Surge-type glaciers: controls, processes and distribution

Abstract

Glacier surging is an internally triggered instability. Surge-type glaciers periodically alternate between long periods of slow flow (the quiescent phase) and short periods of fast flow (the surge phase). Surging yields down-glacier transport of mass and often results in large and sudden glacier advances.The surging phenomenon has always challenged the notion of normality in glacier flow dynamics. The mechanisms of surging remain poorly understood. Observation of different surge behaviors across the world has been used as evidence for the development of glacier type-specific surge models that lack transferability and representativeness. Although only about 1% of the entire glacier population has been observed to surge, the surge phenomenon questions the completeness of our understanding of glacier dynamics. This thesis uses different perspectives to gain a new understanding on the global, regional and local controls on surging and reconcile the mechanisms of surging under a single model. Through a geodatabase of surge-type glaciers, datasets of climate and glacier geometry variables and a global distribution model we explore the controls on the non-random distribution of surge-type glaciers on a global scale. The highest densities of surge-type glaciers are found in a well-defined climatic envelope bounded by temperature and precipitation thresholds, while glacier geometry exerts a second-order control on their distribution. We introduce the enthalpy cycle model which relates flow oscillations to imbalances between enthalpy gains and losses. Enthalpy balance is satisfied outside of the optimal surge envelope, in cold and dry or warm and wet regions. However, the intermediate conditions of the optimal surge envelope prevent enthalpy balance to be reached, yielding dynamics cycling of glacier flow. Thermal switch models have been used to explain surging of polythermal glaciers. We reconstruct the evolution of the thermal regime of six glaciers in Svalbard from existing and new data. The large and thick surge-type glaciers of our sample do not return to a cold-based conditions between surges, demonstrating that thermal switching cannot apply to surges of large glaciers in Svalbard. On the other hand, the thin and mostly cold glaciers display evidence of former warmbased thermal regimes, showing that switches in climate can make glaciers go in and out of surging. We demonstrate that the concept of enthalpy cycling can explain surge and surge-like behavior in Svalbard. Finally, we investigate the role played by local controls on the initiation and development of the surges of two large polythermal glaciers in Svalbard. First, passive seismics and DEM differencing enabled the reconstruction of the chronology of events that led to the catastrophic surge of the Nathorstbreen glacier system. Removal of backstress by the failure of the frozen glacier terminus triggered the catastrophic collapse of one of the tributaries of the glacier system, source of unusual seismic activity. Secondly, the upward propagating surge of Svalbard tidewater glacier Aavatsmarkbreen is understood in terms of changes in the force balance. Glacier retreat and thinning caused a rapid steepening of the glacier snout, which in turn increased the driving stresses substantially. Development of crevasse fields during the late quiescent and surge phases allowed transfer of surface meltwater to the bed, increasing basal water storage and causing ice acceleration. The increase in driving stress and surface-to-bed drainage both contributed to basal enthalpy production, and controlled the pattern of surge evolution.