Comparing Schmidt-hammer rebound values with terrestrial cosmogenic nuclides-derived ages in the Reißeck Mountains, Hohe Tauern Range, Austria

Knowing the age and evolution of present-day relict rock glaciers help to decipher past landscape evolution. In an Alpine context, this is particularly relevant for the Alpine Lateglacial and early Holocene period. Relative dating of the surface of a relict rock glacier with the Schmidt-hammer exposure-age dating (SHD) approach has the advantage of a cheap, rather easy handling, and fast method in comparison to absolute age dating approaches such as for instance terrestrial cosmogenic nuclides (TCN) using ¹⁰Be. A combination of the two methods at identical sampling sites might help to reduce intrinsic uncertainties of both methods. However, there is still a lack of direct comparisons of dating results based on TCN ages to ages based on Schmidt hammer rebound values. In this study, we compared published TCN ages from 34 sampling sites of relict rock glaciers and neighboring landforms taken from Steinemann et al. (2020) with measured SHD data. The TCN-samples have been taken primarily from two rock glacier systems consisting of gneissic rocks named Tandl and Norbert in the Reißeck Mountains, Carinthia, Austria. At each site where Steinemann et al. (2020) took a sample to quantify the absolute age based on ¹⁰Be, we carried out 100 individual Schmidt-hammer rebound measurements. The results of the two methods were partly consistent but partly difficult to interpret. At the study site Tandl (n=20), a significant correlation between TCN ages and R-values has been detected. The age calibrating curve for the Tandl site, suitable to calculate absolute ages from the relative R-values, is: age[ka] = -1.128ˑx R + 55.642 with an R² of 0.803. In contrast, no significant correlation between R-values measured at the study site Norbert (n=14) in comparison to ages derived by TCN data was revealed. This might be due to a more complex transport history of the sampled boulders in terms of both glacial as well as periglacial transport elements, the influence of a more complex lithology at Norbert, elevation effects (impacting differences in weathering), block instability or exhumation and erosion effects of the sampled boulders. Furthermore, gneiss is more difficult to measure with the Schmidt-hammer approach due to its common anisotropy compared to, for example, granite, which is the lithology mostly used in previous studies where TCN and SHD was compared. Therefore, our study comprises an interesting case study of both successful and problematic direct comparisons of TCN- and SHD-derived age data.

Steinemann O, Reitner JM, Ivy-Ochs S, Christl M, Synal HA (2020) Tracking rockglacier evolution in the Eastern Alps from the Lateglacial to the early Holocene Quaternary Science Reviews 241:106424. https://doi.org/10.1016/j.quascirev.2020.106424