Search

Terrestrial thermal environments can serve as analogs for subsurface environments in the search for life because they regularly host microbial communities, which may leave behind biosignatures. This study focused on an acid-sulfate hydrothermal site as an analog for a potentially habitable environment on Mars. A weathered boulder in the thermal area was dissected, revealing an interior marked with disconnected horizons of differently colored materials, very low pH, and increasing temperature. The mineralogy comprised weathering products from andesite (kaolinite, quartz, clinoptilolite) along with sulfate salts (alunite, jarosite, tschermigite, and copiapite) formed by oxidation of sulfide and ferrous iron. Characterization of organic matter in this boulder and several soil samples yielded interesting but surprising results. Both mass spectrometry and Raman spectroscopy identified organic compounds in portions of the soils and the boulder. Jarosite-associated samples showed more numerous and diverse organic signatures than did Al-bearing silicate samples, despite the lower total organic carbon content of the jarosite-associated soils (0.69 ± 0.07 wt% Corg) compared to the Al-bearing samples (1.28 ± 0.13 wt% Corg). Results from our geochemical, mineralogical, and spectroscopic study of hydrothermal alteration products and salts inform the heterogeneous distribution of inorganic and organic materials that could delineate habitats and demonstrate the limits on organic matter detectability using different analytical techniques. Furthermore, we relate our measurements and results directly to current and upcoming martian missions, and we provide recommendations for detection and characterization of minerals and organics as biosignatures on Mars using instruments on future missions.

The ancient rocks of the Pilbara region of Western Australia have been an important analog site for the study of possible inhabited environments in the search for life on early Mars for over four decades. Here, we review the evidence for Paleo- to Neoarchean life and the habitats that it occupied in the Pilbara Craton and unconformably overlying Fortescue Group of the Mount Bruce Supergroup. Nine major inhabited environments are described, which range from land to sea, and into the subsurface, showing that life had diversified into, and flourished within, a range of different environments early in Earth history. An important additional component in the search for life on Mars involves the manner in which evidence for early life is preserved. From the examples studied here, early mineralization of organic matter is key to the preservation of reliable biosignatures, in either silica, carbonate, or pyrite, but burial by volcanic ash can also provide excellent preservation.

Recent discoveries of geyserite and siliceous sinter with textural biosignatures in the ∼3.5 Ga Dresser Formation of the Pilbara Craton, Western Australia, extended the record of inhabited subaerial hot springs on Earth by ∼3 billion years, back to the time when siliceous sinter deposits are known to have formed on Mars (

Ooids are accretionary grains commonly reported from turbulent, shallow-water environments. They have long been associated with microbially dominated ecosystems and often occur in close proximity to, or embedded within, stromatolites, yet have historically been thought to form solely through physicochemical processes. Numerous studies have revealed both constructive and destructive roles for microbes colonizing the surfaces of modern calcitic and aragonitic ooids, but there has been little evidence for the operation of these processes during the Archean and Proterozoic, when both ooids and microbially dominated ecosystems were more widespread. Recently described carbonate ooids from the 2.9 Ga Pongola Supergroup, South Africa, include well-preserved examples composed of diagenetic dolomite interpreted to have formed from a high-Mg-calcite precursor. Spatial distributions of organic matter and elements associated with metabolic activity (N, S, and P) were interpreted as evidence for a biologically induced origin. Here, we describe exceptionally well-preserved ooids composed of calcite, collected from Earth's oldest known carbonate lake system, the ~2.72 Ga Meentheena Member (Tumbiana Formation), Fortescue Group, Western Australia. We used optical microscopy, Raman spectroscopy, XRD, SEM-EDS, LA-ICP-MS, EA-IRMS, and a novel micro-XRF instrument to investigate an oolite shoal deposited between stromatolites that preserve abundant evidence for microbial activity. We report an extremely fine, radial-concentric, calcitic microfabric that is similar to the primary and early diagenetic fabrics of calcitic ooids reported from modern temperate lakes. Early diagenetic silica has trapped isotopically light and thermally mature organic matter. The close association of organic matter with mineral phases and microfabrics related to primary and early diagenetic processes suggest incorporation of organic matter occurred during accretion, likely due to the presence of microbial biofilms. We conclude that the oldest known calcitic ooids were likely formed through processes similar to those that mediate the accretion of ooids in similar environments today, including formation within a microbial biosphere.

This report reviews how terrestrial hot spring systems can sustain diverse and abundant microbial communities and preserve their fossil records. Hot springs are dependable water sources, even in arid environments. They deliver reduced chemical species and other solutes to more oxidized surface environments, thereby providing redox energy and nutrients. Spring waters have diverse chemical compositions, and their outflows create thermal gradients and chemical precipitates that sustain diverse microbial communities and entomb their remnants. These environments probably were important habitats for ancient benthic microbial ecosystems, and it has even been postulated that life arose in hydrothermal systems. Thermal spring communities are fossilized in deposits of travertine, siliceous sinter, and iron minerals (among others) that are found throughout the geological record back to the oldest known well-preserved rocks at 3.48 Ga. Very few are known before the Cenozoic, but it is likely that there are many more to be found. They preserve fossils ranging from microbes to trees and macroscopic animals. Features on Mars whose morphological and spectroscopic attributes resemble spring deposits on Earth have been detected in regions where geologic context is consistent with the presence of thermal springs. Such features represent targets in the search for evidence of past life on that planet.

Identifying microbial fossils in the rock record is a difficult task because they are often simple in morphology and can be mimicked by non-biological structures. Biosignatures are essential for identifying putative fossils as being definitively biological in origin, but are often lacking due to geologic effects which can obscure or erase such signs. As such, there is a need for robust biosignature identification techniques. Here we show new evidence for the application of trace elements as biosignatures in microfossils. We found elevated concentrations of magnesium, aluminum, manganese, iron, and strontium colocalized with carbon and sulfur in microfossils from Drummond Basin, a mid-Paleozoic hot spring deposit in Australia. Our results also suggest that trace element sequestrations from modern hot spring deposits persist through substantial host rock alteration. Because some of the oldest fossils on Earth are found in hot spring deposits and ancient hot spring deposits are also thought to occur on Mars, this biosignature technique may be utilized as a valuable tool to aid in the search for extraterrestrial life.

The ca. 2.45–2.22 Ga Turee Creek Group, Western Australia, contains carbonate-rich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2–2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn- and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate–siliciclastic shelf. These carbonates exhibit moderate δ13C values of −2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

Early in Mars’ history, life may have flourished in lacustrine environments developed during a comparatively warmer and wetter climate regime. Sustained subaqueous environments are high priority landing sites for the Mars 2020 mission, which is likely to focus on in situ analysis of geological features present in these environments, and on sample return. However, the community lacks experience investigating ancient, microbially dominated lacustrine environments, in part due to their rarity. Among the rocks that were formed during the Archean and that host biosignatures, only the Fortescue Group (2775–2630 Ma) is known to contain well-preserved lake deposits. Intervals within this group preserve extensive fluvio-lacustrine depositional facies and several categories of biosignatures recording the presence of early, microbially dominated ecosystems. The broader depositional environment of lacustrine units within the Fortescue Group, which formed on plateaus renewed by episodes of ongoing basaltic volcanism, is analogous to the broader depositional setting of paleolakes that may be encountered on Mars. Repeated lake high stands and drying events provide an analogue for climate change related to obliquity variations in early Martian environments, insights into the effect of climate changes on the lacustrine microbial biosphere, and an exploration model for taphonomic windows preserving microbial biosignatures in elements of these systems.

The ca. 3.48 Ga Dresser Formation, Pilbara Craton, Western Australia, is well known for hosting some of Earth’s earliest convincing evidence of life (stromatolites, fractionated sulfur/carbon isotopes, microfossils) within a dynamic, low-eruptive volcanic caldera affected by voluminous hydrothermal fluid circulation. However, missing from the caldera model were surface manifestations of the volcanic-hydrothermal system (hot springs, geysers) and their unequivocal link with life. Here we present new discoveries of hot spring deposits including geyserite, sinter terracettes and mineralized remnants of hot spring pools/vents, all of which preserve a suite of microbial biosignatures indicative of the earliest life on land. These include stromatolites, newly observed microbial palisade fabric and gas bubbles preserved in inferred mineralized, exopolymeric substance. These findings extend the known geological record of inhabited terrestrial hot springs on Earth by ∼3 billion years and offer an analogue in the search for potential fossil life in ancient Martian hot springs.

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member d 13 Ccarb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of 13 C-depleted organic matter. d 13 Corg values average 36.7% ,c onsistent with Neoarchean marine units reported from elsewhere, but significantly less 13 C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.

The hydrocarbons preserved in an Archean rock were extracted, and their composition and distribution in consecutive slices from the outside to the inside of the rock were examined. The 2.7 Ga rock was collected from the Fortescue Group in the Pilbara region, Western Australia. The bitumen I (solvent-extracted rock) and bitumen II (solvent-extracted hydrochloric acid-treated rock) fractions have different hydrocarbon compositions. Bitumen I contains only trace amounts of aliphatic hydrocarbons and virtually no aromatic hydrocarbons. In contrast, bitumen II contains abundant aliphatic and aromatic hydrocarbons. The difference seems to reflect the weathering history and preservational environment of the investigated rock. Aliphatic hydrocarbons in bitumen I are considered to be mainly from later hydrocarbon inputs, after initial deposition and burial, and are therefore not indigenous. The lack of aromatic hydrocarbons in bitumen I suggests a severe weathering environment since uplift and exposure of the rock at the Earth's surface in the Cenozoic. On the other hand, the high abundance of aromatic hydrocarbons in bitumen II suggests that bitumen II hydrocarbons have been physically isolated from removal by their encapsulation within carbonate minerals. The richness of aromatic hydrocarbons and the relative scarcity of aliphatic hydrocarbons may reflect the original compositions of organic materials biosynthesised in ancient organisms in the Archean era, or the high thermal maturity of the rock. Cyanobacterial biomarkers were observed in the surficial slices of the rock, which may indicate that endolithic cyanobacteria inhabited the surface outcrop. The distribution of aliphatic and aromatic hydrocarbons implies a high thermal maturity, which is consistent with the lack of any specific biomarkers, such as hopanes and steranes, and the prehnite-pumpellyite facies metamorphic grade.

The 2.72Ga Tumbiana Formation is a succession of clastic and carbonate rocks outcropping along the southern margin of the Pilbara Craton in Western Australia. It hosts abundant, diverse and exceptionally well-preserved stromatolites and has provided the setting for numerous investigations focussing on the Archaean biosphere. Despite its palaeobiological significance, the overall depositional setting of the Tumbiana Formation remains unclear. Here we present the results of stratigraphic, sedimentological and geochemical investigation of the Tumbiana Formation in the well-known Redmont/"Knossos" area and at several localities in the northwestern Pilbara sub-basin. We suggest these data are best explained by deposition in fluvial and lacustrine environments of an inward-draining continental basin. δ13Corg values vary from -49.9‰ to -15.0‰. Conical stromatolite morphologies, commonly attributed to cyanobacteria, are anomalously little depleted in 13Corg, implying a higher relative contribution of organic matter from phototrophic versus methane cycling metabolisms.

Some modern filamentous oxygenic photosynthetic bacteria (cyanobacteria) form macroscopic tufts, laminated cones and ridges that are very similar to some Archean and Proterozoic stromatolites. However, it remains unclear whether microbes that constructed Archean clumps, tufts, cones and ridges also produced oxygen. Here, we address this question by examining the physiology of cyanobacterial clumps, aggregates ~0.5 mm in diameter that initiate the growth of modern mm- and cm-scale cones. Clumps contain more particulate organic carbon in the form of denser, bowed and bent cyanobacterial filaments, abandoned sheaths and non-cyanobacterial cells relative to the surrounding areas. Increasing concentrations of oxygen in the solution enhance the bending of filaments and the persistence of clumps by reducing the lateral migration of filaments away from clumps. Clumped mats in oxic media also release less glycolate, a soluble photorespiration product, and retain a larger pool of carbon in the mat. Clumping thus benefits filamentous mat builders whose incorporation of inorganic carbon is sensitive to oxygen. The morphogenetic sequence of mm-scale clumps, reticulate ridges and conical stromatolites from the 2.7 Ga Tumbiana Formation likely records similar O2-dependent behaviors, preserving currently the oldest morphological signature of oxygenated environments on Early Earth., United States. National Aeronautics and Space Administration (NASA Astrobiology Institute NNA08CN84A), National Science Foundation (U.S.) (EAR-0843358)

The Flinders Ranges and its surroundings in South Australia comprise an impressive rugged terrain that rises abruptly from piedmont plains to the east and west and merges into the plains of the Cenozoic Lake Eyre Basin to the north. Folded and faulted Neoproterozoic-Cambrian clastic and carbonate sedimentary rocks of the Adelaide Geosyncline (Adelaide Rift Complex) form the predominant geology of the ranges and record varied depositional environments and metamorphic overprints and have had a complex landscape history, resulting in a diverse regolith. This ancient, arid terrain represents some of the best analogue landscapes and settings in Australia to observe features and processes fundamental to the evolution of the Earth. The strata of the Flinders Ranges record the evolution of terrestrial surface environments and the biosphere through the Cryogenian, Ediacaran and Cambrian periods, including evidence for Neoproterozoic glaciations, orbital and rotational dynamics and asteroid impact. The diverse assemblages of stromatolites, ancient and modern hydrothermal systems, and alteration assemblages provide field laboratories for astrobiological and hyperspectral research and training. For these reasons the northern Flinders Ranges near Arkaroola have been selected as a site for multi-disciplinary Mars analogue research and space education.

The macroscopic fossil record of the Archean consists solely of stromatolites and other microbialites, which seldom offer compelling clues to the identities of the organisms that formed them. Tufted microbial mats are an exception because their formation is known to require a suite of morphological and behavioural characteristics from which the behavioural and biological affinities of early microbialite-constructing microbes can be inferred. Here, the oldest yet reported convincing fossil tufted microbial mats are described and discussed in the context of other ancient and modern examples. Significantly, cyanobacteria dominate all known modern occurrences and may also have been the builders of ancient examples, the oldest of which predate by several hundred million years the earliest convincing cyanobacterial microfossils and most geochemical evidence for an oxygenated atmosphere.

Of various problems that have hindered progress in documenting the Precambrian history of life, the difficulty in distinguishing between bona fide microbial fossils and nonbiological microscopic pseudofossils has been among the most serious. Though errors in the interpretation of putative Precambrian fossil microbes have diminished greatly over recent years, mistakes continue to be made. We suggest that such errors can be avoided by the use of a multifaceted strategy based on a specified series of biologically definitive characteristics that document the presence of interrelated biological morphology and biologically derived chemistry. To illustrate this promising approach, we use optical microscopy, confocal laser scanning microscopy, and Raman spectroscopy, together, to distinguish between authentic microbial fossils and microscopic “look-alikes,” both coccoidal and filamentous, rock-embedded in five Proterozoic and two Archean geological units: bona fide fossils of the ∼800 Ma Bitter Springs Formation of central Australia and ∼3050 Ma Farrel Quartzite of northwestern Australia; and objects we regard to be pseudofossils from the ∼770 Ma Chanda Limestone of southern India, ∼800 Ma Myrtle Springs Formation of South Australia, ∼1020 Ma Lakhanda Formation of southeastern Siberia, ∼1700 Ma Vempalle Formation of central India, and ∼2629 Ma Marra Mamba Iron Formation of northwestern Australia. The results demonstrate that no single criterion, by itself, is sufficient to establish the biological origin of such objects. Instead, as shown here, this problem appears solvable by the use of an interdisciplinary approach that combines the data and techniques of geology, biology, and chemistry.

The ca. 1.8 Ga Duck Creek Formation, Western Australia, preserves 1000 m of carbonates and minor iron formation that accumulated along a late Paleoproterozoic ocean margin. Two upward-deepening stratigraphic packages are preserved, each characterized by peritidal precipitates at the base and iron formation and carbonate turbidites in its upper part. Consistent with recent studies of Neoarchean basins, carbon isotope ratios of Duck Creek carbonates show no evidence for a strong isotopic depth gradient, but carbonate minerals in iron formations can be markedly depleted in [superscript 13]C. In contrast, oxygen isotopes covary strongly with depth; delta[superscript 18]O values as positive as 2%. VPDB in peritidal facies systematically decline to values of 6 to 16% in basinal rocks, reflecting, we posit, the timing of diagenetic closure. The Duck Creek Formation contains microfossils similar to those of the Gunflint Formation, Canada; they are restricted to early diagenetic cherts developed in basinal facies, strengthening the hypothesis that such fossils capture communities driven by iron metabolism. Indeed, X-ray diffraction data indicate that the Duck Creek basin was ferruginous throughout its history. The persistence of ferruginous waters and iron formation deposition in Western Australia for at least several tens of millions of years after the transition to sulfidic conditions in Laurentia suggests that the late Paleoproterozoic expansion of sulfidic subsurface waters was globally asynchronous.

Australia has numerous landforms and features, some unique, that provide a useful reference for interpreting the results of spacecraft orbiting Mars and exploring the martian surface. Examples of desert landforms, impact structures, relief inversion, long-term landscape evolution and hydrothermal systems that are relevant to Mars are outlined and the relevant literature reviewed. The Mars analogue value of Australia's acid lakes, hypersaline embayments and mound spring complexes is highlighted along with the Pilbara region, where the oldest convincing evidence of life guides exploration for early life on Mars. The distinctive characteristics of the Arkaroola Mars Analogue Region are also assessed and opportunities for future work in Australia are outlined.

Rare earth elements and selected trace elements were measured in 48 samples of carbonate and chert from stromatolites and associated facies in the 3.45 billion year old Strelley Pool Formation, Pilbara Craton, Western Australia. The samples show coherent REE+Y patterns that vary systematically with sedimentary facies. Chert samples from bedded cherts beneath the Strelley Pool Formation and from the upper bedded chert members in the formation show REE+Y patterns consistent with originating by precipitation from hydrothermal and mixed marine-hydrothermal fluids. In contrast, carbonates and cherts from the stromatolitic reef member share the essential shale-normalized characteristics of other Archean marine precipitates (LREE depletion, positive La and Gd anomalies, absence of a negative Ce anomaly and a strongly superchondritic Y/Ho ratio). The close correspondence between REE+Y signatures and independent sedimentary facies interpretations is viewed as strong evidence for the primary nature of REE+Y patterns. They can thus be used as a proxy for the fluids from which sediments precipitated. Mixing hyperbolae can be constructed that reproduce the chemistry of cherts and carbonates by mixing of hydrothermal and marine fluid endmembers throughout the entire vertical succession from beneath the Strelley Pool Formation to the uppermost cherts. The mixing hyperbolae provide semi-quantitative confirmation that the trace element compositions across the suite of cherts represent different mixtures of ambient seawater and hydrothermal fluids. Our results indicate that the Earth's oldest supracrustal carbonates and associated cherts record important aspects of the REE geochemistry of the waters in which they precipitated, and provide valuable information on possible habitats of some of Earth's earliest biota.

Microstructures recently reported from an Archaean sedimentary succession (ca. 3.0 Ga) in the Mount Goldsworthy–Mount Grant area in the northeastern Pilbara Craton meet the criteria for compelling evidence of biogenicity [Sugitani, K., Grey, K., Allwood, A., Nagaoka, T., Mimura, K., Minami, M., Marshall, C.P., Van Kranendonk, M.J., Walter, M.R., 2007. Diverse microstructures from Archaean chert from the Mount Goldsworthy–Mount Grant area, Pilbara Craton, Western Australia: microfossils, dubiofossil, or pseudofossils. Precambrian Res. 158, 228–262]. The structures are morphologically diverse. Although they were tentatively classified into five major morphological types (thread-like, film-like, small ( 15 μm) spheroidal, and spindle-like), the possible taxonomic significance of these groups was not discussed. Building on our earlier analysis, we focus on the morphology of the larger spheroids, as well as presenting further evidence relating spindles and several bizarre forms, and attempt to group them taxonomically and adduce additional evidence for their biogenicity. Taphonomic features were identified in each of the various morphological groups, but the range of morphological diversity of the spheroids cannot be attributed solely to taphonomic alteration. Four subdivisions of spheroids are proposed: (1) simple single-walled spheroids, (2) thin-walled spheroids having a diffuse envelope, (3) thick-walled spheroids, and (4) spheroids having an extensively folded wall. Simple single-walled spheroids, 15–60 μm in diameter, with various wall textures but commonly lacking envelopes or appendages form the dominant subgroup. Other complex morphologies are present and include aligned or associated bodies of thin-walled spheroids with diffuse envelopes, and spindle-like structures containing inner spheroidal bodies. The degree of morphological complexity and associations between structures suggest the presence of reproductive phases. If correct, this implies that the early Earth (ca. 3.0 Ga) showed a higher level of biodiversity than is currently postulated.

Detailed mapping and C and O stable isotopic data from sedimentary carbonate in units both above and below the paleo-erosion surface on the Bitter Springs Formation (BSF) in the northeastern Amadeus Basin, Australia, have clarified the stratigraphy of the area. Isotopic data indicate that the top of the Loves Creek Member of the Bitter Springs Formation is preserved near Corroboree Rock, and is overlain by fenestrate-carbonate-clast breccia, and dolomitic quartz sandstone and chert-pebble conglomerate of the Pioneer Sandstone. The isotopic data, as well as lithologic data, indicate the presence of a 1–2 m-thick cap carbonate preserved between Corroboree Rock and areas 10 km to the northeast. In many places the cap carbonate layer is mostly a syn-sedimentary dolomite-clast breccia, consistent with deposition and disturbance in shallow water. C and O isotopic data also indicate that thin-bedded sandstone and dolomite above the Bitter Springs Formation at Ellery Creek, and a newly discovered massive chert-bearing dolomite at Ross River could both belong to the glaciogenic Olympic Formation. Detailed mapping also provides a more detailed context for the famous black chert microfossil locality in the Bitter Springs Formation at Ross River.

Biodegraded bitumens associated with quartz and calcite veins in the Cambrian Fish River Subgroup sediments of the Nama Group of southern Namibia have a geochemical signature diagnostic for organic matter that was deposited in a saline lacustrine palaeoenvironment. In particular, they contain abundant gammacerane, β-carotane and 3β-methylhopanes while 24-isopropyl cholestanes and dinosteroids are not detectable. Sealed tube hydrous pyrolysis of asphaltene and polar fractions yielded saturated hydrocarbons amenable to C isotopic analysis, and these analyses show unusually low δ 13 C values. These combined characteristics are also present in immature bitumens from the Permian Irati Formation of Brazil and a saline lacustrine facies of the Whitehill Formation in the Karoo Basin, South Africa. We conclude that the bitumens originated from Whitehill equivalent strata of the Kalahari Basin deposited in what was an extensive saline lacustrine basin in southwestern Gondwana during the Early Permian. In southern Africa, source rocks of the Whitehill Formation are generally immature for petroleum generation and it is therefore likely that the Nama bitumens were expelled by contact metamorphism during emplacement of Karoo dolerite sills and dykes in the Jurassic.

The 3.43 billion-year-old Strelley Pool Chert, Pilbara Craton, Western Australia, contains compelling evidence of Early Archaean life in the form of kilometre-sized remnants of an ancient stromatolitic carbonate platform. Reviewing and building on earlier studies, we examine the fossilized remains of the platform to seek ecosystem-scale insights to Earth's early biosphere, examining the evidence for biosedimentation, and the importance and effect of different environmental processes on biological activity. Both vertical and lateral trends show that stromatolite abundance and diversity are greatest in the area interpreted as an isolated, partially restricted, peritidal marine carbonate platform, or reef, where there is virtually no trace of hydrothermal or terrigenous clastic input. In contrast, stromatolites are poorly developed or absent among hydrothermal, volcaniclastic or terrigenous clastic sedimentary facies, and are absent in deeper marine settings that are laterally equivalent to shallow marine stromatolitic facies. Hydrothermal veins, some of which were previously interpreted as vents that exhaled fluids from which the stromatolitic structures precipitated, are shown to postdate the stromatolites. On the platform, stromatolite facies associations varied between different palaeoenvironments, but some stromatolite types occurred across different palaeoenvironments, highlighting the combined influence of biological and environmental processes on stromatolite formation. The regional distribution of stromatolites in the palaeoenvironment suggests a biological response to variations in water depth, sediment influx and hydrothermal activity with stromatolite formation favoured by relatively ‘normal’ shallow marine environments with low clastic/chemical sedimentation rates and no direct input from high temperature hydrothermal systems. The lithology, structure and fabrics of the stromatolites, and their close association with abundant evaporite crystal pseudomorphs, indicate that evaporitic precipitation was probably the dominant non-biological process that contributed to stromatolite formation. The study supports a biological interpretation for the origin of the stromatolites, and reveals compelling evidence for the conditions that favoured biological activity on the early Earth and formation of macroscopic biosignatures that could be preserved for most of Earth's history.

A diverse assemblage of indigenous carbonaceous microstructures, classified here as highly probable microfossils to pseudomicrofossils, is present in the >ca. 2.97 Ga Farrel Quartzite (Gorge Creek Group) at Mount Grant and Mount Goldsworthy, Pilbara Craton, Western Australia. The microstructures are an integral part of the primary sedimentary fabrics preserved in black chert beds. The interbedding of chert with layers of large silicified crystal pseudomorphs and fine to coarse grained volcaniclastic/clastic beds indicate deposition in a partially evaporitic basin with terrigenous clastic and volcaniclastic input. Similar associations of microstructures are present at the same stratigraphic level in outcrops more than 2 km apart. Four major microstructural types are present: thread-like, film-like, spheroidal and lenticular to spindle-like, each of which can be further subdivided into several sub-types. Most of the microstructures were deposited as part of an assemblage of chemical and clastic sediments, although there are some thread-like microstructures present for which a synsedimentary origin cannot be confirmed. Many specimens appear to have originally had flexible but breakable walls and some occur in colony-like aggregations. Size distributions for the four major types are generally narrow, a feature typical of biogenic structures. The microstructures are composed of disordered carbon (as revealed by Raman spectroscopy) and the bulk isotopic composition of the carbon is δ 13 C The combined morphological and geological evidence suggests that the film-like structures, small spheres associated with films, large spheroids and spindle-like structures are probable to highly probable fossil remains of microorganisms. The morphological variety among the microstructures suggests that a diverse microbial ecosystem flourished in the Pilbara region during the Archaean.

An apparent sine wave pattern of columns in a single specimen of the stromatolite Anabaria juvensis (subsequently identified as Kotuikania) from a Neoproterozoic dolomite unit, originally assigned to the ∼850 Ma Bitter Springs Formation, in the Amadeus Basin, central Australia, was interpreted previously as recording heliotropic growth, that is, the non-vertical growth of columns throughout the year controlled by averaged incident solar radiation [Vanyo, J.P., Awramik, S.M., 1985. Stromatolites and Earth– Sun–Moon dynamics. Precambrian Research 29, 121–142]. The model of heliotropic growth was used to estimate obliquity of the ecliptic (Earth's axial tilt) and days/year at 850 Ma. Subsequent work, however, casts strong doubt on the heliotropic interpretation. Further field observations and the study of 11 additional specimens of Anabaria=Kotuikania juvensis from the original locality confirm that the columns typically display strong branching, which produces a common divergence and convergence of columns that is incompatible with heliotropic growth. The rare, apparent sinuosity of columns is seen as the fortuitous product of column irregularity and column branching. Moreover, stratigraphic studies indicate that the host dolomite unit does not belong to the Bitter Springs Formation but caps the younger Cryogenian glaciogenic succession in the Amadeus Basin and hence is ∼600 Ma. The previous estimate of ∼435 (range 409–485) days/year based on extrapolated counts of laminae in the original specimen of A.=K. juvensis conflicts with the figure of 400±7 days/year indicated by high-quality palaeotidal data obtained from the late Cryogenian (∼640–600 Ma) Elatina–Reynella tidal rhythmites in South Australia. We conclude that inferences concerning Neoproterozoic obliquity and palaeorotation cannot be drawn from the non-vertical growth patterns of the columnar stromatolite A.=K. juvensis. © 2007 Elsevier B.V. All rights reserved.

The 1,640 Ma HYC (Here’s Your Chance) deposit at McArthur River, Northern Territory, Australia, is one of the largest and least metamorphosed Proterozoic stratiform lead-zinc-silver deposits in the world. The thermal history of the deposit is a currently not well understood, both low and high temperature mechanisms have been proposed. From our study we were able to estimate (from both kerogen and bitumen thermal maturity indices which concur) the thermal maturity to be equivalent to Ro 1.1–2.0%, corresponding to the wet gas generation zone, with a maximum relatively low burial temperature range of 120–180 °C in the ore samples. Regionally, temperatures were not further constrained due to the complex and dynamic nature of the sedimentary environment.

Raman spectra of carbonaceous materials in one of the world's oldest sedimentary rock formations – the Strelley Pool Chert (Pilbara Craton, Western Australia) – are analysed to determine whether primary structural characteristics of organic molecules may have survived to the present day. We use Raman spectral parameters to identify variations in molecular structure of the carbon and determine whether original characteristics of the carbonaceous materials have been completely thermally overprinted, as would be expected during c.3.5 billion years of geologic history. To the contrary, we find that the molecular structure of the carbonaceous materials varies depending on the sedimentary layer from which the sample came and the inferred original palaeoenvironmental setting of that layer, as determined by other geochemical and geological data. Thus, we argue that the spectral characteristics of the carbonaceous materials reflect original palaeoenvironments that varied through time from warm hydrothermal settings to cooler marine conditions and a return to hydrothermal conditions. Raman spectroscopy is also used to show that organic matter is present in trace amounts in association with putative stromatolites (laminated sedimentary structures possibly formed by microorganisms) in the Strelley Pool Chert, which were previously thought to be devoid of organic remains. Furthermore, the Raman spectra of carbon associated with stromatolites indicate lower thermal maturity compared to the carbon in (non-stromatolitic) hydrothermal deposits above (younger) and below (older). Significantly, this indicates that the stromatolites are not abiotic hydrothermal precipitates – as previously proposed – but were formed in a cooler marine environment that may have been more favorable to life.

The International Union of Geological Sciences has approved a new addition to the geologic time scale: the Ediacaran Period. The Ediacaran is the first Proterozoic period to be recognized on the basis of chronostratigraphic criteria and the first internationally ratified, chronostratigraphically defined period of any age to be introduced in more than a century. In accordance with procedures established by the International Commission on Stratigraphy, the base of the Ediacaran Period is defined by a Global Stratotype Section and Point (GSSP) placed at the base of the Nuccaleena Formation cap carbonate directly above glacial diamictites and associated facies at Enorama Creek in the Flinders Ranges of South Australia. Its top is defined by the initial GSSP of the Cambrian Period. The new Ediacaran Period encompasses a distinctive interval of Earth history that is bounded both above and below by equally distinctive intervals. Both chemostratigraphic and biostratigraphic data indicate that the subdivision of the period into two or more series is feasible, and this should be a primary objective of continuing work by the Ediacaran Subcommission of the ICS.

Micro-scale analytical techniques permit correlation of chemistry with morphology of individual Proterozoic acritarchs (organic-walled microfossils), and thus provide new approaches for elucidating their biological affinities. A combination of micro-Fourier transform infrared (FTIR) spectroscopy and laser micro-Raman spectroscopy was used to investigate the organic structure and composition of individual acritarchs. Well preserved Neoproterozoic acritarchs from the Tanana Formation, Australia (ca. 590–565 Ma), and Mesoproterozoic acritarchs from the Roper Group (1.5–1.4 Ga), Australia, and Ruyang Group, China (1.4–1.3 Ga, age poorly resolved but certainly >1000 Ma and H stretching bands in the 2900 cm−1 region relative to the C C aromatic ring stretching band at 1600 cm−1. This FTIR spectrum is consistent with the FTIR spectra obtained from algaenans isolated from extant chlorophyte and eustigmatophyte microalgae. FTIR spectra of Leiosphaeridia sp. from the Tanana Formation contain a less intense aliphatic C H stretching band relative to the C C aromatic ring stretching band. By comparison, the spectra acquired from the Mesoproterozoic acritarchs were dominated by C C aromatic ring stretching bands at 1600 cm−1 relative to moderate-weak CH3 terminal groups (1345 cm−1), C H aliphatic stretching (3000–2700 cm−1), and C O (1710 cm−1), although some differences in biopolymer composition occurred between species. Curve-fitting of the aliphatic C Hx stretching region provides greater insight into the aliphatic structures of the acritarchs. The CH2/CH3 intensity ratio can be used to assess the relative chain length and degree of branching. Organic material in the Tanarium conoideum consists of straight long chain hydrocarbons, while the other acritarchs contain hydrocarbons consisting of short chains that are highly branched. In this study it was found that Raman spectroscopy does not provide additional information about biopolymer composition of Proterozoic acritarchs, but rather offers complementary data regarding the aromaticity and degree of saturation of the macromolecular structure of acritarch cysts.

A petrographic, mineralogical, and geochemical investigation reveals evidence for selective preservation of microfacies in siliceous sinters. Plio-Pleistocene sinters from Steamboat Springs, Nevada, contain identical but fewer petrographic textures than Pleistocene sinters from Artist Point, Yellowstone National Park, Wyoming. The latter are preserved in altered sediments of unknown origin. A sequence of pore-filling and mineralogical changes explains the excellent preservation of nine microfacies and two petrographic textures. Plio-Pleistocene Steamboat Springs sinters are preserved in successions of andesitic basalt flows and were likely subjected to a more extreme thermal and a different hydrological environment than those from Yellowstone National Park. Most microfacies were obliterated during postdepositional heating of Steamboat Spring sinters. Differences in diagenetic histories related to depositional environment, water chemistry, and/or subsequent burial must account for the loss of textural evidence between the diagenetic stage represented by Artist Point sinters and that of Steamboat Springs sinters. Hence, early postdepositional history affects both the likelihood and quality of microfossil preservation.

Rover missions to the rocky bodies of the Solar System and especially to Mars require lightweight, portable instruments that use minimal power, require no sample preparation, and provide suitably diagnostic mineralogical information to an Earth-based exploration team. Short-wave infrared (SWIR) spectroscopic instruments such as the Portable Infrared Mineral Analyser (PIMA, Integrated Spectronics Pty Ltd., Baulkham Hills, NSW, Australia) fulfill all these requirements. We describe an investigation of a possible Mars analogue site using a PIMA instrument. A survey was carried out on the Strelley Pool Chert, an outcrop of stromatolitic, silicified Archean carbonate and clastic succession in the Pilbara Craton, interpreted as being modified by hydrothermal processes. The results of this study demonstrate the capability of SWIR techniques to add significantly to the geological interpretation of such hydrothermally altered outcrops. Minerals identified include dolomite, white micas such as illite-muscovite, and chlorite. In addition, the detection of pyrophyllite in a bleached and altered unit directly beneath the succession suggests acidic, sulfur-rich hydrothermal activity may have interacted with the silicified sediments of the Strelley Pool Chert.

Biomarker molecular fossils in 2770 Ma shales suggest that the Eucarya diverged from other principal domains early in Earth history. Nonetheless, at present, the oldest fossils that can be assigned to an extant eukaryotic clade are filamentous red algae preserved in ca. 1200 Ma cherts from Arctic Canada. Between these records lies a rich assortment of potentially protistan microfossils. Combined light microscopy, scanning electron microscopy, and transmission electron microscopy on 1500-1400 Ma fossils from the Roper Group, Australia, and broadly coeval rocks from China show that these intermediate assemblages do indeed include a moderate diversity of eukaryotic remains. In particular, preserved cell wall ultrastructure, observed using transmission electron microscopy (TEM), can help to bridge the current stratigraphic gap between the unambiguous eukaryotic morphologies of later Proterozoic assemblages and molecular biomarkers in much older rocks.

Polycyclic aromatic hydrocarbons (PAHs) in ore and mudstone within the McArthur River ore deposit show compound distribution patterns similar to those of hydrothermally generated petroleum in the Guaymas Basin and significantly different from those found in conventional oil. PAH abundances and their isomer distributions result from a temperature gradient between the source of mineralizing fluids and the sediments fringing the ore system during ore formation. Along with other geochemical, geological, paleobiological and mineralogical lines of evidence, these data provide strong evidence that the ore formed within partially lithified sediments under marine conditions. Given that the McArthur River ore body is an exquisitely preserved example of a sediment-hosted base-metal deposit, these results may be widely applicable. The McArthur deposit is also a rich repository of paleobiological information, allowing studies of the microbiology of ore formation and the paleobiology of an ancient hydrothermal system, as is discussed elsewhere.

The Neoproterozoic Earth experienced at least two, probably global, glaciations. Each glaciation was superceded by deposition of a layer of carbonate (‘cap-carbonate’) that has a distinctive lithology and depletion in 13 C ( δ 13 C carbonate ∼ −5‰). The ∼700 Ma Sturtian glaciation is followed by deposition of a cap-carbonate and post-glacial succession which contain bacterially produced sulfides extremely enriched in 34 S (average δ 34 S sulfide ∼ +30‰) with maximum values up to +60‰. This level of 34 S enrichment in sulfides is unique to the Sturtian post-glacial succession and recognised in Australia, Canada, and China. In the Neoproterozoic of the Nama Basin, Namibia, the Gobabis Member is the basal unit of the Court Formation, which overlies the glacial Blaubeker Formation. δ 13 C carbonate analyses from the Gobabis Member range from −5.2 to −2.2‰ (average = −3.7‰; n = 10). δ 34 S sulfide ranges from +16.1 to +61.1‰ (average = +37.6‰; n = 8). These results are consistent with a Sturtian age for the Blaubeker Formation and overlying Gobabis Member, which have previously been interpreted as Sturtian. The sulfur isotopic results are comparable with δ 34 S sulfide in Sturtian post-glacial units of Australia, Canada and China. This adds to the evidence for correlation of the Blaubeker Formation with Sturtian glaciations on other continents. The cause of such elevated δ 34 S sulfide is enigmatic. Geochemical evidence suggests the sulfide was not formed from low sulfate waters nor in euxinic conditions, which discounts any known modern analogue. 34 S enrichment in sulfides is therefore postulated to be caused by enrichment of 34 S in contemporaneous seawater ( δ 34 S sulfate up to +60‰?). The rise in seawater δ 34 S sulfate is considered to be the result of intense bacterial sulfate reduction in an anoxic ocean during the Sturtian glaciation.

Nature Communications 8: Article number: 15263 (2017); Published: 9 May 2017; Updated: 16 August 2017. The original manuscript presented evidence that the deposits studied were fluvial rather than marine to support the conclusion that evidence of life on land had been discovered in the 3.4 Ga Dresser Formation, Pilbara Craton, Australia.

The formation of the McArthur River lead-zinc deposit involves thermogenic or biologic oxidation of sedimentary organic matter, the products of which generated a massive stratiform sulfide ore body, and secondary carbonate and silica precipitates formed within the sediment pile down the flow pathway and above the reaction zone. The fine-grained texture of the mineralization indicates that primary ore texture is preserved, and coupled with the regional thermal maturity, indicate that this deposit is an ideal location to study organic matter signals related to ore formation and the sedimentary environment. Biomarker data point to a marine environment of deposition and are consistent with data previously collected from the host Barney Creek Formation in the adjacent Glyde Subbasin. An unusual biomarker distribution found in some samples from within two-orebody is considered to be related to the presence of sulfide-oxidizing bacteria. These organisms flourished after turbidite deposition, when oxygen in the upper part of the water column was mixed down to the sediment water interface. The biomarker data are supported by micropalaeontologic observations from the same samples and are consistent with intermittent oxygenation of the water column to the sediment water interface. This observation suggests an extension of the known occurrence of sulfide-oxidizing bacteria back in time by 800 million years, to 1640 Ma.

Carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual Australian Neoproterozoic basins, in a tectonic and sedimentologic framework, have allowed the construction of isotopic records for the interval 830–750 Ma. This, combined with the discovery of a restricted stratigraphic range for the acritarch Cerebrosphaera buickii in mid-Neoproterozoic strata of Australia, makes possible correlation with successions in Canada, Namibia and Spitsbergen. That part of the mid-Neoproterozoic succession in Spitsbergen which contains C. buickii was previously considered younger than the Sturtian glacial epoch, but this study has indicated that the Akademikerbreen Group is older than that. A correlation scheme is proposed along five tie lines, and is based on a correlation between Australia and Canada at ∼830 Ma, and Australia, Spitsbergen and Namibia at ∼760 Ma. The lowest ever recorded seawater 87 Sr/ 86 Sr ratios in the upper Shaler Supergroup of Canada (0.70561–0.70670) compare with ratios in evaporites of the ∼830 Ma Bitter Springs Formation, Amadeus Basin, Australia (0.70568–0.70720). Comparable δ 13 C carb records support a correlation. At about 760 Ma, a correlation is made between the heaviest mid-Neoproterozoic δ 13 C carb values of 7.2‰ (upper Burra Group, Adelaide Rift Complex) and 8.1‰ (Kanpa Formation, western Officer Basin) in Australia, 8.5‰ in Spitsbergen (Backlundtoppen Formation, Akademikerbreen Group), and 8.5‰ in Namibia (Ombombo Subgroup, Congo Craton). The stratigraphic range of C. buickii in Australia and Spitsbergen supports this correlation. A compilation of δ 13 C carb and 87 Sr/ 86 Sr records between about 830 and 750 Ma from Australia, Canada, Namibia and Spitsbergen reveals numerous negative excursions, which are not associated with preserved glacial deposits. It is necessary, therefore, to consider additional models to the ‘Snowball Earth’ to explain the large and rapid carbon isotopic excursions.

We construct a time scale for the 840–544 Ma Neoproterozoic interval from isotopic variation of δ13Ccarbonate and δ13Corganic, 87Sr/86Sr, and δ34Ssulfate in seawater measured from reference columns in Canada and Australia. We distinguish 18 features (Z–I) in the δ13Ccarbonate and δ13Corganic curves: two intervals of well-defined variation in 87Sr/86Sr; and two peaks in the variation of δ34Ssulfate. Newly acquired isotopic data in Australia enable correlation with Canada: the Gillen Member of the Bitter Springs Formation, estimated to be about 840 Ma, is correlated with the upper Shaler Supergroup; the Sturtian glacials, about 700 Ma, with the Rapitan glacials; and the Marinoan glacials, about 600 Ma, with the Ice Brook glacials. We recognize only these two major glaciations, and possibly a third minor glaciation, at 570 Ma. Columns in Poland, Namibia, Iran, and Siberia, and possibly Oman and Mongolia provide correlation by δ13C, and in Svalbard, Siberia, Oman, and Mali by 87Sr/86Sr. The inter-glacial (700–600 Ma) peak of δ34Ssulfide enables correlation among Australia, Namibia, and China. These correlations allow calibration of the resultant stratigraphy against time using the best available dates from a number of regions; we make simple linear interpolations between those dates. While recognizing that this can be no more than a rough approximation of the true ages away from the calibration points, the resultant age estimates have the merit of suggesting numerous tests of our stratigraphic scheme. The Neoproterozoic part of the 87Sr/86Sr curve resembles in range the Phanerozoic part, but of δ13C and δ34Ssulfate do not: the 20.5‰ amplitude of δ13C and 26.5‰ of δ34S greatly exceed the Phanerozoic 7.5 and 17‰, reflecting radically reduced net carbon and sulfur fluxes in younger times. The earliest Phanerozoic explosion of organisms with carbonate skeletons and the proliferation of bioturbating organisms are coincident with the onset of a C-cycle with isotopic fluctuations damped in both frequency and amplitude. The Neoproterozoic glaciations at 700 and 600 Ma are marked by negative δ13Ccarbonate and correspondingly depleted δ13Corganic and lower 87Sr/86Sr. The minimum 87Sr/86Sr at 840 Ma reflects the input to an ephemeral epeiric sea of excess 86Sr from extensive mafic volcanics in Australia and possibly Canada. The onset of higher values at 600 Ma corresponds to the early Pan-African and Cadomian amalgamation, and internal deformation, uplift, and erosion in Antarctica–Australia. The wealth of new data and the time framework they suggest allow us to build on the work of earlier authors and make a fresh attempt at explaining some of the major features of Neoproterozoic history, particularly in the interval from 700 Ma to the base of the Cambrian. Quantitative modeling is beyond the scope of our study, but we offer explanations for some of the isotopic features we have documented. Two icehouse states were preceded by massive sequestering of CO2 and accompanied by catastrophic declines in biological productivity. During and immediately after the older, Sturtian, glaciation, the deeper parts of the ocean were anoxic and contained sufficient ferrous iron to sequester very large amounts of sulfur derived from bacterial reduction of sulfate; the evidence suggests that the resultant huge shift in the sulfur isotropic composition was global and accompanied by the reduction of as much as half the sulfate in the anoxic parts of the oceans. The effects of the Pan-African orogeny include mountain building and a high rate of sedimentation, which resulted in the burial of large amounts of organic matter and concomitant oxygenation of the hydrosphere and atmosphere. This reinforced a trend in oxygenation that began before the second, Marinoan, glaciation. A second huge sulfur isotope anomaly accompanies the tectonism, and has been explained previously as possibly resulting from the desiccation and flushing of evolving ocean basins. This may be linked to a remarkable carbon isotope anomaly immediately preceding the Cambrian: the anomaly could be due to release of methane from oceanic clathrates de-stabilized by combined sea level fall and global warming resulting from volcanic release of CO2.

Seven sulfur-isotope stages are apparent in Australian Neoproterozoic sections. (1) From 840 Ma to the Sturtian glaciation (700 Ma) δ34Ssulfate varied little (+19 to +17.5‰) and δ34Ssulfide ranged from −20 to +23‰. (2) The Sturtian glaciation was followed by (3) a rise in δ34Ssulfide to an average of +30‰ (seen also in China, Namibia and Canada) and in δ34Ssulfate at least to +26‰ and possibly to +45‰. The sharp rise in δ34S of all sulfur fractions (sulfate, sulfide, and organic) leads us to speculate that sulfide depleted in 34S was deposited on the abyssal plain and that residual sulfate enriched in 34S remained beneath a stagnant, ice-covered ocean during the Sturtian glaciation and was brought to the shallow continental margin during the post-glacial transgression. (4) δ34Ssulfide and δ34Ssulfate fall from 640 to 607 Ma (δ34Ssulfate down to +17‰) probably from oxygen excess (through massive organic carbon burial), which causes weathering of sulfide depleted in 34S continental sulfide and previously deposited sulfide on the abyssal plain. (5) The oxic environment of the Marinoan (605–595 Ma) glaciation minimises sulfate-reduction (unlike the Sturtian) so that δ34Ssulfate is unvarying. (6) The post-glacial environment has sulfide depleted in 34S (average δ34Ssulfide

A combined sedimentological and biogeochemical study has been conducted on several Terminal Proterozoic mid-shelf microbial mat facies from the Centralian Super-basin. Isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from 'planktonic' and 'benthic microbial-mat' populations contributed to the sediment. The 'planktonic' source provided a suite of n-alkanes withC20 predominance and the odd n-alkanesC20, whereas, the 'benthic' source contributed an overlay of n-alkanesC20 with a strong even preference, together with mid-chain methyl alkanes. Kerogen and biomarkers derived from the microbial mat were found to be depleted in 13C relative to planktonic material. Pyrite in the microbial mats was also found to be depleted in 34S compared to surrounding facies. The combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria. These organisms have specific environmental tolerances that set limits on palaeo-environment. Their requirement for oxygen indicates that the water column above the mid-shelf could not have been anoxic. Accordingly, from the results and age determinations reported here, it would appear that mid-shelf environments of the Centralian Superbasin of Australia were seeing significant levels of oxygen through the Ediacarian.

Molecular phylogenetic studies of extant organisms have shown that those branches of the Bacteria and Archaea that lie closest to the “last common ancestor” of all life are occupied by hyperthermophiles. At the same time, the search for former life on Mars has focussed on thermal spring deposits. For these reasons there is interest in the palaeobiology of ancient thermal spring deposits on Earth. Many such deposits are known but very few have been studied by palaeobiologists. The Devonian sinters of the Drummond Basin, Australia, rank with the Rhynie cherts of Scotland as the oldest well established examples of fossil subaerial hot springs. The Drummond Basin sinters are closely comparable with modem examples in Yellowstone National Park, Wyoming, and elsewhere. It is possible to recognise a range of palaeoenvironments from high temperature vents through former hot-water channelways and terraces to ambient temperature marsh deposits. Cyanobacterial stromatolites and microfossils are abundant in those palaeoenvitronments characterised by temperatures well above ambient, and the marsh and other ambient temperature palaeoenvironments contain abundant herbaceous lycopsids and sphenopsids encrusted by microbial overgrowths. © 1998 Association of Australasian Palaeontologists.

An airborne hyperspectral remote sensing dataset was obtained of the North Pole Dome region of the Pilbara Craton in October 2002. It has been analyzed for indications of hydrothermal minerals. Here we report on the identification and mapping of hydrothermal minerals in the 3.459 Ga Panorama Formation and surrounding strata. The spatial distribution of a pattern of subvertical pyrophyllite rich veins connected to a pyrophyllite rich palaeohorizontal layer is interpreted to represent the base of an acid-sulfate epithermal system that is unconformably overlain by the stromatolitic 3.42 Ga Strelley Pool Chert., 29 pages, 9 figures, 2 tables

A visible and near infrared (VNIR) to shortwave infrared (SWIR) hyperspectral dataset of the Early Archaean North Pole Dome, Pilbara Craton, Western Australia, has been analysed for indications of hydrothermal alteration. Occurrence maps of hydrothermal alteration minerals were produced. It was found that using a spatial resolution on the ground of approximately 5 m and spectral coverage from 0.4 to 2.5 mm was sufficient to delineate several hydrothermal alteration zones and associated veins, including phyllic, serpentinitic and chloritic alteration. These results suggest this level of spectral and spatial resolution would be ideal for localising shallow epithermal activity, should such activity have existed, on the surface of Mars.

The basement of central Australia includes the Arunta Block and the 1200-1100 Ma Musgrave Block succeeded by 1075-1000 Ma bimodal volcanic rifts and dolerite dykes. A regional hiatus at 1000-800 Ma, interpreted as reflecting a central post-volcanic (underplated) upland, was followed at 800 Ma by a second swarm of dykes in the Musgrave Block, Gawler Craton and Stuart Shelf, associated with the crustal sagging that initiated the Centralian Superbasin. The Centralian Superbasin is defined here to encompass the Neoproterozoic fill of the Amadeus, Georgina, Ngalia, Officer and Savory Basins. The intra-cratonic superbasin was disrupted internally 540–600 Ma ago (“Petermann Ranges Orogeny”) by a central uplift and associated thrusts, and by mid-Carboniferous (320 Ma) tectonism (“Alice Springs Orogeny”) to form the structural basins listed above. The stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions. New results from acritarch biostratigraphy and isotope chemostratigraphy, in conjunction with conventional lithostratigraphy and sequence analysis, allow better stratigraphic resolution than previously possible. Field observations in the recently discovered Savory Basin in Western Australia provide a basis for correlations with the adjacent basins.

The Savory Basin in central Western Australia was recognized in the mid‐1980s during regional mapping of very poorly exposed Proterozoic rocks previously assigned to the Bangemall Basin. All of the sedimentary rock units in the Savory Basin have been included in the Savory Group, which unconformably overlies the Mesoproterozoic Yeneena and Bangemall Groups. Correlation with adjacent basins is impeded by poor outcrop and the lack of subsurface information. Possible correlations have been investigated with the much better known Amadeus Basin to the east, and with the Officer Basin. Two correlations now clarify the age and relationships of the Savory Group. First, the Skates Hills Formation contains distinctive stromatolites previously recorded from the Bitter Springs Formation of the Amadeus Basin. In addition, the Skates Hills and Bitter Springs Formations have many lithological features in common. This correlation is strengthened by comparison with surface and subsurface units in the northern Officer Basi...

A microfossil assemblage recently discovered from the Farrel Quartzite (ca. 3.0 Ga) in the Goldsworthy greenstone belt in the Pilbara Craton comprises five main morphological types (thread like, film-like, spheroidal [>15 and

Morphologically diverse structures that may constitute organic microfossils are reported from three remote and widely separated localities assigned to the ca. 3400 Ma Strelley Pool Formation in the Pilbara Craton, Western Australia. These localities include the Panorama, Warralong, and Goldsworthy greenstone belts. From the Panorama greenstone belt, large (40 μm) lenticular to spindle-like structures, spheroidal structures, and mat-forming thread-like structures are found. Similar assemblages of carbonaceous structures have been identified from the Warralong and Goldsworthy greenstone belts, though these assemblages lack the thread-like structures but contain film-like structures. All structures are syngenetic with their host sedimentary black chert, which is associated with stromatolites and evaporites. The host chert is considered to have been deposited in a shallow water environment. Rigorous assessment of biogenicity (considering composition, size range, abundance, taphonomic features, and spatial distributions) suggests that cluster-forming small (15 μm) spheroids, lenticular to spindle-like structures, and film-like structures with small spheroids are probable microfossils. Thread-like structures are more likely fossilized fibrils of biofilm, rather than microfossils. The biogenicity of solitary large (15 μm) spheroids and simple film-like structures is less certain. Although further investigations are required to confirm the biogenicity of carbonaceous structures from the Strelley Pool Formation, this study presents evidence for the existence of morphologically complex and large microfossils at 3400 Ma in the Pilbara Craton, which can be correlated to the contemporaneous, possible microfossils reported from South Africa. Although there is still much to be learned, they should provide us with new insights into the early evolution of life and shallow water ecosystems.