Climate warming and the persistence of buried ice in the Pyrenees: Multi-Proxy evidence from Clots de la Menera cirque (Andorra)

Abstract

The Central-Eastern Pyrenees experienced extensive glaciation during the Late Pleistocene's cold phases, with the final deglaciation occurring during Termination-1 (∼19-11 ka). This period saw multiple glacial advances and retreats, leaving abundant geomorphic evidence in valleys and cirques. This is the case of the Clots de la Menera (CM) Cirque, located in Andorra, a cirque formed below relatively low peaks (ca. 2600–2800 m a.s.l.) in the transition zone between the Central and Eastern Pyrenees, which contains a wide variety of glacial and periglacial landforms. This study aims to refine our understanding of the timing and processes that shape the present-day mountain landscape at the cirque level by presenting a multi-approach study of the environmental evolution from the last glaciation to the present conditions in the CM Cirque. Cosmic-ray exposure (CRE) dating of a moraine and a polished surface reveals the final deglaciation of the cirque at 14.9 ± 0.1 ka and 12.8 ± 1.3 ka, respectively. As the glaciers disappeared, two rock glaciers (RG1 and RG2) formed shortly thereafter at 9.7 ± 0.5 ka and 11.1 ± 0.6 ka, respectively. Differential synthetic interferometric aperture radar (InSAR) data suggest that these rock glaciers remain active, moving at 1.1–4.1 cm yr⁻¹. Geophysical surveys, including Vertical Electrical Sounding and Nuclear Magnetic Resonance, suggest the presence of frozen masses within the core of the rock glacier, which likely corresponds to permafrost conditions and buried ice. Thermal measurements further support the probable occurrence of frozen bodies within these rock glaciers, recording winter temperatures of −4 °C at the snow/ground interface. Therefore, CRE ages suggest that the still-moving rock glaciers in the CM Cirque formed during the Early Holocene and continued to move due to the preservation of frozen masses within these features at elevations of about 2400–2500 m, well below the regional 0 °C annual isotherm. The results confirm the formation of rock glaciers during deglaciation and demonstrate the persistence of frozen masses in a permafrost environment that no longer receives contributions of ice or debris; with minimal displacement, this landform maintains the stability of the boulders. Integrating these findings enhances our understanding of permafrost distribution in the Pyrenees, its relationship with past environmental change, and the significance of CRE dating in unveiling past rock-glacier dynamics.