A meso-mechanical approach to time-dependent deformation and fracturing of partially saturated sandstone

In the present paper, we investigate how water acts to weaken rock in two complementary ways: mechanically through the generalized effective stress principle, and chemically through time-dependent rock-fluid reactions that allow subcritical crack growth. These processes, together with capillary suction and stress corrosion, were incorporated into a three-dimensional discrete element grain-based model to investigate both the time-independent and the time-dependent mechanical behavior of partially saturated sandstone at the mesoscale. The capillary parameters related to capillary suction and subcritical parameters related to stress corrosion in the model were calibrated to match the deformation behavior of partially saturated sandstone observed in laboratory. Following this calibration, numerical simulations of partially saturated sandstone with different levels of saturation were performed in uniaxial compression. The simulations show that both the peak strength and elastic modulus of the sandstone decrease as a function of increasing saturation and that the relationships between these properties can be expressed by negative exponential functions. The simulations are in good agreement with the experimental results. Second, the long-term brittle deformation of partially saturated sandstone with different levels of saturation under a constant stress level was modeled. The results show that time-to-failure during brittle creep decreases, and the initial strain and the minimum creep strain rate increases, as a function of increasing saturation, as also observed in laboratory. The simulations also highlight the formation of tensile cracks as the main deformation mechanism during brittle creep. Finally, brittle creep in partially saturated sandstone samples with different levels of saturation was studied under different stress levels. These simulations show that the minimum creep strain rate and the time-to-failure as a function of stress can be well described by exponential relations. We conclude that the proposed model permits a deeper understanding of time-independent and time-dependent deformation and failure of partially saturated sandstone at the mesoscale

A toolbox for identifying the expression of dome-forming volcanism on exoplanets

The presence of volcanism is often anecdotally used to define a “living planet”. Since dome-building volcanism on Earth occurs primarily at plate boundaries, the identification of such domes could inform on exoplanetary development. Lava domes form when extruded magma is too viscous to flow from a vent, and their morphology on Earth varies from flat, pancake lobes to steep, blocky domes. Identification of lava domes on other terrestrial planets in our Solar System indicates that they likely also exist on rocky exoplanets. Here we show, using particle-based modelling, that the diversity of lava dome morphology in our Solar System is dwarfed by the diversity expected for exoplanets. Specifically, the height-to-diameter ratio of a dome decreases as a function of increasing gravity (i.e., planetary mass and radius). For example, lava domes on high-gravity super-Earths will be extremely wide and flat and a volcanic origin may not be immediately apparent. Creating a toolbox to help identify exoplanetary volcanism will allow us to make initial estimations as to the development and habitability of these alien worlds as images become available

The Formation and Evolution of the First Massive Black Holes

The first massive astrophysical black holes likely formed at high redshifts (z>10) at the centers of low mass (~10^6 Msun) dark matter concentrations. These black holes grow by mergers and gas accretion, evolve into the population of bright quasars observed at lower redshifts, and eventually leave the supermassive black hole remnants that are ubiquitous at the centers of galaxies in the nearby universe. The astrophysical processes responsible for the formation of the earliest seed black holes are poorly understood. The purpose of this review is threefold: (1) to describe theoretical expectations for the formation and growth of the earliest black holes within the general paradigm of hierarchical cold dark matter cosmologies, (2) to summarize several relevant recent observations that have implications for the formation of the earliest black holes, and (3) to look into the future and assess the power of forthcoming observations to probe the physics of the first active galactic nuclei.Comment: 39 pages, review for "Supermassive Black Holes in the Distant Universe", Ed. A. J. Barger, Kluwer Academic Publisher

Analysis of capillary water imbibition in sandstone via a combination of nuclear magnetic resonance imaging and numerical DEM modeling

The physics of water imbibition into initially unsaturated sandstone is critical to the understanding of displacement processes and fluid transport in the vadose zone. The distribution of water within rock is important due to its significant influence on rock mechanical behavior. Here, therefore, we used nuclear magnetic resonance (NMR) technology to visualize and quantify the dynamics of water infiltration and distribution in initially unsaturated sandstone. The progression of water imbibition in sandstone specimens under the following two conditions were analyzed: (1) specimens soaked in water for different durations and (2) specimens soaked in water for the same duration and then held in this state for different durations. An analytic function was developed to estimate the sandstone moisture profile and to determine the unsaturated flow within the sandstone when the water distribution matched laboratory observations. Finally, a three-dimensional discrete element grain-based model was formulated that incorporates the local parallel-plate method, the unsaturated flow function, and the generalized effective stress principle. We used this model to effectively reproduce the process of water imbibition in the laboratory sandstone specimens. The effect of water imbibition on the mechanical properties of the studied sandstone was also evaluated. These results show that the strength of the core of the sample was reduced as water migrated from its surface to its center, resulting in a decrease in bulk sample strength as standing duration (i.e. water distribution uniformity) increased. The results of this study aid in our understanding of the influence of water imbibition on the mechanical behavior of sandstone, which is important for rock slope stability assessments following rainfall

Fracture and damage localization in volcanic edifice rocks from El Hierro, Stromboli and Tenerife

© 2018 The Author(s). We present elastic wave velocity and strength data from a suite of three volcanic rocks taken from the volcanic edifices of El Hierro and Tenerife (Canary Islands, Spain), and Stromboli (Aeolian Islands, Italy). These rocks span a range of porosity and are taken from volcanoes that suffer from edifice instability. We measure elastic wave velocities at known incident angles to the generated through-going fault as a function of imposed strain, and examine the effect of the damage zone on P-wave velocity. Such data are important as field measurements of elastic wave tomography are key tools for understanding volcanic regions, yet hidden fractures are likely to have a significant effect on elastic wave velocity. We then use elastic wave velocity evolution to calculate concomitant crack density evolution which ranges from 0 to 0.17: highest values were correlated to the damage zone in rocks with the highest initial porosity

Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour

Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments

Biogenesis and functions of bacterial S-layers.

The outer surface of many archaea and bacteria is coated with a proteinaceous surface layer (known as an S-layer), which is formed by the self-assembly of monomeric proteins into a regularly spaced, two-dimensional array. Bacteria possess dedicated pathways for the secretion and anchoring of the S-layer to the cell wall, and some Gram-positive species have large S-layer-associated gene families. S-layers have important roles in growth and survival, and their many functions include the maintenance of cell integrity, enzyme display and, in pathogens and commensals, interaction with the host and its immune system. In this Review, we discuss our current knowledge of S-layer and related proteins, including their structures, mechanisms of secretion and anchoring and their diverse functions

Broad Resistance to ACCase Inhibiting Herbicides in a Ryegrass Population Is Due Only to a Cysteine to Arginine Mutation in the Target Enzyme

BACKGROUND: The design of sustainable weed management strategies requires a good understanding of the mechanisms by which weeds evolve resistance to herbicides. Here we have conducted a study on the mechanism of resistance to ACCase inhibiting herbicides in a Lolium multiflorum population (RG3) from the UK. METHODOLOGY/PRINCIPAL FINDINGS: Analysis of plant phenotypes and genotypes showed that all the RG3 plants (72%) that contained the cysteine to arginine mutation at ACCase codon position 2088 were resistant to ACCase inhibiting herbicides. Whole plant dose response tests on predetermined wild and mutant 2088 genotypes from RG3 and a standard sensitive population indicated that the C2088R mutation is the only factor conferring resistance to all ten ACCase herbicides tested. The associated resistance indices ranged from 13 for clethodim to over 358 for diclofop-methyl. Clethodim, the most potent herbicide was significantly affected even when applied on small mutant plants at the peri-emergence and one leaf stages. CONCLUSION/SIGNIFICANCE: This study establishes the clear and unambiguous importance of the C2088R target site mutation in conferring broad resistance to ten commonly used ACCase inhibiting herbicides. It also demonstrates that low levels "creeping", multigenic, non target site resistance, is not always selected before single gene target site resistance appears in grass weed populations subjected to herbicide selection pressure

Conjugative Botulinum Neurotoxin-Encoding Plasmids in Clostridium botulinum

Clostridium botulinum produces seven distinct serotypes of botulinum neurotoxins (BoNTs). The genes encoding different subtype neurotoxins of serotypes A, B, F and several dual neurotoxin-producing strains have been shown to reside on plasmids, suggesting that intra- and interspecies transfer of BoNT-encoding plasmids may occur. The objective of the present study was to determine whether these C. botulinum BoNT-encoding plasmids are conjugative.C. botulinum BoNT-encoding plasmids pBotCDC-A3 (strain CDC-A3), pCLJ (strain 657Ba) and pCLL (strain Eklund 17B) were tagged with the erythromycin resistance marker (Erm) using the ClosTron mutagenesis system by inserting a group II intron into the neurotoxin genes carried on these plasmids. Transfer of the tagged plasmids from the donor strains CDC-A3, 657Ba and Eklund 17B to tetracycline-resistant recipient C. botulinum strains was evaluated in mating experiments. Erythromycin and tetracycline resistant transconjugants were isolated from donor:recipient mating pairs tested. Transfer of the plasmids to the transconjugants was confirmed by pulsed-field gel electrophoresis (PFGE) and Southern hybridizations. Transfer required cell-to-cell contact and was DNase resistant. This indicates that transfer of these plasmids occurs via a conjugation mechanism.This is the first evidence supporting conjugal transfer of native botulinum neurotoxin-encoding plasmids in C. botulinum, and provides a probable mechanism for the lateral distribution of BoNT-encoding plasmids to other C. botulinum strains. The potential transfer of C. botulinum BoNT-encoding plasmids to other bacterial hosts in the environment or within the human intestine is of great concern for human pathogenicity and necessitates further characterization of these plasmids

Group II Intron-Anchored Gene Deletion in Clostridium

Clostridium plays an important role in commercial and medical use, for which targeted gene deletion is difficult. We proposed an intron-anchored gene deletion approach for Clostridium, which combines the advantage of the group II intron “ClosTron” system and homologous recombination. In this approach, an intron carrying a fragment homologous to upstream or downstream of the target site was first inserted into the genome by retrotransposition, followed by homologous recombination, resulting in gene deletion. A functional unknown operon CAC1493–1494 located in the chromosome, and an operon ctfAB located in the megaplasmid of C. acetobutylicum DSM1731 were successfully deleted by using this approach, without leaving antibiotic marker in the genome. We therefore propose this approach can be used for targeted gene deletion in Clostridium. This approach might also be applicable for gene deletion in other bacterial species if group II intron retrotransposition system is established