Multi-method constraints on late Pleistocene glacier fluctuations in the Ossau valley (SW France), with wider implications for the Pyrenean icefield during the last glacial maximum

Current evidence throughout the Pyrenees indicates that the local last glacial maximum (LLGM) of the Late Pleistocene occurred earlier than the globally recognised LGM period (26–19 ka). Some LGM glaciers subsequently grew to sizes similar to those of their LLGM predecessors, but others fell short. These apparent differences have been attributed either to peculiar palaeoprecipitation patterns across the mountain range or to artifacts among the different dating methods underpinning the glacier fluctuation chronologies. Here we test these two alternatives by presenting 33 new 10 Be and 36 Cl surface exposure ages from the terminal moraines of the Ossau valley, by elaborating 3D glacial reconstructions fitted to successive generations of landforms, and by discussing the data in light of published 14 C ages from the nearby Estarrès threshold-lake glaciolacustrine sequence and from several archaeological sites. Results indicate that a first major glacier advance occurred at some time before 35 ka cal BP, after which the Ossau glacier receded by 4 km between 34–30 ka and 31–27 ka cal BP. A subsequent readvance generated LLGM moraines ca. 22.7 ka, followed by post-LLGM recessional moraines (age: 20.2 ka) positioned 2 km to the south. Around 19 ka cal BP, the glacier retreated by a further 30 km. By revealing an interval of maximum glacier growth during the LGM, this new chronology suggests that the distinction between LGM and LLGM in the Pyrenees may increasingly become obsolete as multi-method approaches help to refine Late Pleistocene icefield fluctuation patterns in other valleys. • Late Pleistocene glacier fluctuations in the Pyrenees are unevenly documented. •New 10 Be and 36 Cl exposure ages indicate maximum advances before and during the LGM. • The most recent maximum extent of glaciation in the Ossau valley occurred ca. 23 ka. • Ca. 20 ka, the Ossau glacier had already receded by 2 km, melting rapidly after 19 ka. • Results match glacier limits inferred from 59 14 C-dated lake and prehistoric deposits.