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Integration of geomechanical, geological, geochemical, and petrophysical characterization is critical to enhance production from organic-rich mudrocks. This paper introduces an integrated rock classification method in the Eagle Ford Formation shales and marls in southern Texas, consisting of organic-rich fossiliferous marine shale deposited during the Late Cretaceous. A joint inversion of triple-combo, spectral gamma-ray, and elemental capture spectroscopy logs was initially conducted to estimate volumetric concentrations of minerals, porosity, and fluid saturation. Effective elastic properties such as Young's modulus and Poisson's ratio as well as minimum horizontal stress were then estimated as a function of depth. Rock classification was finally performed based on geologic texture and geochemical properties, as well as well log-based estimates of petrophysical and geomechanical properties. The introduced method was applied to two wells located in the oil window of the Eagle Ford Formation (average gas-to-oil ratio of less than 2000 MCF/STB). The results of the integrated rock classification demonstrated similar organic richness and petrophysical properties in both wells. However, the geomechanical rock classification derived from in-situ stress profiles suggested higher proportions (55% vs. 34% of net thickness, respectively) of the better completions quality rock class, with lower minimum horizontal stress (MHS), in one of the wells. A 90-day report on the cumulative oil production of this well shows an additional 11,000 bbl (i.e., 20% more oil production) compared to the second well. This observation was accounted for by geomechanical properties and the distribution of rock classes that differentiate reservoir quality and production.

Summary Reuse of flowback water in hydraulic fracturing is usually used by industry to reduce consumption, transportation, and disposal cost of water. However, because of complex interactions between injected water and reservoir rocks, induced fractures may be blocked by impurities carried by flowback and mineral precipitation by water/rock interactions, which causes formation damage. Therefore, knowledge of flowback water/rock interactions is important to understand the changes within the formation and effects on hydraulic fracturing performance. This study focuses on investigating flowback water/rock interactions during hydraulic fracturing in Marcellus Shale. Simple deionized water (DI)/rock interactions and complicated flowback water/rock interactions were studied under static and dynamic conditions. In static experiments, crushed reservoir rock samples were exposed to water for 3 weeks at room condition. In the dynamic experiment, continuous water flow interacted with rock samples through the coreflooding experimental system for 3 hours at reservoir condition. Before and after experiments, rock samples were characterized to determine the change on the rock surfaces. Water samples were analyzed to estimate the particle precipitation tendency and potential to modify flow pathway. Surface elemental concentrations, mineralogy, and scanning electron microscope (SEM) images of rock samples were characterized. Ion contents, particle size, total dissolved solids (TDS), and zeta-potential in the water samples were analyzed. After flowback water/rock interaction, the surface of the rock sample shows changes in the compositions and more particle attachment. In produced water, Na, Sr, and Cl concentrations are extremely high because of flowback water contamination. Water parameters show that produced water has the highest precipitation tendency relative to all water samples. Therefore, if flowback water without any treatment is reused in hydraulic fracturing, formation damage is more likely to occur from blockage of pores. Flowback water management is becoming very important due to volumes produced in every hydraulic fracturing operation. Deep well injection is no longer a favorable option because it results in disposal of high volumes of water that cannot be used for other purposes. A second option is the reuse of waste water for fracturing purposes, which reduces freshwater use significantly. However, the impurities present in flowback water may deteriorate the fracturing job and reduce or block the hydraulic fracturing apertures. This study shows that a simple filtration process applied to the flowback water allows for reinjection of the flowback water without further complication to the water/rock interaction, and does not cause significant formation damage in the fractures.

Early diagenetic precipitation of authigenic carbonate has been a globally significant carbon sink throughout Earth history. In particular, SO4 2– and Fe3+ reduction and CH4 production create conditions in pore fluids that promote carbonate mineral precipitation; however, these conditions may be modified by the presence of acid-base buffers such as clay minerals. We integrated the acid-base properties of clay minerals into a biogeochemical model that predicts the evolution of pore-water pH and carbonate mineral saturation during O2, Fe3+, and SO42– reduction and CH4 production. Key model inputs were obtained using two natural clay mineral–rich sediments from the Integrated Ocean Drilling Program as well as from literature. We found that clay minerals can enhance carbonate mineral saturation during O2 and SO42– reduction and moderate saturation during Fe3+ reduction and CH4 production if the pore-fluid pH and clay mineral pKa values are within ~2 log units of one another. We therefore suggest that clay minerals could significantly modify the environmental conditions and settings in which early diagenetic carbonate precipitation occurs. In Phanerozoic marine sediments—where O2 and SO42– have been the main oxidants of marine sedimentary organic carbon—clay minerals have likely inhibited carbonate dissolution and promoted precipitation of authigenic carbonate.

Stack, K. et al., The Mars 2020 Perseverance rover landing site is located within Jezero crater, a ∼50km diameter impact crater interpreted to be a Noachian-aged lake basin inside the western edge of the Isidis impact structure. Jezero hosts remnants of a fluvial delta, inlet and outlet valleys, and infill deposits containing diverse carbonate, mafic, and hydrated minerals. Prior to the launch of the Mars 2020 mission, members of the Science Team collaborated to produce a photogeologic map of the Perseverance landing site in Jezero crater. Mapping was performed at a 1:5000 digital map scale using a 25 cm/pixel High Resolution Imaging Science Experiment (HiRISE) orthoimage mosaic base map and a 1 m/pixel HiRISE stereo digital terrain model. Mapped bedrock and surficial units were distinguished by differences in relative brightness, tone, topography, surface texture, and apparent roughness. Mapped bedrock units are generally consistent with those identified in previously published mapping efforts, but this study’s map includes the distribution of surficial deposits and sub-units of the Jezero delta at a higher level of detail than previous studies. This study considers four possible unit correlations to explain the relative age relationships of major units within the map area. Unit correlations include previously published interpretations as well as those that consider more complex interfingering relationships and alternative relative age relationships. The photogeologic map presented here is the foundation for scientific hypothesis development and strategic planning for Perseverance’s exploration of Jezero crater., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Planetary Instrument for X-ray Lithochemistry (PIXL) is a micro-focus X-ray fluorescence spectrometer mounted on the robotic arm of NASA’s Perseverance rover. PIXL will acquire high spatial resolution observations of rock and soil chemistry, rapidly analyzing the elemental chemistry of a target surface. In 10 seconds, PIXL can use its powerful 120 μm-diameter X-ray beam to analyze a single, sand-sized grain with enough sensitivity to detect major and minor rock-forming elements, as well as many trace elements. Over a period of several hours, PIXL can autonomously raster-scan an area of the rock surface and acquire a hyperspectral map comprised of several thousand individual measured points. When correlated to a visual image acquired by PIXL’s camera, these maps reveal the distribution and abundance variations of chemical elements making up the rock, tied accurately to the physical texture and structure of the rock, at a scale comparable to a 10X magnifying geological hand lens. The many thousands of spectra in these postage stamp-sized elemental maps may be analyzed individually or summed together to create a bulk rock analysis, or subsets of spectra may be summed, quantified, analyzed, and compared using PIXLISE data analysis software. This hand lens-scale view of the petrology and geochemistry of materials at the Perseverance landing site will provide a valuable link between the larger, centimeter- to meter-scale observations by Mastcam-Z, RIMFAX and Supercam, and the much smaller (micron-scale) measurements that would be made on returned samples in terrestrial laboratories.

Reuse of flowback water in hydraulic fracturing is usually used by industry to reduce consumption, transportation and disposal cost of water. But because of complex interactions between injected water and reservoir rocks, induced fractures may be blocked by impurities carried by flowback and mineral precipitation by water-rock interactions, which causes formation damage. Therefore, knowledge of flowback water-rock interactions is important to understand the changes within the formation and effects on hydraulic fracturing performance. This study focuses on investigating flowback water-rock interactions during hydraulic fracturing in Marcellus Shale Formation. Simple deionized water-rock interactions and complicated flowback water-rock interactions were studied under static and dynamic conditions. In static experiments, crushed reservoir rock samples were exposed to water for three weeks at room condition. In the dynamic experiment, continuous water flow interacted with rock samples through the core-flooding experimental system for three hours at reservoir condition. Before and after experiments, rock samples were characterized to determine the change on the rock surfaces. Water samples were analyzed to estimate the particle precipitation tendency and potential to modify flow pathway. Surface elemental concentrations, mineralogy and SEM images of rock samples were characterized. Ion contents, particle size, TDS and Zeta potential in the water samples were analyzed. In both static and dynamic experiments, compared with deionized water-rock interaction, rock samples after being exposed to flowback water show change in the elemental compositions, more fine particles attachment and new minerals detected on rock surface due to effect of flowback water. In produced water, Na, Sr and Cl are extremely high after flowback water-rock interactions because of flowback water contamination. Water parameter analysis indicates after flowback water-rock interactions, suspensions in produced water have highest precipitation tendency because of extremely highest TDS, largest particle size and lowest absolute Zeta potential relative to all water samples. Therefore, according to rock and water characterization, if flowback water without any treatment would be reused in the field hydraulic fracturing operation, created flow pathways and pores are more likely to be blocked and formation damage will be caused. This study provides information on comparison between deionized water-rock interaction and flowback water interaction. The information enhances the understanding of basic water-rock interaction mechanisms and evaluates formation damage caused by reuse of flowback water.

Hydraulic transformations in turbidity currents are commonly driven by or reflected in changes in suspended sediment concentrations, but changes preceding transformations can be difficult to diagnose because they do not produce qualitative changes in resultant deposits. This study integrates particle settling experiments and in situ detection of hydraulically contrasting particles in turbidites in order to infer changes in suspended sediment concentration during deposition of massive (Bouma Ta) sandstone divisions. Because grains of contrasting density are differentially sorted during hindered settling from dense suspensions, relative grading patterns can be used to estimate suspended sediment concentrations and interpret hydraulic evolution of the depositing turbidity currents. Differential settling of dense particles (aluminum ballotini) through suspensions of hydraulically coarser light particles (silica ballotini) with volumetric concentration, Cv, were studied in a thin vessel by using particle-image-velocimetry. At high Cv, aluminum particles were less retarded than co-sedimenting silica particles, and effectively settled as hydraulically coarser grains. This was because particles were entrained into clusters dominated by the settling behavior of the silica particles. Terminal settling velocities of both particles converged at Cv ≥ 25%, and particle sorting was diminished. The results of settling experiments were applied to understand settling of analogous feldspar and zircon grains in natural turbidity flows. Distributions of light and heavy mineral grains in massive sandstones, Bouma Ta divisions, of turbidites from the Middle Permian Brushy Canyon Formation were observed in situ by X-ray fluorescence microscopy (μXRF). Hydraulic sorting of these grains resulted in characteristic patterns of zirconium abundance that decreased from base to top within Ta divisions. These profiles resulted from upward fining of zircon grains with respect to co-occurring feldspar grains. Although calculated settling velocity distributions for zircon grains in structureless sandstones were slower than those for feldspar grains at infinite dilution, calculated settling velocity distributions for zircon and feldspar grains in overlying black siltstone layers were identical. This evidence suggests that these sandstone divisions were deposited from hyperconcentrated suspensions where particle segregation was diminished and hydraulically fine grains were entrained with hydraulically coarse particles. Hydraulic fining of zircon grains during deposition implies that the suspended sediment concentration at the bases of turbidity currents increased even as the overall current evolved toward lower density as reflected by cessation of Ta deposition and by hydraulic equivalence of zircon and feldspar grains in overlying low-density turbiditic siltstones. This evolution likely resulted from volumetric collapse of the turbidity currents.

Thinly laminated siltstone and sandy siltstone are major components of the Upper Permian Brushy Canyon Formation, west Texas and south New Mexico. These rocks have been variously interpreted as the deposits of low-density turbidity currents or as windblown sediment deposited over water. Nevertheless, all models agreed that this lithology was deposited without subsequent reworking by bottom currents or burrowing organisms. These siltstones, thus, are ideal test units for quantitatively estimating hydraulic properties of the flows that formed them. In particular, the Zr/Ti ratio was tested as a geochemical proxy for flow size and transport distance. In situ geochemical abundance and grain size of particles with contrasting susceptibility to erosion—Zr- and Ti-rich particles—were mapped and measured by X-ray fluorescence analytical microscopy, μXRF. Lamination thickness was measured from Fe fluorescence intensity, which increased sharply at the top of each layer. Within the same sample, zircon grains were systematically finer than rutilated quartz grains. Zr/Ti fluorescence ratio positively correlated with lamination thickness, not particle sizes. In other words, Zr/Ti fluorescence ratio fluctuations resulted from variations in mineral abundance. Therefore, variations of Zr/Ti fluorescence ratio in these siltstones are likely caused by fluctuations in the intensity of erosional events rather than transport distance. High Zr/Ti ratios and thick laminations reflect periods of enhanced erosion. The average wind velocity during typical events was estimated to be at least 150 km?hr?1, or the equivalent of a Category 1 hurricane. The method used here could be applied to both outcrop and subsurface strata correlation.

Some microbial carbonates are robust biosignatures due to their distinct morphologies and compositions. However, whether carbonates induced by microbial iron reduction have such features is unknown. Iron-reducing bacteria use various strategies to transfer electrons to iron oxide minerals (e.g., membrane-bound enzymes, soluble electron shuttles, nanowires, as well as different mechanisms for moving over or attaching to mineral surfaces). This diversity has the potential to create mineral biosignatures through manipulating the microenvironments in which carbonate precipitation occurs. We used Shewanella oneidensis MR-1, Geothrix fermentans, and Geobacter metallireducens GS-15, representing three different strategies, to reduce solid ferric hydroxide in order to evaluate their influence on carbonate and micropore formation (micro-size porosity in mineral rocks). Our results indicate that electron transfer strategies determined the morphology (rhombohedral, spherical, or long-chained) of precipitated calcium-rich siderite by controlling the level of carbonate saturation and the location of carbonate formation. Remarkably, electron transfer strategies also produced distinctive cell-shaped micropores in both carbonate and hydroxide minerals, thus producing suites of features that could potentially serve as biosignatures recording information about the sizes, shapes, and physiologies of iron-reducing organisms. Key Words: Microbial iron reduction-Micropore-Electron transfer strategies-Microbial carbonate. Astrobiology 18, 28-36.

Microbial life was well established and widespread by the Paleoarchean; however, the degree of evolutionary advancement such as microbial motility, intra- and inter-species interactions, phototropism, or oxygenic photosynthesis by that time remains highly debated. The 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB, South Africa) are Earth's oldest well-preserved siliciclastic tidal deposits. They exhibit a unique assemblage of microbial mats, providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings. The fossil mats are preserved as kerogenous laminations (0.5–1 mm thick) that can be traced laterally for ∼15 km in a ∼1000 m-thick succession of fine- to coarse-grained tidal sandstones and conglomerates. We here present a detailed stratigraphic and depositional facies analysis, documenting the association of the three principal mat morphotypes with specific environmental settings: (1) planar-type in coastal floodplain, (2) wavy-type in intertidal, and (3) tufted-type in upper inter- to supratidal facies. All mat types indicate a flourishing phototrophic biota; moreover, the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments.

The Cretaceous to early Paleogene (ca. 140–50 Ma) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO_2. Hypotheses for the elevated CO_2 concentrations invoke an increase in volcanic CO_2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO_2 during this time interval. We show here that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. In particular, many of these continental arcs, unlike island arcs, intersected ancient continental platform carbonates stored on the continental upper plate. We show that the greater length of Cretaceous–Paleogene continental arcs, specifically carbonate-intersecting arcs, could have increased global production of CO_2 by at least 3.7–5.5 times that of the present day. This magmatically driven crustal decarbonation flux of CO_2 through continental arcs exceeds that delivered by Cretaceous oceanic crust production, and was sufficient to drive Cretaceous–Paleogene greenhouse conditions. Thus, carbonate-intersecting continental arc volcanoes likely played an important role in driving greenhouse conditions in the Cretaceous–Paleogene. If so, the waning of North American and Eurasian continental arcs in the Late Cretaceous to early Paleogene, followed by a fundamental shift in western Pacific subduction zones ca. 52 Ma to an island arc–dominated regime, would have been manifested as a decline in global volcanic CO_2 production, prompting a return to an icehouse baseline in the Neogene. Our analysis leads us to speculate that long-term (>50 m.y.) greenhouse-icehouse oscillations may be linked to fluctuations between continental- and island arc–dominated states. These tectonic fluctuations may result from large-scale changes in the nature of subduction zones, changes we speculate may be tied to the assembly and dispersal of continents. Specifically, dispersal of continents may drive the leading edge of continents to override subduction zones, resulting in continental arc volcanism, whereas assembly of continents or closing of large ocean basins may be manifested as large-scale slab rollback, resulting in the development of intraoceanic volcanic arcs. We suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.

Crustal xenoliths (pyroxenites and plagioclaseþ quartzþ pyroxene lithologies) from the Quaternary Big Pine volcanic field on the eastern flank of the Sierra Nevada Batholith in California (USA) represent the products of metasomatic reaction between the margins of a Cretaceous granodioritic pluton and Paleozoic marbles, possibly at mid-crustal depths based on the equilibration temperatures recorded byTi-in-quartz geothermometry.This interpretation is based on the presence of plagioclase showing relict plutonic textures, pyroxenite characterized by nearly pure diopside clinopyroxene, recrystallized plagioclase with anomalously high anorthite content, textures indicating replacement of plagioclase by clinopyroxene (and vice versa), ‘ghost’ plagioclase rare earth element signatures in some clinopyroxenes, and the presence of phlogopite endmember micas at the contact between clinopyroxene-rich and plagioclase-rich zones.These observations suggest that the xenoliths represent fragments of an ‘endoskarn’ , the outer sheath of a pluton that chemically reacted with carbonate country-rock. Mass transfer between the carbonate country-rock and the pluton involved transfer of Ca and Mg from the carbonate into the pluton and transfer of Na, K, Al and Si from the pluton to the carbonate, the latter generating extensive endoskarns.The Ca metasomatism of the pluton converted alkali feldspar components into anorthite-rich plagioclase, releasing Na and K, which left the plutonic system. K, in particular, migrated towards the carbonate and precipitated phlogopite upon entering clinopyroxene-rich lithologies. Mass-balance calculations, based on theory and residual enrichments in immobile elements such as Ti, suggest that the pluton experienced net mass loss (415%) in the form of Si, Al, Na and K to the surrounding country-rock, but a net gain in Ca and Mg.

Much of the Archean record of microbial communities consists of fossil mats and stromatolites. Critical physical emergent properties governing the evolution of large-scale (centimeters to meters) topographic relief on the mat landscape are (a) mat surface roughness relative to the laminar sublayer and (b) cohesion. These properties can be estimated for fossil samples under many circumstances. A preliminary analysis of Archean mat cohesion suggests that mats growing in shallow marine environments from throughout this time had cohesions similar to those of modern shallow marine mats. There may have been a significant increase in mat strength at the end of the Archean.

This paper presents a speculative scenario for the evolution of the earth's surface environment and biological community 3.5–2.4 Ga based on the geologic record and its interpretation. Available geologic evidence suggests that the earth's climate before 3.2 Ga was hot, probably 60–73 °C, as a result of a CO 2 and CH 4 greenhouse. The early biological community was probably dominated by anoxygenic photosynthetic thermophiles. Cyanobacteria, if they evolved before 3.1 Ga, would have struggled to survive at such high temperatures. As a result, the atmosphere contained extremely low levels of O 2 and Δ 33 S shows wide departures from 0 indicating mass independent fractionation of S isotopes in the upper atmosphere. This hot early climate collapsed 3.1–3.0 Ga due to growth of large new blocks of continental crust (cratons), the weathering of which resulted in the depletion of atmospheric CO 2 and eventual drawdown of CH 4 due to formation and rainout of methane aerosols. Cooling may have culminated in glaciation about 3.0–2.9 Ga. This climatic catastrophe and attendant changes in atmospheric composition drove a major biological revolution 3.0–2.7 Ga characterized by the emergence of new low-temperature taxa, including cyanobacteria, and their spread throughout surface environments at the expense of extreme thermophiles, including methanogens. During a period of general tectonic stability, 3.0–2.7 Ga, the earth's surface was characterized by overall clement conditions with temperatures 2 after 2.75 Ga and by 2.7 Ga surface temperatures had returned to >60 °C. Cyanobacteria were again suppressed, O 2 production waned, and atmospheric mass independent fractionation is again indicated by extreme variations in Δ 33 S. These conditions persisted, perhaps intermittently, until about 2.5–2.4 Ga, when weathering and erosion of vast new blocks of continental crust formed about 2.65–2.6 Ga caused the second, and probably last collapse of a >60 °C surface climate. Broad tectonic, climatic, and biological events 3.5–2.9 Ga are remarkably parallel to those of the Late Archean and Paleoproterozoic 2.75–2.2 Ga and Neoproterozoic 1.0–0.50 Ga. These ∼500-myr long cycles of greenstone volcanism and crustal generation; climatic and atmospheric instability; and biological innovation reflect the long-term interaction of the tectonic, atmospheric, climatic, and biological components of the earth's surface system. They suggest that the evolution of the earth's interior, expressed through its control on the formation of large blocks of continental crust, has influenced atmospheric composition and climate which in turn have provided a fundamental control on the timing and directions of biological evolution throughout earth history.

The geological record of carbonaceous matter from at least 3.5 Ga to the end of the Precambrian is fundamentally continuous in terms of carbonaceous matter structure, composition, environments of deposition/preservation, and abundance in host rocks. No abiotic processes are currently known to be capable of producing continuity in all four of these properties. Although this broad view of the geological record does not prove that life had arisen by 3.5 Ga, the end of the early Archean, it suggests a working hypothesis: most if not all carbonaceous matter present in rocks older than 3.0 Ga was produced by living organisms. This hypothesis must be tested by studies of specific early geological units designed to explore the form, distribution, and origin of enclosed carbonaceous matter. The carbonaceous, environmentally diverse 3416 Ma Buck Reef Chert (BRC) of the Barberton greenstone belt, South Africa, provides an opportunity for such a study. Upward facies progressions in the BRC reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm- and wave-active shelf to a deep, low-energy basinal setting below storm wave base. Abundances and ratios of Al2O3, Zr, TiO2, and Cr track inputs of various types of volcaniclastic and terrigenous clastic materials. In particular, Zr/Al2O3 and Zr serve as proxies for concentration of windblown dust and, indirectly, as proxies for sedimentation rate. Cu, Zn, Ni, and FeO were concentrated in the most slowly deposited transitional and basinal sediments, inconsistent with a hydrothermal setting but consistent with a normal marine setting. The distribution of microfacies defined by associations and layering of clastic, ferruginous, and carbonaceous grains correlates with facies transitions. Fine carbonaceous laminations, which occur only in shallow platform settings, represent photosynthetic microbial mats. These were ripped up and the debris widely redistributed in shallow and deep water by waves and storms. The isotopic composition of carbonaceous matter ranges from − 35‰ to − 30‰ in shallow-water settings and to − 20‰ in deep-water units. The heavier δ13C in deep-water carbonaceous matter is thought to reflect microbial processing, possibly by fermentation and methanogenesis, of organic matter originally produced in shallow water. Hydrothermal origins for BRC carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole, an inference supported by field, petrographic, and geochemical analysis. We suggest that the biological model proposed here for BRC carbonaceous matter is the best currently available. The hypothesis that “at least some carbonaceous matter present in rocks older than 3.0 Ga was produced by living organisms” should be regarded as likely until extraordinary contradictory evidence is presented.

There is considerable debate about the mode and age of formation of large (up to � 200 m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite d 18 O values range from � 0.7 to +1.0& and dD from � 125 to � 146&, which is consistent with formation from modern meteoric waters at 20 to 25 C. Hematite d 18 O values range from � 0.7 to � 2.0&, which is consistent with formation at low to moderate temperatures (40–55 C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.

Recent re-evaluations of the geological record of the earliest life on Earth have led to the suggestion that some of the oldest putative microfossils1 and carbonaceous matter were formed through abiotic hydrothermal processes2,3. Similarly, many early Archaean (more than 3,400-Myr-old) cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes2,4,5. Here we present the results of a field, petrographic and geochemical study testing these hypotheses for the 3,416-Myr-old Buck Reef Chert, South Africa. From sedimentary structures and distributions of sand and mud, we infer that deposition occurred in normal open shallow to deep marine environments. The siderite enrichment that we observe in deep-water sediments is consistent with a stratified early ocean6,7. We show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments. We find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments. Instead, we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica8,9 3,416 Myr ago.

Iron-bearing early diagenetic carbonate cements are common in sedimentary rocks, where they are thought to be associated with microbial iron reduction. However, little is yet known about how local environments around actively iron-reducing cells affect carbonate mineral precipitation rates and compositions. Precipitation experiments with the iron-reducing bacterium Shewanella oneidensis MR-1 were conducted to examine the potential role of cells in promoting precipitation and to explore the possible range of precipitate compositions generated in varying fluid compositions. Actively iron-reducing cells induced increased carbonate mineral saturation and nucleated precipitation on their poles. However, precipitation only occurred when calcium was present in solution, suggesting that cell surfaces lowered local ferrous iron concentrations by adsorption or intracellular iron oxide precipitation even as they locally raised pH. Resultant precipitates were a range of thermodynamically unstable calcium-rich siderites that would likely act as precursors to siderite, calcite, or even dolomite in nature. By modifying local pH, providing nucleation sites, and altering metal ion concentrations around cell surfaces, iron-reducing micro-organisms could produce a wide range of carbonate cements in natural sediments.

Valley-filling deposits of the Nama Group, southern Namibia, record two episodes of erosional downcutting and backfill, developed close together in time near the Ediacaran-Cambrian boundary. Geochronological constraints indicate that the older valley fill began 539.4 ± 1 Ma or later; the younger of these deposits contains unusually well-preserved populations of the basal Cambrian trace fossil Treptichnus pedum. Facies analysis shows that T. pedum is closely linked to a nearshore sandstone deposit, indicating a close environmental or taphonomic connection to very shallow, mud-draped sandy seafloor swept by tidal currents. Facies restriction may limit the biostratigraphic potential of T. pedum in Namibia and elsewhere, but it also illuminates functional and ecological interpretation. The T. pedum tracemaker was a motile bilaterian animal that lived below the sediment-water interface—propelling itself forward in upward-curving projections that breached the sediment surface. The T. pedum animal, therefore, lived infaunally, perhaps to avoid predation, surfacing regularly to feed and take in oxygen. Alternatively, the T. pedum animal may have been a deposit feeder that surfaced largely for purposes of gas exchange, an interpretation that has some support in the observed association of T. pedum with mud drapes. Treptichnus pedum provides our oldest record of animals that combined anatomical and behavioral complexity. Insights from comparative biology suggest that basal Cambrian T. pedum animals already possessed the anatomical, neurological, and genetic complexity needed to enable the body plan and behavioral diversification recorded by younger Cambrian fossils.

Three morphotypes of microbial mats are preserved in rocks deposited in shallow-water facies of the 3.42 Ga Buck Reef chert (BRC). Morphotype alpha consists of fine anastomosing and bifurcating carbonaceous laminations, which loosely drape underlying detrital grains or form silica-filled lenses. Morphotype beta consists of meshes of fine carbonaceous strands intergrown with detrital grains and dark laminations, which loosely drape coarse detrital grains. Morphotype gamma consists of fine, even carbonaceous laminations that tightly drape underlying detrital grains. Preservation of nearly uncompacted mat morphologies and detrital grains deposited during mat growth within a well-characterized sedimentary unit makes quantitative correlation between morphology and paleoenvironment possible. All mats are preserved in the shallowest-water interval of those rocks deposited below normal wave base and above storm wave base. This interval is bounded below by a transgressive lag formed during regional flooding and above by a small condensed section that marks a local relative sea-level maximum. Restriction of all mat morphotypes to the shallowest interval of the storm-active layer in the BRC ocean reinforces previous interpretations that these mats were constructed primarily by photosynthetic organisms. Morphotypes alpha and beta dominate the lower half of this interval and grew during deposition of relatively coarse detrital carbonaceous grains, while morphotype gamma dominates the upper half and grew during deposition of fine detrital carbonaceous grains. The observed mat distribution suggests that either light intensity or, more likely, small variations in ambient current energy acted as a first-order control on mat morphotype distribution. These results demonstrate significant environmental control on biological morphogenetic processes independent of influences from siliciclastic sedimentation.

The widely held view that the Archaean climate around 3.5 billion years ago was remarkably warm — with ocean temperatures perhaps as high as 80 °C — has been questioned on the grounds that the established method of estimating ancient ocean temperature (from the oxygen isotope ratios of sedimentary deposits) is subject to significant uncertainty. Michael Hren et al. have adopted a different approach to estimating ocean temperature, based on an analysis of both oxygen and hydrogen isotopes of 3.4 billion-year-old Buck Reef Chert sediments from South Africa. The isotopes sampled are consistent with formation in waters no warmer than about 40 °C, suggesting that Earth's early oceans may have been far cooler than previously thought. The study of stable oxygen isotope ratios (δ18O) of Precambrian cherts suggests that ocean temperatures during the Archaean era (about 3.5 billion years ago) were between 55 °C and 85 °C, but uncertainty about the δ18O of the primitive ocean has led to considerable debate regarding this conclusion. Here, a combined analysis of oxygen and hydrogen istopes sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa indicates that the ancient ocean was much cooler than previously thought. Stable oxygen isotope ratios (δ18O) of Precambrian cherts have been used to establish much of our understanding of the early climate history of Earth1,2,3 and suggest that ocean temperatures during the Archaean era (∼3.5 billion years ago) were between 55 °C and 85 °C (ref. 2). But, because of uncertainty in the δ18O of the primitive ocean, there is considerable debate regarding this conclusion. Examination of modern and ancient cherts indicates that another approach, using a combined analysis of δ18O and hydrogen isotopes (δD) rather than δ18O alone, can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters4,5. Here we show that δ18O and δD sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa are consistent with formation from waters at varied low temperatures. The most 18O-enriched Buck Reef Chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below ∼40 °C. Geochemical and sedimentary evidence suggests that the Buck Reef Chert was formed in shallow to deep marine conditions, so our results indicate that the Palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler (≤40 °C) than previously thought2.

Thin carbonaceous laminations preserved in shallow-water facies of the 3416 Ma Buck Reef Chert, South Africa, have been interpreted to represent some of the oldest-known mats constructed by photosynthetic microbes. Preservation of these mats within a unit containing facies deposited at water depths ranging from 0 m to >200 m provides an opportunity to explore the electron donors employed in early microbial photosynthesis. The presence of siderite (FeCO3) as a primary sediment, lack of hematite (Fe2O3), and lack of cerium anomalies throughout the Buck Reef Chert imply that the entire water column was anoxic despite the presence of photosynthetic organisms. Authigenic uranium (Ua = U–Th/3) correlates inversely with siderite abundance, suggesting that variations in carbonate rather than oxygen activity controlled uranium mobility. The inferred lack of oxygen and ferric minerals and the presence of dissolved Fe2+ in the water column imply that H2O, Fe2+, and H2S could not have served as primary electron donors for carbon fixation. It is most likely that Buck Reef Chert bacteria utilized H2 as the primary reductant for photosynthesis.

Raman spectra of carbonaceous material were collected in situ from samples of cherts of the Onverwacht and Fig Tree Groups in the central Barberton greenstone belt. The spectra feature two dominant peaks characteristic of disordered carbon: the D peak at ;1310 cm 21 and the O peak at 1580-1600 cm 21 . D peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above. No correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration, implying that metamorphism in the central Barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids. Samples from the Marble Bar Chert of the Pilbara block, Western Australia, have been heated to the same extent as samples from Barberton. This study demonstrates the use of Raman spectra of carbonaceous material as a sensitive geother- mometer for low-temperature metamorphic facies. This application could also be used to establish the antiquity of putative microfossils from metamorphic terranes.

The 3.5-3.2 Ga sedimentary record shows evidence of surface temperatures of 70 ′ 15 °C, nahcolite (NaHCO 3 ) as a primary evaporitic mineral, and an aggressive weathering regime even in the absence of land vegetation. These features are best explained by a mixed CH 4 and CO 2 atmospheric greenhouse in which CH 4 /CO 2 ratios were «1 and pCO 2 was at least 100-1000 times the present value and perhaps as high as several bars. The formation of large areas of continental crust at 3.2-3.0 Ga, including the Kaapvaal and Pilbara cratons, resulted in the gradual depletion of atmospheric CO 2 through weathering. By 2.9-2.7 Ga, declining pCO 2 was associated with climatic cooling and siderite-free soils. Transitory CH 4 /CO 2 ratios of ∼1 may have resulted in the sporadic formation of organic haze from atmospheric CH 4 , reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 C-depleted organic C. Surface temperatures of

It has long been recognized that deciphering the relationship between the history of life on Earth and the history of the planet is a profound task. Recent technological innovations in both the earth and life sciences have made this task more tractable than ever before, leading to the emergence of the discipline of geobiology — the study of how organisms have influenced, and been influenced by, the Earth's environment. Along with enthusiasm for this new field, however, has come confusion, as geobiology combines highly specialized and historically separate fields. How does a sedimentologist communicate his/her problems to a cell biologist and vice versa? The fact that geobiology derives from two disparate scientific traditions — those of natural history and experimental science — can make identification of appropriate problems challenging. As C.P. Snow [1] recognized nearly a half century ago in his famous lecture ‘The Two Cultures’, communication between different disciplines often results in “a gulf of mutual incomprehension” that can be difficult to ford. Although Snow was referring to the divide that separates the humanities from the sciences, many of his insights can be applied to the divide that until recently has separated biology from geology. In this Primer, we shall attempt to illustrate the compelling nature of geobiology by highlighting two geobiological problems. Our goal is to introduce molecular and cell biologists to this discipline, and make it clear just how much their skills can contribute to it and their questions benefit from it. We begin with a brief review of what is known about the geochemical evolution of the Earth. From there, we provide two examples of problems relevant to the co-evolution of life and Earth. The first example illustrates how a better understanding of biology — specifically, the distribution and function of sterol-like molecules in bacterial membranes — will inform our understanding of the rise of oxygen, arguably the most important event in the geochemical evolution of the Earth. The second example illustrates the counterpoint: how a better understanding of changes in the Earth's geochemistry over time can affect our interpretations of organelle evolution, specifically, the relationship between hydrogenosomes and mitochondria.