Throstur Thorsteinsson

The role of crystal fabric in flow near an ice divide

. Polycrystalline ice near an ice divide typically shows a crystal fabric (crystal preferred orientation) with c axes clustered vertically. We explore the effect of this fabric on the large-scale flow pattern near an ice divide. We incorporate an analytical formulation for anisotropy into a non-linear flow law within a finite-element ice-sheet flow model. With four different depth profiles of crystal fabric, we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than ∼25°. For a steady-state divide, the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non-linearity of ice flow. A vertically oriented fabric tends to increase the size of the isochrone arch, never to reduce it. Also, non-random fabric has little effect on the ice-divide-flow pattern when ice is modeled as a linear (Newtonian) fluid. Finally, when we use a crystal-fabric profile that closely approximates the measured profile for Siple Dome, West Antarctica, the model predicts concentrated bed-parallel shearing 300 m above the bed.

Bed topography and lubrication inferred from surface measurements on fast-flowing ice streams

Observations of surface elevation (s) and horizontal velocity components (u and v) are inverted to infer the topography (b) and lubrication (c) at the bed of an ice stream, based on a linearized perturbation theory of the transmission of flow disturbances through the ice thickness. Synthetic data are used to illustrate non-uniqueness in the inversion, but also demonstrate that effects of b and c can be separated when s, u and v are specified, even with added noise to simulate measurement errors. We have analyzed prominent short-horizontal-scale (∼2 km) features in topography and velocity pattern in a local 64 km by 32 km area of the surface of Ice Stream E,West Antarctica. Our preferred interpretation of bed conditions beneath the most prominent features on the surface identifies a deep trough in the basal topography with low lubrication in the base of the trough.

A survey of early health effects of the Eyjafjallajökull 2010 eruption in Iceland: a population-based study

Objective To estimate physical and mental health effects of the Eyjafjallajökull volcanic eruption on nearby residents. Design Cross-sectional study. Setting The Icelandic volcano Eyjafjallajökull erupted on 14 April 2010. The eruption lasted for about 6 weeks and was explosive, ejecting some 8 million tons of fine particles into the atmosphere. Due to prevailing winds, the ash spread mostly to the south and south-east, first over the rural region to the south, later over the Atlantic Ocean and Europe, closing European air space for several days. Participants Residents (n=207) of the most ash-exposed rural area south and east of the volcano. Methods The study period was from 31 May to 11 June 2010. Participants were examined by a physician. To ascertain respiratory health, standardised spirometry was performed before and after the use of a bronchodilator. All adult participants answered questionnaires about mental and physical health, their childrens health and the use of protective equipment. Results Every other adult participant reported irritation in eyes and upper airway when exposed to volcanic ash. Adults (n=26) and children (n=5) with pre-existing asthma frequently reported worsening of their symptoms. No serious health problems requiring hospitalisation could be attributed to the eruption. The majority of the participants reported no abnormal physical or mental symptoms to the examining physician. Compared to an age- and gender-matched reference group, the ash-exposed participants reported lower smoking rates and were less likely to have ventilation impairment. Less than 10% of the participants reported symptoms of stress, anxiety or depression. Conclusions Short-term ash exposure was associated with upper airway irritation symptoms and exacerbation of pre-existing asthma but did not contribute to serious health problems. The exposure did not impair respiratory function compared to controls. Outdoor use of protective glasses and face masks was considered protective against irritation in eyes and upper airway.

High Latitude Dust in the Earth System

Natural dust is often associated with hot, subtropical deserts, but significant dust events have been reported from cold, high latitudes. This review synthesizes current understanding of high-latitude (≥50°N and ≥40°S) dust source geography and dynamics and provides a prospectus for future research on the topic. Although the fundamental processes controlling aeolian dust emissions in high latitudes are essentially the same as in temperate regions, there are additional processes specific to or enhanced in cold regions. These include low temperatures, humidity, strong winds, permafrost and niveo-aeolian processes all of which can affect the efficiency of dust emission and distribution of sediments. Dust deposition at high latitudes can provide nutrients to the marine system, specifically by contributing iron to high-nutrient, low-chlorophyll oceans; it also affects ice albedo and melt rates. There have been no attempts to quantify systematically the expanse, characteristics, or dynamics of high-latitude dust sources. To address this, we identify and compare the main sources and drivers of dust emissions in the Northern (Alaska, Canada, Greenland, and Iceland) and Southern (Antarctica, New Zealand, and Patagonia) Hemispheres. The scarcity of year-round observations and limitations of satellite remote sensing data at high latitudes are discussed. It is estimated that under contemporary conditions high-latitude sources cover >500,000 km2 and contribute at least 80–100 Tg yr−1 of dust to the Earth system (~5% of the global dust budget); both are projected to increase under future climate change scenarios.

An analytical approach to deformation of anisotropic ice-crystal aggregates

Deformation rates of single hexagonal crystals, deforming by glide on the basal plane, are described as a function of stress state and crystal orientation. These results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous. Analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric. This type of fabric is approximated by a uniform distribution of c-axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis. For simple shear stress acting on a single-maximum fabric there is a slight de-enhancement for cone angles of 60-90°. In uniaxial compression a maximum enhancement of ∼1.7 occurs at a cone angle of 57°. A pure shear stress state has similar features, with the additional complication that it causes a non-zero transverse strain rate, except for perfect vertical alignment of crystals and isotropic fabric. In combined states of stress the contribution of each stress component to the strain rate depends on fabric. A single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states.

The crossover stress, anisotropy and the ice flow law at Siple Dome, West Antarctica

We used observations and modeling of Siple Dome, West Antarctica, a ridge ice divide, to infer the importance of linear deformation mechanisms in ice-sheet flow. We determined the crossover stress (a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms) by combining measurements of ice properties with in situ deformation rate measurements and a finite-element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal-orientation fabric. We found that a crossover stress of 0.18 bar produces the best match between predicted and observed deformation rates. For Siple Dome, this means that including a linear term in the flow law is necessary, but generally the flow is still dominated by the nonlinear (Glen; n =3 ) term. The pattern of flow near the divide at Siple Dome is also strongly affected by crystal fabric. Measurements of sonic velocity, which is a proxy for vertically oriented crystal fabric, suggest that a bed-parallel shear band exists several hundred meters above the bed within the Ice Age ice.

Health effects following the Eyjafjallajökull volcanic eruption : a cohort study

Objectives The study aimed to determine whether exposure to a volcanic eruption was associated with increased prevalence of physical and/or mental symptoms. Design Cohort, with non-exposed control group. Setting Natural disasters like volcanic eruptions constitute a major public-health threat. The Icelandic volcano Eyjafjallajökull exposed residents in southern Iceland to continuous ash fall for more than 5 weeks in spring 2010. This study was conducted during November 2010–March 2011, 6–9 months after the Eyjafjallajökull eruption. Participants Adult (18–80 years of age) eruption-exposed South Icelanders (N=1148) and a control population of residents of Skagafjörður, North Iceland (N=510). The participation rate was 72%. Main outcome measures Physical symptoms in the previous year (chronic), in the previous month (recent), General Health Questionnaire (GHQ-12) measured psychological morbidity. Results The likelihood of having symptoms during the last month was higher in the exposed population, such as; tightness in the chest (OR 2.5; 95% CI 1.1 to 5.8), cough (OR 2.6; 95% CI 1.7 to 3.9), phlegm (OR 2.1; 95% CI 1.3 to 3.2), eye irritation (OR 2.9; 95% CI 2.0 to 4.1) and psychological morbidity symptoms (OR 1.3; 95% CI 1.0 to 1.7). Respiratory symptoms during the last 12 months were also more common in the exposed population; cough (OR 2.2; 95% CI 1.6 to 2.9), dyspnoea (OR 1.6; 95% CI 1.1 to 2.3), although the prevalence of underlying asthma and heart disease was similar. Twice as many in the exposed population had two or more symptoms from nose, eyes or upper-respiratory tract (24% vs 13%, p<0.001); these individuals were also more likely to experience psychological morbidity (OR 4.7; 95% CI 3.4 to 6.5) compared with individuals with no symptoms. Most symptoms exhibited a dose–response pattern within the exposed population, corresponding to low, medium and high exposure to the eruption. Conclusions 6–9 months after the Eyjafjallajökull eruption, residents living in the exposed area, particularly those closest to the volcano, had markedly increased prevalence of various physical symptoms. A portion of the exposed population reported multiple symptoms and may be at risk for long-term physical and psychological morbidity. Studies of long-term consequences are therefore warranted.

Ice microstructure and fabric: an up to date approach for measuring textures

Automatic c-axes analyzers have been developed over the past few years, leading to a large improvement in the data available for analysis of ice crystal texture. Such an increase in the quality and quantity of data allows for stricter statistical estimates. The current textural parameters, i.e. fabric (crystallographic orientations) and microstructure (grain-boundary networks), are presented. These parameters define the state of the polycrystal and give information about the deformation undergone by the ice. To reflect the findings from automatic measurements, some parameter definitions are updated and new parameters are proposed. Moreover, a MATLAB® toolbox has been developed to extract all the textural parameters. This toolbox, which can be downloaded online, is briefly described.

Sliding versus till deformation in the fast motion of an ice stream over a viscous till

The partitioning between till deformation and sliding in the fast flow of ice streams with active basal melting is examined assuming no adhesion of the till to the ice base and incompressible viscous fluid behavior for the till. For deforming-till thickness of 10 m or less the predicted contribution to basal motion by sliding is larger than shearing in the till unless there is short-scale roughness with wavelengths less than order 0.1 m on the ice sole. At such short scales strain heating within the till and focused melting on the ice sole would quickly eliminate the roughness. Thus, fast flow over a till bed would be expected to be mostly by sliding over the subglacial till. More realistic continuum behavior of the till including non-linear and compressible deformation strengthens the conclusion. If sliding is not dominant, then there must be adhesion of the till to the ice base, some mechanism that continuously generates short-scale roughness on the ice-till interface, or very weak internal slip boundaries within the till.

Folding in strongly anisotropic layers near ice-sheet centers

Abstract Conditions for passive folding near ice-sheet centers are derived treating the ice as an anisotropic viscous medium. Vertical uniaxial compression at a dome, or pure shear stress near a ridge divide, both tend to stretch and flatten folds, while horizontal simple shear deformation tends to overturn folds. Overturned folding is likely in a given vicinity in a steady-state flow field, if the initial slope of a layer disturbance exceeds the ratio of compressive/extensive deformation to shear deformation. Analytical equations for particle tracks in steady state allow us to model the evolution of layers with initial slope disturbances (``wrinkles’’). the effects of anisotropy are explored using an analytical solution for the strain rate as a function of a vertically symmetric c-axis orientation distribution, called a cone fabric. Stronger anisotropy (small cone angle) makes the material softer in horizontal shear, and facilitates folding, i.e. elements with smaller slopes can be overturned for the same stress. the relation between anisotropy and folding is complicated by the fact that, for a range of cone angles, the material is also softer in compression, which opposes folding. Simulating a layer with spatially variable tilt of cone symmetry axes, and accounting for fabric development, demonstrates that variations in the fabric cause localized flow variations that could create the initial perturbations.