THE ORIGIN OF ENGLACIAL SEDIMENT STRUCTURES, WESTERN SKEIĐARÁRJÖKULL, ICELAND

Processes of englacial sediment entrainment are hotly debated. Previous research 1998 at the western margin of temperate glacier Skeiðarárjökull, Iceland, in 1998 identified a single population of debris-filled crevasses infilled with glaciofluvial sediment, interpreted as the product of supraglacial drainage at the end of a glacial surge in 1991. However, Skeiðarárjökull was also subject to a ‘linearly-rising’ jökulhlaup in November 1996, during which elevated basal water pressure generated hydraulic fracturing and resultant englacial sediment deposition.This research i) examines the relationship of glaciohydraulic events during the 1991 surge and the November 1996 jökulhlaup to sedimentary structures on the western margin of Skeiðarárjökull, ii) presents new evidence related to these events, and iii) suggests a new explanation for the origin of these features. Fieldwork was undertaken at Skeiðarárjökull from 1996-2002. Analysis of aerial photograph and video footage, field descriptions, and sedimentological and geochemical analyses were used to constrain the origin of sediment structures.

Multiple lines of evidence preclude supraglacial drainage during the 1991 surge as an origin for the debris structures on western Skeiðarárjökull. During the 1991 surge, no turbid water flowed over the glacier surface. Surge-generated crevasses penetrated only 20m into the glacier. Knowledge of ablation rates on Skeiðarárjökull confirm that the ice surface in 1998 would have been 70m below the base of the crevasses, any surge-signature within them having melted away. We assert that hydraulic processes during the 1996 jökulhlaup resulted in the formation of the sediment structures continuing to outcrop on the western margin of Skeiðarárjökull. Relict debris-filled fractures from supraglacial jökulhlaup outlets can be traced into the western margin. Although some fractures did not break the surface during the flood, sediment deposition occurred in blind englacial fractures. Sediment structures present in recent years are not present in immediate post-flood aerial footage; they have been revealed by ablation. This research illustrates that jökulhlaups can be a potent agent of englacial sediment deposition and should be incorporated into models of englacial sediment entrainment.