Amino acid geochronology of raised beaches in south west Britain

Abstract

Based on (1) the epimerization of l:isoleucine to d:alloisoleucine (DL ratios) in Patella vulgata, Littorina littorea, L. littoralis, L. saxatilis, Littorina species and Nucella lapillus from raised beaches in south west Britain, (2) statistical analysis of the DL ratios, and (3) lithostratigraphic and geomorphic evaluation, three (DL) Stages are proposed. The DL ratios for all the species measured are converted to a Patella vulgata standard.The three (DL) Stages are: 1.(1) The Minchin Hole (DL) Stage, DL ratios 0.175 ± 0.014, defined at a stratotype in Minchin Hole Cave, Gower, Wales.2.(2) A provisionally defined, but as yet, unamed (DL) Stage, because of the current unavailability of a suitable stratotype, with DL ratios of 0.135 ± 0.0143.(3) The Pennard (DL) Stage, DL ratios 0.105 ± 0.016, defined at a stratotype in Minchin Hole Cave, Gower, Wales.Two geochronological models of the three high sea-level events representing the DL Stages are constrained by uranium-series age determinations on stalagmite interbedded with marine beds in Minchin Hole and Bacon Hole Caves, Gower, Wales. A potential ‘fixed point’ in model evaluation is an age determination which is equivalent to Oxygen Isotope Sub-stage 5e (122 ka). The two models are: Oxygen Isotope Stage and age (ka BP)(DL) StageDL RatioModel 1aModel 1bModel 2Pennard0.105 ± 0.0165a(ca. 80)5e(122)5e(122)5c(ca. 100)Unnamed0.135 ± 0.0145e(122)7(194 ?)Minchin Hole0.175 ± 0.0147(186–245)7(186–245)7(216 ?)The ages of the raised beaches are used to constrain the timing and extent of glaciation in the area. The extent of the Late Devensian glaciation is fixed by reference to localities where raised beaches of the Pennard (DL) Stage outcrop, as well as by using (unpublished) data on molluscs incorporated into glacial deposits. Two pre-Devensian glaciations antedate the Minchin Hole (DL) Stage. The Irish Sea Glaciation of the Bristol Channel (Irish Sea ‘older drift’ of South Wales/ Fremington Till of North Devon) is ascribed to Oxygen Isotope 10 or earlier, while the Paviland (Welsh) Glaciation is ascribed to Oxygen Isotope Stage 8 or earlier. An earlier glaciation (Berwyn Glaciation) is inferred from erratics of known Welsh provenance in Essex.