Evaluating post-glacial bedrock erosion and surface exposure duration by coupling in-situ OSL and <sup>10</sup>Be dating

Abstract

Assessing the impact of Quaternary glaciation at the Earth's surface implies understanding of the long-term evolution of alpine landscapes. In particular, it requires simultaneous quantification of the impact of climate variability on past glacier fluctuations and on bedrock erosion. Here we present a new approach for evaluating post-glacial bedrock surface erosion in mountainous environments by combining in-situ cosmogenic 10 Be (TCN) and optically stimulated luminescence (OSL) surface exposure dating. Using a numerical approach, we show how it is possible to simultaneously invert bedrock OSL 15 signals and 10 Be concentrations into quantitative estimates of post-glacial exposure duration and bedrock surface erosion. By exploiting the fact that OSL and TCN data are integrated over different timescales, this approach can be used to estimate how bedrock erosion rates vary spatially and temporally since glacier retreat in an alpine environment.

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

“…The OSL signal was bleached near the surface and reached a plateau at depth. As a reference profile, a model was computed (Lehmann et al, 2019, their equation 2) using t 0 and considering no erosion (dashed black line in Figs. 3A-3C).…”

“…The 10 Be concentrations provided us with constraints on the time since the rocks were exposed to cosmic rays and with a temporal framework for the possible erosion histories since exposure. OSL profiles constrained the erosion history since the rock exposure to light following ice decay (Lehmann et al, 2019). Note that it is the difference in sensitivity between 10 Be and OSL that makes it possible to quantify surface erosion rate histories over short (<10 2 yr) and long (>10 4 yr) time scales (Sohbati et al, 2018;Lehmann et al, 2019).…”

“…The evolution in time of the OSL bleaching front into a rock surface depends on exposure age, surface erosion, electron trapping and detrapping 1 GSA Data Repository item 2020042, supplemental details on sample preparation, measurement, and analysis, as well as a sample list with their characteristics and measured 10 Be concentrations, calibration details, and luminescence signal, is available online at http://www.geosociety.org/datarepository/2020/, or on request from editing@geosociety.org. (bleaching) rates, and athermal loss (Lehmann et al, 2019, their equation 1). While the latter three variables can be constrained from laboratory measurements, the erosion history is unknown.…”

“…In this inversion, we assumed that erosion rates evolved as step functions from no erosion to erosion going forward in time ( Fig. 2; Lehmann et al, 2019). A reference OSL profile was established by taking the TCN exposure age with no erosion correction (t 0 ).…”

“…To address these issues, we adopted a new method that combined optically stimulated luminescence (OSL) and TCN surface exposure dating (Sohbati et al, 2018;Lehmann et al, 2019). In this study, we applied this methodology to investigate how erosion rates have evolved over time scales of 10 1 -10 4 yr on bedrock surfaces of the Mer de Glace (Mont Blanc massif, European Alps) and how morphometric and climatic factors control their evolution.…”