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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.

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 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.

To investigate the function of 2-methylhopanoids in modern cyanobacteria, the hpnP gene coding for the radical S-adenosyl methionine (SAM) methylase protein that acts on the C-2 position of hopanoids was deleted from the filamentous cyanobacterium Nostoc punctiforme ATCC 29133S. The resulting ΔhpnP mutant lacked all 2-methylhopanoids but was found to produce much higher levels of two bacteriohopanepentol isomers than the wild type. Growth rates of the ΔhpnP mutant cultures were not significantly different from those of the wild type under standard growth conditions. Akinete formation was also not impeded by the absence of 2-methylhopanoids. The relative abundances of the different hopanoid structures in akinete-dominated cultures of the wild-type and ΔhpnP mutant strains were similar to those of vegetative cell-dominated cultures. However, the ΔhpnP mutant was found to have decreased growth rates under both pH and osmotic stress, confirming a role for 2-methylhopanoids in stress tolerance. Evidence of elevated photosystem II yield and NAD(P)H-dependent oxidoreductase activity in the ΔhpnP mutant under stress conditions, compared to the wild type, suggested that the absence of 2-methylhopanoids increases cellular metabolic rates under stress conditions., NASA Astrobiology Institute (Award NNA13AA90A)

There is concern in many developed countries that school students are turning away from science. However, students may be choosing not to study science and dismissing the possibility of a scientific career because, in the junior secondary years, they gain a false view of science and the work of scientists. There is a disparity between science as it is portrayed at school and science as it is practiced. This paper describes a study to explore whether engaging in science through astrobiology outreach activities may improve students' understanding of the nature and processes of science, and how this may influence their interest in a career in science. The results suggest that the students attending these Mars research–related outreach activities are more interested in science than the average student but are lacking in understanding of aspects of the nature of science. A significant difference was detected between pre- and posttest understandings of some concepts of the nature of science. Key Words: Science education—School science—Creativity—Nature and processes of science—Attitudes—Astrobiology. Astrobiology 12, 1143–1153.

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)

Families of closely related chemical compounds, which are relatively resistant to degradation, are often used as biomarkers to help trace the evolutionary history of early groups of organisms and the environments in which they lived. Biomarkers derived from hopanoid variations are particularly useful in determining bacterial community compositions. 2-Methylhopananoids have been thought to be diagnostic for cyanobacteria, and 2-methylhopanes in the geological record are taken as evidence for the presence of cyanobacteria-containing communities at the time of sediment deposition. Recently, however, doubt has been cast on the validity of 2-methylhopanes as cyanobacterial biomarkers, since non-cyanobacterial species have been shown to produce significant amounts of 2-methylhopanoids. This study examines the diversity of hpnP, the hopanoid biosynthesis gene coding for the enzyme that methylates hopanoids at the C2 position. Genomic DNA isolated from stromatolite-associated pustular and smooth microbial mat samples from Shark Bay, Western Australia, was analysed for bacterial diversity, and used to construct an hpnP clone library. A total of 117 partial hpnP clones were sequenced, representing 12 operational taxonomic units (OTUs). Phylogenetic analysis showed that 11 of these OTUs, representing 115 sequences, cluster within the cyanobacterial clade. We conclude that the dominant types of microorganisms with the detected capability of producing 2-methylhopanoids within pustular and smooth microbial mats in Shark Bay are cyanobacteria.

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.

Responses to changes in external salinity were examined in Halobacterium salinarum NRC-1. H. salinarum NRC-1 grows optimally at 4.3 M NaCl and is capable of growth between 2.6 and 5.1 M NaCl. Physiological changes following incubation at 2.6 M NaCl were investigated with respect to growth behavior and proteomic changes. Initial observations indicated delayed growth at low NaCl concentrations (2.6 M NaCl), and supplementation with different sugars, amino acids, or KCl to increase external osmotic pressure did not reverse these growth perturbations. To gain a more detailed insight into the adaptive responses of H. salinarum NRC-1 to changes in salinity, the proteome was characterized using iTRAQ (amine specific isobaric tagging reagents). Three hundred and nine differentially expressed proteins were shown to be associated with changes in the external sodium chloride concentration, with proteins associated with metabolism revealing the greatest response.

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 extraction of nucleic acids from a given environment marks a crucial and essential starting point in any molecular investigation. Members of Halococcus spp. are known for their rigid cell walls, and are thus difficult to lyse and could potentially be overlooked in an environment. Furthermore, the lack of a suitable lysis method hinders subsequent molecular analysis. The effects of six different DNA extraction methods were tested on Halococcus hamelinensis, Halococcus saccharolyticus and Halobacterium salinarum NRC-1 as well as on an organic rich, highly carbonated sediment from stromatolites spiked with Halococcus hamelinensis. The methods tested were based on physical disruption (boiling and freeze/thawing), chemical lysis (Triton X-100, potassium ethyl xanthogenate (XS) buffer and CTAB) and on enzymatic lysis (lysozyme). Results showed that boiling and freeze/thawing had little effect on the lysis of both Halococcus strains. Methods based on chemical lysis (Triton X-100, XS-buffer, and CTAB) showed the best results, however, Triton X-100 treatment failed to produce visible DNA fragments. Using a combination of bead beating, chemical lysis with lysozyme, and thermal shock, lysis of cells was achieved however DNA was badly sheared. Lysis of cells and DNA extraction of samples from spiked sediment proved to be difficult, with the XS-buffer method indicating the best results. This study provides an evaluation of six commonly used methods of cell lysis and DNA extraction of Halococcus spp., and the suitability of the resulting DNA for molecular analysis.

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.

Recently, halite and sulfate evaporate rocks have been discovered on Mars by the NASA rovers, Spirit and Opportunity. It is reasonable to propose that halophilic microorganisms could have potentially flourished in these settings. If so, biomolecules found in microorganisms adapted to high salinity and basic pH environments on Earth may be reliable biomarkers for detecting life on Mars. Therefore, we investigated the potential of Resonance Raman (RR) spectroscopy to detect biomarkers derived from microorganisms adapted to hypersaline environments. RR spectra were acquired using 488.0 and 514.5 nm excitation from a variety of halophilic archaea, including Halobacterium salinarum NRC-1, Halococcus morrhuae, and Natrinema pallidum. It was clearly demonstrated that RR spectra enhance the chromophore carotenoid molecules in the cell membrane with respect to the various protein and lipid cellular components. RR spectra acquired from all halophilic archaea investigated contained major features at approximately 1000, 1152, and 1505 cm(-1). The bands at 1505 cm(-1) and 1152 cm(-1) are due to in-phase C=C (nu(1) ) and C-C stretching ( nu(2) ) vibrations of the polyene chain in carotenoids. Additionally, in-plane rocking modes of CH(3) groups attached to the polyene chain coupled with C-C bonds occur in the 1000 cm(-1) region. We also investigated the RR spectral differences between bacterioruberin and bacteriorhodopsin as another potential biomarker for hypersaline environments. By comparison, the RR spectrum acquired from bacteriorhodopsin is much more complex and contains modes that can be divided into four groups: the C=C stretches (1600-1500 cm(-1)), the CCH in-plane rocks (1400-1250 cm(-1)), the C-C stretches (1250-1100 cm(-1)), and the hydrogen out-of-plane wags (1000-700 cm(-1)). RR spectroscopy was shown to be a useful tool for the analysis and remote in situ detection of carotenoids from halophilic archaea without the need for large sample sizes and complicated extractions, which are required by analytical techniques such as high performance liquid chromatography and mass spectrometry.

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.

We thank Dvořak et al. for their comment (1) on our paper (2), in which we compare sulfur-cycling ∼1.8- and ∼2.3-Ga fossil communities with their modern counterparts and report that the community fabric of the fossil and modern microbes, as well as their organismal and cellular morphology, their interlinked energy-production via anaerobic sulfate-reduction and sulfur species oxidation, and their use of sulfate and nitrate to fuel this sulfur cycle appear to have remained unchanged over a segment of geological time equivalent to half the age of the Earth. Given these observations, our paper suggests that the apparent long-term stasis of the form, function, and metabolic requirements of this ecosystem may be attributable to the seemingly unchanging physical-biological characteristics of its subseafloor environment, a possible example of evolution’s null hypothesis.

Significance An ancient deep-sea mud-inhabiting 1,800-million-year-old sulfur-cycling microbial community from Western Australia is essentially identical both to a fossil community 500 million years older and to modern microbial biotas discovered off the coast of South America in 2007. The fossils are interpreted to document the impact of the mid-Precambrian increase of atmospheric oxygen, a world-changing event that altered the history of life. Although the apparent 2-billion-year-long stasis of such sulfur-cycling ecosystems is consistent with the null hypothesis required of Darwinian evolution—if there is no change in the physical-biological environment of a well-adapted ecosystem, its biotic components should similarly remain unchanged—additional evidence will be needed to establish this aspect of evolutionary theory.

Hamelin Pool in Western Australia is one of the two major sites in the world with active marine stromatolite formation. Surrounded by living smooth and pustular mats, these ancient laminated structures are associated with cyanobacterial communities. Recent studies have identified a wide diversity of bacteria and archaea in this habitat. By understanding and evaluating the microbial diversity of this environment we can obtain insights into the formation of early life on Earth, as stromatolites have been dated in the geological record as far back as 3.5 billion years. Automated ribosomal intergenic spacer analysis (ARISA) patterns were shown to be a useful method to genetically discriminate halophilic archaea within this environment. Patterns of known halophilic archaea are consistent, by replicate analysis, and the halophilic strains isolated from stromatolites have novel intergenic spacer profiles. ARISA-PCR, performed directly on extracted DNA from different sample sites, provided significant insights into the extent of previous unknown diversity of halophilic archaea within this environment. Cloning and sequence analysis of the spacer regions obtained from stromatolites confirmed the novel and broad diversity of halophilic archaea in this 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.

Paralana is an active, radon-containing hot spring situated in a region of South Australia's Flinders Ranges with a long history of hydrothermal activity. Gas bubbling into the pool is composed of radon (from the radioactive decay of radium), nitrogen, carbon dioxide, and trace helium and hydrogen. The microbial composition of mat and biofilm samples from Paralana was determined using culture-independent 16S rRNA techniques. We have previously demonstrated that the hot spring contains a diverse bacterial community. Here we summarise these findings and report on theArchaeaidentified in Paralana. Archaeal inhabitants include members of the Crenarchaeota and Euryarchaeota kingdoms.