Josep Ventura

In the highest tributaries of the Upper Garonne Basin, Central Pyrenees, cirques up to 2600 m a.s.l. were already deglaciated by 15–14 ka. The long‐term deglaciation during Termination‐1 (T‐1) was interrupted by glacial advances within the cirques during the Bølling–Allerød (B‐A) interstadial and the Younger Dryas stadial. The cirques preserve a variety of glacial and periglacial landforms whose chronologies are poorly known. This study is focused on the Lòcampo cirque (42°38′06″N and 0°59′10″E), Upper Garonne Basin, where a detailed geomorphological map and ¹⁰Be terrestrial cosmic ray exposure (CRE) dating allowed us to constrain the chrono‐sequence between the glacial and periglacial domains. In the small Lòcampo cirque, a glacier formed a cirque moraine between 2200 and 2300 m a.s.l., which surrounds a relict rock glacier encompassing several transversal ridges. Additionally, longitudinal ridges typically observed in debris‐covered glaciers are preserved between the moraine and the rock glacier. The eight‐sample data set of CRE ages indicates the formation of the cirque moraine during the second half of the B‐A, by 13.2±1.1 ka. Exposure ages from the rock glacier boulders show a range between 13.6±0.9 and 11.9±0.7 ka, which did not allow its formation to be chronologically constrained. Therefore, the environmental evolution following the moraine stabilization could follow the formation of a debris‐covered glacier at the bottom of the Lòcampo cirque, with the subsequent formation of the rock glacier. After the rock glacier formation, its front rapidly ceased at 13.6±0.9 ka, while the upper ridges gradually stabilized until it became definitively relict at 11.9±0.7 ka or afterwards. These results show evidence of the complex glacial to periglacial transition that needs more robust chronological data sets to better understand the role of climate forcing and local topography during the deglaciation in mid‐latitude mountain environments.

The Upper Garonne Basin hosted the largest glacier of the Pyrenees during Late Pleistocene glaciations. Glacial retreat started after the Last Glacial Maximum (20-19 ka) of the last glacial cycle, and by the onset of the Bølling–Allerød interstadial (B-A; 15-14 ka) ice masses were already confined within the glacial cirques and highest valleys. In the Aran Valley, headwaters of the Garonne Basin, the low glacial cirques (with maximum elevations of ~2600 m) preserve a significant variety of glacial and periglacial landforms of unknown age. This study focuses in the Locampo Cirque, where we conducted a detailed geomorphological mapping of the glacial and periglacial landforms and dated a set of 8 samples using 10Be Cosmic-Ray Exposure (CRE). This approach allowed to establish the chronology and identify the mechanisms involved in the transition from glacial to periglacial dynamics. CRE ages reveal that the external moraine closing the cirque (2210-2335 m) probably formed during the second half of the B-A at 13.7 ± 1.5 ka. Subsequently, glaciers retreated, and a rock glacier developed inside the external moraine (2210-2310 m) during the paraglacial phase. The final stabilization of this rock glacier most likely occurred during the Younger Dryas (YD)-Holocene transition, at 11.9 ± 0.7 ka. These results report the first chronology on the formation of rock glaciers in the northern slope of the Central Pyrenees and confirm the pattern observed in other mid-latitude mountains, showing that the final deglaciation occurred during the (cold but dry) YD, accompanied by the development and stabilization of rock glaciers.