Tracking rockglacier evolution in the Eastern Alps from the Lateglacial to the early Holocene

Abstract

Relict rockglaciers are distinctive indicators of past permafrost occurrence. Their lower limit is attributed to a former mean annual air temperature (MAAT) of below −2 °C. This study provides a comprehensive dataset of 34 10Be exposure ages from boulders along two complex series of relict rockglaciers, called Tandl rockglaciers and Norbert rockglaciers (Carinthia, Austria). The lowest Tandl rockglacier complex stabilised around 14 ka at an elevation of 1350 m a.s.l., the lowest Norbert rockglaciers (1580 and 1730 m a.s.l.) stabilised around 15.7 ka. Additionally, in both study sites the low elevation relict rockglaciers interacted with glacial deposits of the local pre-Bølling glaciers (Gschnitz stadial glacier). Temperature lowering based on our data of the Gschnitz rockglaciers ranges between 6.3 and 4.5 °C compared to modern MAAT. The cross-cutting relationships of the rockglaciers and the glacial deposits together with the exposure ages of the rockglaciers, indicate that these rockglaciers, and therewith also permafrost, developed shortly after or even simultaneously with retreat of the Gschnitz stadial glaciers. This is the first permafrost formation in the Alpine areas after the retreat of the (warm-based) Last Glacial Maximum glaciers. The Tandl and Norbert rockglacier lobes located at higher elevations, up to about 2300 m a.s.l., finally stabilised in the early Holocene; ages of several dated lobes lie between 12-10 ka. At this time, which corresponds to the Egesen stadial (Younger Dryas) cold phase, rockglaciers and glaciers co-existed. From the lowest position of the Egesen rockglacier lobe at the Tandl site (1700 m a.s.l.), a temperature lowering for the Egesen stadial of −4.6 °C was calculated. This study highlights the potential of relict rockglacier deposits as an independent paleoclimate archive and their usefulness for reconstruction of past permafrost development and distribution in high mountain areas when they can be placed in a temporal framework.