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

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.