Palaeogeography and 10Be exposure-age chronology of Middle and Late Pleistocene glacier systems in the northern Pyrenees: Implications for reconstructing regional palaeoclimates

We present constraints on the chronology, palaeogeography and palaeoclimatic implications for western Europe of Middle and Late Pleistocene glaciation in the Ariege catchment, north-eastern Pyrenees. Based on 37 cosmogenici Be-10 exposure ages obtained from moraines and glacially scoured bedrock exposures, we have dated six regional glacial stades. The chronology emphasises several conclusions that are new to the region: (i) in the Pyrenees, the global LGM occurred several thousands of years after the maximum ice extent of the Wurmian glaciation: (ii) almost full glacier extent occurred again during the global LGM, but only in the eastern Pyrenees: (iii) valley glaciers such as in the Ariege trunk valley attained lengths of similar to 25 km during the Oldest Dryas Stadial. The cosmogenic exposure ages additionally provide the first direct evidence of similar to 10 km-scale fluctuations of ice-front positions during the Late Pleistocene glacial cycle, probably at the time of Marine Isotope Stage 3, as well as rare insights into Middle Pleistocene ice-margin positions. Compared with existing evidence of glacial palaeogeography in other catchments of the Pyrenan range, the data also reveal a sharp contrast in relative ice extent between the glacierized catchments of the eastern Pyrenees (Ariege, Carol-Malniu, Tet), where Wurmian MIE and LGM ice fronts reached similar positions, and the rest of the range, where ice fronts at the time of the MIS 2 and global LGM palaeotemperature minima lagged far behind the Wurmian MIE terminal moraines. We ascribe this non-uniform behaviour among catchments to non-uniform palaeoclimatic conditions along the strike of the Pyrenees. A range of palaeoenvironmental proxies reviewed in the literature highlight a sharp difference in sea-surface temperatures between the warmer Western Mediterranean and the cooler Bay of Biscay. The consequences of such an E-W temperature gradient on weather patterns during the global LGM are compatible with a strengthening of Mediterranean low-pressure systems (particularly the semi-permanent Balearic Low) generating greater precipitation over the eastern Pyrenees compared to Atlantic weather systems further west. (C) 2011 Elsevier B.V. All rights reserved.