Early oxygenation of the terrestrial environment during the Mesoproterozoic.

Geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of Earth's atmosphere and oceans. Key stages in increasing oxygenation are postulated for the Palaeoproterozoic era (∼2.3 billion years ago, Gyr ago) and the late Proterozoic eon (about 0.8 Gyr ago), with the latter implicated in the subsequent metazoan evolutionary expansion. In support of this rise in oxygen concentrations, a large database shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of Δ(34)S < 25‰ before 1 Gyr to ≥50‰ after 0.64 Gyr. This change in Δ(34)S has been interpreted to represent the evolution from single-step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation, largely bacterially mediated. This evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non-photosynthetic sulphide-oxidizing bacteria. Here we report Δ(34)S values exceeding 50‰ from a terrestrial Mesoproterozoic (1.18 Gyr old) succession in Scotland, a time period that is at present poorly characterized. This level of fractionation implies disproportionation in the sulphur cycle, probably involving sulphide-oxidizing bacteria, that is not evident from Δ(34)S data in the marine record. Disproportionation in both red beds and lacustrine black shales at our study site suggests that the Mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen-rich atmosphere, but had also penetrated into subsurface sediment.

Pond-derived organic carbon driving changes in arsenic hazard found in Asian groundwaters.

Microbially mediated reductive processes involving the oxidation of labile organic carbon are widely considered to be critical to the release of arsenic into shallow groundwaters in South and Southeast Asia. In areas where there is significant pumping of groundwater for irrigation the involvement of surface-derived organic carbon drawn down from ponds into the underlying aquifers has been proposed but remains highly controversial. Here we present isotopic data from two sites with contrasting groundwater pumping histories that unequivocally demonstrate the ingress of surface pond-derived organic carbon into arsenic-containing groundwaters. We show that pond-derived organic carbon is transported to depths of up to 50 m even in an arsenic-contaminated aquifer in Cambodia thought to be minimally disturbed by groundwater pumping. In contrast, in the extensively exploited groundwaters of West Bengal, we show that pond-derived organic carbon is transported in shallow groundwater to greater depths, in excess of 100 m in the aquifer. Intensive pumping of groundwaters may potentially drive secular increases in the groundwater arsenic hazard in this region by increasing the contribution of bioavailable pond-derived dissolved organic carbon drawn into these aquifer systems and transporting it to greater depths than would operate under natural flow conditions.

Coevolution of diagenetic fronts and fluid-fracture pathways.

Diagenetic boundaries are paleo-reaction fronts, which have the potential to archive the termination of metasomatic processes in sedimentary rocks. They have not been extensively studied, perhaps because they appear simple in outcrop. Recent work has demonstrated the significance of paleo-reaction fronts to decipher multiphase recrystallization processes and provide high porosity zones. This paper provides a detailed documentation of reaction front evolution in a tectonically active salt basin and reveals a high level of complexity, associated with multiple fluid flow and tectonic events. Here, consistent patterns of increasing dolomite stoichiometry and ordering, along with a change from seawater-derived, fabric-retentive dolomite to fracture-controlled, fabric-destructive hydrothermal dolomite are observed vertically across the stratabound dolomite bodies. These patterns, coupled with a decrease in porosity, increase in ∆47 temperature and δ18Owater values indicate multiphase recrystallization and stabilization by warm, Mg-rich fluids. The stratabound dolomite bodies apparently terminated at a fracture-bound contact, but the presence of dolomite fragments within the fracture corridor suggests that fracturing post-dated the first dolomitization event. The termination of dolomite formation is therefore interpreted to be associated with a decrease in the capacity of the magnesium-rich fluids to dolomitize the rock, as indicated by the presence of non-stoichiometric and poorly ordered dolomite at the reaction fronts. The fracture corridors are interpreted to exploit dolostone-limestone boundaries, forming prior to a later, higher temperature, hydrothermal dolomitization event, which coincided with the formation and growth of the anticline. Karstification subsequently exploited these fracture corridors, widening fractures and leading to localized collapse and brecciation. The results demonstrate that an apparently simple reaction front can have a complex history, governed by the inheritance of prior diagenetic events. These events modified rock properties in such a way that fluid flow was repeatedly focused along the original dolomite-limestone boundary, overprinting much of its original signature. These findings have implications to the prediction of structurally controlled diagenetic processes and the exploration of naturally fractured carbonate reservoirs for energy exploration globally.

Sulfur isotopes of hydrothermal vent fossils and insights into microbial sulfur cycling within a lower Paleozoic (Ordovician-early Silurian) vent community.

Symbioses between metazoans and microbes involved in sulfur cycling are integral to the ability of animals to thrive within deep-sea hydrothermal vent environments; the development of such interactions is regarded as a key adaptation in enabling animals to successfully colonize vents. Microbes often colonize the surfaces of vent animals and, remarkably, these associations can also be observed intricately preserved by pyrite in the fossil record of vent environments, stretching back to the lower Paleozoic (Ordovician-early Silurian). In non-vent environments, sulfur isotopes are often employed to investigate the metabolic strategies of both modern and fossil organisms, as certain metabolic pathways of microbes, notably sulfate reduction, can produce large sulfur isotope fractionations. However, the sulfur isotopes of vent fossils, both ancient and recently mineralized, have seldom been explored, and it is not known if the pyrite-preserved vent organisms might also preserve potential signatures of their metabolisms. Here, we use high-resolution secondary ion mass spectrometry (SIMS) to investigate the sulfur isotopes of pyrites from recently mineralized and Ordovician-early Silurian tubeworm fossils with associated microbial fossils. Our results demonstrate that pyrites containing microbial fossils consistently have significantly more negative δ34 S values compared with nearby non-fossiliferous pyrites, and thus represent the first indication that the presence of microbial sulfur-cycling communities active at the time of pyrite formation influenced the sulfur isotope signatures of pyrite at hydrothermal vents. The observed depletions in δ34 S are generally small in magnitude and are perhaps best explained by sulfur isotope fractionation through a combination of sulfur-cycling processes carried out by vent microbes. These results highlight the potential for using sulfur isotopes to explore biological functional relationships within fossil vent communities, and to enhance understanding of how microbial and animal life has co-evolved to colonize vents throughout geological time.

The Winchcombe meteorite, a unique and pristine witness from the outer solar system.

Direct links between carbonaceous chondrites and their parent bodies in the solar system are rare. The Winchcombe meteorite is the most accurately recorded carbonaceous chondrite fall. Its pre-atmospheric orbit and cosmic-ray exposure age confirm that it arrived on Earth shortly after ejection from a primitive asteroid. Recovered only hours after falling, the composition of the Winchcombe meteorite is largely unmodified by the terrestrial environment. It contains abundant hydrated silicates formed during fluid-rock reactions, and carbon- and nitrogen-bearing organic matter including soluble protein amino acids. The near-pristine hydrogen isotopic composition of the Winchcombe meteorite is comparable to the terrestrial hydrosphere, providing further evidence that volatile-rich carbonaceous asteroids played an important role in the origin of Earth's water.

Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust.

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth's mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth's surface.

Sulphur isotopes of alkaline magmas unlock long-term records of crustal recycling on Earth.

Earth's surface and mantle sulphur reservoirs are connected via subduction, crustal recycling and volcanism. Although oceanic hotspot lavas currently provide the best constraints on the deep sulphur cycle, their restricted age range (<200 Ma) means they cannot reveal temporal variations in crustal recycling over Earth history. Sulphur-rich alkaline magmas offer the solution because they are associated with recycled sources (i.e. metasomatized lithospheric mantle and plumes) and, crucially, are found throughout the geological record. Here, we present a detailed study of sulphur isotope fractionation in a Mesoproterozoic alkaline province in Greenland and demonstrate that an enriched subduction-influenced source (δ34S of +1 to +5‰) can be reconstructed. A global δ34S compilation reveals secular variation in alkaline magma sources which support changes in the composition of the lithospheric mantle and/or Ga timescales for deep crustal recycling. Thus, alkaline magmas represent a powerful yet underutilized repository for interrogating crustal recycling through geological time.

Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters.

Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwaters in which. Arsenic release to is widely attributed largely to reductive dissolution of arsenic-bearing iron minerals, driven by metal reducing bacteria using bioavailable organic matter as an electron donor. However, the nature of the organic matter implicated in arsenic mobilization, and the location within the subsurface where these processes occur, remains debated. In a high resolution study of a largely pristine, shallow aquifer in Kandal Province, Cambodia, we have used a complementary suite of geochemical tracers (including 14C, 3H, 3He, 4He, Ne, δ18O, δD, CFCs and SF6) to study the evolution in arsenic-prone shallow reducing groundwaters along dominant flow paths. The observation of widespread apparent 3H-3He ages of <55years fundamentally challenges some previous models which concluded that groundwater residence times were on the order of hundreds of years. Surface-derived organic matter is transported to depths of >30m, and the relationships between age-related tracers and arsenic suggest that this surface-derived organic matter is likely to contribute to in-aquifer arsenic mobilization. A strong relationship between 3H-3He age and depth suggests the dominance of a vertical hydrological control with an overall vertical flow velocity of ~0.4±0.1m·yr-1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08±0.03µM·yr-1 is broadly comparable to previous estimates from other researchers for similar reducing aquifers in Bangladesh. Although apparent arsenic groundwater accumulation rates varied significantly with site (e.g. between sand versus clay dominated sequences), rates are generally highest near the surface, perhaps reflecting the proximity to the redox cline and/or depth-dependent characteristics of the OM pool, and confounded by localized processes such as continued in-aquifer mobilization, sorption/desorption, and methanogenesis.

No significant boron in the hydrated mantle of most subducting slabs.

Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling.

Biogeochemical probing of microbial communities in a basalt-hosted hot spring at Kverkfjöll volcano, Iceland.

We investigated bacterial and archaeal communities along an ice-fed surficial hot spring at Kverkfjöll volcano-a partially ice-covered basaltic volcano at Vatnajökull glacier, Iceland, using biomolecular (16S rRNA, apsA, mcrA, amoA, nifH genes) and stable isotope techniques. The hot spring environment is characterized by high temperatures and low dissolved oxygen concentrations at the source (68°C and <1 mg/L (±0.1%)) changing to lower temperatures and higher dissolved oxygen downstream (34.7°C and 5.9 mg/L), with sulfate the dominant anion (225 mg/L at the source). Sediments are comprised of detrital basalt, low-temperature alteration phases and pyrite, with <0.4 wt. % total organic carbon (TOC). 16S rRNA gene profiles reveal that organisms affiliated with Hydrogenobaculum (54%-87% bacterial population) and Thermoproteales (35%-63% archaeal population) dominate the micro-oxic hot spring source, while sulfur-oxidizing archaea (Sulfolobales, 57%-82%), and putative sulfur-oxidizing and heterotrophic bacterial groups dominate oxic downstream environments. The δ13 Corg (‰ V-PDB) values for sediment TOC and microbial biomass range from -9.4‰ at the spring's source decreasing to -12.6‰ downstream. A reverse effect isotope fractionation of ~3‰ between sediment sulfide (δ34 S ~0‰) and dissolved water sulfate (δ34 S +3.2‰), and δ18 O values of ~ -5.3‰ suggest pyrite forms abiogenically from volcanic sulfide, followed by abiogenic and microbial oxidation. These environments represent an unexplored surficial geothermal environment analogous to transient volcanogenic habitats during putative "snowball Earth" scenarios and volcano-ice geothermal environments on Mars.

Subsurface absorption of anthropogenic warming of the land surface: the case of the world's largest brickworks (Stewartby, Bedfordshire, UK).

Stewartby works, for a time the world's largest brickworks, began operation around the start of the twentieth century and closed in 2008. Subsurface temperature measurements are available in its vicinity, obtained as part of monitoring of an adjacent landfill in one of the former quarries for the Oxford Clay, which was the raw material for brick manufacture. A striking subsurface temperature anomaly, an increment of ~12°C, was first measured in 2004, and has subsequently decayed over time. The anomaly is centred beneath one of the former brick kilns, which operated between 1935 and 1991. To investigate processes of heat absorption by the shallow subsurface, this anomaly has been modelled as a consequence of conductive heat flow into the ground due to the operation of the ~3000 m(2) kiln. This modelling indicates that a very large amount of heat energy was transported into the subsurface; we estimate the typical downward surface heat flow during operation of the kiln as ~1 W m(-2) and the energy stored in the subsurface beneath it at its time of shutdown as ~6 TJ, or ~0.03% of that released by the fuel for heating the kiln, such that the total heat energy stored beneath this multi-kiln site peaked at ~200TJ. The proportion of heat energy transported into the subsurface was relatively low due to the nature of the Oxford Clay, which has a low thermal conductivity (~0.8 W m(-1)°C(-1)) and diffusivity (~0.3mm(2)s(-1)); in a more conductive lithology it might well have been three times greater. After kiln shutdown this subsurface thermal anomaly began to dissipate by upward heat conduction and release of heat into the atmosphere; at present about half of the peak energy stored remains, decreasing at ~1% per year, the maximum temperature anomaly being currently ~7°C at a depth of ~30 m and the typical upward heat flow during this span of time having exceeded the regional ~40 mW m(-2) background by roughly an order of magnitude. We believe this to be the first documented case whereby a subsurface thermal anomaly associated with operation of industrial plant has been related in detail to the history of site operations. This case study thus bears upon the controversial topic of the development of subsurface heat islands in general, and the associated perturbation of the thermal state of the subsurface as a result of anthropogenic warming of the atmosphere. It has previously been suggested that the worldwide heat gain in the subsurface over recent decades has exceeded that in the atmosphere by a factor of three. We show that this result is subject to some uncertainty, for example because it does not factor in lateral variations in thermal properties. Nonetheless, our case study demonstrates dissipation of a subsurface thermal anomaly by heat transport into the atmosphere. This indicates that warming of the atmosphere will be sustained in the future by dissipation of the large amount of energy stored in pre-existing subsurface thermal anomalies on a global scale, an issue of major societal implications that demands more detailed investigation.

Long term geological record of a global deep subsurface microbial habitat in sand injection complexes.

There is extensive evidence from drilling into continental margins for microbial colonization of a deep biosphere. However it is difficult to prove deep biosphere activity in the geological record, where evidence for life is dominated by the remains of organic matter buried after deposition at the surface. Nevertheless we propose that natural injections of sand into muddy strata at continental margins represent an excellent habitat opportunity for deep microbial activity down to several kilometres' present day depth. Sulphur isotope data for iron sulphides precipitated soon after injection indicate consistent microbial sulphate reduction through the geological record. The complexes are favourable sites for colonization, because high permeability and extensive sand/mud interface allow ready availability of electron donors and nutrients. The measured examples of iron sulphide in injected sands extend back to the Proterozoic, and show that injected sand complexes have been a long-term environment for deep subsurface microbial colonization.

Sulphur stable isotope systematics in diagenetic pyrite from the North Sea hydrocarbon reservoirs revealed by laser combustion analysis.

Our study focuses on pyrite nodules developed in the Brent Group sandstones, which host the Brent Oilfield, one of the North Sea's greatest oil and gas producers. Timing of nodule formation is equivocal, but due to the forceful, penetrative textures that abound, it is considered late. This pyrite offers a research opportunity because it records the development of the supply of H(2)S in a hydrocarbon reservoir and its sulphur isotopic composition. Laser-based analysis of δ(34)S reveals an extraordinary diversity in values and patterns. The values range from-27 to+72‰, covering half the terrestrial range, with large variations at the submillimetre scale. Isotopically heavy (δ(34)S ∼+30‰ or higher) sulphide is endemic, but low δ(34)S pyrite is also present and appears to represent a temporally though not spatially (on the ∼cm scale) distinct pyritisation event. The distribution of δ(34)S values within individual concretions can be normal (Gaussian), but in some cases may reflect progressive isotope fractionation process(es), conceivably of Rayleigh type. The source of the sulphur and the identity of the isotope fractionation process(es) remain enigmatic.

Geochemical and stable isotopic constraints on the generation and passive treatment of acidic, Fe-SO4 rich waters.

Reducing and Alkalinity Producing Systems (RAPS) remediate net-acidic metalliferous mine drainage by creating anoxic conditions in which bacterial sulfate reduction (BSR) raises alkalinity and drives the precipitation of iron and other chalcophilic elements as sulfides. We report chemical and stable isotopic data from a study monitoring the biogeochemical processes involved in the generation of mine waters and their remediation by two RAPS. Sulfur isotopes show that sulfate in all mine waters has a common source (pyrite oxidation), whilst oxygen isotopes show that oxidation of pyritic sulfur is mediated by Fe(III)(aq). The isotopic composition of dissolved sulfide, combined with the sulfur and oxygen isotopic composition of sulfate in RAPS effluents, proves BSR and details its dual isotope systematics. The occurrence and isotopic composition of solid phase iron sulfides indicate the removal of reduced sulfur within the RAPS, with significant amounts of elemental sulfur indicating reoxidation steps. However, only 0 to 9% of solid phase iron occurs as Fe sulfides, with approximately 70% of the removed iron occurs as Fe(III) (hydr)oxides. Some of the (hydr)oxide is supplied to the wetland as solids and is simply filtered by the wetland substrate, playing no role in alkalinity generation or proton removal. However, the majority of iron is supplied as dissolved Fe(II), indicating that acid generating oxidation and hydrolysis reactions dominate iron removal. The overall contribution of BSR to the sulfur geochemistry in the RAPS is limited and sulfate retention is dominated by sulfate precipitation, comparable to aerobic treatment systems, and show that the proton acidity resulting from iron oxidation and hydrolysis must be subsequently neutralised by calcite dissolution and/or BSR deeper in the RAPS sediments. BSR is not as important as previously thought for metal removal in RAPS. The results have practical consequences for the design, treatment performance and long-term functionality of such systems.