Maximum glacier extent of the Penultimate Glacial Cycle in the Upper Garonne Basin (Pyrenees): new chronological evidence

Abstract

<p>The Upper Garonne Basin constituted the longest glacier of the Pyrenean ice field during the Late Pleistocene. From the peaks of the axial Pyrenees that exceed 2,800-3,000 m, the Garonne palaeoglacier flowed along ~80 km northwards during the major glacial advances reaching only 420-440 m. This palaeoglacier reached the Pyrenean foreland, at the Loures-Barouse-Barbazan basin (LBBB) where it formed a terminal moraine complex that is examined in this work. We have constrained the timing of the maximum glacial extent as well as the onset of the deglaciation from the end of the Last Glacial Cycle (LGC) based on the geomorphological observations and a 12-sample dataset of 10Be Cosmic-Ray Exposure (CRE) ages. There are two moraine systems at the LBBB, where the first is composed of weathered ridges at the outermost part of the basin and the second encompasses well-preserved ridges stretching across the innermost part of the basin. Chronological data shows that the external moraines were abandoned by the ice at the end of the Penultimate Glacial Cycle (PGC) and the onset of the Eemian Interglacial, at ~129 ka. The few existing reliable boulders to date in the internal moraine showed inconsistent ages as they were probably affected by post-glacial processes and therefore, this work adds no evidence of subsequent glacial advances or standstills during the LGC in the LBBB. However, the terminal basin was already deglaciated during the global Last Glacial Maximum (GLGM) at 24-21 ka, as revealed by exposure ages from polished surfaces at the confluence of the Garonne-la Pique valleys, 13 km south of the entrance of the LBBB. This study introduces the first solid CRE database in the Pyrenees for the glacial advance that occurred during the PGC and provides also new evidence from the GLGM when the Garonne palaeoglacier had already significantly shrunk.</p>