Description: Calving mechanisms are still poorly understood and stress states in the vicinity of ice-shelf fronts are insufficiently known for the development of physically motivated calving laws that match observations. A calving model requires the knowledge of maximum tensile stresses. These stresses depend on different simulation approaches and material models. Therefore, this study compares results of a two-dimensional (2-D) continuum approach using finite elements with results of a one- dimensional (1-D) beam model elaborated in Reeh (1968). A purely viscous model, as well as a viscoelas- tic Maxwell model, is applied for the 2-D case. The maximum tensile stress usually appears at the top surface of an ice shelf. Its location and magnitude are predominantly influenced by the thickness of the ice shelf and the height of the freeboard, the traction-free part at the ice front. More precisely, doub- ling the thickness leads to twice the stress maximum, while doubling the freeboard, based on changes of the ice density, results in an increase of the stress maximum by 61%. Poisson’s ratio controls the evolu- tion of the maximum stress with time. The viscosity and Young’s modulus define the characteristic time of the Maxwell model and thus the time to reach the maximum principal stress.

Description: To understand the dynamics of ice shelves, a knowledge of their internal and basal structure is very important. As the capacity to perform local surveys is limited, remote sensing provides an opportunity to obtain the relevant information. We must prove, however, that the relevant information can be obtained from remote sensing of the surface. That is the aim of this study. The Jelbart Ice Shelf, Antarctica, exhibits a variety of surface structures appearing as stripe-like features in radar imagery. We performed an airborne geophysical survey across these features and compared the results to TerraSAR-X imagery. We find that the stripe-like structures indicate surface troughs coinciding with the location of basal channels and crevasse-like features, revealed by radio-echo sounding. HH and VV polarizations do not show different magnitude. In surface troughs, the local accumulation rate is larger than at the flat surface. Viscoelastic modelling is used to gain an understanding of the surface undulations and their origin. The surface displacement, computed with a Maxwell model, matches the observed surface reasonably well. Our simulations show that the surface troughs develop over decadal to centennial timescales.

Description: Ice shelves are floating ice masses, which are sensitive to climate changes. The main mechanisms for the mass loss of ice shelves around Antarctica are basal melting and calving. For an understanding of the mechanisms of calving the influence of environmental parameters needs to be investigated. We use a fracture mechanical approach to examine the nature and frequency of calving events. Ice responses to load in two ways: on long time scales ice reacts like a viscous fluid, and on short time scale like an elastic solid. As calving is a representation of the solid nature of ice, the elastic response is important and linear elastic fracture mechanics can be applied. However, gravity remains a long time load and hence, a viscous component needs to be taken into account as well. Therefore, we use a Kelvin-Voigt model for analyzing the transient response of an ice shelf to a calving event. In a simplified 2D-model the ice shelf is treated as a rectangular block, in which the gravity force is the only load in a first analysis. The stresses on the surface in the vicinity of the calving front are computed with the finite element software COMSOL. The boundary conditions are the water pressure at the front and bottom of the ice shelf and a constant displacement at the inflow. A stationary state will reappear until eventually the subsequent calving event occurs, the termination time is around 175days. Based on this time interval and the flow velocity of the ice shelf we estimate the calving rate. Different parameter studies reveal the influence of geometry and material parameters on the stresses for an elastic material model. The literature and measurements at the Ekstroem Ice Shelf, East Antarctica, provides the relevant parameter range. Due to the depth-dependent water pressure at the ice front, a bell shaped distribution of stresses on the surface is found. For this reason the location of the maximal stress denotes the most likely position for a calving event and is arranged in between 0.65H and 0.85H, with H the thickness at the ice front. The results of these studies are compared to the results for two cross-sections of measured geometries of the Ekstroem Ice Shelf.

Description: Disintegration events in ice shelves have been the subject of extensive investigations in the past years, however comprehensive explanations applicable to a majority of events are still missing. A popular assumption made by Scambos et al. (2000) [1] links disintegration events to a general thinning of the ice shelf in conjunction with growing melt-water ponds leading to hydro fractures. This explanation seems reasonable for break-up events that happened in Antarctic summers. Large parts of the Wilkins Ice Shelf, however broke-up in fall and winter periods. Therefore, the aim of the present study is to analyse the possibility of frost wedging of water filled surface crevasses in an ice shelf as a source of break-up events. Configurational forces are used to assess crack criticality. The simulations are performed on a 2-dimensional single crack with a mode-I type load, body forces and additional crack-face pressure due to freezing of the water. Depth-dependent density profiles are considered. The relevant parameters, Young’s modulus, Poisson’s ratio and external loading are obtained from literature, remote sensing data analysis and modelling of the ice dynamics. The investigation is performed using the finite element software COMSOL. The simulations show that in comparison to water filled crevasses without ice, thin layers of frozen water may lead to a decreasing criticality at the crack tip as long as the ice ‘bridge’ is allowed to take tensile loads. An increasing crack criticality can be seen for thicker layers of ice. The results are compared to findings from previous finite element analyses of dry and water filled cracks as presented in Plate et al. (2012) [2]. [1] Scambos, T., Hulbe, C., Fahnestock, M., & Bohlander, J. (2000). The link between climate warming and break-up of ice shelves in the Antarctic Peninsula. Journal of Glaciology, 46(154), 516–530. [2] Plate, C., Müller, R., Humbert, A., & Gross, D. (2012). Evaluation of the criticality of cracks in ice shelves using finite element simulations. The Cryosphere, 6(5), 973–984.

Description: The dynamics of the Antarctic Ice Sheet can be well seen and studied on the behavior of Pine Island Glacier. Despite the long time believe in a slow response of the ice sheet to changing atmospheric and oceanic forcing, Pine Island has shown acceleration, thinning and a significant grounding line retreat in the past decades. These ongoing processes are coinciding with a concentrated mass loss in the area around Pine Island Glacier, the Amundsen Sea Embayment. The area is of additional interest due to its retrograde bed slope below the glacier. The postulated instability of the setting turns the glacier into an even more suitable object for modeling studies. Plenty of working groups have conducted modeling studies of Pine Island Glacier with varying model complexity and diverse focuses. We want to add to this by conducting model experiments with a diagnostic 3D full-stokes model of Pine Island Glacier. The model is thermo-mechanically coupled and implemented with the commercial finite-element package COMSOL Multiphysics©. We use remotely sensed surface velocity data to validate our results. The focus of our work lies on the basal properties below the glacier and the connection to sliding behav- ior. We believe that this is a crucial part, as different basal conditions might cause different responses to ongoing changes in the area. Recent studies presented evidence for the existence of a water saturated sediment basin below the main trunk of the glacier. We conduct a variety a numerical experiments with which we test different approaches of combining information about the basal properties with sliding laws.

Description: At the base of a thick ice sheet the temperature locally reaches the pressure melting point and melting generates a thin subglacial water layer. The basal water lubricates the base and thus enhances the sliding of the ice sheet. As a consequence of sliding, the heat source of internal strain heating decreases and the basal ice cools down over time. When frictional heat and heat advection do not counterbalance this, the ice will become frozen to the bedrock again. In addition, strain heating within a temperate ice layer generates a liquid water fraction in the ice, leading to a softer material and enhanced deformation. If the horizontal or vertical advection of cold ice to the base is weak, this positive feedback will lead to a local creep instability. The effect of basal water is thus twofold: it affects the sliding, as well as the rheology and via both ways the ice dynamics. Subglacial water is therefore a crucial component in the dynamic evolution of ice sheets. We present numerical simulations of the present day ice flow using the three-dimensional thermo-coupled full- Stokes model TIM FD 3 on a 2.5 km horizontal grid in the area of the western Dronning Maud Land, Antarc- tica, including the three ice streams Stancomb-Wills, Veststraumen and Plogbreen and the adjacent Brunt and Riiser- Larsen ice shelves. Three different flux routing algorithms for the subglacial meltwater and a modified Weertman-type sliding relation were implemented in the model to account for higher sliding velocities under wet basal conditions. Subsequent to spin-up simulations different sliding simulations considering wet and dry basal conditions were performed. The simulations show a cyclic behaviour on millennial time scale at distinct locations in the model domain. We estimate the distribution of subglacial water based on different flux routing methods and the effect on the ice flow and the basal thermal regime. We further present our analysis of the involved feedback mechanism between ice flow, temperature and rheology, that are related to the simulated cyclic behaviour.

Description: We study the presence and effect of subglacial water on the motion of the inland ice in western Dronning Maud Land. A full-Stokes model including three routing schemes for a thin film of subglacial water and a modification of a Weertman-type sliding relation to account for higher sliding velocities under wet basal conditions were used to perform 200 ka spin-up simulations on a 2.5 km grid. Subsequent 30 ka simulations with wet and dry basal conditions were analysed for the effects of sliding on the thermal regime and velocities. The occurrence of the major ice streams in this area is mainly controlled by the ice and bedrock geometry. Smaller glaciers only appear as pronounced individual glaciers, when subglacial water is taken into account. The thermal regime is affected by creep instabilities produced by an ice rheology including a microscopic water content, leading to a cyclic behaviour on millennial time scales of the ice flow and occurrence of temperate ice at the base.