In situ10Be production-rate calibration from a 14C-dated late-glacial moraine belt in Rannoch Moor, central Scottish Highlands

Abstract

An objective of terrestrial in situ cosmogenic nuclide research is to obtain precise and accurate production-rate estimates on the basis of geological calibration sites from a diverse range of latitudes and altitudes. However, a challenge has been to establish production rates on the basis of landforms for which independent ages have been determined directly using absolute isotopic dating techniques. Here we present a 10Be production-rate calibration from a recessional moraine belt located in Rannoch Moor, central Scottish Highlands (56.63°N, 4.77°W; ∼310 330 m a.s.l.). This moraine belt was deposited at the margin of the disintegrating late-glacial West Highland ice field (WHIF) during the final stages of deglaciation. Minimum-limiting 14C dates on macrofossils of the earliest terrestrial vegetation to arrive on the landscape place the timing of moraine abandonment, and hence exposure of morainal boulder surfaces to the cosmic-ray flux, to no later than 12,480 ± 100 calendar years before C.E. 1950 (cal yrs BP). Maximum-limiting 14C dates on marine shells incorporated into basal tills deposited during expansion of the WHIF to its full late-glacial extent place the onset of deglaciation, and thus deglaciation of Rannoch Moor, to no earlier than 12,700 ± 100 cal yrs BP. After removal of a single high-concentration outlier, surface 10Be concentrations of 11 boulders rooted in two sub-parallel moraine ridges exhibit a high degree of internal consistency and affords an arithmetic mean of 6.93 ± 0.24 [x104] atoms g−1 (1σ). This data set yields a site-specific 10Be production rate of 5.50 ± 0.18 at g−1 yr−1, based on the midpoint age 12,590 ± 140 cal yrs BP of the bracketing 14C chronology. Transforming this result to sea-level/high-latitude (SLHL) neutron-spallation 10Be production-rate values using Version 3 of the University of Washington (UW) Online Production-Rate Calculator yields upper and lower bounds, and a mid-point rate. Maximum-limiting SLHL 10Be production rates, based on minimum-limiting 14C age control, are 3.95 ± 0.11 (2.7%) at g−1 yr−1 for the commonly used Lm and St scaling protocols. The corresponding (non-dimensional) correction factor for a reference production rate determined by the LSDn scaling model is 0.79 ± 0.02 (2.7%). Minimum-limiting SLHL reference 10Be production rates, based on maximum-limiting 14C age control, are 3.88 ± 0.11 (2.7%) at g−1 yr−1 (St) and 3.89 ± 0.11 (2.7%) at g−1 yr−1 (Lm). The corresponding correction factor for LSDn scaling is 0.77 ± 0.02 (2.7%). SLHL reference production-rate values based on a midpoint age of 12,590 ± 140 yrs are 3.91 ± 0.11 (2.8%) at g−1 yr−1 (St) and 3.92 ± 0.11 (2.8%) at g−1 yr−1 (Lm). The corresponding correction factor for LSDn scaling is 0.78 ± 0.02. The production-rate calibration data set presented here for Scotland yields SLHL values that agree with those determined from calibration data sets based on directly dated landforms from northeastern North America, the Arctic, the Swiss Alps, the Southern Hemisphere middle latitudes, and from the high tropical Andes. We suggest that this production-rate calibration data set from the central Scottish Highlands, used together with the UW online calculators, will produce accurate 10Be surface-exposure ages in the British Isles.