Compositional variation during monogenetic volcano growth and its implications for magma supply to continental volcanic fields

Individual volcanoes of continental monogenetic volcanic fields are generally presumed to erupt single magma batches during brief eruptions. Nevertheless, in two unrelated volcanic fields (the Waipiata volcanic field, New Zealand, and the Miocene-Pliocene volcanic field in western Hungary), we have identified pronounced and systematic compositional differences among products of individual volcanoes. We infer that this indicates a two-stage process of magma supply for these volcanoes. Each volcano records: (1) intrusion of a basanitic parent magma to lower- to mid-crustal levels and its subsequent fractionation to form a tephritic residual melt;, (2) subsequent transection of this reservoir by a second batch of basanitic melt, with tephrite rising to the surface at the head of the propagating basanite dyke. Eruption at the surface then yields initial tephrite, typically erupted as pyroclasts, followed by eruption and shallow intrusion of basanite from deeper in the dyke. By analogy with similar tephrite-basanite eruptions along rift zones of intraplate ocean-island volcanoes, we infer that fractionation to tephrite would have required decades to centuries. We conclude that the two studied continental monogenetic volcanic fields demonstrate a consistent history of early magmatic injections that fail to reach the surface, followed by capture and partial eruption of their evolved residues in the course of separate and significantly later injections of basanite that extend to the surface and erupt. This systematic behaviour probably reflects the difficulty of bringing small volumes of dense, primitive magma to the surface from mantle source regions. Ascent through continental crust is aided by the presence in the dyke head of buoyant tephrite captured during transection of the earlier-emplaced melt bodies

Investigating uptake of N2O in agricultural soils using a high-precision dynamic chamber method

Uptake (or negative flux) of nitrous oxide (N2O)in agricultural soils is a controversial issue which has proved difficult to investigate in the past due to constraints such as instrumental precision and methodological uncertainties. Using a recently developed high-precision quantum cascade laser gas analyser combined with a closed dynamic chamber, a well-defined detection limit of 4 μg N2O-N m could be achieved for individual soil flux measurements. 1220 mea- surements of N2O flux were made from a variety of UK soils using this method, of which 115 indicated uptake by the soil (i.e. a negative flux in the micrometeorological sign convention). Only four of these apparently negative fluxes were greater than the detection limit of the method, which suggests that the vast majority of reported negative fluxes from such measurements are actually due to instrument noise. As such, we suggest that the bulk of negative N2O fluxes reported for agricultural fields are most likely due to limits in detection of a particular flux measurement methodology and not a result of microbiological activity consuming atmospheric N2O

Potential risks of induced seismicity from high volume hydraulic fracturing of shales in Northern Ireland

Hydraulic fracturing (HF) has made it possible to economically produce hydrocarbons directly from low‐permeability reservoirs such as shales by injecting high pressure fluids to create fracture networks. However, over the last decade the number of observations of induced earthquakes caused by HF operations around the world has increased as the shale gas industry has developed. Data from the US and Canada suggest that on average around 1% of HF wells can be linked to earthquakes with magnitudes of 3 or greater. Earthquakes of this size are large enough to be felt by people. However, in some areas of the US and Canada the percentage of wells associated with induced earthquakes is much higher (>30%). This variability is often explained in terms of geological factors such as proximity to existing faults. In a small number of cases, HF operations have triggered earthquakes large enough to cause potentially damaging ground motions. Such earthquakes cannot be confidently predicted in advance of operations. These observations suggest that the risk from induced seismicity during HF operations is not negligible. Earthquakes with magnitudes greater than around 2 result from slip on existing faults that is triggered by stress changes caused by the injection of fluid during the HF process. The size of the earthquake will depend on both the area of the ruptured part of the fault and the amount of slip. Since such faults may extend outside the hydraulically fractured zone, the maximum magnitude will be controlled by local geology and tectonics, not operational parameters such as the amount of injected fluid. As a result, the maximum magnitude is highly uncertain. Induced earthquakes have been observed in wide variety of geological settings and in areas where there are relatively few tectonic earthquakes. In some areas, the resulting hazard from induced earthquakes due to HF operations is significantly greater than the hazard from tectonic earthquakes. As a result, the low hazard from tectonic earthquakes in Northern Ireland does not guarantee that the hazard from induced seismicity will also be low. Induced earthquakes are likely to be clustered in space and time around the locus of HF operations. Hazard is likely to increase with the number of wells and will be highest during or shortly after HF operations. Hazard may also be a function of total injected volume, with larger injected volumes leading to more earthquakes and increasing the probability of larger events. Operations that target shallow formations may pose a higher hazard, since for a given magnitude, the intensity of ground motions at the surface will be greater. The potential for actual damage depends on the intensity of motions and both the number and vulnerability of buildings exposed to ground shaking. As a result, the risk of damage to buildings will be higher in densely populated urban areas than in rural areas. Risk studies for the UK have shown that cosmetic and minor structural damage may occur for earthquakes with magnitudes as low as 3. Higher resolution geophysical data is needed to identify fault structures and depth to basement in sedimentary basins with hydrocarbon potential in Northern Ireland in order to help mitigate risk of induced seismicity from hydraulic fracturing. Improved regional seismic monitoring should also be considered. Similarly, the present-day stress regime and stress state of faults in both the Lough Allen and Rathlin basins is poorly known. Further work is needed to address this. Current risk-mitigation strategies have had limited success. There may be insufficient data to identify geological faults prior to operations and even where high resolution data are available, there may still be hidden faults. Similarly, traffic light systems based on specific earthquake magnitude thresholds have often failed. Statistical methods that relate the volume of injected fluid or the injection rate to induced earthquake activity may allow useful probabilistic forecasts in the future but may be associated with considerable uncertainties without calibration for local conditions

Nitrous oxide emission sources from a mixed livestock farm

The primary aim of this study was to identify and compare the most significant sources of nitrous oxide (N2O) emissions from soils within a typical mixed livestock farm in Scotland. The farm area can be considered as representative of agricultural soils in this region where outdoor grazing forms an important part of the animal husbandry. A high temporal resolution dynamic chamber method was used to measure N2O fluxes from the featureless, general areas of the arable and pasture fields (general) and from those areas where large nitrogen additions are highly likely, such as animal feeding areas, manure heaps, animal barns (features). Individual N2O flux measurements varied by four orders of magnitude, with values ranging from −5.5 to 80,000 μg N2O-N m−2 h−1. The log-normal distribution of the fluxes required the use of more complex statistics to quantify uncertainty, including a Bayesian approach which provided a robust and transparent method for “upscaling” i.e. translating small-scale observations to larger scales, with appropriate propagation of uncertainty. Mean N2O fluxes associated with the features were typically one to four orders of magnitude larger than those measured on the general areas of the arable and pasture fields. During warmer months, when widespread grazing takes place across the farm, the smaller N2O fluxes of the largest area source – the general field (99.7% of total area) – dominated the overall N2O emissions. The contribution from the features should still be considered important, given that up to 91% of the fluxes may come from only 0.3% of the area under certain conditions, especially in the colder winter months when manure heaps and animal barns continue to produce emissions while soils reach temperatures unfavourable for microbial activity (<5 °C)

Development of Alditol Acetate Derivatives for the Determination of 15N-Enriched Amino Sugars by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry

Amino sugars can be used as indices to evaluate the role of soil microorganisms in active nitrogen (N) cycling in soil. This paper details the assessment of the suitability of gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS) for the analysis of ¹⁵N-enriched amino sugars as alditol acetate derivatives prior to application of a novel ¹⁵N stable isotope probing (SIP) approach to amino sugars. The efficient derivatization and cleanup of alditol acetate derivatives for GC was achieved using commercially available amino sugars, including glucosamine, mannosamine, galactosamine, and muramic acid, as laboratory standards. A VF-23ms stationary phase was found to produce optimal separations of all four compounds. The structure of the alditol acetate derivatives was confirmed using gas chromatography/mass spectrometry (GC/MS). For GC–C–IRMS determinations, implementation of a two-point normalization confirmed the optimal carrier gas flow rate to be 1.7 mL min^–1. Linearity of δ¹⁵N value determinations up to δ¹⁵N_t of 469 ± 3.1‰ (where δ¹⁵N_t is the independently measured δ¹⁵N value) was confirmed when 30 nmol N was injected on-column, with the direction of deviation from δ¹⁵N_t at low sample amount dependent on the ¹⁵N abundance of the analyte. Observed between- and within-run memory effects were significant (<i>P</i> < 0.007) when a highly enriched standard (469 ± 3.1‰) was run; therefore, analytical run order and variation in ¹⁵N enrichment of analytes within the same sample must be considered. The investigated parameters have confirmed the isotopic robustness of alditol acetate derivatives of amino sugars for the GC–C–IRMS analysis of ¹⁵N-enriched amino sugars in terms of linearity over an enrichment range (natural abundance to 469 ± 3.1‰) with on-column analyte amount over 30 nmol N

Nitrogen fertiliser interactions with urine deposit affect nitrous oxide emissions from grazed grasslands

Cattle excreta deposited on grazed pastures are responsible for one fifth of the global anthropogenic nitrous oxide (N2O) emissions. One of the key nitrogen (N) sources is urine deposited from grazing animals, which contributes to very large N loadings within small areas. The main objective of this plot study was to establish whether the application of N fertiliser and urine deposit from dairy cows synergistically interacts and thereby increases N2O emissions, and how such interaction is influenced by the timing of application. The combined application of fertiliser (calcium ammonium nitrate) and urine significantly increased the cumulative N2O emissions as well as the N2O emission factor (EF) from 0.35 to 0.74 % in spring and from 0.26 to 0.52 % in summer. By contrast, EFs were lower when only fertiliser (0.31 % in spring, 0.07 % in summer) or urine was applied (0.33 % in spring, 0.28 % in summer). In autumn, N2O emissions were larger than in other seasons and the emissions from the combined application were not statistically different to those from either the separately applied urine or N fertiliser (EF ranging from 0.72 to 0.83, p-value < 0.05). The absence of significant synergistic effect could be explained by weather conditions, particularly rainfall during the three days prior to and after application in autumn. This study implies that the interactive effects of N fertilisation and urine deposit, as well as the timing of the application on N2O emission need to be taken into account in greenhouse gas emission inventories

Scientists and public: is the information flow direction starting to change?

Over half of the population of the UK own a smartphone, and about the same number of people uses social media such as Twitter. For the British Geological Survey (BGS) this means millions of potential reporters of real-time events and in-the-field data capturers, creating a new source of scientific information that could help to better understand and predict natural processes