Quaternary glaciations in the central Cantabrian mountainsnew geomorphological and geochronological contributions

Résumé

The Cantabrian Mountains (42¿43ºN) extend along the northern coast of the Iberian Peninsula for more than 450 km, displaying broad land areas above 2000 m altitude at just 20-150 km inland from the Atlantic Ocean. Glacial landforms and deposits are widespread, suggesting that this mountain region could have been particularly sensitive to past climate fluctuations. Previous studies suggest that glaciers developed asymmetrically in both sides of the range. Available geochronological framework suggests at least 2 glacial advances occurring during the Last Glacial Cycle: (i) a glacial maximum stage that took place at a minimum age of 36¿45 ka BP (MIS 3) and (ii) a second glacial advance at 19¿23 ka BP (MIS 2), for which glaciers would have attained comparable dimensions. Until date, asynchronous glacial maximum conditions have been reported only for the Castro Valnera (MIS 4) and Queixa-Invernadoiro (MIS 6) massifs based on a limited number of ages. These chronological data are mainly based on radiocarbon and optically stimulated luminescence applied to glacial related deposits and secondarily on cosmogenic nuclides applied directly to glacial sediments. Despite glacial studies have been continuously developing since the 19th century, the number, extent and timing of past glacial oscillations and the paleoclimate significance of this terrestrial record is not fully understood yet, partly due to the scarcity of numerical ages and to the lack of detailed glacial reconstructions in many valleys. The main goal of this work is to provide new geomorphological and geochronological constrain that may help to understand better regional landscape evolution and establish correlations with other terrestrial and marine paleoclimate proxies. The study area corresponds to a 60 km-long segment of the Central Cantabrian Mountains (930 km2), corresponding to the Nalón-Aller-Ponga and Curueño-Porma-Esla upper basins. Some specific aims were: (i) improving the geomorphological map; (ii) reconstruct numerically the palaeo-glaciers topography for estimating former equilibrium lines and characterize the asymmetric distribution of glaciers between mountain slopes; (iii) providing additional time constrain through the application of 10Be surface exposure dating to glacial and periglacial erosion surfaces, and (iv) correlating this record with other paleoclimate proxies. The reconstruction of former glaciers revealed a strong asymmetric development of glaciers, with about a third of the total glaciated area located in the north side of the range during the local glacial maximum, where glaciers barely reached 6 km-long and 150 m-thick. In contrast, glaciers from the southern slope recorded up to 19 km-long and more than 300 m-thick. Paleo-ELA estimations clearly show a lowering gradient in the eastward direction, fitting current precipitation pattern and suggesting similar landscape influence on precipitation in the past than nowadays. A minimum exposure age of 106 ka was obtained for the local glacial maximum, which suggests a response of local glaciers to the sharpest summertime insolation minima recorded in the northern hemisphere during the last 150 ka. Glaciers would have attained comparable outline margins during Glacial Stage 1, at a minimum exposure age of 50.8 ka (MIS 3) and probably also at ca. 24 ka (global LGM). A progressive shortening and thinning of glacier tongues characterized subsequent Glacial Stages 2 and 3, indicating a glacier retreating pattern from 16¿15 ka to 14 ka that matches the Late Glacial evolution observed for other mountain settings of the Iberian Peninsula. Finally, the latest glacial oscillation or Glacial Stage 4 corresponds to a glacial cirque readvance which would have culminated with rock glacier formation. Minimum exposure ages bracketed this last advance between 12.5 and 11.6 ka, coevally with the Younger Dryas cold interval.