Constraining Quaternary ice covers and erosion rates using cosmogenic 26Al/10Be nuclide concentrations. (1st February 2018)

• Record Type: Journal Article
• Authors: Knudsen, Mads Faurschou; Egholm, David Lundbek
• Is Part Of: Quaternary science reviews. Volume 181(2018)
• Page Start: 65
• Page End: 75
• Publication Date: 2018-02-01
• Subjects: Cosmogenic nuclides -- 26Al/10Be ratios -- Monte Carlo modelling -- Variable erosion rates -- Glacial plucking -- Ice-cover history -- Quaternary; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2017.12.012
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Paired cosmogenic nuclides are often used to constrain the exposure/burial history of landforms repeatedly covered by ice during the Quaternary, including tors, high-elevation surfaces, and steep alpine summits in the circum-Arctic regions. The approach generally exploits the different production rates and half-lives of 10 Be and 26 Al to infer past exposure/burial histories. However, the two-stage minimum-limiting exposure and burial model regularly used to interpret the nuclides ignores the effect of variable erosion rates, which potentially may bias the interpretation. In this study, we use a Monte Carlo model approach to investigate systematically how the exposure/burial and erosion history, including variable erosion and the timing of erosion events, influence concentrations of 10 Be and 26 Al. The results show that low 26 Al/ 10 Be ratios are not uniquely associated with prolonged burial under ice, but may as well reflect ice covers that were limited to the coldest part of the late Pleistocene combined with recent exhumation of the sample, e.g. due to glacial plucking during the last glacial period. As an example, we simulate published 26 Al/ 10 Be data from Svalbard and show that it is possible that the steep alpine summits experienced ice-free conditions during large parts of the late Pleistocene and varying amounts of glacial erosion. This scenario, which contrasts with the original interpretation of more-or-less continuous burial under non-erosive ice over the last ∼1 Myr, thus challenge the conventional interpretation of such data. On the other hand, high 26 Al/ 10 Be ratios do not necessarily reflect limited burial under ice, which is the common interpretation of high ratios. In fact, high 26 Al/ 10 Be ratios may also reflect extensive burial under ice, combined with a change from burial under erosive ice, which brought the sample close to the surface, to burial under non-erosive ice at some point during the mid-Pleistocene. Importantly, by allowing for variable erosion rates, the model results may reconcile spatially varying 26 Al/ 10 Be data from bedrock surfaces preserved over multiple glacial cycles, suggesting that samples from the same high-elevation surface or neighbouring alpine summits may have experienced similar long-term burial under ice, but varying amounts of glacial erosion. Highlights: The timing of erosion is strongly affects concentrations of 10 Be and 26 Al. Variable erosion rates challenge the conventional interpretation of 26 Al/ 10 Be data. Very low 26 Al/ 10 Be ratios may reflect extensive exposure and recent exhumation. Very high 26 Al/ 10 Be ratios may reflect extensive ice cover and a transition in erosion regime. Spatially-varying 26 Al/ 10 Be data may reflect similar long-term burial under ice but varying erosion.