Structural evolution triggers a dynamic reduction in active glacier length during rapid retreat: Evidence from Falljökull, SE Iceland

Abstract

Over the past two decades Iceland's glaciers have been undergoing a phase of accelerated retreat set against a backdrop of warmer summers and milder winters. This paper demonstrates how the dynamics of a steep outlet glacier in maritime SE Iceland have changed as it adjusts to recent significant changes in mass balance. Geomorphological evidence from Falljökull, a high-mass turnover temperate glacier, clearly shows that between 1990 and 2004 the ice front was undergoing active retreat resulting in seasonal oscillations of its margin. However, in 2004However, in -2006 this glacier crossed an important dynamic threshold and effectively reduced its active length by abandoning its lower reaches to passive retreat processes. A combination of ice surface structural measurements with radar, lidar, and differential Global Navigation Satellite Systems data are used to show that the upper active section of Falljökull is still flowing forward but has become detached from and is being thrust over its stagnant lower section. The reduction in the active length of Falljökull over the last several years has allowed it to rapidly reequilibrate to regional snowline rise in SE Iceland over the past two decades. It is possible that other steep, mountain glaciers around the world may respond in a similar way to significant changes in their mass balance, rapidly adjusting their active length in response to recent atmospheric warming.

Skaftafellsjökull, Iceland: glacial geomorphology recording glacier recession since the Little Ice Age

“…Such conditions appear to have prevailed since recession into prominent overdeepenings (Cook & Swift, 2012) located behind overridden moraine arcs and are therefore likely to dominate for several decades into the future if ice recession continues at its present rate (cf. Bennett, Evans, Carbonneau, & Twigg, 2010;Chandler et al, 2015Chandler et al, , 2016Evans et al, 2015;Phillips, Finlayson, Bradwell, Everest, & Jones, 2014;Phillips, Finlayson, & Jones, 2013).…”

The geomorphology of Svínafellsjökull and Virkisjökull-Falljökull glacier forelands, southeast Iceland

“…The mapping supports other Observatory research being carried out at the glacier (e.g. Bradwell, Sigurðsson, & Everest, 2013;Flett et al, 2017;Krabbendam, Bradwell, Everest, & Eyles, 2017;MacDonald et al, 2016;Phillips, Finlayson, & Jones, 2013, 2014.…”

“…The relative accessibility of southeast Iceland's glaciers and the preservation of their landscape legacy have enabled the development of a deeper understanding of glacial and glacial geomorphic processes and continue to encourage new research (e.g. Boulton & Hindmarsh, 1987;Eyles, 1979;Hannesdóttir, Björnsson, Pálsson, Aðalgeirsdóttir, & Guðmundsson, 2015a, 2015bPhillips, Finlayson, Bradwell, Everest, & Jones, 2014;Spedding & Evans, 2002).…”

Subglacial drumlins and englacial fractures at the surge‐type glacier, Múlajökull, Iceland

“…At that location the fault was associated with a notable (metre‐scale) surface displacement that showed continued development over time (Phillips et al. ). In other surging glaciers, rare down‐glacier dipping fractures that were observed in ice cliff sections have been interpreted as backthrusts, associated with intense longitudinal compression and shortening (Lawson et al.…”

“…Englacial structures, such as fractures and faults, provide an indication of the stress and strain rate in ice, and so can provide insights into glacier dynamics (Moore et al, 2010;Murray and Booth, 2010;Phillips et al, 2013Phillips et al, , 2014Lovell et al, 2015). These structures have also been suggested to play an important role in glacier drainage (Fountain et al, 2005;Harper et al, 2010), and have been linked to dewatering and the evacuation of water-saturated sediment from the bed during glacier surges (Bennett et al, 2000;Woodward and Burke, 2007;Rea and Evans, 2011).…”