The 1.88-Ga Gunflint biota is one of the most famous Precambrian microfossil lagerstätten and pro... more The 1.88-Ga Gunflint biota is one of the most famous Precambrian microfossil lagerstätten and provides a key record of the biosphere at a time of changing oceanic redox structure and chemistry. Here, we report on pyritized replicas of the iconic autotrophic Gunflintia-Huroniospora microfossil assemblage from the Schreiber Locality, Canada, that help capture a view through multiple trophic levels in a Paleoproterozoic ecosystem. Nanoscale analysis of pyritic Gunflintia (sheaths) and Huroniospora (cysts) reveals differing relic carbon and nitrogen distributions caused by contrasting spectra of decay and pyritization between taxa, reflecting in part their primary organic compositions. In situ sulfur isotope measurements from individual microfossils (δ 34 S V-CDT +6.7‰ to +21.5‰) show that pyritization was mediated by sulfate-reducing microbes within sediment pore waters whose sulfate ion concentrations rapidly became depleted, owing to occlusion of pore space by coeval silicification. Three-dimensional nanotomography reveals additional pyritized biomaterial, including hollow, cellular epibionts and extracellular polymeric substances, showing a preference for attachment to Gunflintia over Huroniospora and interpreted as components of a saprophytic heterotrophic, decomposing community. This work also extends the record of remarkable biological preservation in pyrite back to the Paleoproterozoic and provides criteria to assess the authenticity of even older pyritized microstructures that may represent some of the earliest evidence for life on our planet. biogeochemistry | taphonomy | paleontology
Exceptional 3-D preservation of Ediacaran rangeomorph fossils is found on a single bedding plane ... more Exceptional 3-D preservation of Ediacaran rangeomorph fossils is found on a single bedding plane at Spaniard's Bay, Newfoundland. This high-quality preservation has previously been explained by entrainment of organisms within the T d-e mudstone division of a distal turbidite, followed by encasement within concretions. Our sedimentological and taphonomic analysis reveals a clear association between these fossils and evidence for erosive unidirectional flows, including scours marks, tool marks, ridge-and-groove marks, parting lineations and current crescents. We suggest an alternative sequence of events that runs broadly as follows: (i) rangeomorph discs were anchored to the seafloor during deposition of planar laminated silts (our unit 2, <10 mm thick; T d ), now bearing pyrite framboids and pyritized organic matter; (ii) rangeomorph fronds were then felled and entrained by high velocity unidirectional currents, to lie within their own erosional scours at the top of unit 2, or to form tool marks; (iii) this topography was then draped and cast by soft-weathering sand (unit 3, T c ) associated with the growth of early diagenetic pyrite around sand grains. Pyrite grains also appear to have replaced clumps of organic matter. Fossil impressions have since been exposed by differential weathering of the ferruginous sands with respect to the silts. This new context now provides a parsimonious explanation for a range of hitherto paradoxical structures. Features previously regarded as microbial mats ('bubble trains') that formed in the lee of sinuous ripples on the top of unit 2 may be explained as load-casts, or by localised gas escape within areas of lowered hydraulic pressure. Rangeomorph fronds remarkably preserved in positive (rather than the more usual negative) epirelief are explained by means of sediment-casting of branches that became ruptured in the high velocity current. Paradoxical structures previously thought to be enclosing biological 'sheaths' around rangeomorph fronds are reinterpreted as scour marks, whereas imbricate overlaps of first order branches in Beothukis, Trepassia and Avalofractus are explained by hydraulic shear, driven by overlying currents across ruptured and deflated fronds. We find that rangeomorph bodies could be deflated, imbricated, folded over, inverted, and infilled with fine sediment. Our hydraulic model provides a null hypothesis against which future observations of rangeomorph fronds can now be tested. It removes some significant anomalies in our understanding of rangeomorph architecture, and provides a better understanding of the physical properties of their body tissues, permitting the possibility of a reasoned consideration of their puzzling biological affinities.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.
The Archean sub-seafl oor has been proposed as an environment for the emergence of life, with sep... more The Archean sub-seafl oor has been proposed as an environment for the emergence of life, with septate clusters of titanite microtextures in pillow lava rims argued to be the earliest traces of microbial microboring. Here we use nanoscale secondary ion mass spectrometry (NanoSIMS) to test possible geochemical traces of life in ca. 3.45 Ga pillow lavas of the Barberton Greenstone Belt, South Africa. Sulfi de inclusions in the titanite microtextures record strongly negative sulfur isotope fractionations of δ 34 S VCDT -39.8‰ to -3.2‰ (VCDT-Vienna Canyon Diablo Troilite). These represent the largest range and most negative δ 34 S values so far reported from the Archean, and are consistent with an early biogenic origin for the sulfi des. Extensive in situ elemental mapping did not fi nd any organic linings associated with the microtextures, despite the high spatial resolution and sensitivity of the NanoSIMS. The absence of organic linings thus excludes a key line of evidence previously used to support the biogenicity of the microtextures. In contrast, in situ sulfur isotope analysis of basalt-hosted sulfi des provides an alternative approach to investigating the existence and nature of an Archean subseafl oor biosphere.
Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records o... more Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records of sediment diagenesis and tracers of pore water chemistry. For over a century, rinded spheroidal structures that exhibit an Fe(III) oxide-rich exterior and Fe-poor core have been described as oxidation products of Fe(II) carbonate concretions. However, mechanisms governing Fe(III) oxide precipitation within these structures remain an enigma. Here we present chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide-rich rind of spheroidal concretions collected from the Jurassic Navajo Sandstone (southwest United States), implicating a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the friable interior. The mean δ 13 C value of organic carbon from the Fe(III) oxidecemented exterior, δ 13 C of −20.55‰, is consistent with a biogenic signature from autotrophic bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology, including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C, and N with bacterial morphotypes demonstrate microorganisms associated with Fe(III) oxides. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and present microbial catalysis as a mechanism of Fe(III) oxide concretion formation. Microbial biosignatures in rinded Fe(III) oxide-rich concretions within an exhumed, Quaternary aquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars, where both Fe(II) carbonate and Fe(III) oxide-rich concretions have been identifi ed.
The study of life remote in space has strong parallels with the study of life remote in time. Bot... more The study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called 'fossils', here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and 'mats'; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding 'waste products'; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.
The study of life remote in space has strong parallels with the study of life remote in time. Bot... more The study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called 'fossils', here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and 'mats'; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding 'waste products'; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.
Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on su... more Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3.5 billion years ago, leading to suggestions that the earliest microbial ecosystems were sulphur-based 1-5 . However, morphological evidence for these sulphur-metabolizing bacteria has been elusive. Here we report the presence of microstructures from the 3.4-billion-year-old Strelley Pool Formation in Western Australia that are associated with micrometre-sized pyrite crystals. The microstructures we identify exhibit indicators of biological affinity, including hollow cell lumens, carbonaceous cell walls enriched in nitrogen, taphonomic degradation, organization into chains and clusters, and δ 13 C values of −33 to −46 Vienna PeeDee Belemnite (VPDB). We therefore identify them as microfossils of spheroidal and ellipsoidal cells and tubular sheaths demonstrating the organization of multiple cells. The associated pyrite crystals have 33 S values between −1.65 and +1.43 and δ 34 S values ranging from −12 to +6 Vienna Canyon Diablo Troilite (VCDT) 5 . We interpret the pyrite crystals as the metabolic by-products of these cells, which would have employed sulphate-reduction and sulphur-disproportionation pathways. These microfossils are about 200 million years older than previously described 6 microfossils from Palaeoarchaean siliciclastic environments.
The context for the emergence of life on Earth sometime prior to 3.5 billion years ago is almost ... more The context for the emergence of life on Earth sometime prior to 3.5 billion years ago is almost as big a puzzle as the definition of life itself. Hitherto, the problem has largely been addressed in terms of theoretical and experimental chemistry plus evidence from extremophile habitats like modern hydrothermal vents and meteorite impact structures. Here, we argue that extensive rafts of glassy, porous, and gas-rich pumice could have had a significant role in the origin of life and provided an important habitat for the earliest communities of microorganisms. This is because pumice has four remarkable properties. First, during eruption it develops the highest surface-area-to-volume ratio known for any rock type. Second, it is the only known rock type that floats as rafts at the air-water interface and then becomes beached in the tidal zone for long periods of time. Third, it is exposed to an unusually wide variety of conditions, including dehydration. Finally, from rafting to burial, it has a remarkable ability to adsorb metals, organics, and phosphates as well as to host organic catalysts such as zeolites and titanium oxides. These remarkable properties now deserve to be rigorously explored in the laboratory and the early rock record.
A sandstone from the ca. 3400 Ma Strelley Pool Formation of Western Australia contains pristine m... more A sandstone from the ca. 3400 Ma Strelley Pool Formation of Western Australia contains pristine micron-sized pyrite intimately associated with organic material coating framework quartz grains. A synsedimentary to early diagenetic origin for this pyrite is indicated by its occurrence in black, bedded sandstone at the base of the formation, and in reworked black clasts higher up in the formation. High-resolution multiple sulfur isotope analysis ( 32 S, 33 S, 34 S) using secondary ion mass spectrometry (NanoSIMS and large-radius ion microprobe) reveals δ 34 S VCDT (Vienna Canyon Diablo troilite) values between ~-12‰ and +6‰, and Δ 33 S values between -1.65‰ and +1.43‰, from pyrite grains within a single thin section. A large spread of δ 34 S values over only 5-10 µm, together with the spatial association of pyrite with carbon and nitrogen, indicates biological processing of sulfur. The presence of both +Δ 33 S and -Δ 33 S signals overprinted by signifi cant mass-dependent δ 34 S fractionation in this pyrite population indicates for the fi rst time that both microbial sulfate reduction of aqueous sulfate (-Δ 33 S) and microbial disproportionation of elemental sulfur (+Δ 33 S) were co-occurring in an openmarine, sedimentary hosted ecosystem in the Paleoarchean.
Pyrite (FeS 2 ) oxidation in modern sedimentary environments is neither a purely chemical nor pur... more Pyrite (FeS 2 ) oxidation in modern sedimentary environments is neither a purely chemical nor purely microbial process, but it is significantly enhanced by the activity of microorganisms that use reduced forms of iron and sulphur in their metabolisms. On the early Earth, where oxygen levels were thought to be b 10 −5 of the present atmospheric level and chemical oxidants scarce, such biological mediation may have been critical in the redox cycles of iron and sulphur. Here, we show that detrital sedimentary pyrite grains in a~3.4 billion-year-old sandstone were colonised by microbial communities. The detrital pyrite comes from the basal quartz arenite member of the 3.43-3.35 Ga Strelley Pool Formation (SPF) in the East Strelley greenstone belt of the Pilbara Craton, Western Australia. Rock chips and petrographic thin sections of black sandstones occurring on two ridges close to the SPF type locality of Strelley Pool were investigated using optical microscopy, SEM, TEM, laser Raman and NanoSIMS. The detrital pyrite grains exhibit laminated carbonaceous coatings of early Archean age, with localised enrichments of nitrogen that are interpreted as the in situ remains of biofilms growing on these nutrient-rich minerals. Pyrite surfaces contain spherical pits, chains of pits and channels that are morphologically distinct from abiotic alteration features. The pits and channels are widespread, have a clustered distribution typical of microbial colonisation, and are closely comparable to biologically mediated microstructures in the younger rock record and those created by extant Fe-and S-oxidising microbes in the laboratory. They are thus interpreted as trace fossils formed by the attachment of bacteria to the pyrite surfaces. A nano-layer and discreet nano-grains of secondary mineral precipitates, namely Fe-oxides belonging to the magnetite-maghaemite group, attest to pyrite oxidation. These are intimately associated with the biofilms and trace fossils, and are interpreted to represent the fossilised mineral products of biologically mediated pyrite oxidation. These data extend the geological range of microbes capable of metabolising reduced Fe and/or S compounds back to the early Archean and indicate that pyrite-rich sedimentary rocks provide promising targets in the search for extraterrestrial life.
We here show that nano-scale mapping of elements commonly utilized in biological cycles provides ... more We here show that nano-scale mapping of elements commonly utilized in biological cycles provides a promising new additional line of evidence when evaluating the extent of the contribution of biology to microbialites. Our case study comes from Lake Clifton in Western Australia, a unique environment where living domical and conical microbialites occur in close proximity to £4000-year-old fossilized equivalents. The outer margins of a partially lithified, actively growing Lake Clifton microbialite are characterized by abundant filamentous cyanobacteria within a loosely cemented aragonite matrix. Nano-scale chemical maps have been successfully matched to specific morphological features such as trichomes, sheaths and putative extracellular polymeric substances (EPS). A suite of elements (C, O, Mg, N, Si, S) is concentrated within cyanobacterial sheaths, with carbon, magnesium, nitrogen and sulfur also enriched within trichomes and putative EPS. Calcium distribution highlights the sites of aragonite mineralization. In contrast, the fossilized Lake Clifton microbialite contains only rare, extensively degraded cyanobacterial filaments, the mean diameter of which is <50% of the living equivalents. Nevertheless, nano-scale chemical maps can again be matched with morphological features. Here, poorly preserved filamentous microfossils are highlighted by enrichments in nitrogen and sulfur. Magnesium is no longer concentrated within the filaments, instead it co-occurs with calcium and oxygen in the calcite cement. Extension of this study to a 2720-million-year-old stromatolitic microbialite from the Tumbiana Formation of Western Australia shows that similar nano-scale signals, in particular nitrogen and sulfur enrichments, are characteristic of stromatolite laminations, even when morphological microfossils are absent. The close similarities of nano-scale elemental distributions in organic material from modern and ancient microbialites show that this technique provides a valuable addition to the morphological investigation of such structures, particularly in non-fossiliferous ancient examples.
The 1.88-Ga Gunflint biota is one of the most famous Precambrian microfossil lagerstätten and pro... more The 1.88-Ga Gunflint biota is one of the most famous Precambrian microfossil lagerstätten and provides a key record of the biosphere at a time of changing oceanic redox structure and chemistry. Here, we report on pyritized replicas of the iconic autotrophic Gunflintia-Huroniospora microfossil assemblage from the Schreiber Locality, Canada, that help capture a view through multiple trophic levels in a Paleoproterozoic ecosystem. Nanoscale analysis of pyritic Gunflintia (sheaths) and Huroniospora (cysts) reveals differing relic carbon and nitrogen distributions caused by contrasting spectra of decay and pyritization between taxa, reflecting in part their primary organic compositions. In situ sulfur isotope measurements from individual microfossils (δ 34 S V-CDT +6.7‰ to +21.5‰) show that pyritization was mediated by sulfate-reducing microbes within sediment pore waters whose sulfate ion concentrations rapidly became depleted, owing to occlusion of pore space by coeval silicification. Three-dimensional nanotomography reveals additional pyritized biomaterial, including hollow, cellular epibionts and extracellular polymeric substances, showing a preference for attachment to Gunflintia over Huroniospora and interpreted as components of a saprophytic heterotrophic, decomposing community. This work also extends the record of remarkable biological preservation in pyrite back to the Paleoproterozoic and provides criteria to assess the authenticity of even older pyritized microstructures that may represent some of the earliest evidence for life on our planet. biogeochemistry | taphonomy | paleontology
Exceptional 3-D preservation of Ediacaran rangeomorph fossils is found on a single bedding plane ... more Exceptional 3-D preservation of Ediacaran rangeomorph fossils is found on a single bedding plane at Spaniard's Bay, Newfoundland. This high-quality preservation has previously been explained by entrainment of organisms within the T d-e mudstone division of a distal turbidite, followed by encasement within concretions. Our sedimentological and taphonomic analysis reveals a clear association between these fossils and evidence for erosive unidirectional flows, including scours marks, tool marks, ridge-and-groove marks, parting lineations and current crescents. We suggest an alternative sequence of events that runs broadly as follows: (i) rangeomorph discs were anchored to the seafloor during deposition of planar laminated silts (our unit 2, <10 mm thick; T d ), now bearing pyrite framboids and pyritized organic matter; (ii) rangeomorph fronds were then felled and entrained by high velocity unidirectional currents, to lie within their own erosional scours at the top of unit 2, or to form tool marks; (iii) this topography was then draped and cast by soft-weathering sand (unit 3, T c ) associated with the growth of early diagenetic pyrite around sand grains. Pyrite grains also appear to have replaced clumps of organic matter. Fossil impressions have since been exposed by differential weathering of the ferruginous sands with respect to the silts. This new context now provides a parsimonious explanation for a range of hitherto paradoxical structures. Features previously regarded as microbial mats ('bubble trains') that formed in the lee of sinuous ripples on the top of unit 2 may be explained as load-casts, or by localised gas escape within areas of lowered hydraulic pressure. Rangeomorph fronds remarkably preserved in positive (rather than the more usual negative) epirelief are explained by means of sediment-casting of branches that became ruptured in the high velocity current. Paradoxical structures previously thought to be enclosing biological 'sheaths' around rangeomorph fronds are reinterpreted as scour marks, whereas imbricate overlaps of first order branches in Beothukis, Trepassia and Avalofractus are explained by hydraulic shear, driven by overlying currents across ruptured and deflated fronds. We find that rangeomorph bodies could be deflated, imbricated, folded over, inverted, and infilled with fine sediment. Our hydraulic model provides a null hypothesis against which future observations of rangeomorph fronds can now be tested. It removes some significant anomalies in our understanding of rangeomorph architecture, and provides a better understanding of the physical properties of their body tissues, permitting the possibility of a reasoned consideration of their puzzling biological affinities.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.
The Archean sub-seafl oor has been proposed as an environment for the emergence of life, with sep... more The Archean sub-seafl oor has been proposed as an environment for the emergence of life, with septate clusters of titanite microtextures in pillow lava rims argued to be the earliest traces of microbial microboring. Here we use nanoscale secondary ion mass spectrometry (NanoSIMS) to test possible geochemical traces of life in ca. 3.45 Ga pillow lavas of the Barberton Greenstone Belt, South Africa. Sulfi de inclusions in the titanite microtextures record strongly negative sulfur isotope fractionations of δ 34 S VCDT -39.8‰ to -3.2‰ (VCDT-Vienna Canyon Diablo Troilite). These represent the largest range and most negative δ 34 S values so far reported from the Archean, and are consistent with an early biogenic origin for the sulfi des. Extensive in situ elemental mapping did not fi nd any organic linings associated with the microtextures, despite the high spatial resolution and sensitivity of the NanoSIMS. The absence of organic linings thus excludes a key line of evidence previously used to support the biogenicity of the microtextures. In contrast, in situ sulfur isotope analysis of basalt-hosted sulfi des provides an alternative approach to investigating the existence and nature of an Archean subseafl oor biosphere.
Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records o... more Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records of sediment diagenesis and tracers of pore water chemistry. For over a century, rinded spheroidal structures that exhibit an Fe(III) oxide-rich exterior and Fe-poor core have been described as oxidation products of Fe(II) carbonate concretions. However, mechanisms governing Fe(III) oxide precipitation within these structures remain an enigma. Here we present chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide-rich rind of spheroidal concretions collected from the Jurassic Navajo Sandstone (southwest United States), implicating a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the friable interior. The mean δ 13 C value of organic carbon from the Fe(III) oxidecemented exterior, δ 13 C of −20.55‰, is consistent with a biogenic signature from autotrophic bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology, including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C, and N with bacterial morphotypes demonstrate microorganisms associated with Fe(III) oxides. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and present microbial catalysis as a mechanism of Fe(III) oxide concretion formation. Microbial biosignatures in rinded Fe(III) oxide-rich concretions within an exhumed, Quaternary aquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars, where both Fe(II) carbonate and Fe(III) oxide-rich concretions have been identifi ed.
The study of life remote in space has strong parallels with the study of life remote in time. Bot... more The study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called 'fossils', here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and 'mats'; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding 'waste products'; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.
The study of life remote in space has strong parallels with the study of life remote in time. Bot... more The study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called 'fossils', here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and 'mats'; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding 'waste products'; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.
Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on su... more Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3.5 billion years ago, leading to suggestions that the earliest microbial ecosystems were sulphur-based 1-5 . However, morphological evidence for these sulphur-metabolizing bacteria has been elusive. Here we report the presence of microstructures from the 3.4-billion-year-old Strelley Pool Formation in Western Australia that are associated with micrometre-sized pyrite crystals. The microstructures we identify exhibit indicators of biological affinity, including hollow cell lumens, carbonaceous cell walls enriched in nitrogen, taphonomic degradation, organization into chains and clusters, and δ 13 C values of −33 to −46 Vienna PeeDee Belemnite (VPDB). We therefore identify them as microfossils of spheroidal and ellipsoidal cells and tubular sheaths demonstrating the organization of multiple cells. The associated pyrite crystals have 33 S values between −1.65 and +1.43 and δ 34 S values ranging from −12 to +6 Vienna Canyon Diablo Troilite (VCDT) 5 . We interpret the pyrite crystals as the metabolic by-products of these cells, which would have employed sulphate-reduction and sulphur-disproportionation pathways. These microfossils are about 200 million years older than previously described 6 microfossils from Palaeoarchaean siliciclastic environments.
The context for the emergence of life on Earth sometime prior to 3.5 billion years ago is almost ... more The context for the emergence of life on Earth sometime prior to 3.5 billion years ago is almost as big a puzzle as the definition of life itself. Hitherto, the problem has largely been addressed in terms of theoretical and experimental chemistry plus evidence from extremophile habitats like modern hydrothermal vents and meteorite impact structures. Here, we argue that extensive rafts of glassy, porous, and gas-rich pumice could have had a significant role in the origin of life and provided an important habitat for the earliest communities of microorganisms. This is because pumice has four remarkable properties. First, during eruption it develops the highest surface-area-to-volume ratio known for any rock type. Second, it is the only known rock type that floats as rafts at the air-water interface and then becomes beached in the tidal zone for long periods of time. Third, it is exposed to an unusually wide variety of conditions, including dehydration. Finally, from rafting to burial, it has a remarkable ability to adsorb metals, organics, and phosphates as well as to host organic catalysts such as zeolites and titanium oxides. These remarkable properties now deserve to be rigorously explored in the laboratory and the early rock record.
A sandstone from the ca. 3400 Ma Strelley Pool Formation of Western Australia contains pristine m... more A sandstone from the ca. 3400 Ma Strelley Pool Formation of Western Australia contains pristine micron-sized pyrite intimately associated with organic material coating framework quartz grains. A synsedimentary to early diagenetic origin for this pyrite is indicated by its occurrence in black, bedded sandstone at the base of the formation, and in reworked black clasts higher up in the formation. High-resolution multiple sulfur isotope analysis ( 32 S, 33 S, 34 S) using secondary ion mass spectrometry (NanoSIMS and large-radius ion microprobe) reveals δ 34 S VCDT (Vienna Canyon Diablo troilite) values between ~-12‰ and +6‰, and Δ 33 S values between -1.65‰ and +1.43‰, from pyrite grains within a single thin section. A large spread of δ 34 S values over only 5-10 µm, together with the spatial association of pyrite with carbon and nitrogen, indicates biological processing of sulfur. The presence of both +Δ 33 S and -Δ 33 S signals overprinted by signifi cant mass-dependent δ 34 S fractionation in this pyrite population indicates for the fi rst time that both microbial sulfate reduction of aqueous sulfate (-Δ 33 S) and microbial disproportionation of elemental sulfur (+Δ 33 S) were co-occurring in an openmarine, sedimentary hosted ecosystem in the Paleoarchean.
Pyrite (FeS 2 ) oxidation in modern sedimentary environments is neither a purely chemical nor pur... more Pyrite (FeS 2 ) oxidation in modern sedimentary environments is neither a purely chemical nor purely microbial process, but it is significantly enhanced by the activity of microorganisms that use reduced forms of iron and sulphur in their metabolisms. On the early Earth, where oxygen levels were thought to be b 10 −5 of the present atmospheric level and chemical oxidants scarce, such biological mediation may have been critical in the redox cycles of iron and sulphur. Here, we show that detrital sedimentary pyrite grains in a~3.4 billion-year-old sandstone were colonised by microbial communities. The detrital pyrite comes from the basal quartz arenite member of the 3.43-3.35 Ga Strelley Pool Formation (SPF) in the East Strelley greenstone belt of the Pilbara Craton, Western Australia. Rock chips and petrographic thin sections of black sandstones occurring on two ridges close to the SPF type locality of Strelley Pool were investigated using optical microscopy, SEM, TEM, laser Raman and NanoSIMS. The detrital pyrite grains exhibit laminated carbonaceous coatings of early Archean age, with localised enrichments of nitrogen that are interpreted as the in situ remains of biofilms growing on these nutrient-rich minerals. Pyrite surfaces contain spherical pits, chains of pits and channels that are morphologically distinct from abiotic alteration features. The pits and channels are widespread, have a clustered distribution typical of microbial colonisation, and are closely comparable to biologically mediated microstructures in the younger rock record and those created by extant Fe-and S-oxidising microbes in the laboratory. They are thus interpreted as trace fossils formed by the attachment of bacteria to the pyrite surfaces. A nano-layer and discreet nano-grains of secondary mineral precipitates, namely Fe-oxides belonging to the magnetite-maghaemite group, attest to pyrite oxidation. These are intimately associated with the biofilms and trace fossils, and are interpreted to represent the fossilised mineral products of biologically mediated pyrite oxidation. These data extend the geological range of microbes capable of metabolising reduced Fe and/or S compounds back to the early Archean and indicate that pyrite-rich sedimentary rocks provide promising targets in the search for extraterrestrial life.
We here show that nano-scale mapping of elements commonly utilized in biological cycles provides ... more We here show that nano-scale mapping of elements commonly utilized in biological cycles provides a promising new additional line of evidence when evaluating the extent of the contribution of biology to microbialites. Our case study comes from Lake Clifton in Western Australia, a unique environment where living domical and conical microbialites occur in close proximity to £4000-year-old fossilized equivalents. The outer margins of a partially lithified, actively growing Lake Clifton microbialite are characterized by abundant filamentous cyanobacteria within a loosely cemented aragonite matrix. Nano-scale chemical maps have been successfully matched to specific morphological features such as trichomes, sheaths and putative extracellular polymeric substances (EPS). A suite of elements (C, O, Mg, N, Si, S) is concentrated within cyanobacterial sheaths, with carbon, magnesium, nitrogen and sulfur also enriched within trichomes and putative EPS. Calcium distribution highlights the sites of aragonite mineralization. In contrast, the fossilized Lake Clifton microbialite contains only rare, extensively degraded cyanobacterial filaments, the mean diameter of which is <50% of the living equivalents. Nevertheless, nano-scale chemical maps can again be matched with morphological features. Here, poorly preserved filamentous microfossils are highlighted by enrichments in nitrogen and sulfur. Magnesium is no longer concentrated within the filaments, instead it co-occurs with calcium and oxygen in the calcite cement. Extension of this study to a 2720-million-year-old stromatolitic microbialite from the Tumbiana Formation of Western Australia shows that similar nano-scale signals, in particular nitrogen and sulfur enrichments, are characteristic of stromatolite laminations, even when morphological microfossils are absent. The close similarities of nano-scale elemental distributions in organic material from modern and ancient microbialites show that this technique provides a valuable addition to the morphological investigation of such structures, particularly in non-fossiliferous ancient examples.
The rock record provides us with unique evidence for testing models as to when and where cellular... more The rock record provides us with unique evidence for testing models as to when and where cellular life first appeared on Earth. Its study, however, requires caution. The biogenicity of stromatolites and 'microfossils' older than 3.0 Gyr should not be accepted without critical analysis of morphospace and context, using multiple modern techniques, plus rejection of alternative non-biological (null) hypotheses. The previous view that the co-occurrence of biology-like morphology and carbonaceous chemistry in ancient, microfossil-like objects is a presumptive indicator of biogenicity is not enough. As with the famous Martian microfossils, we need to ask not 'what do these structures remind us of ?', but 'what are these structures?' Earth's oldest putative 'microfossil' assemblages within 3.4-3.5 Gyr carbonaceous cherts, such as the Apex Chert, are likewise self-organizing structures that do not pass tests for biogenicity.
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