Nicola McLoughlin

A fresh look at the fossil evidence for early Archaean cellular life

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?’ Earths 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. There is a preservational paradox in the fossil record prior to ca 2.7 Gyr: suitable rocks (e.g. isotopically light carbonaceous cherts) are widely present, but signals of life are enigmatic and hard to decipher. One new approach includes detailed mapping of well-preserved sandstone grains in the ca 3.4 Gyr Strelley Pool Chert. These can contain endolithic microtubes showing syngenicity, grain selectivity and several levels of geochemical processing. Preliminary studies invite comparison with a class of ambient inclusion trails of putative microbial origin and with the activities of modern anaerobic proteobacteria and volcanic glass euendoliths.

Growth of synthetic stromatolites and wrinkle structures in the absence of microbes – implications for the early fossil record

Stromatolites and wrinkle structures are often taken to be an important indicator for early life. While both may be shaped by microbial mat growth, this can be open to doubt, so that the contribution of abiotic processes in their construction always needs to be established (Grotzinger & Knoll, 1999). We here report laboratory spray deposition experiments that can generate stromatolites and wrinkle structures in the absence of microbes. These minicolumnar and sometimes branched stromatolites are produced artificially by the aggregation of a synthetic colloid in a turbulent flow regime. They self-organize at the relatively low particle concentrations found in the outer parts of a spray beam. This contrasts with adjacent stratiform deposits that are produced by high rates of colloid deposition and relatively low sediment viscosities found in the centre of a spray beam. These stratiform laminae become subsequently wrinkled during hardening of the colloid. These results support numerical models that together suggest that physicochemical processes are capable of generating laminated sedimentary structures without the direct participation of biology. Geological environments where comparable abiogenic stromatolites and wrinkle structures may be found include: splash-zone silica sinters, desert varnish crusts and early Archean cherts formed from silica gel precursors.

Two coexisting sulfur metabolisms in a ca. 3400 Ma sandstone

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 (32S, 33S, 34S) using secondary ion mass spectrometry (NanoSIMS and large-radius ion microprobe) reveals δ34SVCDT (Vienna Canyon Diablo troilite) values between ∼–12‰ and +6‰, and Δ33S values between –1.65‰ and +1.43‰, from pyrite grains within a single thin section. A large spread of δ34S 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 +Δ33S and –Δ33S signals overprinted by significant mass-dependent δ34S fractionation in this pyrite population indicates for the first time that both microbial sulfate reduction of aqueous sulfate (–Δ33S) and microbial disproportionation of elemental sulfur (+Δ33S) were co-occurring in an open-marine, sedimentary hosted ecosystem in the Paleoarchean.

Use of NanoSIMS in the search for early life on Earth: ambient inclusion trails in a c. 3400 Ma sandstone

Ambient inclusion trails (AIT) are enigmatic microtubular structures created by the migration of mineral crystals through a lithified substrate. The decomposition of organic material has been suggested as the driving force for the crystal migration, but has yet to be rigorously tested. AIT may hold potential as a biosignature for investigating early life on Earth if the associated organic material can be shown to be biological. This paper attempts to test the formation mechanism and biogenicity of AIT from the c. 3400 Ma Strelley Pool sandstone of Western Australia using NanoSIMS technology. In doing so, we demonstrate the unique ability of the NanoSIMS to combine sub-micron scale imaging with in situ chemical and isotopic data, thereby enhancing our ability to evaluate the biogenicity criteria for Archaean microstructures. Enrichments of a suite of major elements (C, N, P, S) and trace elements (Co, Fe, Ni, Zn), often associated with biological processes, are found within several AIT in this sandstone. C and N enrichments are most common along AIT margins, and correlate with depletions of Si, O, Ca and Mg, indicating that this material is indeed organic in nature. δ13C values of this carbonaceous material average −26‰. Petrographic observations show that some of the AIT occur in the centre of detrital sandstone grains where they were sealed from later fluid flow and therefore preserve primary Archaean (bio)geochemistry. In contrast, AIT found around the outer edges of sandstone grains may contain more recent organic material introduced by later fluid migration and are an unreliable biosignature. Using the petrographic and geochemical data a multi-stage model for AIT formation and subsequent diagenetic modification is proposed. The possible sources of the primary organic material are discussed and we conclude that the data are consistent with a biological origin for these AIT. An abiogenic origin is more difficult to sustain but cannot yet be completely excluded for AIT in general.

Oceanic Pillow Lavas and Hyaloclastites as Habitats for Microbial Life Through Time – A Review

This chapter summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. We report textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust and compare these to data obtained from ophiolite and greenstone belts dating back to c. 3.8 Ga. Petrographic descriptions of the granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias are provided and contrasted with textures produced by abiotic alteration (palagonitization). The geological setting in particular the degree of deformation and metamorphism experienced by each study site is documented in outcrop photographs, geological maps and stratigraphic columns (where possible). In addition, X-ray mapping evidence and carbon isotopic data that are consistent with a biogenic origin for these alteration textures is explained and a model for their formation is presented. Lastly, the petrographic observations and direct radiometric dating techniques that have been used to establish the antiquity and syngenicity of these microbial alteration textures are reviewed.

Mechanisms of microtunneling in rock substrates: distinguishing endolithic biosignatures from abiotic microtunnels

Rock-dwelling, endolithic micro-organisms can create tubular microcavities (TMCs) by the dissolution of rock substrates. Microtunnels can also conceivably be formed by abiotic processes, and collectively, these structures are here termed tubular microcavities. A textural record of life in subseafloor environments is provided by biological TMCs, and it is imperative to distinguish these from abiological tunnels. To this end, the morphologies and petrographic context of tunnels formed by chemical solution, physical abrasion, and biological processes are here described. Biological TMCs in volcanic glass are restricted to sites that were connected to early fluid circulation. Their shapes, distribution, and the absence of intersections exclude an origin by chemical dissolution of pre-existing heterogeneities such as, radiation damage trails, gas-escape structures, or fluid inclusion trails. Rather their characteristics are best explained by microbial dissolution, involving perhaps, cellular extensions that provide a mechanism of localizing and directing microtunnel formation as observed in terrestrial soils. Biological TMCs are contrasted with ambient inclusion trails (AITs) found in cherts and authigenic minerals. These differ in exhibiting longitudinal striae, a constant diameter, and polygonal cross-section, sometimes with terminal inclusions. The origin(s) of AITs remain unclear but they are hypothesized to form by migration of crystalline or organic inclusions in sealed substrates, in contrast to biotic TMCs that form in open systems. We present diagnostic morphological and petrographic criteria for distinguishing these different types of TMCs. Moreover, we argue that AIT-type processes are not viable in volcanic glass because of the absence of crystalline millstones, localized chemical solution agents, and elevated fluid pressures, necessary to drive this process.

Nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the ∼1.9-Ga Gunflint chert

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 (δ34SV-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.

Tubular Compression Fossils from the Ediacaran Nama Group, Namibia

Abstract Abundant tubular macrofossils occur in finely laminated siltstones and shales of the 548–542 Ma Schwarzrand Subgroup, Nama Group, Namibia. The Nama tubes occur in both the Vingerbreek and Feldschuhhorn members commonly in dense populations and always in fine-grained, lower shore-face lithologies deposited below fair-weather wave base. The tubes are preserved mostly as compressed casts and molds that range in width from 0.6 to 2.1 mm; apparently incomplete specimens reach lengths up to 10 cm. All specimens show sinuous bending and occasional brittle fracture, indicating an original construction of strong but flexible organic matter. Feldschuhhorn specimens preserve fine longitudinal pleats or folds that record pliant organic walls, but the older Vingerbreek populations do not. Similarly, some specimens in the Feldschuhhorn Member display branching, while Vingerbreek tubes do not. The abundant Feldschuhhorn tubes are assigned to the widespread Ediacaran problematicum Vendotaenia antiqua; however, the distinctive Vingerbreek population remains in open nomenclature. The most abundant fossils in Nama rocks, these tubes resemble populations in Ediacaran successions from Russia, China, Spain, and elsewhere. Beyond their local importance, then, such tubes may turn out to be the most abundant record of Ediacaran life.

Sulfur isotope evidence for a Paleoarchean subseafloor biosphere, Barberton, South Africa

The Archean sub-seafloor 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. Sulfide 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 sulfides. Extensive in situ elemental mapping did not find 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 sulfides provides an alternative approach to investigating the existence and nature of an Archean subseafloor biosphere.

The ∼3.4 billion-year-old strelley pool sandstone: A new window into early life on Earth

The recognition and understanding of the early fossil record on Earth is vital to the success of missions searching for life on other planets. Despite this, the evidence for life on Earth before ~3.0 Ga remains controversial. The discovery of new windows of preservation in the rock record more than 3.0 Ga would therefore be helpful to enhance our understanding of the context for the earliest life on Earth. Here we report one such discovery, a ~3.4 Ga sandstone at the base of the Strelley Pool Formation from the Pilbara of Western Australia, in which micrometre-sized tubular structures preserve putative evidence of biogenicity. Detailed geological mapping and petrography reveals the depositional and early diagenetic history of the host sandstone. We demonstrate that the depositional environment was conducive to life and that sandstone clasts containing putative biological structures can be protected from later metamorphic events, preserving earlier biological signals. We conclude from this that sandstones have an exciting taphonomic potential both on early Earth and beyond.