Geomorphology of icy debris fans: Delivery of ice and sediment to valley glaciers decoupled from icecaps

摘要

The pace and volume of mass flow processes contributing ice and sediment to icy debris fans (IDFs) were documented at sites in Alaska and New Zealand by integrating field observations, drone and time-lapse imagery, ground penetrating radar, and terrestrial laser scanning. Largely unstudied, IDFs are supraglacial landforms at the mouths of bedrock catchments between valley glaciers and icecaps. Time-lapse imagery recorded 300–2300 events reaching 15 fans during intervals from nine months to two years. Field observations noted hundreds of deposits trapped within catchments weekly that were later remobilized onto fans. Deposits were mapped on images taken three to four times per day. Most events were ice avalanches (58%–100%). Slush avalanches and/or flows were common in spring and fall (0%–65%). Icy debris flows were 4000%. Annual contributions by mass flows ranged from 133,200 to 5,200,000 m 3 , representing 3%–56% of fan volume. Although ablation occurred, mainly during summers, significant ice transfer occurred through fan subsurface areas to adjacent valley glaciers. Icy debris fans annually contributed 5–10 m/yr over the hundreds of meters of valley glacier adjacent to the IDFs studied. Some IDFs lengthened in response to thinning of valley glaciers. Icy debris fans supplied significant ice and sediment to valley glaciers, slowing the rate of deglaciation. Results of this study have implications toward managing hazards and predicting glacial mass balance in alpine regions. For example, having quantitative information about the role of ice contribution from IDFs to valley glaciers may result in forecasting a lower rate of deglaciation than traditionally recognized for some glaciers decoupled from icecaps.