Geomorphic Response of Glacial Decoupling in Alpine Regions in Response to Climate Warming: Icy Debris Fans and Early Paraglacial Landscape Evolution

Trop, Jeffrey

Icy debris fans are largely unexplored landforms that occur along the margins of valley glaciers in high alpine regions where glaciers have become disconnected from high-level icecaps as climate warmed. Ice and sediment shed from icecaps and escarpment walls moves through small bedrock catchments and is delivered to the fans, and ultimately flows into the subjacent valley glacier. Flow from icy debris fans may represent a major mode of ice supply to the annual budget of glaciers lacking tributaries or direct connections to icecaps. Direct field observations documented exceptional rates of depositional activity at McCarthy Creek Glacier in the Wrangell Mountains of Alaska (in Wrangell-St. Elias National Park and Preserve, Fig. 1) and at Mueller, La Perouse, and Douglas Glaciers in the Southern Alps of New Zealand (in Aoraki/Mt. Cook and Westland/Tai Poutini National Parks). Our study approach integrated multiple modern techniques that produced a 4-D characterization of icy debris fans for the first time, including: 1) time-lapse photography over periods of 1-2 years, 2) ground penetrating radar (GPR), and 3) terrestrial laser scanning (TLS). Collectively, our results document: 1) higher frequency and volume of mass flow events than previously recognized (ice avalanches, slush flows, icy debris flows, and rockfalls), including more activity during winter months (fans received between 200 – 2,300 significant depositional events per year, dominated by ice avalanches); 2) variations between mass flow processes (flow transformations) related to relative proportions of ice, snow, water, and lithic material; 3) more significant flow of ice and lithic (sediment) material to subjacent valley glaciers than previously documented, improving our knowledge of glacier budgets; 4) significant variations between the geometry of feeder catchments and the nature and pace of depositional processes – which results in differences in the evolution of icy debris fans. TLS surveys between 2013 and 2015 indicate that icy debris fans contribute 1.5% – 5% of glacier volume annually to their valley glaciers. This contribution, in some cases, appears to be slowing thinning and deglaciation of valley glaciers and represents an important contribution to annual ice budgets. For example, McCarthy Glacier, the smallest glacier studied that has large icy debris fans, thinned about 1 m/yr while New Zealand glaciers lost 5 – 10 m/yr during our study (Fig. 5). Our results improve models for landscape evolution in deglaciating alpine regions and have also provided guidance to parks in management of geologic hazards in these areas frequented by the general public.