Lithostratigraphic analysis of a new stromatolite-thrombolite reef from across the rise of atmospheric oxygen in the Paleoproterozoic Turee Creek Group, Western Australia

Distinctive microfossil supports early Paleoproterozoic rise in complex cellular organisation

The great oxidation event (GOE), ~2.4 billion years ago, caused fundamental changes to the chemistry of Earth's surface environments. However, the effect of these changes on the biosphere is unknown, due to a worldwide lack of well-preserved fossils from this time. Here, we investigate exceptionally preserved, large spherical aggregate (SA) microfossils permineralised in chert from the c. 2.4 Ga Turee Creek Group in Western Australia. Field and petrographic observations, Raman spectroscopic mapping, and in situ carbon isotopic analyses uncover insights into the morphology, habitat, reproduction and metabolism of this unusual form, whose distinctive, SA morphology has no known counterpart in the fossil record. Comparative analysis with microfossils from before the GOE reveals the large SA microfossils represent a step-up in cellular organisation. Morphological comparison to extant micro-organisms indicates the SAs have more in common with coenobial algae than coccoidal bacteria, emphasising the complexity of this microfossil form. The remarkable preservation here provides a unique window into the biosphere, revealing an increase in the complexity of life coinciding with the GOE.

A late Paleoproterozoic (1.74 Ga) deep-sea, low-temperature, iron-oxidizing microbial hydrothermal vent community from Arizona, USA

Modern marine hydrothermal vents occur in a wide variety of tectonic settings and are characterized by seafloor emission of fluids rich in dissolved chemicals and rapid mineral precipitation. Some hydrothermal systems vent only low-temperature Fe-rich fluids, which precipitate deposits dominated by iron oxyhydroxides, in places together with Mn-oxyhydroxides and amorphous silica. While a proportion of this mineralization is abiogenic, most is the result of the metabolic activities of benthic, Fe-oxidizing bacteria (FeOB), principally belonging to the Zetaproteobacteria. These micro-organisms secrete micrometer-scale stalks, sheaths, and tubes with a variety of morphologies, composed largely of ferrihydrite that act as sacrificial structures, preventing encrustation of the cells that produce them. Cultivated marine FeOB generally require neutral pH and microaerobic conditions to grow. Here, we describe the morphology and mineralogy of filamentous microstructures from a late Paleoproterozoic (1.74 Ga) jasper (Fe-oxide-silica) deposit from the Jerome area of the Verde mining district in central Arizona, USA, that resemble the branching tubes formed by some modern marine FeOB. On the basis of this comparison, we interpret the Jerome area filaments as having formed by FeOB on the deep seafloor, at the interface of weakly oxygenated seawater and low-temperature Fe-rich hydrothermal fluids. We compare the Jerome area filaments with other purported examples of Precambrian FeOB and discuss the implications of their presence for existing redox models of Paleoproterozoic oceans during the "Boring Billion."

Mars Rover Techniques and Lower/Middle Cambrian Microbialites from South Australia: Construction, Biofacies, and Biogeochemistry

The Perseverance rover (Mars 2020) is equipped with an instrumental and analytical payload capable of identifying a broad range of organic molecules in geological samples. To determine the efficacy of these analytical techniques in recognizing important ecological and environmental signals in the rock record, this study utilized analogous equipment, including gas chromatography/mass spectrometry, Raman spectroscopy, X-ray fluorescence (XRF), Fourier transform infrared spectroscopy, along with macroscopic and petrographic observations, to examine early-middle Cambrian microbialites from the Arrowie Basin, South Australia. Morphological and petrographic observations of these carbonate successions reveal evidence of hypersaline-restricted environments. Microbialites have undergone moderate diagenesis, as supported by XRF data that show mineral assemblages, including celestine and the illitization of smectite. Raman spectral data, carbon preference indices of ∼1, and the methylphenanthrene index place the samples in the prehnite/pumpellyite metamorphic facies. Pristane and phytane are the only biomarkers that were detected in the least thermally mature samples. This research demonstrates a multitechnique approach that can yield significant geological, depositional, paleobiological, and diagenetic information that has important implications for planning future astrobiological exploration.

Constraining the rise of oxygen with oxygen isotopes

After permanent atmospheric oxygenation, anomalous sulfur isotope compositions were lost from sedimentary rocks, demonstrating that atmospheric chemistry ceded its control of Earth’s surficial sulfur cycle to weathering. However, mixed signals of anoxia and oxygenation in the sulfur isotope record between 2.5 to 2.3 billion years (Ga) ago require independent clarification, for example via oxygen isotopes in sulfate. Here we show <2.31 Ga sedimentary barium sulfates (barites) from the Turee Creek Basin, W. Australia with positive sulfur isotope anomalies of ∆³³S up to + 1.55‰ and low δ¹⁸O down to −19.5‰. The unequivocal origin of this combination of signals is sulfide oxidation in meteoric water. Geochemical and sedimentary evidence suggests that these S-isotope anomalies were transferred from the paleo-continent under an oxygenated atmosphere. Our findings indicate that incipient oxidative continental weathering, ca. 2.8–2.5 Ga or earlier, may be diagnosed with such a combination of low δ¹⁸O and high ∆³³S in sulfates.

The loss of anomalous sulfur isotope compositions from sedimentary rocks has been considered a symptom of permanent atmospheric oxygenation. Here the authors show sulfur and oxygen isotope evidence from < 2.31 Ga sedimentary barium sulphates (barites) from the Turee Creek Basin, W. Australia, demonstrating the influence of local non-atmospheric processes on anomalous sulfur isotope signals.

Snapshot of an early Paleoproterozoic ecosystem: Two diverse microfossil communities from the Turee Creek Group, Western Australia

Eighteen microfossil morphotypes from two distinct facies of black chert from a deep-water setting of the c. 2.4 Ga Turee Creek Group, Western Australia, are reported here. A primarily in situ, deep-water benthic community preserved in nodular black chert occurs as a tangled network of a variety of long filamentous microfossils, unicells of one size distribution and fine filamentous rosettes, together with relatively large spherical aggregates of cells interpreted as in-fallen, likely planktonic, forms. Bedded black cherts, in contrast, preserve microfossils primarily within, but also between, rounded clasts of organic material that are coated by thin, convoluted carbonaceous films interpreted as preserved extracellular polymeric substance (EPS). Microfossils preserved within the clasts include a wide range of unicells, both much smaller and larger than those in the nodular black chert, along with relatively short, often degraded filaments, four types of star-shaped rosettes and umbrella-like rosettes. Large, complexly branching filamentous microfossils are found between the clasts. The grainstone clasts in the bedded black chert are interpreted as transported from shallower water, and the contained microfossils thus likely represent a phototrophic community. Combined, the two black chert facies provide a snapshot of a microbial ecosystem spanning shallow to deeper-water environments, and an insight into the diversity of life present during the rise in atmospheric oxygen. The preserved microfossils include two new, distinct morphologies previously unknown from the geological record, as well as a number of microfossils from the bedded black chert that are morphologically similar to-but 400-500 Ma older than-type specimens from the c. 1.88 Ga Gunflint Iron Formation. Thus, the Turee Creek Group microfossil assemblage creates a substantial reference point in the sparse fossil record of the earliest Paleoproterozoic and demonstrates that microbial life diversified quite rapidly after the end of the Archean.

Complex patterns in fossilized stromatolites revealed by hyperspectral imaging (400-2496 nm)

Hyperspectral imaging (400-2496 nm) was used to quantitatively map surface textures and compositional variations in stromatolites to determine whether complexity of textures could be used as evidence to support biogenicity in the absence of preserved biomarkers. Four samples of 2.72-2.4 Ga stromatolites from a variety of settings, encompassing marine and lacustrine environments, were selected for hyperspectral imaging. Images of the sawn surfaces of samples were processed to identify reflectance and mineral absorption features and quantify their intensity (as an index of mineral abundance) using automated feature extraction. Amounts of ferrous iron were quantified using a ratio of reflectance at 1650 and 1299 nm. Visible near infrared imagery (400-970 nm) did not reveal additional textural patterns to those obtained from visual inspection. Shortwave infrared imagery (1000-2496 nm), however, revealed complex laminar and convoluted patterns, including a distinctive texture of sharp peaks and broad, low troughs in one sample, similar to living tufted microbial mats. Spectral analysis revealed another sample to be composed of dolomite. Two other samples were dominated by calcite or chlorite ± illite. Large variations in amounts of ferrous iron were found, but ferric iron was exclusively located in the oxidation crust. Hyperspectral imaging revealed large differences between parts of a sample of biogenic and non-biogenic origin. The former was characterized by calcite with varying amounts of ferrous iron, distributed in lenticular, convoluted patterns; the latter by Mg-Fe chlorite with large amounts of aluminium silicate, distributed as fine laminar layers. All minerals identified by hyperspectral imaging were confirmed by thin section petrography and XRD analyses. Spatial statistics generated from quantitative minerals maps showed different patterns between these different parts of the sample. Thus, hyperspectral imaging was shown to be a powerful tool for detecting structures in stromatolites that could be used, together with other lines of evidence, to support biogenicity.