Rock glacier development in the Northern Calcareous Alps at the Pleistocene-Holocene boundary

Relict rock glaciers provide information on past discontinuous permafrost and former mean annual air temperatures. A lack of records showing former permafrost distribution along the northern Alpine fringe prompted the investigation and numerical dating of a belt of relict rock glaciers in the Karwendel Mountains of the Northern Calcareous Austrian Alps. In two neighbouring cirques that were still glaciated during the early Younger Dryas, eleven 36Cl exposure ages from boulder surfaces were obtained. The ages imply the onset of rock glacier activity around ~12.3ka with subsequent stabilization and permafrost melt out no later than ~10.1ka. Hence, rock glacier formation coincided with glacier retreat in the cirques around the mid-Younger Dryas and continued into the early Holocene. As permafrost induced features, the rock glacier termini indicate the local past lower limit of discontinuous permafrost in open cirque floors at ~2000masl, which is around 400m lower than during the mid-twentieth century at comparable locations in the Karwendel Mountains. Thus, a mean annual air temperature reduction of ~−2.6 to −3.8°C relative to the mid-twentieth century is inferred. Based on a minimum glacier equilibrium line altitude in the cirques, a summer temperature reduction of less than −2.6 to −1.8°C is shown, suggesting an increased seasonality at the time of rock glacier activity.