Cosmogenic 10Be and 26Al ages of Holocene moraines in southern Norway II: evidence for individualistic responses of high-altitude glaciers to millennial-scale climatic fluctuations

Abstract

Abstract:10Be and 26Al exposure ages are reported for boulder moraine ridges in front of two high-altitude cirque glaciers (Austanbotnbreen, Jotunheimen and Østre Tundradalskyrkjabre, Breheimen) in southern Norway. Ages and 1σ external uncertainties for Austanbotnbreen moraines range from 7.4 ± 0.75 ka to 8.7 ± 0.9 ka when no adjustment for snow shielding is made. With the maximum conceivable snow cover effect (2 m thick, a density of 0.2 g/cm3 , 6 months duration) at the exposed site, the resulting adjusted (7.7 ± 0.8 to 10.9 ± 1.3 ka) as well as unadjusted ages suggest moraine formation possibly during both the 8.2 ka event (Finse Event in southern Norway) and the Erdalen Event ( c. 10.0 ka). In contrast, ages obtained for the outer ridges of the large moraine complex in front of Østre Tundradalskyrkjabre range from 1.1 ± 0.3 ka to 1.9 ± 0.4 ka assuming no snow shielding. Assuming extreme snow-lie conditions (4 m thick, 0.2 g/cm3 density, 6 months duration) at this more sheltered site, the adjusted ages (1.4 ± 0.4 to 2.4 ± 0.5 ka) still indicate late-Holocene, pre-`Little Ice Age' ridge formation. Less likely alternative explanations of the late-Holocene ages, including avalanching of debris onto the moraine, inventories of inherited nuclides in the samples and possible episodic `push-deformation' disturbance of the ice-cored moraine complex, are discussed. The results point to the potential of surface exposure dating in reconstructing Holocene glacier variation chronologies, particularly with respect to high-altitude glaciers and to individualistic responses of these glaciers to Holocene millennial-scale climatic fluctuations. Difficulties with dating moraines in front of small high-altitude glaciers using cosmogenic nuclides, including uncertain snow shielding effect, short glacial transport distance, possibly not `zeroing' an inherited cosmogenic signal and repeated disturbance of a moraine complex, are discussed.