Paired ¹⁰Be sampling of polished bedrock and erratic boulders to improve dating of glacial landforms: an example from the Western Alps

Abstract

Cosmogenic nuclide dating of glacial landforms may lead to ambiguous results for ice retreat histories. The persistence of significant cosmogenic concentrations inherited from previous exposure may increase the apparent exposure ages for polished bedrocks affected by limited erosion under ice and for erratic boulders transported by glaciers and previously exposed in high‐altitude rock walls. In contrast, transient burying by moraines, sediments and snow decreases the apparent exposure age. We propose a new sampling strategy, applied to four sites distributed in the Arc and Arve valleys in the Western Alps, to better constrain the factors that can bias exposure ages associated with glacial processes. We used the terrestrial cosmogenic nuclide 10Be (TCN) to estimate the exposure time from paired sampling of depth profiles in polished bedrock and on overlying erratic boulders. For a given sampling site, the exposure ages for both the polished bedrock and boulder are expected to be the same. However, in six cases out of seven, boulders had significantly higher 10Be surface concentrations than those of the associated polished surfaces. In present and past glacial processes, the 10Be distribution with depth for boulders and bedrocks implies the presence of an inheritance concentration of 10Be. Our study suggests that 10Be concentrations in erratic boulders and in polished bedrocks provide maximum and minimum exposure ages of the glacial retreat, respectively. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Paleogeographical reconstruction of the western French Alps foreland during the last glacial maximum using cosmogenic exposure dating

“…Our results are also in agreement with previous dating of the glaciers position at the entrance of the Romanche and Arc valleys at 17.5 ka and 15 ka, respectively (Schwartz et al, 2017;Prud'homme et al, 2020), and between 17.5-16 ka in the Arve valley (Coutterand and Nicoud, 2005;Perret, 2014).…”

“…The current lack of quantitative data for the LGM period raises the question of glacier paleogeography in the western French Alps foreland during this period. In the western French Alps, 10 Be surface-exposure dating spanning the late glacial and the Holocene period has only been applied on the inner valleys (Coutterand and Nicoud, 2005; Cossart et al, 2008, 2012; Perret, 2014; Wirsig et al, 2016; Le Roy et al, 2017; Schwartz et al, 2017; Protin, 2019; Protin et al, 2019; Prud'homme et al, 2020) and not in the foreland area.…”

Timing and extent of glaciation since the Last Glacial Maximum in the upper Kalguty basin, Russian Altai Mountains

“…On the other hand, the roches moutonnées at Kalgutinskii Rudnik could have been buried by till for some time, and by a thin snow cover. Prud'homme et al (2020) suggested that apparent exposure ages in erratic boulders and polished bedrock should be considered as maximum and minimum ages of glacial retreat, respectively. As we discarded the two oldest ages based on discordant 26 Al data, the remaining oldest age of c .…”

Cosmogenic 3He paleothermometry on post-LGM glacial bedrock within the central European Alps

“…First, our approach relies on the assumption that bedrock surfaces have experienced a simple exposure history along the time period recorded by 10 Be concentrations, without pre-exposure or episodic coverage (i.e., non-erosive cold-based ice). Depth profiles of 10 Be measure-ments on glacially polished bedrocks in the western Alps, with apparent exposure ages of 10-20 ka, indicate that an inherited 10 Be concentration due to insufficient glacial erosion may persist and could lead to up to 9 % age overestimates (Prud'homme et al, 2020). Similarly, Wirsig et al (2016b) suggested potential but limited pre-LGM (less than a few ka overestimate) inheritance for some GELM samples.…”