Single-crystal X-ray diffraction study of synthetic sodium–hydronium jarosite

Na–H₃O jarosite was synthesized hydrothermally at 413 K for 8 days and investigated using single-crystal X-ray diffraction (XRD) and electron microprobe analysis (EMPA). The chemical composition of the studied crystal is [Na_0.57(3) (H₃O)_0.36 (H₂O)_0.07]^A Fe_2.93(3) (SO₄)₂ (OH)_5.70 (H₂O)_0.30, and Fe deficiency was confirmed by both EMPA and XRD analysis. The single-crystal XRD data were collected at 298 and 102 K, and crystal structures were refined in space group \( R\overline{3}m\). The room-temperature data match structural trends of the jarosite group, which vary linearly with the c axis. The low-temperature structure at 102 K shows an anisotropic decrease in the unit cell parameters, with c and a decreasing by 0.45 and 0.03 %, respectively. Structural changes are mainly confined to the A site environment. Only minor changes occur in FeO₆ and SO₄ polyhedra. The structure responds upon cooling by increasing bond length distortion and by decreasing quadratic elongation of the large AO₁₂ polyhedra. The structural parameters at low temperature follow very similar patterns to structural changes that correspond to compositional variation in the jarosite group, which is characterised by the flexibility of AO₁₂ polyhedra and rigidity of Fe(OH)₄O₂–SO₄ layers. The most flexible areas in the jarosite structure are localized at AO₁₂ edges that are not shared with neighbouring FeO₆ octahedra. Importantly, for the application of XRD in planetary settings, the temperature-related changes in jarosite can mimic compositional change.     

The cost of achieving South Africa’s ‘fair share’ of global climate change mitigation

Global climate change mitigation action is hampered by systematic under-assessment of national ‘fair shares’, largely on the basis of perceived national interests. This paper aims to inform discussions centred on South Africa’s nationally determined contribution (NDC) by estimating (1) emissions reduction pathways for the country using the Climate Equity Reference Calculator (CERC) assuming a maximum 2°C aggregate warming target and (2) the likely economy-wide net mitigation costs or savings associated with reaching these pathways if known lower-cost mitigation measures, identified through the national mitigation potential analysis, are prioritised. The cumulative net savings associated with achieving the CERC ‘fair share’ emissions pathway, assuming the moderate use of low carbon power generation measures, would reach $5.3 billion by 2030. Net savings could be substantially greater reaching $46.8 billion by 2030 assuming power generation focuses on moving towards full decarbonisation. An unconditional commitment to the mitigation action implied by the ‘fair share’ emissions pathway therefore seems reasonable and prudent purely from the point of view of net country-wide savings. Only if power generation moves towards full decarbonisation would there be a reasonable chance of achieving the more ambitious CERC domestic emissions pathway. However, the significant additional cost associated with achieving the domestic emissions pathway should be conditional on international assistance.

Key policy insights

• South Africa can only achieve its ‘fair share’ of the global mitigation effort if greater use is made of renewable energy options, and can realise significant net savings if it does so.

• Further emissions reductions would incur costs and require significant upscaling of the share of renewable energy and full implementation of all non-power generation mitigation measures available.

• Committing to this further mitigation action contingent on international finance would both strengthen the nation’s position in climate negotiations and support the provision of finance for those vulnerable developing nations that bear little or no responsibility for climate change.

     

Taxonomic distinctness in the diet of two sympatric marine turtle species

Marine turtles are considered keystone consumers in tropical coastal ecosystems and their decline through overexploitation has been implicated in the deterioration of reefs and seagrass pastures in the Caribbean. In the present study, we analysed stomach contents of green (Chelonia mydas) and hawksbill turtles (Eretmochelys imbricata) harvested in the legal turtle fishery of the Turks and Caicos Islands (Caribbean) during 2008–2010. Small juveniles to adult‐sized turtles were sampled. Together with data from habitat surveys, we assessed diet composition and the taxonomic distinctness (and other species diversity measures) in the diets of these sympatric marine turtle species. The diet of green turtles (n = 92) consisted of a total of 47 taxa: including three species of seagrass (present in 99% of individuals), 29 species of algae and eight sponge species. Hawksbill turtles (n = 45) consumed 73 taxa and were largely spongivorous (16 species; sponges present in 100% of individuals) but also foraged on 50 species of algae (present in 73% of individuals) and three species of seagrass. Plastics were found in trace amounts in 4% of green turtle and 9% of hawksbill turtle stomach samples. We expected to find changes in diet that might reflect ontogenetic shifts from small (oceanic‐pelagic) turtles to larger (coastal‐benthic) turtles. Dietary composition (abundance and biomass), however, did not change significantly with turtle size, although average taxonomic distinctness was lower in larger green turtles. There was little overlap in prey between the two turtle species, suggesting niche separation. Taxonomic distinctness routines indicated that green turtles had the most selective diet, whereas hawksbill turtles were less selective than expected when compared with the relative frequency and biomass of diet items. We discuss these findings in relation to the likely important trophic roles that these sympatric turtle species play in reef and seagrass habitats.     

Evaluation of groundwater residence time in a high mountain aquifer system (Sacramento Mountains,USA): insights gained from use of multiple environmental tracers

The New Mexico Bureau of Geology and Mineral Resources (USA) has conducted a regional investigation of groundwater residence time within the southern Sacramento Mountains aquifer system using multiple environmental tracers. Results of the tracer surveys indicate that groundwater in the southern Sacramento Mountains ranges in age from less than 1 year to greater than 50 years, although the calculated ages contain uncertainties and vary significantly depending on which tracer is used. A distinctive feature of the results is discordance among the methods used to date groundwater in the study area. This apparent ambiguity results from the effects of a thick unsaturated zone, which produces non-conservative behavior among the dissolved gas tracers, and the heterogeneous character and semi-karstic nature of the aquifer system, which may yield water from matrix porosity, fractures, solution-enlarged conduits, or a combination of the three. The data also indicate mixing of groundwater from two or more sources, including recent recharge originating from precipitation at high elevations, old groundwater stored in the matrix, and pre-modern groundwater upwelling along fault zones. The tracer data have also been influenced by surface-water/groundwater exchange via losing streams and lower elevation springs (groundwater recycling). This study highlights the importance of using multiple tracers when conducting large-scale investigations of a heterogeneous aquifer system, and sheds light on characteristics of groundwater flow systems that can produce discrepancies in calculations of groundwater age.     

A method to map riparian exotic vegetation (Salix spp.) area to inform water resource management

Hydrological processes within riparian environments worldwide are impacted when introduced species invade. Monitoring and management at substantial expense, are subsequently required to combat deleterious effects on the environment and stream hydrology. Willow species (Salicaceae: Salix spp.) introduced into Australia have spread throughout many riparian systems causing adverse environmental impacts, with high rates of water extraction when located within stream beds (in‐stream willows) thus altering hydrology. Strategies exist to manage willows; however, simpler, more cost‐effective methods are required to map and monitor spatial and temporal distributions. A method is presented to discriminate willow stands from surrounding native riparian vegetation using a single, very high 2 m resolution multispectral WorldView‐2 satellite image. A combination of spectral bands ‘coastal blue’ (400–450 nm), ‘red’ (630–690 nm), ‘red edge’ (705–745 nm) and ‘near infrared2’ (860–1040 nm), minimum noise fraction transformation, median filtering and maximum likelihood supervised classification provided the highest discriminatory power within a 25 km² study area. Of the spectral bands, coastal blue, red edge and near infrared2 are new bands that are not available in other multispectral sensors. These bands proved critical to the success of discriminating willows from other land cover categories (overall accuracy of 97%). Stream channels were defined by incorporating a LiDAR‐derived digital elevation model to discriminate between willows on stream banks and within stream beds. Canopy area estimates of in‐stream willows, coupled with water savings estimates from willow removal, suggest removal of 10.4 ha of Salix fragilis canopy from within river channels in the study area will potentially return 41 ML year^?1 to the environment. The method presented improves our understanding of willow impacts on hydrology and aids decisions regarding willow removal for water resource management. Copyright © 2013 John Wiley & Sons, Ltd.     

Mid‐late Holocene sea‐level variability in eastern Australia

A re‐analysis of sea‐level data from eastern Australia based on 115 calibrated C‐14 ages is used to constrain the origin, timing and magnitude of sea‐level change over the last 7000 years. We demonstrate that the Holocene sea‐level highstand of +1.0–1.5 m was reached ～7000 cal yr bp and fell to its present position after 2000 yr bp . These findings are in contrast to most previous studies that relied on smaller datasets and did not include the now common conversion of conventional C‐14 ages to calendar years. During this ～5000 year period of high sea level, growth hiatuses in oyster beds and tubeworms and lower elevations of coral microatolls are interpreted to represent short‐lived oscillations in sea‐level of up to 1 m during two intervals, beginning c. 4800 and 3000 cal yr bp . The rates of sea‐level rise and fall (1–2 mm yr^?1) during these centennial‐scale oscillations are comparable with current rates of sea‐level rise. The origin of the oscillations is enigmatic but most likely the result of oceanographic and climatic changes, including wind strengths, ice ablation, and melt‐water contributions of both Greenland and Antarctic ice sheets.     

Ediacaran terrane accretion within the Arabian–Nubian Shield

The Ad Dawadimi Terrane is an Ediacaran basin of the Arabian Nubian Shield (ANS), Saudi Arabia. This basin terrane is situated in the far eastern part of the ANS and represents the youngest accretion event of the exposed ANS. Therefore, the timing of events within the basin is key to understanding both the closure of the Mozambique Ocean and the amalgamation of Gondwana along the northern East African Orogen. Here we present U/Pb detrital zircon data for the Abt Formation, the principle basin sediments of the Ad Dawadimi Terrane, along with ⁴⁰Ar/³⁹Ar ages on muscovite and whole rock Sm/Nd data. These data indicate that deep-water deposition in the Abt Basin did not end until after ca. 620 Ma and that deformation and greenschist-facies metamorphism of the Abt Formation occurred at 620 ± 3 (2σ) Ma along an active margin. This is the youngest terrane amalgamation event reported so far in the Arabian–Nubian Shield, but we suggest even younger sutures lie further east beneath the Phanerozoic cover of eastern Saudi Arabia. Our results suggest that the Ediacaran basins of the eastern ANS were not part of the Huqf basin in Oman, which was instead part of a passive margin of Neoproterozoic India, separated from the active margin of Africa by the Mozambique Ocean that probably did not close until the late Ediacaran or early Cambrian.     

Energy deposition and primary chemical products in Titan’s upper atmosphere

Cassini results indicate that solar photons dominate energy deposition in Titan’s upper atmosphere. These dissociate and ionize nitrogen and methane and drive the subsequent complex organic chemistry. The improved constraints on the atmospheric composition from Cassini measurements demand greater precision in the photochemical modeling. Therefore, in order to quantify the role of solar radiation in the primary chemical production, we have performed detailed calculations for the energy deposition of photons and photoelectrons in the atmosphere of Titan and we validate our results with the Cassini measurements for the electron fluxes and the EUV/FUV emissions. We use high-resolution cross sections for the neutral photodissociation of N₂, which we present here, and show that they provide a different picture of energy deposition compared to results based on low-resolution cross sections. Furthermore, we introduce a simple model for the energy degradation of photoelectrons based on the local deposition approximation and show that our results are in agreement with detailed calculations including transport, in the altitude region below 1200 km, where the effects of transport are negligible. Our calculated, daytime, electron fluxes are in good agreement with the measured fluxes by the Cassini Plasma Spectrometer (CAPS), and the same holds for the measured FUV emissions by the Ultraviolet Imaging Spectrometer (UVIS). Finally, we present the vertical production profiles of radicals and ions originating from the interaction of photons and electrons with the main components of Titan’s atmosphere, along with the column integrated production rates at different solar zenith angles. These can be used as basis for any further photochemical calculations.