Centurial‐millenial ice‐rafted debris pulses from ablating marine ice sheets

We use an ice‐sheet model to show that (i) margins of marine ice‐sheets can be expected to be frozen to the bed, except where ice‐streams discharge; (ii) 20–50km retreats induced by ablation rates of 2 m/yr provide sufficient debris flux through the grounding line to produce large sedimentation events. Such ablation would reduce ice‐shelf extent markedly, permitting debris to reach the calving front and be transported by icebergs leading to ice‐rafted debris (IRD) events. Ice shelf break‐up takes around a century (start of IRD pulse), while the creation of warm‐based conditions (end of IRD pulse) due to upwards motion of warm ice takes a few more centuries. Such IRD pulses are unlikely to explain Heinrich events, which are associated with relatively cold periods within glaciations. Surges are not necessary conditions for the production of large IRD events

Consistent generation of ice-streams via thermo-viscous instabilities modulated by membrane stresses

Accurate computation of ice-stream location and dynamics is a key aspiration for theoretical glaciology. Ice-sheet models with thermo-viscous coupling have been shown to exhibit stream-like instabilities using shallow-ice approximation mechanics, but the location and width of these streams depends on the numerical implementation and are not unique. We present results from thermo-viscously coupled ice-sheet models incorporating membrane stresses. Spontaneous generation of fast-flowing linear features still occurs under certain parameter regimes, with computed stream widths between 20 km to 100 km, comparable with observations. These features are maintained as the grid-size is decreased. The thermo-viscous feedback mechanism that generates ice-streams under the shallow ice approximation still operates, now selecting a unique stream size. Computations of thermo-viscous ice flows should include membrane stresses when the bed is approximately flat, e. g. parts of Antarctica and former ice-sheets of the Northern hemisphere. Previous calculations of spontaneous ice-stream generation using the shallow ice approximation should be reassessed. Citation: Hindmarsh, R. C. A. (2009), Consistent generation of ice-streams via thermo-viscous instabilities modulated by membrane stresses, Geophys. Res. Lett., 36, L06502, doi:10.1029/2008GL036877

Modernity Contextualises New Zealand's Royal Commission on Genetic Modification: A Discourse Analysis

The New Zealand's Royal Commission on Genetic Modification (RCGM)'s report was released in the year 2001. RCGM's findings supports the ongoing development of genetic engineering in New Zealand and recommends the recommencement of genetic modification field trials

Generalized Semilocal Theories and Higher Hopf Maps

\def\mon{S^3\stackrel{S^1}{\rightarrow}S^2} \def\inst{S^7\stackrel{S^3}{\rightarrow}S^4} \def\octo{S^{15}\stackrel{S^7}{\rightarrow}S^8} In semilocal theories, the vacuum manifold is fibered in a non-trivial way by the action of the gauge group. Here we generalize the original semilocal theory (which was based on the Hopf bundle \mon) to realize the next Hopf bundle \inst, and its extensions $S^{2n+1}\stackrel{S^3}\rightarrow \H P^n$. The semilocal defects in this class of theories are classified by $\pi_3(S^3)$, and are interpreted as constrained instantons or generalized sphaleron configurations. We fail to find a field theoretic realization of the final Hopf bundle \octo, but are able to construct other semilocal spaces realizing Stiefel bundles over Grassmanian spaces.Comment: 15 pages, uses amssymbols.st

A large-scale numerical model for computing isochrone geometry

A finite-difference model for the calculation of radar layer geometries in large ice masses is presented. Balance velocities are used as coefficients in the age equation and in the heat equation. Solution of the heat equation allows prediction of sliding areas and computation of basal melt rates. Vertical distributions of velocity are parameterized using shape functions. These can be set uniformly, or allowed to vary in space according to the distribution of sliding. The vertical coordinate can either be uniformly distributed within the thickness of the ice, or be uniformly distributed within the flux. The finite-difference scheme results in a large set of linear equations. These are solved using a nested factorization preconditioned conjugate gradient scheme. The convergence properties of some other iteration solution schemes are studied. The output is computations of age and temperature assuming steady state, in large ice masses at high resolution. Age calculations are used to generate isochrones which show the best fit to observed layers. Comparisons with analytical solutions are made, and the influence of the order of the finite-difference approximation and the choice of vertical coordinate on solution accuracy is considered

Flow speed within the Antarctic ice sheet and its controls inferred from satellite observations

Accurate dynamical models of the Antarctic ice sheet with carefully specified initial conditions and well-calibrated rheological parameters are needed to forecast global sea level. By adapting an inverse method previously used in electric impedance tomography, we infer present-day flow speeds within the ice sheet. This inversion uses satellite observations of surface velocity, snow accumulation rate, and rate of change of surface elevation to estimate the basal drag coefficient and an ice stiffness parameter that influences viscosity. We represent interior ice motion using a vertically integrated approximation to incompressible Stokes flow. This model represents vertical shearing within the ice and membrane stresses caused by horizontal stretching and shearing. Combining observations and model, we recover marked geographical variations in the basal drag coefficient. Relative changes in basal shear stress are smaller. No simple sliding law adequately represents basal shear stress as a function of sliding speed. Low basal shear stress predominates in central East Antarctica, where thick insulating ice allows liquid water at the base to lubricate sliding. Higher shear stress occurs in coastal East Antarctica, where a frozen bed is more likely. Examining Thwaites glacier in more detail shows that the slowest sliding often coincides with elevated basal topography. Differences between our results and a similar adjoint-based inversion suggest that inversion or regularization methods can influence recovered parameters for slow sliding and finer scales; on broader scales we recover a similar pattern of low basal drag underneath major ice streams and extensive regions in East Antarctica that move by basal sliding

Bound States Can Stabilize Electroweak Strings

We show that the electroweak $Z-$string can be stabilized by the presence of bound states of a complex scalar field. We argue that fermions coupled to the scalar field of the string can also make the string stable and discuss the physical case where the string is coupled to quarks and leptons. This stabilization mechanism is expected to work for other embedded defects and also for unstable solutions such as the sphaleron.Comment: 11 pages (1 figure available on request), Tufts preprint# TU-92-1

The relationship between sticky spots and radar reflectivity beneath an active West Antarctic ice stream

Isolated areas of high basal drag, or ‘sticky spots’, are important and poorly understood features in the force balance and dynamics of West Antarctic ice streams. Characterizing sticky spots formed by thin or drying subglacial till using ice-penetrating radar is theoretically possible, as high radar bed-returned power (BRP) is commonly related to an abundance of free water at the ice/bed interface, provided losses from englacial attenuation can be estimated. In this study we use airborne radar data collected over Evans Ice Stream to extract BRP profiles and test the sensitivity of BRP to the adopted englacial attenuation correction. We analyse 11 �20km profiles in four fast-flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions. In the majority of profiles we note that the increase in basal drag is accompanied by a decrease in BRP and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence. A comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation, and our findings generally support previous studies that advocate a modelling approach