Glacier dynamics and subsurface classification of Austfonna, Svalbard: : Inferences from observations and modelling

Abstract

Ice loss from glaciers and ice caps in the Arctic constitute a major contribution to eustatic sea-level rise. Climate change is more pronounced in the Arctic than in other regions, because strong feedback mechanisms such as the albedo feedback lead to enhancement of the initial warming trend. Glaciers and ice caps serve as valuable indicators of past and present climate. However, extraction of climate signals from glaciers is not straightforward. The history, current state and future evolution of glaciers result from external factors, e.g. air temperature or precipitation, and intrinsic glacier dynamics, such as changes in thermal structure or subglacial drainage. Climatically-driven and dynamic processes may interact with each other and either moderate or amplify the glacier’s response to climate change. This thesis is a contribution to the project GLACIODYN: ’The dynamic response of Arctic glaciers to global warming’, under the framework of the International Polar Year (IPY) 2007–10. It addresses the dynamic behaviour and mass balance regime of Austfonna by combining geophysical techniques for glacier observation and numerical modelling. Austfonna is one of the largest ice caps in the Arctic and by far the largest land-ice mass on the highly glacierized Svalbard archipelago. Observed interior thickening and marginal thinning of Austfonna during the last decade is presumably related to slow glacier dynamics of several basins that are known to have surged in the past and are currently in the quiescent phase. The observed changes may also partly be imposed by changes in the surface mass balance (SMB). Austfonna’s total mass balance during the last decade was negative. Ice-mass loss was primarily attributed to calving from marine terminating outlets and retreat of the marine ice margin. The calving flux depends on the rate at which ice is transported towards the calving front. Previous estimates rely on ice-surface velocity maps that represent snapshots during mid-1990s winter months. The spatial and temporal variability in snow accumulation is inferred by groundpenetrating radar (GPR). The previously suggested snow-accumulation pattern of Austfonna with about twice as much snow accumulation in the southeast compared to the northwest and a large interannual variability in the total amount of snow is confirmed for the observational period. Rigorous GPR data processing reveals distinct radar zones and allow to distinguish between firn and superimposed ice underneath the last-summer surface, as well as pure glacier ice, representing multi-year-mean accumulation and ablation, respectively. Repeated GPR surveys along coincident transects in spring 2004–07 are utilized to derive time series of radar zones and detect temporal and spatial changes in the SMB regime. Following an initial decrease in 2003–04 the firn area grew in subsequent years of observations, both in terms of the areal extent and thickness, creating favorable conditions for the retention of surface melt in form of internal accumulation and superimposed ice, which constitute a large contribution to the accumulation of Austfonna. Continuous GPS measurements over the period 2008–10 are utilized to investigate seasonal velocity changes along the central flowlines of two marine-terminating fast-flow units, Basin-3 and Duvebreen. Results from low-frequency GPR and a nearby AutomaticWeather Station (AWS) provide supplementary information with respect to glacier geometry and timing of surface melt. The analysis gives insights into mechanisms that may influence basal motion, characteristic bedrock topography above/below sea level, timing and volume of meltwater input into the subglacial drainage system and drainage characteristics. At Basin-3, the observed annual mean ice-surface velocities exceed those measured by InSAR in the mid 1990s by a factor 3, indicting that ice loss due to calving from this outlet may be significantly larger then previously anticipated. A marked speed-up following the onset of the summer melt period is recorded at both Basin-3 and Duvebreen, implying enhanced basal motion through input of meltwater into the subglacial drainage system. Air temperatures during summer 2009 were significantly higher than in 2008 and the relation between observed ice-surface velocity and melt periods is more complex. This suggests a transition to a hydraulically more efficient drainage system that can accommodate repeated meltwater input without increased basal water pressure. To gain a better knowledge of the present-day dynamics of Austfonna and surge-type behaviour of individual basins in particular, the existing ice-sheet model SICOPOLIS is employed. Implementation of the new model domain involves compilation of a comprehensive model-input dataset, comprising surface and bedrock topography, and the climatic forcing in terms of precipitation and air temperature. Steady-state model experiments are performed with the present-day geometry as initial conditions and the present-day climate is applied as external forcing. Mechanisms that may govern the dynamic characteristics are investigated by systematically changing the description of basal motion. Depending on the chosen parameter combinations, simulated fast-flow units operate in a mode of steady fast-flow or cyclic surge behaviour. The model results suggest that a change in the basal thermal regime, ultimately controlled by the long-term evolution of the glacier geometry, is decisive for surge-type behaviour, but not sufficient. The model successfully generates surge-type behaviour if the transition from no-slip to full basal sliding occurs within a narrow temperature range near the pressure-melting point (PMP), and if combined with enhanced sliding of ice regions grounded below sea level, e.g. representing plastic deformation of water-saturated sediments. This scenario is in agreement with previous observations of unconsolidated sediments beneath the marine grounded ice and statistical model results, suggesting a thermally controlled soft-bed surge mechanism for Svalbard glaciers. Fast flow is accomplished by basal motion over a temperate bed. Irrespective of the dynamic regime, considerable volumes of temperate ice are, however absent, both in the observed and simulated ice cap. The simulated steady states of Austfonna provide idealized initial conditions for prognostic model runs that allow to assess the uncertainties in the dynamic response of the ice cap to climate change. Finally, prognostic model runs are performed under the framework of the model initiative GlacMod2010 that aims at investigating the response of an appreciable number of simulated glaciers and ice caps to a set of simple climate-change scenarios. Due to the short simulation period from 2010–2100 (90 years), compared to the principle period of simulated surge-type behaviour (200–500 years), the model was set-up with flow units in steady fast-flow. The prescribed increase in precipitation is found insufficient to compensate for the non-linear increase in mass loss associated with the warming scenario.