Unraveling paraglacial activity on Sierra de Gredos, Central Spain: A study based on geomorphic markers, stratigraphy and OSL

- Paraglacial activity in Central Spain is assessed at Gredos Gorge, Sierra de Gredos.
- OSL, stratigraphy and fieldwork analyses are useful to evaluate paraglacial activity.
- Gredos Gorge has two phases of paraglacial activity before and after deglaciation.
- Mediterranean climate in Gredos Gorge prevents glacigenic sediment stabilization.

We study the paraglacial activity in Gredos Gorge, a glaciated valley of Sierra de Gredos (Central Spain), using geomorphic markers, stratigraphy and an approach based on OSL. We use luminescence signals from debris flow and fluvial deposits as well as OSL dating of three deposits to detect the main paraglacial processes in Gredos Gorge. We identify two major phases of paraglacial activity during and after the retreat of Last Glacial Maximum (LGM) glaciers. The first phase consisted of high discharge debris flows, sourced up-valley, triggered during the deglaciation. The second phase started ~ 11 ka and is characterized by two processes: (1) the triggering of low discharge debris flows sourced on the flanks of moraines, and (2) the reworking of paraglacial deposits by rivers. Our results of OSL signals, stratigraphy markers and sediment volume indicate that during and after the deglaciation of Gredos Gorge, paraglacial sediment was depleted as theory predicts. However, glacigenic sediments have not been transferred to sediment sinks as long as ~ 70% of glacial deposits are still stored in the valley. The transition of the environmental conditions prevailing during the LGM to current Mediterranean climate seems to play an important role in delaying the exhaustion model of glacially-conditioned sediment release. We propose that the poor coverage of vegetation driven by the Mediterranean climate prevents the stabilization of glacigenic sediment that is only mobilized during high discharge events responding to extreme rainfalls.