The fluvial system response to abrupt climate change during the last cold stage: the Upper Pleistocene River Thames fluvial succession at Ashton Keynes, UK

The last interglacial-glacial cycle (125-10 ka BP) is characterised by nume rous rapid shifts in global climate on sub-Milankovitch timescales, recorde d in the ocean and ice core records. These climatic fluctuations are clearl y recorded in those European terrestrial sedimentary sequences that span th is time period without interruption. In the UK, only fragmentary Upper Plei stocene sequences exist, mainly within the fluvial archive of the major riv er systems such as the Thames. The response of the upper River Thames to ab rupt fluctuations in climate is documented in the fluvial sediments beneath the Floodplain Terrace (Northmoor Member of the Upper Thames Formation) at Ashton Keynes, Wiltshire. A number of criteria are set out by which signif icant changes in the fluvial system may be established from the sedimentolo gical, palaeoecological and geochronological information contained within t he succession. The sedimentary succession is divisible into four facies ass ociations, on the basis of their sedimentology and bounding surface charact eristics. These represent distinct phases of fluvial activity at the site a nd allow changes in fluvial style to be inferred. Palaeoecological reconstr uctions from pollen analysis of peats within the sequence provides an indic ation of the nature and direction of Late Glacial environmental change and optically stimulated luminescence and radiocarbon dating methods provide ch ronological control on the sequence. These data suggest that major changes in fluvial style are recorded within the succession, which can be related t o the climatic fluctuations that took place on the oxygen isotope stage 5a/ 4 transition (approximately 70 ka BP) and the Devensian Late Glacial climat ic warm-cold-warm oscillation (13-11 ka BP). The changes in fluvial style a re a result of variations in sediment supply to the river resulting from ch anges in slope stability, vegetation cover and cold-climate mass movement p rocesses and variations in discharge regime caused by changes in precipitat ion patterns, snow cover, permafrost distribution and vegetation cover. (C) 2001 Elsevier Science B.V. All rights reserved.