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

The deglaciation history of the European Alps after the Last Glacial Maximum (LGM) is generally thought to be well established, however, the timing of glacier retreat in the eastern Alps remains poorly constrained. Here, we present the first 10 Be exposure ages from the Klagenfurt Basin (Carinthia, Austria), which was covered by the piedmont lobe of the Drau glacier during the LGM. The 10 Be ages were obtained from glacially polished quartz veins located 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). Our findings indicate that the deglaciation of the Klagenfurt Basin occurred during the late stage of the Oldest Dryas stadial, which is consistent with published 10 Be ages (mean age: 15.0 ± 1.2 ka) from the flat tops of the ∼2000-m-high Nock Mountains that are located NW of the Klagenfurt Basin. Both datasets challenge previous interpretations, which inferred that the southeastern Alps were already ice-free by ∼19-18 ka based on 14 C ages from two postglacial lakes (Lakes Längsee and Jeserzersee). Our reassessment of the sediment and pollen records from the two lakes shows, however, that the marked transition from non-arboreal to boreal pollen occurred at the end of the Oldest Dryas after deglaciation of the Klagenfurt Basin and that the 14 C ages from bulk sediment overestimate the true sedimentation age due to a lake reservoir effect. A later deglaciation than previously assumed is further supported by our re-interpretation of two published 10 Be datasets from the internal part of the eastern Alps, which indicate LGM ice-surface lowering between ∼18.6 and ∼14.8 ka and rock-glacier stabilization at ∼16-14 ka, respectively. The prolonged existence of the Drau glacier until ∼16 ka was likely promoted by enhanced autumn and winter precipitation during the LGM and until ∼17 ka and, afterwards, by Northern Hemisphere cooling during Heinrich Event 1. The combination of dryer conditions and climate amelioration toward the end of the Oldest Dryas then led to the final retreat of the Drau glacier from the Klagenfurt Basin. • 10 Be exposure ages constrain deglaciation of the Klagenfurt Basin ∼16 ka ago. • Exposure ages were obtained from glacially polished quartz veins. • Data refute ice-free conditions at 19-18 ka inferred from 14 C ages from bulk sediment. • 14 C ages from Lakes Längsee and Jeserzersee overestimate true sedimentation age. • Our findings underline the necessity of dating glacial landforms in the eastern Alps.