Postglacial erosion of bedrock surfaces and deglaciation timing: New insights from the Mont Blanc massif (western Alps)

Abstract

Since the Last Glacial Maximum, ∼20 k.y. ago, Alpine glaciers have retreated and thinned. This transition exposed bare bedrock surfaces that could then be eroded by a combination of debuttressing or local frost cracking and weathering. Quantification of the respective contributions of these processes is necessary to understand the links between long-term climate and erosion in mountains. Here, we quantified the erosion histories of postglacial exposed bedrock in glacial valleys. Combining optically stimulated luminescence and terrestrial cosmogenic nuclide (TCN) surface exposure dating, we estimated the erosion rate of bedrock surfaces at time scales from 101 to 104 yr. Bedrock surfaces sampled from the flanks of the Mer de Glace (Mont Blanc massif, European Alps) revealed erosion rates that vary from 3.5 ± 1.2 ⋅ 10−3 mm/yr to 4.3 ± 0.6 mm/yr over ∼500 m of elevation, with a negative correlation between erosion rate and elevation. The observed spatial variation in erosion rates, and their high values, reflect morphometric (elevation and surface slope) and climatic (temperature and snow cover) controls. Furthermore, the derived erosion rates can be used to correct the timing of deglaciation based on TCN data, potentially suggesting very rapid ice thinning during the Gschnitz stadial.

Erosion rates in a wet, temperate climate derived from rock luminescence techniques

“…For BALL03, transient erosion rates were derived using the IR50 (6 -460 mm/ka), pIRIR150 (14 -100 mm/ka) and pIRIR225 (11 -180 mm/ka) signals. These modelled transient erosion rates were broadly comparable to erosion rates inferred from luminescence depth profiles over comparable timeframes in previous studies: (i) rates between <0.038 ± 0.002 and 1.72 ± 0.04 mm/ka for glacial boulders and landslides (granite gneiss, granodorite and quartzite) in the Eastern Pamirs, China (Sohbati et al 2018); and (ii) between 3.5 ± 1.2 mm/ka and 4,300 ± 600 mm/ka for glacially-modified, granitic bedrock in the French Alps (Lehmann et al, 2019b). This latter study modelled higher erosion rates (>100 mm/ka) over timescales from 10 1 to 10 3 a and lower erosion rates (<100 mm/ka) over longer time scales of 10 3 to 10 4 a.…”

“…3.3 to 12.0 mm/ka). Lehmann et al (2019b) noted that their modelled steady-state erosion rates were one to two orders of magnitude higher than suggested by a global compilation of bedrock surface erosion rates based on 10 Be (Portenga and Bierman, 2011), and measurements of upstanding, resistant lithic components (ca. 0.2 -5.0 mm/ka) in crystalline rock surfaces in Arctic Norway (André, 2002).…”

“…Ballantyne and Whittington, 1987), particularly around edges, and thus may have experienced higher erosion rates than the crystalline rocks (e.g. gneiss, granite) used in the studies of Sohbati et al (2018) and Lehmann et al, 2019b. A major advantage of applying this new erosion-meter technique to boulders of the Beinn Alligin rock avalanche was the existing constraints on Holocene erosion rates (~3.3 to mm/ka) for Torridonian sandstones in NW Scotland inferred from boulder edge roundness measurements (Kirkbride and Bell, 2009).…”

“…However, its application has been limited to few studies (e.g. Sohbati et al 2018;Lehmann et al 2019b) validated against long-term erosion rates of landscape evolution from global or regional datasets rather than local, independently-constrained erosion rates derived from the same rock type.…”

Linking Danube River activity to Alpine Ice-Sheet fluctuations during the last glacial (ca. 33–17 ka BP): Insights into the continental signature of Heinrich Stadials

“…18-16.5 ka; , if it existed at all in the Alps (e.g. Lehmann et al, 2020), impacted only a restricted area and volume of ice. This implies that the forcing that caused the initial retreat of the FIS at ca.…”