Arsenic incorporation into FeS2 pyrite and its influence on dissolution: A DFT study

FeS₂ pyrite can incorporate large amounts of arsenic (up to ca. 10 wt%) and hence has a strong impact on the mobility of this toxic metalloid. Focussing on the lowest arsenic concentrations for which the incorporation occurs in solid solution, the substitution mechanisms involved have been investigated by assuming simple incorporation reactions in both oxidising and reducing conditions. The solution energies were calculated by Density Functional Theory (DFT) calculations and we predict that the formation of AsS dianion groups is the most energetically favourable mechanism. The results also suggest that the presence of arsenic will accelerate the dissolution and thus the generation of acid drainage, when the crystal dissolves in oxidising conditions.     

Investigation of the Ablation Behaviour of Andradite-Grossular Garnets and Rutile with Implications for U-Pb Geochronology

U-Pb dating of andradite-grossular garnet (grandite) and rutile by LA-ICP-MS can be used to constrain various metamorphic, metasomatic and igneous geological processes. In this study, we examine and compare the impact of different analytical conditions (fluence, pulse width, laser beam size and ablation frequency) on the ablation crater morphology, ablation rates, down-hole fractionation and U-Pb ages of grandite and rutile samples of different compositions. The shapes of grandite ablation craters suggest the mineral ablates by classical evaporation with significant melting that cannot be eliminated even at fluences just above the ablation threshold. Grandite garnets with higher andradite proportions have faster ablation rates. The overall low U contents of grandite require using large laser beam sizes to obtain acceptable precision of U-Pb ages. At such conditions and crater depths < 10 μm, fluences of 2.1 and 3.5 J cm^-2, laser pulse width of 5 ns and 20 ns, and ablation frequencies between 3.5 and 6.5 Hz, obtain similar and reproducible ages when the proportion of grossular is < 35%. Rutile ablation crater morphology shows evidence of melt splashing and thermal stress cracking. They have significant crater bottom features, which increase in relief with lower fluences and a higher number of laser shots, indicating the features are probably energy-related and making higher fluences, such as 5 J cm^-2, necessary for uniform ablation when using 193 nm excimer lasers. The slow ablation rate at low fluences and then steep increase at around 2.0 J cm^-2 suggests a transition in the ablation mechanism from exfoliation to classical vaporisation. Crater bottom features and other ablation behaviour vary between samples, which could be related to their difference in colour. Although the down-hole fractionation patterns of the samples are similar at 5 J cm^-2, the U-Pb ages of some samples vary significantly with different analytical conditions and/or measurement sessions, particularly when using laser beam sizes of 30 μm, suggesting differences in mass bias and more variable ablation behaviour. A laser beam size of at least 60 μm is recommended for reproducible U-Pb dating of rutile.     

Prairie or planted? Using time-series NDVI to determine grassland characteristics in Montana

Temperate grasslands are a highly threatened global biome. Complicating management and conservation strategy development, modern grasslands can be difficult to characterize across landscapes since they range from native and semi-native to completely non-native species compositions such as those found in heavily managed pastures. Similar to methods used to differentiate C3 and C4 grasses, we investigate the ability of using temporal variations in growth characteristics as an alternative pathway to predicting native versus introduced species composition across grassland landscapes. To do this, we conducted an exploratory analysis using a time-series of Normalized Difference Vegetation Index values as a measure of vegetation greenness with Landsat 5 TM imagery across a growing season and performed an unsupervised classification. Results from the classification were compared with field observation to determine if we can differentiate between native and introduced grassland types in the Northwest Glaciated Plains subecoregion of northeastern Montana. Our results indicated that we predicted grassland cover with 81% accuracy within our 200 km² study area and 71% accuracy in our 5000 km² secondary study area. Further extrapolation of our methodology, combined with the refinement of vegetation indices of time-series imagery, classification algorithms and the availability of data from planned Landsat and Sentinel missions, may provide the spatial detail necessary to improve grassland monitoring and rangeland management over large areas.     

Vegetation controls on channel stability in the Bell River,Eastern Cape,South Africa

Channel instability has occurred in the Bell River in the form of meander cutoffs, a number of which have occurred since 1952. Increased sediment loading from widespread gully erosion in the catchment has been proposed as the trigger for this instability. Willow species of the Salix family, in particular S. caprea, have been planted along the banks in an effort to prevent further channel shifting. This study reports the results of an investigation into the effect of vegetation on channel form and stability over a 17 km stretch of channel. Results indicate that riparian vegetation has significant effects on channel form which have implications for channel stability. Riparian vegetation increases bank stability and reduces channel cross-sectional area, thereby inducing stability at flows less than bankfull. Evidence indicates that narrow stable stretches are associated with relatively high levels of riparian vegetation. Wider, unstable channels are associated with relatively less riparian vegetation. The effectiveness of riparian vegetation relative to bank sediments was investigated. A dense growth of willows was found to have an equivalent effect to banks with a silt-clay ratio of about 70 per cent. The channel narrowing induced by vegetation may contribute to channel shifting at high flows. The reduced channel capacity is thought to result in more frequent overbank flooding which may ultimately lead to channel avulsion. Thus where increased sediment loading is pushing the channel towards instability, vegetation may be effective in imparting local stability, but it is unable to prevent long-term channel shifts, and may rather help to push the system towards more frequent avulsions. Copyright © 1999 John Wiley & Sons, Ltd.     

Growing new generations of critical zone scientists

Critical zone (CZ) science is entering its second decade. A new generation of scientists is emerging trained specifically in CZ science and are contributing to advances in environmental science across disciplines. Concurrently, the global scope of CZ science is being elevated as new countries invest in CZ observatories. Global CZ science has great potential to address a diverse array of questions beyond any single discipline. In this commentary we discuss a series of CZ science grand challenges that should be targeted by early‐career researchers: understanding water availability in the CZ; expanding CZ science into new environments; communicating the societal relevance of CZ science including earthcasting to the public; seamlessly integrating biological sciences within the CZ framework; and scaling CZ processes over large spatial and temporal gradients. Targeting these grand challenges will push CZ science well into the future. We also highlight mechanisms for increased leadership within the CZ community. Copyright © 2017 John Wiley & Sons, Ltd.     

Late spring ultraviolet levels over the United Kingdom and the link to ozone

Erythemally-weighted ultraviolet (UV_ery) levels measured over southern England, during anticyclonic weather between 30 April and 2 May, 1997, were almost 50 higher than normally expected for clear skies and were similar to mid-summer values for the first time since measurements began in 1990. Investigation of this episode suggests that a combination of both meteorological and chemical effects were responsible for generating record low ozone amounts for the time of year. Further, comparisons between the A band ultraviolet (315 to 400 nm wavelength) amounts, and radiative calculations confirm that the high UV_ery was primarily due to the reduction in total ozone. These results are contrasted with a similar period for 1998, in which near climatological ozone amounts were measured. The prospects for enhanced UV_ery levels in future years are briefly reviewed in the light of expected increases in stratospheric halogen levels and greenhouse gases.     

Shades of green in local authority policy-making: a regional case study

Summary Local authorities have been seen as an important agent in the development of more sustainable economies. This paper explores the level and form of 'greening' taking place amongst local authorities in the Midland region of the UK. Particular attention is paid to intra-regional variations in environmental policy-making.     

Development of an adaptive disturbance rejection system for the rapidly deployable stable platform—Part 2: Controller design and closed loop response

Currently, both military and civilian operations that require at-sea cargo transfers are severely limited by environmental conditions and loading forces that induce vessel motions. To increase the robustness of at-sea cargo transfer to these environmental conditions and loading forces, efforts have recently been made toward an actively controlled, rapidly deployable stable platform (RDSP). The purpose of the research presented here is to implement an output feedback adaptive controller and adaptive disturbance rejection scheme that will mitigate the effect of environmental conditions and reject disturbances caused by various loading situations. Because of the controller's distinct ability to adapt to various operating conditions, anticipate and reject load disturbances of unknown magnitude, and adjust to stay within input saturation constraints, the framework is a good fit for the RDSP. Three missions are considered using a previously developed 3 degree of freedom simulation of a 1/10th scale RDSP prototype. Results show successful mitigation of load disturbances and a significant reduction in pitch motions using a control command that remains within the given amplitude and rate constraints. In the case of cargo transfer operations, the adaptive control system is able to significantly increase the cargo throughput by rejecting the disturbances before they are able to cause large pitching dynamics.