Spatial distribution and morphometry of permafrost-related landforms in the Central Pyrenees and associated paleoclimatic implications

Abstract

Present and past permafrost distribution in the Pyrenees is still under discussion. As in other mid-latitude mountain regions, rock glaciers and protalus lobes are the main indicators of permafrost conditions. In this paper we examine the distribution of these landforms in Aran and Boí valleys, formerly glaciated U-shaped valleys ranging from 600 to 3000 m in the southern and northern sides of the Central Pyrenees, respectively. The spatial distribution of these landforms and their chronostratigraphic position within the valley allow a better understanding of the climatic and environmental conditions necessary for their development. Up to 151 permafrost-related landforms were identified in the Boí valley, including 56 rock glaciers and 95 protalus lobes. In the case of the northern Aran valley, 76 rock glaciers and 89 protalus lobes were cataloged corresponding to 165 landforms. Most of the landforms (93% for rock glaciers and 95% for protalus lobes) are located inside the glacial cirques, with a few distributed in the slopes of the formerly glaciated valleys. The lowest elevation of rock glaciers is situated at 1744 m (Aran) and 2007 m (Boí), whereas the lowest protalus lobes are located at 1740 m and 2033 m, respectively. Therefore, this altitude may be indicative of the lowest level recording permafrost conditions during the period in which these landforms formed, with paleotemperatures ca. 7-8 °C lower than present-day. Between 60 and 70% of the protalus lobes and rock glaciers are located between 2300 and 2600 m in Boí valley and 2100 and 2400 m in Aran valley. The aspect shows a prevailing orientation of NW, N and NE for both cases of rock glaciers and protalus lobes, being almost absent in the S, SE and SW aspects. The average slope of both landforms lies between 11 and 27°, with a maximum of 35° for rock glaciers and 29° for protalus lobes. The amplitude/length ratio reveals that rock glaciers placed at lower altitudes are more elongated. The chronostratigraphic position of these landforms within the valley and with respect to the moraine complexes suggests the existence of three generations of permafrost-related landforms which are associated to the massive deglaciation process between the maximum glacial advance of the Last Glaciation and the Holocene.