The influence of ground ice distribution on geomorphic dynamics since the Little Ice Age in proglacial areas of two cirque glacier systems

Abstract

Holocene glaciers have contributed to an abundance of unstable sediments in mountainous environments. In permafrost environments, these sediments can contain ground ice and are subject to rapid geomorphic activity and evolution under condition of a warming climate. To understand the influence of ground ice distribution on this activity since the Little Ice Age (LIA), we have investigated the Pierre Ronde and Rognes proglacial areas, two cirque glacier systems located in the periglacial belt of the Mont Blanc massif. For the first time, electrical resistivity tomography, temperature data loggers and differential global positioning systems (dGPS) are combined with historical documents and glaciological data analysis to produce a complete study of evolution in time and space of these small landsystems since the LIA. This approach allows to explain spatial heterogeneity of current internal structure and dynamics. The studied sites are a complex assemblage of debris-covered glacier, ice-rich frozen debris and unfrozen debris. Ground ice distribution is related to former glacier thermal regime, isolating effect of debris cover, water supply to specific zones, and topography. In relation with this internal structure, present dynamics are dominated by rapid ice melt in the debriscovered upper slopes, slow creep processes in marginal glacigenic rock glaciers, and weak, superficial reworking in deglaciated moraines. Since the LIA, geomorphic activity is mainly spatially restricted within the proglacial areas. Sediment exportation has occurred in a limited part of the former Rognes Glacier and through water pocket outburst flood and debris flows in Pierre Ronde. Both sites contributed little sediment supply to the downslope geomorphic system, rather by episodic events than by constant supply. In that way, during Holocene and even in a paraglacial context as the recent deglaciation, proglacial areas of cirque glaciers act mostly as sediment sinks, when active geomorphic processes are unable to evacuate sediment downslope, especially because of the slope angle weakness.

Internal Structure and Current Evolution of Very Small Debris-Covered Glacier Systems Located in Alpine Permafrost Environments

“…The investigated sites are located within permafrost areas, as shown by regional permafrost maps (BAFU, 2005;Bodin et al, 2008;Lambiel et al, 2009;Boeckli et al, 2012). The local occurrence of permafrost conditions was also shown in talus rock glaciers near les Rognes (Bosson et al, 2015; Figure 2) and Tsarmine Micheletti et al, 2015), or in frozen talus slopes near Entre la Reille (Reynard et al, 1999).…”

“…Because of these debris accumulations, the low slope angle, the weak glacial dynamics and the reduced meltwater streams, sediment evacuation is negligible. These systems are thus sediment sinks, decoupled from downslope sediment transfer systems (Shroder et al, 2000;Benn et al, 2003;Bosson et al, 2015). The investigated sites are located within permafrost areas, as shown by regional permafrost maps (BAFU, 2005;Bodin et al, 2008;Lambiel et al, 2009;Boeckli et al, 2012).…”

“…Firstly, the deglaciation of rockwalls and permafrost degradation increase the debris supply to glacier systems (Bosson et al, 2015;Deline et al, 2015). Secondly, because the driving stress is positively related to ice thickness, thinning glaciers have weaker flow and, therefore, less ability to evacuate the sediment load to the hydrosystem (Benn et al, 2003;Paul et al, 2007).…”

“…Moreover, the height and surface of the latter can be lopsided in comparison with the restricted glacier surface (Maisch et al, 1999;Grunewald and Scheithauer, 2010). Finally, the debris evacuation can be limited because the weak driving stress of these thin ice masses is commonly reduced by the presence of cold ice (Gilbert et al, 2012) and/or large and possibly frozen sediment accumulations in the distal zone, such as moraine dam (sensu Benn et al, 2003), ice-cored moraines or rock glaciers (Shroder et al, 2000;Kirkbride and Deline, 2013;Bosson et al, 2015).…”

“…To our knowledge, no modeling considers their evolution. Furthermore, recent contributions investigated the transition from very small debris-covered glaciers to rock glaciers at the site-specific scale (e.g., Bosson et al, 2015;Emmer et al, 2015;Monnier and Kinnard, 2015;Seppi et al, 2015), but quantitative data on internal structure and on the operating processes are still lacking. In particular, the definition of the distinct components of these systems, the understanding of their origin and the identification of their responses to climatic variations is needed.…”