Testing current estimates of the in situ cosmogenic 10Be production rate in the north-western British Isles, with implications for ice sheet behaviour during Termination

Abstract

Cosmogenic nuclide surface-exposure dating (SED) is a rapidly growing tool in geoscience owing to its unrivalled potential for directly dating rock surfaces and thus the geomorphic and climatic events they represent. Fundamental to the efficacy of the SED method is reliable constraint of the in situ production rate, which is typically calculated via calibration experiments: cosmogenic nuclide concentrations are measured in surfaces for which the true exposure age is known independently, allowing the production rate to be derived (in atoms g−1 yr−1) for the specific calibration site. This value can then be extrapolated to distal field sites using numerical scaling methods designed to account for spatial and elevational differences in geomagnetic and atmospheric shielding. Thanks to successive and increasingly co-ordinated calibration efforts, production rate estimates for the most widely used cosmogenic nuclide, beryllium-10 (10Be), have improved in recent decades, with the majority converging on sea-level high-latitude (SLHL) values of ∼ 3.8–4.1 atoms g−1 yr−1 (“St” scaling). Nonetheless, there remains sufficient variability among production rates to undermine the reliability of derived surface-exposure ages, particularly for applications to short-lived events such as the abrupt climate shifts of the last glacial termination. To help address this uncertainty, this paper reports new 10Be concentrations from deglacial surfaces on the Redpoint Peninsula in north-west Scotland that were exposed during retreat of the last British ice sheet. By comparing the surface-exposure results from eight current 10Be production rates to local radiocarbon constraint for deglaciation, we (1) evaluate the viability of each production rate for this site and (2) report a maximum SLHL value of 3.925 ± 0.07 atoms g−1 yr−1 (“St” scaling), above which resulting surface-exposure ages will be too young with respect to the Redpoint radiocarbon chronology. This study also demonstrates that the Rannoch Moor 10Be production rate, calibrated against independently dated glacial landforms in the central Scottish Highlands, gives the best match with the 14C control and thus is appropriate for Late Pleistocene applications at these geomagnetic latitudes.