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systems (dGPS) are combined with historical documents and glaciological data analysis to produce a complete study of evolution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.