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

Marcelo Fernandes,Karim Keddadouche,Irene Schimmelpfennig,Marc Oliva,Josep Ventura,Georges Aumaître,David Palacios,José María Fernández-Fernández,Gonçalo Vieira,Julia García-Oteyza,Magali Delmas,Aster Team

doi:10.1007/s12665-021-10022-z

Abstract

The Upper Garonne Basin included the longest glacier in the Pyrenees during the Late Pleistocene. During major glacial advances, the Garonne palaeoglacier flowed northwards along ~ 80 km from peaks of the axial Pyrenees exceeding 2800–3000 m until the foreland of this mountain range at the Loures–Barousse–Barbazan basin (LBBb), at 420–440 m. Here, the palaeoglacier formed a terminal moraine complex that is examined in this work. Based on geomorphological observations and a 12-sample data set of 10Be Cosmic-Ray Exposure (CRE) ages, the timing of the maximum glacial extent was constrained as well as the onset of the deglaciation from the end of the Last Glacial Cycle (LGC). Chronological data shows evidence that the external moraines in this basin were abandoned by the ice at the end of the Penultimate Glacial Cycle (PGC) and the onset of the Eemian Interglacial, at ~ 129 ka. No evidence of subsequent glacial advances or standstills occurred during the LGC in this basin were found, as the few existing datable boulders provided in the internal moraine showed inconsistent ages, thus probably being affected by post-glacial processes. The terminal basin was already deglaciated during the global Last Glacial Maximum at 24–21 ka, as revealed by exposure ages of 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 CRE ages in the Pyrenees for the glacial advance occurred during the PGC and provides also new evidence that glaciers had already significantly shrunk during the LGM.

Highlights

High and middle mountain landscapes in mid-latitude ranges have been mostly shaped by glaciers during Pleistocene glacial cycles
The dimensions of Pyrenean glaciers at the time of the gLGM are well established in the eastern part of the range, where 10Be, 36Cl and Schmidt hammer exposure ages have been obtained from boulders embedded in moraines and from ice-scoured rock steps protruding from valley floors (Fig. 1a; Pallàs et al 2006, 2010; Delmas et al 2008; Palacios et al 2015a; Andrés et al 2018; Tomkins et al 2018; Synthesis in Delmas et al 2021b)
Their timing is constrained by the 10Be exposure ages obtained from moraine boulders and polished surfaces (Tables 2, 3)

Summary

Introduction

High and middle mountain landscapes in mid-latitude ranges have been mostly shaped by glaciers during Pleistocene glacial cycles. In the Pyrenees, where this research focuses on, currently available data indicate that the most extensive Pyrenean glaciation of the Late Pleistocene occurred prior to the global Last Glacial Maximum (gLGM: 26–19 ka; Clark et al 2009) This outermost stadial position (local Last Glacial Maximum; lLGM) was reached not just once, but several times between MIS 5b (or MIS 4) and MIS 3 (Fig. 1a; Mardones and Jalut 1983; Andrieu et al 1988; García-Ruiz et al 2003, 2013; Lewis et al 2009; Pallàs et al 2010; Delmas et al 2011; Turu et al 2016; Sancho et al 2018; Tomkins et al 2018; Synthesis in Delmas et al 2021a). Other periods of glacial advance and retreat during Termination I are well-known for northern and southern valleys

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Discussion

Conclusion