The challenging application of cosmogenic dating methods in residual glacial landforms: the case of Sierra Nevada (Spain)

Abstract

An accurate review of the literature on surface exposure dating methods shows evidence of the difficulty in applying cosmogenic dating methods to old moraines because of the intensity of late Quaternary erosion processes. Moreover, as in some previous cases, we also found special difficulties in applying these methods to LIA moraines, caused by the intensity of current paraglacial processes. The objective of this study is to apply cosmogenic dating methods to very old and very young moraines, which in both cases have been or are being affected intensively by erosion. With this purpose, we collected samples of boulders from moraines corresponding to (i) the penultimate glaciation and (ii) the Little Ice Age (LIA), both from Sierra Nevada in the south of the Iberian Peninsula. The sampling strategy was based on a preliminary accurate analysis of the geomorphological settings of two valley sites that resulted in the collection of only four boulder samples from an old moraine and three more from a very recent moraine. Using in situ-produced cosmogenic 10Be to date these boulders, the old samples yielded an age of ca. 130–135 ka for moraine stabilization. The younger samples indicate that the LIA moraine accretion probably occurred between the fourteenth and seventeenth centuries, with a subsequent stage of accumulation during the nineteenth century as suggested by historical documents. Dating a glaciation that occurred prior to the last Pleistocene glacial cycle and dating LIA glacial stages are novel in the context of Iberian glaciations and agree with other palaeoenvironmental studies in Iberian and in other European mountains. The limited number of boulders adequate for cosmic-ray exposure dating prevents statistical methods to be applied, and therefore highlights the need to improve geomorphological criteria in sample selection.