CRITICAL LIFELINES FOR ALPINE GLACIERS DECOUPLED FROM ICECAPS: MASS FLOW CONTRIBUTION FROM ICY DEBRIS FANS IN ALASKA AND NEW ZEALAND

Pace and volume of mass flow processes contributing ice and lithic sediment to icy debris fans (IDFs) were documented at four sites in Alaska and New Zealand by integrating field observations, drone and time-lapse imagery, ground penetrating radar (GPR) and terrestrial laser scanning (TLS). IDFs are supra-glacial landforms at the mouths of bedrock catchments where valley glaciers decoupled from icecaps. Time-lapse imagery recorded 300–2,300 events reaching 15 fans during intervals from 0.75–2 years. Field observations noted 100’s of deposits trapped within catchments weekly, later remobilized onto fans. Deposits were mapped on images taken 3-4 times/day. Most events were ice avalanches (58–100%). Slush avalanches/flows were common in Spring and Fall (0–65%). Icy debris flows were <5% of the events; observed only at sites with geomorphically complex catchments. Rockfalls were common within catchments; only one reached a fan. Site selection provided a spectrum of catchment relationships between icecaps and fans. Unlike alluvial settings where larger fans are associated with larger catchments, there are no relationships between IDF area and catchment area. The largest most active IDFs occur below hanging glaciers or short chutes between the icecap and glacier; dominated by ice avalanches and slush avalanches. Larger, complex catchments allowed temporary storage of ice and sediment; later remobilized into ice and slush avalanches and debris flows.

IDFs are exceptionally dynamic compared to alluvial fans, with resurfacing rates from 300—4,306% annually. Annual contributions by mass flows ranged from 133,200—5,200,000 m³; ranging from 3—56% of fan volume. Although ablation occurs, mainly during summers, significant ice transfer occurs through the fan subsurface to glaciers. Estimates of glacier volume from GPR-TLS data indicate that IDFs annually contributed 2—5% of glacier mass. Small glaciers (McCarthy, AK) showed minor thinning (< 1 m/yr) compared to larger glaciers (La Perouse, Douglas, Mueller Glaciers, NZ) that lost > 5—10m/yr. IDFs lengthen to keep pace with thinning glaciers. Supply to valley glaciers through IDFs contributes material to glaciers decoupled from icecaps, slowing deglaciation; data potentially useful in resource management and predicting glacial mass balance in alpine regions.