Heterogeneous Fe(II) oxidation and zeta potential

The determination of electrophoretic mobility and zeta potential was used as a diagnostic tool, alongside kinetic experiments, to delineate between three plausible mechanisms for the heterogeneous oxidation of Fe(II) by dissolved oxygen. One of these mechanisms is dependant on the positive surface charge that exists on Fe(III) (oxy)hydroxide surfaces at pH values below the Iso-Electric Point (IEP). However, this mechanism can be disputed as catalysis is observed on Fe(III) (oxy)hydroxide surfaces above the IEP despite a negative zeta potential. As well as an IEP shift an overall reduction of the magnitude of the zeta potential is observed in samples of field Fe(III) (oxy)hydroxide collected from the Taff Merthyr mine water treatment site in South Wales, UK. Low zeta potentials determined in mine water treatment systems will have beneficial effects for particle coagulation and settling in passive mine water treatment systems.     

The Contribution of Variability of Lift-induced Upwash to the Uncertainty in Vertical Winds Determined from an Aircraft Platform

Aircraft-based vertical flux measurements fill a gap in the spatial domain for studies of biosphere–atmosphere exchange. To acquire valid flux data, a determination of the deviation from the mean vertical wind, w′, is essential. When using aircraft platforms, flux measurements are subject to systematic and random errors from airflow distortion caused by the lift-induced upwash ahead of the aircraft. Although upwash is typically considered to be a constant quantity over periods used for calculating fluxes, it can vary significantly over short (and longer) periods due to changes in aircraft lift. The characterization of such variations in upwash are of undeniable importance to flux measurements, especially when real-time computations of w′ are required. In this paper, the variability in upwash was compared to the calculated upwash from the model of Crawford et al. (Boundary-Layer Meteorol, 80:79–94, 1996) using data taken during a long-period (phugoid mode) free oscillation of the aircraft. The cyclic variation of lift during the free oscillation offers an ideal scenario in which to acquire in-flight data on the upwash that is present, as well as to test the capability of upwash correction models. Our results indicate that while this model corrects for much of the mean upwash, there can be significant variations in upwash on a time scale that is important to flux measurements. Our results suggest that use of the measured load factor could be an easily implemented operational constraint to minimize uncertainty in w′ due to changing upwash from changing aircraft lift. We estimate, using the phugoid data, and from variations in aircraft attitude and airspeed in flux-measurement configuration, that the uncertainty in w caused by variable upwash is approximately ± 0.05 m s⁻¹.     

The distribution of platinum group elements (PGE) and other chalcophile elements among sulfides from the Creighton Ni–Cu–PGE sulfide deposit, Sudbury, Canada, and the origin of palladium in pentlandite

Concentrations of platinum group elements (PGE), Ag, As, Au, Bi, Cd, Co, Mo, Pb, Re, Sb, Se, Sn, Te, and Zn, have been determined in base metal sulfide (BMS) minerals from the western branch (402 Trough orebodies) of the Creighton Ni–Cu–PGE sulfide deposit, Sudbury, Canada. The sulfide assemblage is dominated by pyrrhotite, with minor pentlandite, chalcopyrite, and pyrite, and they represent monosulfide solid solution (MSS) cumulates. The aim of this study was to establish the distribution of the PGE among the BMS and platinum group minerals (PGM) in order to understand better the petrogenesis of the deposit. Mass balance calculations show that the BMS host all of the Co and Se, a significant proportion (40–90%) of Os, Pd, Ru, Cd, Sn, and Zn, but very little (<35%) of the Ag, Au, Bi, Ir, Mo, Pb, Pt, Rh, Re, Sb, and Te. Osmium and Ru are concentrated in equal proportions in pyrrhotite, pentlandite, and pyrite. Cobalt and Pd (∼1 ppm) are concentrated in pentlandite. Silver, Cd, Sn, Zn, and in rare cases Au and Te, are concentrated in chalcopyrite. Selenium is present in equal proportions in all three BMS. Iridium, Rh, and Pt are present in euhedrally zoned PGE sulfarsenides, which comprise irarsite (IrAsS), hollingworthite (RhAsS), PGE-Ni-rich cobaltite (CoAsS), and subordinate sperrylite (PtAs₂), all of which are hosted predominantly in pyrrhotite and pentlandite. Silver, Au, Bi, Mo, Pb, Re, Sb, and Te are found predominantly in discrete accessory minerals such as electrum (Au–Ag alloy), hessite (Ag₂Te), michenerite (PdBiTe), and rhenium sulfides. The enrichment of Os, Ru, Ni, and Co in pyrrhotite, pentlandite, and pyrite and Ag, Au, Cd, Sn, Te, and Zn in chalcopyrite can be explained by fractional crystallization of MSS from a sulfide liquid followed by exsolution of the sulfides. The early crystallization of the PGE sulfarsenides from the sulfide melt depleted the MSS in Ir and Rh. The bulk of Pd in pentlandite cannot be explained by sulfide fractionation alone because Pd should have partitioned into the residual Cu-rich liquid and be in chalcopyrite or in PGM around chalcopyrite. The variation of Pd among different pentlandite textures provides evidence that Pd diffuses into pentlandite during its exsolution from MSS. The source of Pd was from the small quantity of Pd that partitioned originally into the MSS and a larger quantity of Pd in the nearby Cu-rich portion (intermediate solid solution and/or Pd-bearing PGM). The source of Pd became depleted during the diffusion process, thus later-forming pentlandite (rims of coarse-granular, veinlets, and exsolution flames) contains less Pd than early-forming pentlandite (cores of coarse-granular).     

The Kabanga Ni sulfide deposits, Tanzania: II. Chalcophile and siderophile element geochemistry

The Kabanga deposit constitutes one of the most significant Ni sulfide discoveries of the last two decades (indicated mineral resource 23 Mt of ore at 2.64% Ni, inferred resource 28.5 Mt at 2.7% Ni, November 2008). The sulfides are hosted by predominantly harzburgitic and orthopyroxenitic intrusions that crystallized from magnesian basaltic and picritic magmas. However, compared with other sulfide ores that segregated from such magmas (e.g., Jinchuan, Pechenga, Raglan), most Kabanga sulfides have low Ni (<1–3%), Cu (∼0.1–0.4%), and PGE contents (≪1 ppm), high Ni/Cu (5–15), and low Ni/Co (10–15) and Pd/Ir (2–20). Sulfides with higher metal contents (up to ∼5% Ni, 0.8% Cu, 10 ppm PGE) are found in only one unit from Kabanga North. The observed metal contents are consistent with segregation of magmatic sulfides from fertile to strongly metal-depleted magmas, at intermediate to very low mass ratios of silicate to sulfide liquid (R factors) of approximately 10–400. Sulfide saturation was triggered prior to final emplacement, by assimilation of up to 50% of the total sulfur in the intrusions from sulfide-bearing metasedimentary country rocks. Immiscible sulfide liquid was entrained by the magma and ultimately precipitated in dynamic magma conduits that formed tubular and sill-like mafic–ultramafic bodies characterized by abundant magmatic breccias, highly irregular layering, and frequent compositional reversals. The unusually large degree of crustal contamination and the low R factors render Kabanga an end-member in the spectrum of magmatic Ni sulfide ores.     

Titan's surface: Search for spectral diversity and composition using the Cassini VIMS investigation

The surface composition of Titan is of great importance for understanding both the internal evolution of Titan and its atmosphere. The Visual and Infrared Mapping Spectrometer (VIMS) investigation on Cassini is observing Titan from 0.35 to 5.11 μm with spatial resolution down to a few kilometers during each flyby of the spacecraft as it orbits Saturn. Our search for spectral diversity using seven methane transmission windows in the near infrared suggests that spectrally distinct units exist on the surface of Titan and that most of the surface can be modeled using only a few distinct spectral units: water frost, CO₂ frost, atmospheric scattering, and an unknown material bright at 2 μm. A dark, spectrally neutral material is also implied. Use of an atmospheric scattering component with spectral mixing analysis may provide a method for partially removing atmospheric effects. In some locations, atmospheric scattering accounts for the majority of the signal. There are also small regions with unusual spectra that may be due to low signal and high noise and/or may be exotic materials of interest. Further, we searched within the methane windows for spectral features associated with Titan's surface. Only the 5-μm and, to a lesser extent, the 2-μm window provide a reasonable opportunity for this, as the shorter-wavelength windows are too narrow and the 2.8-μm window is cluttered with an unknown atmospheric constituent. We find evidence for only one spectral feature: near 4.92 μm for the 5-μm bright Tui Regio region. CO₂ frost with grains smaller than about 10 μm is the best candidate we have found so far to explain this absorption as well as the feature's spectral contrast between the 2.7- and the 2.8-μm atmosphere subwindows. This suggested CO₂ identification is supported by the presence of an endmember in the spectral mixture analysis that is consistent with CO₂ frost with large grain sizes. We find no other absorption features that are statistically significant, including those reported earlier by others. These results are consistent with but greatly extend our early analysis that treated only the Ta data set [McCord, T.B., et al., 2006a. Planet. Space Sci. 54, 1524-1539]. In the spectral feature search process, we explored in detail the noise characteristics of the VIMS data within the 5-μm window, which has generally very low signal (4-20 DN), due to the measurement conditions and low illumination levels. We find noise of nearly Gaussian statistics except for some erratic darks and noise spikes, and the data set seems generally well behaved. We present examples of our attempt to improve on the standard VIMS pipeline data calibration.     

Projected risks to groundwater‐dependent terrestrial vegetation caused by changing climate and groundwater abstraction in the Central Perth Basin,Western Australia

The effect of potential climate change on groundwater‐dependent vegetation largely depends on the nature of the climate change (drying or wetting) and the level of current ecosystem dependence on groundwater resources. In south‐western Australia, climate projections suggest a high likelihood of a warmer and drier climate. The paper examines the potential environmental impacts by 2030 at the regional scale on groundwater‐dependent terrestrial vegetation (GDTV) adapted to various watertable depths, on the basis of the combined consideration of groundwater modelling results and the framework for GDTV risk assessment. The methodology was tested for the historical period from 1984 to 2007, allowing validation of the groundwater model results' applicability to such an assessment. Climate change effects on GDTV were evaluated using nine global climate models under three greenhouse gas emission scenarios by applying the climate projections to groundwater models. It was estimated that under dry climate scenarios, GDTV is likely to be under high and severe risk over more than 20% of its current habitat area. The risk is also likely to be higher under an increase in groundwater abstraction above current volumes. The significance of climate change risk varied across the region, depending on both the intensity of the change in water regime and the sensitivity of the GDTV to such change. Greater effects were projected for terrestrial vegetation dependent on deeper groundwater (6–10 m). Copyright © 2013 John Wiley & Sons, Ltd.     

Developing carbon budgets for UK agriculture, land-use, land-use change and forestry out to 2022

This paper derives a notional future carbon budget for UK agriculture, land use, land use change and forestry sectors (ALULUCF). The budget is based on a bottom-up marginal abatement cost curve (MACC) derived for a range of mitigation measures for specified adoption scenarios for the years 2012, 2017 and 2022. The results indicate that in 2022 around 6.36 MtCO₂e could be abated at negative or zero cost. Furthermore, in the same year, over 17% of agricultural GHG emissions (7.85 MtCO₂e) could be abated at a cost of less than the 2022 Shadow Price of Carbon (£34 (tCO₂e)^???1). The development of robust MACCs faces a range of methodological hurdles that complicate cost-effectiveness appraisal in ALULUCF relative to other sectors. Nevertheless, the current analysis provides an initial route map of efficient measures for mitigation in UK agriculture.     

Recommendations on methods for the detection and control of biological pollution in marine coastal waters

Adverse effects of invasive alien species (IAS), or biological pollution, is an increasing problem in marine coastal waters, which remains high on the environmental management agenda. All maritime countries need to assess the size of this problem and consider effective mechanisms to prevent introductions, and if necessary and where possible to monitor, contain, control or eradicate the introduced impacting organisms. Despite this, and in contrast to more enclosed water bodies, the openness of marine systems indicates that once species are in an area then eradication is usually impossible. Most institutions in countries are aware of the problem and have sufficient governance in place for management. However, there is still a general lack of commitment and concerted action plans are needed to address this problem. This paper provides recommendations resulting from an international workshop based upon a large amount of experience relating to the assessment and control of biopollution.     

Redox evolution in glacial drift aquifers: role of diamicton units in reduction of Fe(III)

High iron concentrations create water quality problems for municipal use in glacial drift aquifer units. The chemical evolution of oxic groundwater in shallow aquifer units to anoxic groundwater in deeper aquifer units, in which soluble Fe(II) is stable, is attributed to coupled reduction of Fe(III) on aquifer solids with oxidation of organic carbon. The objective of this study was to characterize the distribution of organic carbon in aquifer and aquitard sediments to determine the availability of potential electron donors to drive these reactions. To do this, four complete rotasonic cores in a glacial aquifer/aquitard system were sampled at close intervals for analyses of grain-size distribution and organic carbon content. The results indicate significantly higher organic carbon concentrations in diamicton (till) units that function as aquitards, relative to coarse-grained aquifer units. In addition, readily reducible iron content in the diamicton units and lower aquifer unit materials is sufficient to produce far more dissolved iron than is present in the aquifer. Groundwater evolves to the level of iron reduction as a terminal electron-accepting process as it moves downward through aquitard units along flow paths from upland recharge areas to downgradient discharge areas. Deeper aquifer units are therefore unlikely to contain groundwater with low iron concentration.