The sequestration of CO 2 in the deep geosphere is one potential method for reducing anthropogenic emissions to the atmosphere without a drastic change in our energy-producing technologies. Immediately after injection, the CO 2 will be stored as a free phase within the host rock. Over time it will dissolve into the local formation water and initiate a variety of geochemical reactions. Some of these reactions could be beneficial, helping to chemically contain or 'trap' the CO 2 as dissolved species and by the formation of new carbonate minerals; others may be deleterious, and actually aid the migration of CO 2. It will be important to understand the overall impact of these competing processes. However, these processes will also be dependent upon the structure, mineralogy and hydrogeology of the specific lithologies concerned and the chemical stability of the engineered features (principally, the cement and steel components in the well completions). Therefore, individual storage operations will have to take account of local geological, fluid chemical and hydrogeological conditions. The aim of this paper is to review some of the possible chemical reactions that might occur once CO 2 is injected underground, and to highlight their possible impacts on long-term CO 2 storage.If sequestration of CO 2 is to be a practicable largescale disposal method, the CO 2 must remain safely underground, and not return to the atmosphere within relatively short timescales (e.g. thousands of years), so that natural buffering processes (e.g. oceanic and forestry sinks) have sufficient time to reduce global atmospheric CO 2 levels to environmentally acceptable levels. Indeed, acceptable performance will need to be demonstrated in order to satisfy operational, regulatory and public acceptance criteria. The track record of CO2-assisted enhanced oil recovery (EOR) operations and purposedesigned underground storage of natural gas shows that underground storage can be practicable and leakage minimized over 'industrial' time periods (e.g. tens of years). However, there is much less information for longer-term processes, and these must be understood, especially as CO 2 is more chemically reactive than methane.The injection of a relatively reactive substance such as CO 2 into the deep subsurface will result in chemical disequilibria and the initiation of various chemical reactions. It is important to understand the direction, rate and magnitude of such reactions both in terms of their impact upon the ability of a host formation to contain the injected CO 2 safely, and in terms of the longevity of CO 2 containment (e.g. Rochelle et al. 1999). Some reactions, such as the precipitation of CO 2 in secondary carbonate minerals, may be beneficial and aid containment. However, other reactions may result in mineral dissolution -facilitating the formation of migration pathways and so act to reduce containment. The aim of this paper is to highlight some of these processes, and to illustrate their possible impact on long-term CO 2 storage. It is hoped that...

Abstract. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The high spatial resolution of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional isotope dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-millimetre scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and “hit or miss”. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; our examples are drawn from the dating of fracture-filling calcite, but our discussion is relevant to all carbonate applications. We review several limitations to the method, including open-system behaviour, variable initial-lead compositions, and U–daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging and image-based dating that utilises LA-ICP-MS elemental and isotopic map data.

Hydrogen is a key energy source for subsurface microbial processes, particularly in subsurface environments with limited alternative electron donors, and environments that are not well connected to the surface. In addition to consumption of hydrogen, microbial processes such as fermentation and nitrogen fixation produce hydrogen. Hydrogen is also produced by a number of abiotic processes including radiolysis, serpentinization, graphitization, and cataclasis of silicate minerals. Both biotic and abiotically generated hydrogen may become available for consumption by microorganisms, but biotic production and consumption are usually tightly coupled. Understanding the microbiology of hydrogen cycling is relevant to subsurface engineered environments where hydrogen-cycling microorganisms are implicated in gas consumption and production and corrosion in a number of industries including carbon capture and storage, energy gas storage, and radioactive waste disposal. The same hydrogen-cycling microorganisms and processes are important in natural sites with elevated hydrogen and can provide insights into early life on Earth and life on other planets. This review draws together what is known about microbiology in natural environments with elevated hydrogen, and highlights where similar microbial populations could be of relevance to subsurface industry.

The 2010 Eyjafjallajökull lasted 39 days and had 4 different phases, of which the first and third (14–18 April and 5–6 May) were most intense. Most of this period was dominated by winds with a northerly component that carried tephra toward Europe, where it was deposited in a number of locations and was sampled by rain gauges or buckets, surface swabs, sticky‐tape samples and air filtering. In the UK, tephra was collected from each of the Phases 1–3 with a combined range of latitudes spanning the length of the country. The modal grain size of tephra in the rain gauge samples was 25 μm, but the largest grains were 100 μm in diameter and highly vesicular. The mass loading was equivalent to 8–218 shards cm−2, which is comparable to tephra layers from much larger past eruptions. Falling tephra was collected on sticky tape in the English Midlands on 19, 20 and 21st April (Phase 2), and was dominated by aggregate clasts (mean diameter 85 μm, component grains <10 μm). SEM‐EDS spectra for aggregate grains contained an extra peak for sulphur, when compared to control samples from the volcano, indicating that they were cemented by sulphur‐rich minerals e.g. gypsum (CaSO4⋅H2O). Air quality monitoring stations did not record fluctuations in hourly PM10 concentrations outside the normal range of variability during the eruption, but there was a small increase in 24‐hour running mean concentration from 21–24 April (Phase 2). Deposition of tephra from Phase 2 in the UK indicates that transport of tephra from Iceland is possible even for small eruption plumes given suitable wind conditions. The presence of relatively coarse grains adds uncertainty to concentration estimates from air quality sensors, which are most sensitive to grain sizes <10 μm. Elsewhere, tephra was collected from roofs and vehicles in the Faroe Islands (mean grain size 40 μm, but 100 μm common), from rainwater in Bergen in Norway (23–91 μm) and in air filters in Budapest, Hungary (2–6 μm). A map is presented summarizing these and other recently published examples of distal tephra deposition from the Eyjafjallajökull eruption. It demonstrates that most tephra deposited on mainland Europe was produced in the highly explosive Phase 1 and was carried there in 2–3 days.

A sequential extraction methodology, designed to measure the solid phase partitioning of metals in soils and sediments, is described. The method uses centrifugation to pass increasing concentrations of HNO 3 through the sample, followed by ICP-AES analysis of major and trace elements of the extracts. A data-processing algorithm is used to identify the number of physico-chemical components extracted, their composition and the proportion of each in each extract. The algorithm has been successfully tested on a synthetic data set and the combination of the extraction methodology and data-processing algorithm have been tested on a contaminated soil sample (NIST SRM 2710). The 14 extracts from each duplicate experiment were analysed for 19 elements and data analysis identified nine chemically distinct soil components: pore-water residual solutes; organic, easily exchangeable; a Cu–Zn dominated phase; a Pb-dominated phase; amorphous Fe oxide/oxyhydroxide; crystalline Fe oxide; Fe–Ti oxide; and Mn oxide.