit and c. 2 to −5‰ in the younger carbonate unit. Current age constraints indicate that the Loch Maree Group is too young to be fully coeval with the Lomagundi–Jatuli Event but is within the age range of the Shunga Event. This revives consideration of a straightforward mass-balance process involving burial of organic carbon as an explanation for at least some of the C-cycle perturbations of Palaeoproterozoic time. Supplementary Material: Sample descriptions from the Loch Maree Group, geochemical data, sample preparation and geochronology data are available at http://www.geolsoc.org.uk/SUP18865.

Archaean Soils, Lakes and Springs: Looking for Signs of Life

Microbial life in Archaean non-marine settings like soils, lakes and springs would have faced several challenges. These would have included exposure to UV light; aridity, salinity and temperature changes; and nutrient availability. Current understanding is that none of these challenges would have been insurmountable. Microbial organisms of Archaean marine environments are likely to have been similar in their lifestyles and habits to those of the Archaean terrestrial world. Non-marine stromatolites, microbial filaments, microbial borings and microbially-induced sedimentary structures might therefore have been preserved. But Archaean subaerial surfaces would have been very prone to erosion by wind and rain, so the oldest fossil ‘soils’ of subaerially weathered surfaces (up to 3.47 Ga) are mostly identified using geochemistry. However, some ancient duricrusts like calcretes have been reported. Archaean lacustrine microbial life may have included stromatolites of the Tumbiana Formation of Western Australia. The case that these were lacustrine rather than marine is critically assessed, with the conclusion that the stratigraphy provides the strongest supporting evidence here. Archaean terrestrial hot springs, though often mentioned in origin of life studies, are not yet known from the rock record. In the Palaeoproterozoic to present these silica and carbonate-precipitating environments are commonly found in proximity to volcanic sediments and faults, where the deposits form terraced mounds, fissure ridges and hydrothermal lakes. It remains plausible that life could have existed and even evolved in these hypothesised Archaean hot-spring settings, and there is cause for optimism that the evidence for this might one day be found.

A microbial role in the construction of Mono Lake carbonate chimneys

Lacustrine carbonate chimneys are striking, metre-scale constructions. If these were bioinfluenced constructions, they could be priority targets in the search for early and extraterrestrial microbial life. However, there are questions over whether such chimneys are built on a geobiological framework or are solely abiotic geomorphological features produced by mixing of lake and spring waters. Here, we use correlative microscopy to show that microbes were living around Pleistocene Mono Lake carbonate chimneys during their growth. A plausible interpretation, in line with some recent works by others on other lacustrine carbonates, is that benthic cyanobacteria and their associated extracellular organic material (EOM) formed tubular biofilms around rising sublacustrine spring vent waters, binding calcium ions and trapping and binding detrital silicate sediment. Decay of these biofilms would locally have increased calcium and carbonate ion activity, inducing calcite precipitation on and around the biofilms. Early manganese carbonate mineralisation was directly associated with cell walls, potentially related to microbial activity though the precise mechanism remains to be elucidated. Much of the calcite crystal growth was likely abiotic, and no strong evidence for either authigenic silicate growth or a clay mineral precursor framework was observed. Nevertheless, it seems likely that the biofilms provided initial sites for calcite nucleation and encouraged the primary organised crystal growth. We suggest that the nano-, micro- and macroscale fabrics of these Pleistocene Mono Lake chimneys were affected by the presence of centimetre-thick tubular and vertically stacked calcifying microbial mats. Such carbonate chimneys represent a promising macroscale target in the exploration for ancient or extraterrestrial life.