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

摘要

We present constraints on the chronology, palaeogeography and palaeoclimatic implications for western Europe of Middle and Late Pleistocene glaciation in the Ariège catchment, north-eastern Pyrenees. Based on 37 cosmogenic ~(10)Be 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 Würmian 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 Ariège trunk valley attained lengths of ~25km during the Oldest Dryas Stadial. The cosmogenic exposure ages additionally provide the first direct evidence of ~10km-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 (Ariège, Carol-Malniu, Têt), where Würmian 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 Würmian 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.