Comment on esurf-2022-8

Abstract

Rock glaciers are some of the most frequent cryospheric landforms in mid-latitude high-elevation mountain ranges. Their activity strongly influences alpine environments over short (years to decades) and long (centuries to millennia) timescales. Being conspicuous expressions of mountain permafrost and important water reserves in the form of ground ice, rock glaciers are seen as increasingly important actors in the geomorphological and hydrological evolution of mountain systems, especially in the context of climate change. Over geological timescales, rock glaciers both reflect paleoclimate conditions and transport rock boulders produced by headwall erosion and therefore participate in shaping high mountain slopes. However, the dynamics of rock glaciers and their evolution over different timescales remain under-constrained. In this study, we adopt a multi-method approach including field observations, remote sensing and geochronology, to investigate the rock glacier system of the Vallon de la Route (Combeynot massif, western French Alps). Remote sensing employing image correlation documents the displacement field of the rock glacier over modern timescales (1–101 years). Over longer periods (103–104 years), we employ terrestrial cosmogenic nuclide (10Be in quartz) exposure dating on rock-boulder surfaces located along the central flow line of the rock glacier, targeting different longitudinal positions from the headwall to the rock-glacier terminus. Our results show 10Be surface-exposure ages ranging from 1.88 ± 0.14 to 13.10 ± 0.51 ka. The spatial distribution of rock-glacier boulders reveals a first-order inverse correlation between 10Be surface-exposure age and elevation; and a positive correlation with horizontal distance to the headwall. These observations support the hypothesis of rock boulders falling from the headwall and remaining on the glacier surface as they are transported down valley, which thus can be used to estimate rock-glacier surface velocity over geological timescales. Our results also suggest that the rock glacier is characterized by two major episodes of activity. The first phase, starting around 12 ka, displays a 10Be-age gradient that suggests a rock-glacier surface velocity of about 0.45 m/a. Following a quiescent period between ca. 6.2 ka and 3.4 ka, the present-day active upper two units have been emplaced during climatic conditions favoring rock-glacier motion at around 0.18 m/a. Those results allow us to quantify back erosion rates of the headwall between 1.0 and 2.5 mm/a, higher than catchment-integrated denudation rates estimated over millennial timescales, suggesting that the rock-glacier system promotes the maintenance of high rock-wall erosion.