Deglaciation of the Klagenfurt Basin (Austria): constraints from 10Be exposure dating and implications for the glacial history of the southeastern Alps

The deglaciation history of the European Alps is thought to be well established, however, the timing of glacier retreat in the eastern Alps remains poorly constrained. Here, we present the first ¹⁰Be exposure ages from the Klagenfurt Basin (Carinthia, Austria), which was covered by the piedmont lobe of the Drau glacier during the Last Glacial Maximum (LGM). The ¹⁰Be ages were obtained from glacially polished quartz veins between ~530 and ~800 m a.s.l. and range from 17.4±0.6 to 13.5±0.7 ka (mean age: 15.9±1.0 ka). The age data indicate that deglaciation of the Klagenfurt Basin occurred near the end of the Oldest Dryas stadial and are consistent with published ¹⁰Be ages from the flat tops of the ~2000-m-high Nock Mountains farther north (mean age: 15.0±1.2 ka) (Wölfler et al., 2022). Both data sets refute a widely accepted scenario, in which the southeastern Alps were already ice-free by ~19-18 ka (e.g., van Husen, 1997; Reitner, 2007; Ivy-Ochs et al., 2023). Our reassessment of the underlying age constraints for this still prevailing view shows that the respective ¹⁴C ages were obtained from bulk-sediment samples in two postglacial lakes (Lake Längsee: Schmidt et al., 1998, 2002; Lake Jeserzer See: Schmidt et al, 2012). ¹⁴C ages from bulk lake-sediment samples are, however, known to overestimate the true sedimentation age due to a reservoir effect (e.g., Ilyaschuk et al., 2009; Hou et al., 2012). At Lake Längsee, the overestimation of the true sedimentation ages by the ¹⁴C ages is confirmed by a layer of Neapolitan Yellow Tuff, whose age was independently determined by ⁴⁰Ar/³⁹Ar dating at its origin (Deino et al., 2004). Later deglaciation than previously assumed is further supported by two published ¹⁰Be age data sets from the Hohe Tauern mountains, which indicate LGM ice-surface lowering between ~18.6 and ~14.8 ka (Wirsig et al., 2016) and rock-glacier stabilization at ~16-14 ka (Steinemann et al., 2020), respectively. Our interpretations agree with palaeo-precipitation records derived from cave carbonates, which indicate enhanced autumn and winter precipitation during the LGM and until ~17 ka (Spötl et al., 2021; Warken et al., 2024). The combined evidence presented in our study shows that deglaciation of the southeastern Alps occurred at ~16-15 ka and hence later than previously thought.

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