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

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

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

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

Full-depth englacial vertical ice-sheet velocities measured using phase-sensitive radar

We describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling. Using a ground-based phase-sensitive radio-echo sounder (pRES) during seven Antarctic field seasons, we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on Berkner Island, Roosevelt Island, Fletcher Promontory and Adelaide Island. Recorded changes in reflector positions yield 'full-depth' profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides. We interpret these variations by comparing them to the results of a full-Stokes simulation of ice-divide flow, qualitatively validating the model and demonstrating that we are directly detecting an ice-dynamical phenomenon called the Raymond Effect. Using pRES, englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice. We discuss how these measurements could be used with inverse methods to measure ice rheology, and to improve ice-core dating by incorporating pRES-measured vertical velocities into age modelling

Influence of channelling on heating in ice-sheet flows

Ice‐sheet flows can be channelled by perturbations in the basal topography or in the sliding coefficient. These lead to spatial variation in the steady profile, the flux and the dissipative heating. This paper examines the linearized theory of heating variations, showing that the map plane aspect ratio of the basal perturbation has a dominating effect on the qualitative behaviour. For ribbing transverse to the direction of flow, maximum heating occurs over bedrock and sliding viscosity highs. When flow‐parallel channelling occurs maximum heating occurs over bedrock lows and sliding viscosity lows. These results are used to examine symmetry‐breaking behaviour of numerical thermoviscous ice‐sheet models in terms of a dissipation‐driven creep instability

Flow at ice-divide triple junctions: 1. Three-dimensional full-Stokes modeling

Ice domes are either axisymmetric, high points along ridges, or ridge triple junctions. We model time-dependent isothermal flow near triple junctions, solving the full set of mechanical equations with a nonlinear power law rheology. Forcing is applied through the boundary conditions, which affect flow patterns at outlets. Where such forcing is purely axisymmetric, an axisymmetric dome is formed. If a threefold symmetry in the forcing is applied, the axisymmetric dome breaks up into three ridges subtending angles of 120 degrees. Sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state. Here, in steady state, the ridges join the triple junction at nearly 120 degrees, but one ridge curves to satisfy the boundary forcing. The slope pattern in the immediate dome vicinity depends only on a dimensionless parameter, which is a function of the ice consistency, the accumulation, and the rheological power law index. Attempts to replicate the topography around Summit, Greenland, obtained a good fit with n = 3. At a triple junction the dome is really distinct from the surrounding ridges, contrary to the highest point of a single ridge divide. As a consequence, the Raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center. Along the ridges leaving the dome, the Raymond effect is still present and decreases with the ratio of the flow across and along the ridge. In the vicinity of the dome, horizontal strain rates vary strongly from uniaxial to biaxial. Large-scale effects, represented in our model as fluxes at boundaries, seem to be the primary controls on dome position and shape

Pore-water signal of marine ice-sheets

The pore-water signal left behind by a glacier overriding a porous medium is considered. Important processes are infiltration, tracer diffusion, radio-active decay, penetration of freezing fronts and erosion and deposition. Our knowledge of the basal hydrology of glaciers is so incomplete that we are not able to determine on theoretical grounds how much water should infiltrate the ground; water can drain through aquifers to the margin, through the aquifer to sub-glacial channels, or entirely at the glacier bed. Infiltration could be negligible or affect the whole depth of the aquifer. Diffusion is limited by the tortuosity parameter, which as yet is poorly explained by theory. Diffusion over 20,000 years may only affect a depth of 10 m, which means that the relevant areas are readily accessible by cores but are likely to have been disturbed by surface effects. The influence of sedimentation and unstable tracers is discernible but sometimes difficult to distinguish from the effects of the glaciation history. First steps in an observation programme should be the establishment of the typical depth to which marine sediments are affected. This will constrain the basal hydrology, and reduce the number of possible scenarios