Alpine rock glacier activity over Holocene to modern timescales (western French Alps)

[摘要] Active rock glaciers are some of the most frequent cryospheric landforms in midlatitude high-elevation mountain ranges. Their activity strongly influences the hydrology and geomorphology of alpine environments over short (years to decades) and long (centuries to millennia) timescales. Being conspicuous expressions of mountain permafrost and important waterreserves in the form of ground ice, rock glaciers are seen as increasinglyimportant actors in the geomorphological and hydrological evolution ofmountain systems, especially in the context of current climate change. Overgeological timescales, rock glaciers both reflect paleoclimate conditionsand transport rock boulders produced by headwall erosion, and they 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 fieldobservations, remote sensing, and geochronology, to investigate the rockglacier system of the Vallon de la Route (Combeynot Massif, western French Alps). Remotely sensed images and correlation techniques are used to document the displacement field of the rock glacier over timescales ranging from days to decades. Additionally, to estimate displacement over periods from centuries to millennia, we employ terrestrial cosmogenic nuclide (quartz 10 Be ) surface-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. The remote sensing analysis demonstrates that between 1960 and 2018 the twolower units of the rock glacier were motionless, the transitional unitpresented an integrated surface velocity of 0.03±0.02  m a −1 , and the two upper active units above 2600 m a.s.l. showed a velocity between 0.14±0.08 and 0.15±0.05  m a −1 . Our results show 10 Be surface-exposure ages ranging from 13.10±0.51 to 1.88±0.14  ka. The spatial distribution of dated rock glacier boulders reveals a first-order inverse correlation between 10 Be 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, and they may therefore be used to estimate rock glacier surface velocity over geological timescales. Our results also suggest that the rock glacier is characterized by two major phases of activity. The first phase, starting around 12 ka, displays a 10 Be age gradient with a rock glacier surface velocity of about 0.45 m a −1 , following a quiescent period between ca. 6.2 and 3.4 ka before the emplacement of the present-day upper two active units. Climatic conditions have favored an integrated rock glacier motion of around 0.18 m a −1 between 3.4 ka and present day. These resultsallow us to quantify back-wearing rates of the headwall of between 1.0 and2.5 mm a −1 , higher than catchment-integrated denudation rates estimated over millennial timescales. This suggests that the rock glacier system promotes the maintenance of high rock wall erosion by acting as debris conveyor and allowing freshly exposed bedrock surfaces to be affected by erosion processes.

[学科分类] 土壤学