Theoretical models for interpreting the dielectric behaviour of HF-doped ice

first_imgTo understand the recent dielectric measurements made on HF-doped ice single crystals requires a full knowledge of the concentration of electrical defects present in ice and their subsequent interactions. Previous interpretations of the behaviour of HF-doped ice have concentrated upon specific features in isolation, whereas this paper presents analyses of a data set of 139 temperature and impurity combinations from 17 HF-doped ice single crystals. The interpretation of the behaviour of these crystals is in terms of several possible theoretical models. All models are based upon the common assumptions that HF molecules enter the ice lattice substitutionally and that excess Bjerrum and ionic defects can be formed at the HF sites. They also use the theory of electrical conduction in ice by Jaccard (1959) and the defect equilibria analysis in ice by Kroger (1974). All models yield values for the concentration, mobility, energy of formation and charges for the different types of electrical defect considered to be generated. From the model which assumes that only three fluorine centres exist, the approximate derived values of the mobility and charge for the L-defect and positive ionic defect are as follows: μ L = 5 × 10-8 m2 V-1 s-1 at 273 K, eDL = 0.44e; μ + = 2.7 × 10-8 m2 V-1 s-1 at 273 K, e ± 0.73 e . Finally, using the derived defect conductivities and the Jaccard theory of electrical conduction, the relaxation tune of HF-doped ice has been successfully predicted over a wide range of temperature and fluoride concentration.last_img read more


The formation of vertical edges on Antarctic moss peat banks

first_imgMoss peat banks, which can be up to 3 m deep, greater than 2500 m2 in area, and up to 5000 yr old, occur in the Antarctic. Radiocarbon dates show that the peat accumulation rate has varied from 0.25 to 2.00 mm yr-1. These peat banks have an abrupt vertical edge of exposed peat, which does not appear to be due to erosion of a once-larger peat bank. These edges seem to have arisen because, as peat accumulates and the moss banks become deeper, increased snow-lie prevents outward growth of the banks at their edges.last_img


Oceanic environment of George VI Ice Shelf, Antarctic Peninsula

first_imgA collation of available data shows that sea-water with a temperature 3°C above the in-situ freezing point lies beneath George VI Ice Shelf in the Antarctic Peninsula, and is widespread on the Amundsen-Bellingshausen continental shelf. The presence of warm water is a factor in the recent and continuing disintegration of ice shelves in the region, yet the meteorology and oceanography of the sector are little known. We discuss a plausible link between the present climatic conditions, sea-water characteristics and the warm-water intrusion on to the continental shelf, thereby illustrating an indirect climatic influence on the mass balance of ice shelves.last_img


Ice sheet retreat from the Antarctic Peninsula shelf

first_imgSide-scan sonar and sub-bottom acoustic profiler data and sediment cores reveal the processes that controlled sediment transport and deposition on the continental shelf of the Antarctic Peninsula Pacific margin off Anvers Island, during deglaciation over the last 11,000 years or more. Glacial flutes and striations mark the flow of low-profile ice streams draining the interior, across the middle and outer shelf. Most probably, ice sheets were grounded to the continental shelf edge along this margin during the last glacial maximum. Iceberg furrows overwrite the ice sheet record in areas between 500 and 350 m water depth, and reflect calving from a retreating ice shelf front. Cores show open marine sedimentation replacing diamicton deposition close to the grounding line during this retreat, which rapidly cleared the outer and middle shelf shortly before 11,000 years BP (from AMS14C dates on organic carbon). The shallower, scoured and largely sediment-free inner shelf cleared later, probably before 6000 years BP. Open marine sediments on the middle and outer shelf include a pelagic biogenic component and suspended sediment from modern glacier tongues, supplemented by resuspension of older sediment in shallow shelf regions (by currents and by grounded icebergs). Sedimentation is too slow to be able to fill in the concave-up profile of the continental shelf during a full interglacial, confirming the intense glacial-interglacial cyclicity of sedimentation on the continental slope inferred from seismic reflection profiles. The observed rapid deglaciation of the middle and outer shelf supports published numerical model results that the Antarctic Peninsula’s narrow interior and broad continental shelf make the ice sheet sensitive to imposed eustatic sea-level change. A low-profile marine-based ice sheet over the continental shelf during glacial maximum would have made a major contribution to that sensitivity, in the early stages of deglaciation. It follows that the Antarctic Peninsula ice sheet, and probably most others, are not so sensitive today.last_img read more


Energetic electron precipitation during high-speed solar wind stream driven storms

first_imgElectron precipitation from the Earth’s inner magnetosphere transmits solar variability to the Earth’s upper atmosphere and may affect surface level climate. Here we conduct a superposed epoch analysis of energetic electrons observed by the NOAA POES spacecraft during 42 high-speed solar wind stream (HSS) driven geomagnetic storms to determine the temporal evolution and global distribution of the precipitating flux. The flux of trapped and precipitating E > 30 keV electrons increases immediately following storm onset and remains elevated during the passage of the HSS. In contrast, the trapped and precipitating relativistic electrons (E > 1 MeV) drop out following storm onset and subsequently increase during the recovery phase to levels which eventually exceed the prestorm levels. There is no evidence for enhanced precipitation of relativistic electrons during the MeV flux drop out, suggesting that flux drop outs during the main phase of HSS-driven storms are not due to precipitation to the atmosphere. On average, the flux of precipitating E > 30 keV electrons is enhanced by a factor of similar to 10 during the passage of the high-speed stream at all geographic longitudes. In contrast, the precipitating relativistic electron count rate is observed to peak in the region poleward of the South Atlantic Anomaly. During the passage of the high-speed stream, the flux of precipitating E > 30 keV electrons peaks in the region from 2100 to 1200 magnetic local time at low L (4 < L < 7) and in the prenoon sector at high L (7 < L 30 keV electrons in both regions.last_img read more


Late Holocene changes in precipitation in northwest Tasmania and their potential links to shifts in the Southern Hemisphere westerly winds

first_imgAccurate projections of future climate changes in regions susceptible to drought depend on a good understanding of past climate changes and the processes driving them. In the absence of longer term instrumental data, paleoclimate data are needed. In this study we develop a precipitation reconstruction for Rebecca Lagoon (41°11′S, 144°41′E), northwest Tasmania. First, the relationship between scanning reflectance spectroscopy measurements of sediment cores in the visible spectrum (380–730 nm) and instrumental precipitation record (1912–2009) was used to develop a model to reconstruct precipitation back in time. Results showed that the ratio of reflectance between 660 and 670 nm (i.e., reflectance at 660 nm/reflectance at 670 nm; a measure of pigment diagenesis) was significantly related to annual precipitation. A calibration model was developed (R = − 0.56, pauto < 0.001, RMSEP = 43.0 mm yr− 1, 5 year triangular filtered data, calibration period 1912–2009). Second, this calibration-in-time model was used to reconstruct late Holocene precipitation changes over the last ~ 3000 years. This showed relatively dry conditions from ca. 3100–2800 cal yr BP, wet conditions from ca. 2800–2400 cal yr BP, dry conditions from ca. 2400–2000 calyr BP, and variable conditions after this. Relatively wet conditions occurred from ca. 500 cal yr BP to the late AD 1800 s (ca. 50 cal. yr BP). The precipitation reconstruction indicates that conditions were relatively dry for the 20th century compared to the last ~ 3000 years. In particular, the dry period measured in recent decades is one of the most intense in at least the last 500 years. As precipitation in this region is primarily driven by the Southern Hemisphere westerly winds, these changes are discussed in terms of shifts in westerly wind strength and/or position.last_img read more


Southern Ocean mesopelagic fish communities in the Scotia Sea are sustained by mass immigration

first_imgThe biomass of mesopelagic fish in the Southern Ocean is one of the largest of any ocean region and is dominated (both in terms of diversity and biomass) by myctophids (lantern-fish). Despite their high ecological importance in this region, and globally, our understanding of the life-cycles and distribution of myctophids remains limited. We examined length-frequency data from trawl-nets collected across a major sector of the Southern Ocean (the Scotia-Weddell sector) in different seasons to determine patterns of recruitment and growth. There was an absence of larval myctophids, of any species, in net-catches, while larger, older individuals became increasingly dominant with increasing latitude. Very few specimens were found to contain mature gonads, indicating that individuals do not reach reproductive condition in this region. Most myctophid species that occurred within the survey regions neither recruited locally nor were self-supporting. Myctophids are prey for a large number of higher predators (penguins, seals and cetaceans) in the Scotia Sea and are a major predator of zooplankton and krill. We show that this vital part of the Southern Ocean food-web is dependent on mass immigration from lower latitudes in the region. By implication, the sensitivities of this system depend not only on local conditions but also on levels of connectivity to other oceanic regions.last_img read more


Conflicting science requirements impact on rare moss conservation measures

first_imgThe Antarctic Treaty recognizes the outstanding scientific values of the Antarctic environment through the designation of Antarctic Specially Protected Areas (ASPAs) that have rigorous management plans specific to each site. Deception Island has the largest concentration of rare bryophyte species and communities in Antarctica, while also offering substantial opportunities for research in a range of scientific disciplines due to its volcanic nature. As a result, conflicts between research interests and conservation goals may arise. On the summit ridge of Caliente Hill severe trampling damage to the moss assemblages growing in association with localized geothermal activity was observed. The range of species affected included the entire known population of Schistidium deceptionense, an endemic moss known only from this site, as well as other very rare Antarctic mosses (Ditrichum ditrichoideum, Bryum orbiculatifolium, Bucklandiella subcrispipila, Pohlia wahlenbergii and Dicranella hookeri). A photomapping study was undertaken to characterize further the status of the site and monitor changes within it. Increased awareness, co-ordination of activities and a spatial zoning within the site could help mitigate damage from permitted activities. Nevertheless, prioritization of longer term conservation goals over short-term research interests may ultimately be necessary where local human impact cannot be managed by other means.last_img read more


Comparative population genomics reveals key barriers to dispersal in Southern Ocean penguins

first_imgThe mechanisms that determine patterns of species dispersal are important factors in the production and maintenance of biodiversity. Understanding these mechanisms helps to forecast the responses of species to environmental change. Here we used a comparative framework and genome‐wide data obtained through RAD‐seq to compare the patterns of connectivity among breeding colonies for five penguin species with shared ancestry, overlapping distributions, and differing ecological niches, allowing an examination of the intrinsic and extrinsic barriers governing dispersal patterns. Our findings show that at‐sea range and oceanography underlie patterns of dispersal in these penguins. The pelagic niche of emperor (Aptenodytes forsteri), king (A. patagonicus), Adélie (Pygoscelis adeliae) and chinstrap (P. antarctica) penguins facilitates gene flow over thousands of kilometres. In contrast, the coastal niche of gentoo penguins (P. papua) limits dispersal, resulting in population divergences. Oceanographic fronts also act as dispersal barriers to some extent. We recommend that forecasts of extinction risk incorporate dispersal and that management units are defined by at‐sea range and oceanography in species lacking genetic data.last_img read more


Thicker shells compensate extensive dissolution in brachiopods under future ocean acidification

first_imgOrganisms with long generation times require phenotypic plasticity to survive in changing environments until genetic adaptation can be achieved. Marine calcifiers are particularly vulnerable to ocean acidification due to dissolution and a reduction in shell-building carbonate ions. Long-term experiments assess organisms’ abilities to acclimatise or even adapt to environmental change. Here we present an unexpected compensatory response to extensive shell dissolution in a highly calcium-carbonate-dependent organism after long-term culture in predicted end-century acidification and warming conditions. Substantial shell dissolution with decreasing pH posed a threat to both a polar (Liothyrella uva) and a temperate (Calloria inconspicua) brachiopod after 7 months and 3 months exposure, respectively, with more extensive dissolution in the polar species. This impact was reflected in decreased outer primary layer thickness in the polar brachiopod. A compensatory response of increasing inner secondary layer thickness, and thereby producing a thicker shell was exhibited by the polar species. Less extensive dissolution in the temperate brachiopod did not affect shell thickness. Increased temperature did not impact shell dissolution or thickness. Brachiopod ability to produce a thicker shell when extensive shell dissolution occurs suggests this marine calcifier has great plasticity in calcification providing insights into how similar species might cope under future environmental change.last_img read more