Moraines in the Austrian Alps Record Repeated Phases of Glacier Stabilization through the Late Glacial and the Early Holocene

Abstract

Climate is currently warming due to anthropogenic impact on the Earth’s atmosphere. To better understand the processes and feedbacks within the climate system that underlie this accelerating warming trend, it is useful to examine past periods of abrupt climate change that were driven by natural forcings. Glaciers provide an excellent natural laboratory for reconstructing the climate of the past as they respond sensitively to climate oscillations. Therefore, we study glacier systems and their behavior during the transition from colder to warmer climate episodes, focusing on the period between 15 and 10 ka. Using a combination of geomorphological mapping and beryllium-10 surface exposure dating, we reconstruct ice extents in two glaciated valleys of the Silvretta Massif in the Austrian Alps and find that the general ice retreat during the deglaciation after the Last Glacial Maximum (LGM) was interrupted by glacier stabilization during the Oldest Dryas to Bølling transition (moraine age: 14.4 ± 1.0 ka), during the Younger Dryas (YD; 12.9-11.7 ka), and during the Early Holocene (EH; 12–10 ka). The first moraine age group indicates a lateral stable ice margin that postdates the ‘Gschnitz’ stadial (ca. 17–16 ka) and predates the YD. It shows that local inner-alpine glaciers were larger than during the subsequent YD until the onset of the Bølling warm phase (ca. 14.6 ka), or possibly even into the Bølling. The second age group ca. 80 m below the (pre-)Bølling moraine indicates ice extents during the YD cold phase and captures the spatial and temporal fine structure of glacier retreat during this period. The ice surface lowered by 50–60 m through the YD, which is indicative of milder climate conditions at the end of the YD compared to its beginning. Finally, the third age group falls into a period of more substantial warming, the YD–EH transition, and shows discontinuous warming during the glacial to interglacial transition. The new geochronologies synthesized with pre-existing moraine records from the Silvretta Massif evidence three cold phases that punctuated the general post-LGM warming trend, and illustrate the sensitive response of Silvretta glaciers to abrupt climate oscillations in the past.