Application of a combination of dating techniques to reconstruct the Lateglacial and early Holocene landscape history of the Albula region (eastern Switzerland)

Abstract

Landforms in Val Mulix and the Albula region in eastern Switzerland offer a detailed insight into the period between the Oldest Dryas until the early Holocene. To better understand Lateglacial and Holocene climate change in the central Alps, glacial (moraines, polished bedrock) and periglacial (rock glacier) landforms were dated using a combined approach of numerical (cosmogenic 10Be) and relative (Schmidt-hammer, weathering rind thickness) dating techniques. At high-elevation sites near the Last Glacial Maximum (LGM) trimline, 10Be exposure ages of glacially modified bedrock are between 11.2 ka and 13.5 ka. This suggests the persistence of long-lasting small local ice caps after the breakdown of the LGM ice domes or, alternatively, a reformation of ice perhaps during the Younger Dryas. In Val Mulix we obtained one of the first ages for the Daun-stadial (> 14.7 ka) moraines (14.9 ± 1.8 ka), supporting a pre-Bølling chronological position. The age is in excellent agreement with the age of a boulder from an Egesen I moraine located up-valley which we postulate may be a Daun moraine that was re-occupied during the Egesen stadial. A boulder from an Egesen II moraine gave an age of 10.7 ka, which is similar to ages of Egesen II moraines at other sites in the Alps. 10Be ages from boulders found on a relict rock glacier in Val Mulix indicate that the main active phase lasted from the Lateglacial until the early Holocene. The derived mean annual flow rate is of the order of decimetres, which is in accordance with values stated in the literature based on measuring active rock glaciers in the Alps. Exposure ages from a glacially polished rock barrier showed that this area was ice-free at the end of the Younger Dryas (9.0 ± 0.7 ka and 11.9 ± 0.9 ka). The polished bedrocks are located a few hundred meters down-valley from the Little Ice Age (LIA) moraines. This gives direct evidence of a fast ice retreat towards the end of the Younger Dryas, with glacier length variations that did not exceed the 1850 AD extension (Little Ice Age maximum). Surface exposure dating is, however, limited by several methodological constraints. The choice of suitable snow depths plays a crucial role in the calculation of the 10Be ages. Shielding of surfaces from cosmic rays by snow can significantly influence the exposure age, and variations in the estimated annual snowfall in the Albula region since the LGM is therefore a potential source of considerable uncertainty in our measurements. While the measurement of weathering rind thicknesses turned out to be an appropriate tool to support the reconstruction of Lateglacial landscape evolution, Schmidt-hammer R-values were less helpful. The R-values enabled a temporal distinction of landforms within the Holocene (LIA moraine, active rock glaciers) but not within the Lateglacial. From a methodological point of view, the different dating methods enabled a cross-checking, an extended interpretation of the data and a more accurate estimate of the possible sources of error.