Dead-Ice Under Different Climate Conditions: Processes, Landforms, Sediments and Melt Rates in Iceland and Svalbard

Sammanfattning: Modern dead-ice environments in the glacier forefields of Brúarjökull, Iceland and Holmströmbreen, Svalbard were investigated with focus on landform and sediment genesis, as well as quantification of melting. Field monitoring and studies of multi-temporal aerial photographs, satellite imagery, and Digital Elevation Models (DEMs) provided data for the melting quantification. Sedimentological and geomorphological data were achieved through field investigations and image analyses. Different measures for dead-ice melting (backwasting, downwasting, ice-walled lake area, glacier retreat and thinning) are assessed in relation to local air temperature data going back to the beginning of the instrumental period. A geomorphological map in scale 1:16 000 of the forefield of the surge-type glacier Brúarjökull was produced through digital aerial photograph interpretation and high-resolution DEM analyses. The map was used for the interpretation of landforms and sediments, and provided an overview of the surging glacier landsystem at Brúarjökull. A conceptual model for the formation of transitional-state ice-cored landforms ? ice-cored drumlins ? was also constructed, based on the research in the Brúarjökull forefield. After a complete melting, the model proposes that such drumlins will disintegrate into patches of hummocky dead-ice moraine. Three years of fieldwork combined with analyses of multi-temporal DEMs and aerial photographs revealed that multiple generations of ice-cored moraines are currently exposed to melting at Brúarjökull. Quantifying the melting progression suggests that in the current climate, a complete de-icing of ice-cored landforms is not likely to occur. Some dead-ice bodies are recycled into new ice-cored landforms, because the total melt-out time exceeds the duration of the quiescent period in the surge cycles. Long-term surface lowering due to dead-ice melting takes place with a rate of c. 0.10-0.18 m/yr. At the stagnant snout of Holmströmbreen, an extensive dead-ice area with ice-cored moraines, eskers and kames has developed since the Little Ice Age glacial maximum. Backwasting of ice-cored slopes and mass-movement processes continuously expose new dead-ice and prevents the build-up of an insulating debris-cover. Currently dead-ice melting progresses with a long-term surface lowering rate of c. 0.9 m/yr. The most prominent impact of dead-ice melting is the development of an extensive ice-walled, moraine-dammed lake receiving sediment from the adjacent slopes. Based on a literature review and the results presented here, dead-ice melting in different climatic settings is discussed, with focus on melt rates and sediment-landform genesis. Because identical processes operate with similar rates in different climates, dead-ice deposits provide little information on the climate at the time of deposition. The glaciodynamic significance of dead-ice deposits is that of stagnation of debris-covered glaciers.