Glaciovolcanic evidence for a polythermal Neogene East Antarctic Ice Sheet

A paradigm has existed for more than 30 years that the basal thermal regime of the East Antarctic Ice Sheet in Victoria Land made a fundamental transition from wet-based to cold-based either at ca. 14 Ma or after ca. 2.5 Ma. The basal thermal regime is important because it determines the potential for unstable behavior in an ice sheet. We have studied the environmental characteristics of subglacially erupted volcanic centers scattered along 800 km of the Ross Sea fl ank of the Transantarctic Mountains. The volcanoes preserve evidence for the coeval paleo-ice thicknesses and contain features diagnostic of both wet-based and cold-based ice conditions. By dating the sequences we are able to demonstrate that the basal thermal regime varied spatially and with time between ca. 12 Ma and present. It was polythermal overall and probably comprised a coarse temperature patchwork of frozen-bed and thawed-bed ice, similar to the East Antarctic Ice Sheet today. Thus, an important shift is required in the prevailing paradigm describing its temporal evolution

Role of microstructure and surface defects on the dissolution kinetics of CeO2, a UO2 fuel analogue.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesised CeO2 analogue for UO2 fuel. Dissolution was performed on: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects; and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90°C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce3+ influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long-term performance of spent fuel in geological disposal

Ice-confined construction of a large basaltic volcano—Austurfjöll massif, Askja, Iceland

Austurfjöll is the largest basaltic glaciovolcanic massif at Askja volcano (Central Iceland), and through detailed studies of its volcanological and geochemical characteristics, we provide a detailed account of the sequence and structure of the ice-confined construction of a large Icelandic basaltic volcano. In particular, Austurfjöll represents a geometry of vents, and resulting glaciovolcanic morphology, not previously documented in ice-confined basaltic volcanoes. Austurfjöll was constructed during two major phases of basaltic volcanism, via seven eruptive episodes through disperse fissure-dominated eruptions. The earliest episode involved a rare and poorly exposed example of subaerial activity, and this was succeeded by six episodes involving the eruption of ice-confined pillow lavas and numerous overlapping fissure eruptions of phreatomagmatic tephra. Evidence of local subaerial lavas and tephras indicates the local growth of eruptive centers above englacial lake levels, and subsequent flooding, but no prolonged subaerial activity. Localized ice-contact facies, paleowater levels, and diamictons indicate the position and thickness of the ice was variable during the construction of Austurfjöll, and eruptive activity likely occurred in multiple and variable level meltwater lakes during the last glacial period. Lithofacies evidence including gradational transitions from effusive to explosive deposits, superposition of fragmental facies above coherent facies, and drainage channels suggest that changes in eruptive style were driven largely by external factors such as drainage and the increasing elevation of the massif. This study emphasizes the unique character of Austurfjöll, being composed of large pillow lava sheets, numerous (> 40) overlapping glaciovolcanic tindars, and only localized emergent deposits, as a product of its prolonged ice-confined eruptive history, contrasts with previous descriptions of tuyas and tindars

Complex circular subsidence structures in tephra deposited on large blocks of ice: Varða tuff cone, Öræfajökull, Iceland

Several broadly circular structures up to 16 m in diameter, into which higher strata have sagged and locally collapsed, are present in a tephra outcrop on southwest Öræfajökull, southern Iceland. The tephra was sourced in a nearby basaltic tuff cone at Varða. The structures have not previously been described in tuff cones, and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jökulhlaup that may have been triggered by a subglacial eruption within the Öræfajökull ice cap. They are named ice-melt subsidence structures, and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice-clad volcanoes. The internal structure is better exposed in the Varða examples because of an absence of fluvial infilling and reworking, and erosion of the outcrop to reveal the deeper geometry. The ice-melt subsidence structures at Varða are a proxy for buried ice. They are the only known evidence for a subglacial eruption and associated jökulhlaup that created the ice blocks. The recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice-proximal settings during palaeoenvironmental investigations

Fibromuscular Arterial Disease

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75604/1/j.1440-1673.1970.tb01633.x.pd

Using dissolved H₂O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur(Torfajökull, Iceland)

The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H2O and CO2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to Bláhnúkur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ~0.7 wt.% at the base to ~0.3 wt.% at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption. As Bláhnúkur rises 350 m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1 wt.%) or water-poor (~0.2 wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from Bláhnúkur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on Bláhnúkur’s eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed

Confab - Systematic generation of diverse low-energy conformers

<p>Abstract</p> <p>Background</p> <p>Many computational chemistry analyses require the generation of conformers, either on-the-fly, or in advance. We present Confab, an open source command-line application for the systematic generation of low-energy conformers according to a diversity criterion.</p> <p>Results</p> <p>Confab generates conformations using the 'torsion driving approach' which involves iterating systematically through a set of allowed torsion angles for each rotatable bond. Energy is assessed using the MMFF94 forcefield. Diversity is measured using the heavy-atom root-mean-square deviation (RMSD) relative to conformers already stored. We investigated the recovery of crystal structures for a dataset of 1000 ligands from the Protein Data Bank with fewer than 1 million conformations. Confab can recover 97% of the molecules to within 1.5 Å at a diversity level of 1.5 Å and an energy cutoff of 50 kcal/mol.</p> <p>Conclusions</p> <p>Confab is available from <url>http://confab.googlecode.com</url>.</p

Antistaphylococcal activity of DNA-interactive pyrrolobenzodiazepine (PBD) dimers and PBD-biaryl conjugates

Objectives: pyrrolobenzodiazepine (PBD) dimers, tethered through inert propyldioxy or pentyldioxy linkers, possess potent bactericidal activity against a range of Gram-positive bacteria by virtue of their capacity to cross-link duplex DNA in sequence-selective fashion. Here we attempt to improve the antibacterial activity and cytotoxicity profile of PBD-containing conjugates by extension of dimer linkers and replacement of one PBD unit with phenyl-substituted or benzo-fused heterocycles that facilitate non-covalent interactions with duplex DNA.Methods: DNase I footprinting was used to identify high-affinity DNA binding sites. A staphylococcal gene microarray was used to assess epidemic methicillin-resistant Staphylococcus aureus 16 phenotypes induced by PBD conjugates. Molecular dynamics simulations were employed to investigate the accommodation of compounds within the DNA helix.Results: increasing the length of the linker in PBD dimers led to a progressive reduction in antibacterial activity, but not in their cytotoxic capacity. Complex patterns of DNA binding were noted for extended PBD dimers. Modelling of DNA strand cross-linking by PBD dimers indicated distortion of the helix. A majority (26 of 43) of PBD-biaryl conjugates possessed potent antibacterial activity with little or no helical distortion and a more favourable cytotoxicity profile. Bactericidal activity of PBD-biaryl conjugates was determined by inability to excise covalently bound drug molecules from bacterial duplex DNA.Conclusions: PBD-biaryl conjugates have a superior antibacterial profile compared with PBD dimers such as ELB-21. We have identified six PBD-biaryl conjugates as potential drug development candidate