Holocene environmental change in southern Spain deduced from the isotopic record of a high-elevation wetland in Sierra Nevada

Small lakes and wetlands from high elevation within the Sierra Nevada Range (southern Spain) preserve a complete post-glacial Holocene record. Isotopic, TOC and C/N analyses, carried out on a sediment core, show various stages in the evolution of the Borreguiles de la Virgen, which today constitute a small bog at about 2,950 m above sea level. Glacial erosion generated a cirque depression, which became a small lake during the first phase of infilling (from 8,200 to 5,100 cal yr BP), as suggested by sedimentary evidence, including an atomic C/N ratio generally below 20, low TOC values and the highest δ ¹³C and δ ¹⁵N values of the record. These results imply significant algal productivity, which is confirmed by the microscopic algal remains. Drier conditions became established progressively in this area from 5,100 to 3,700 cal yr BP. Subsequently, the lake evolved into a bog as shown by geochemical evidence (C/N ratios above 20, high TOC content and low δ ¹³C values). Unstable conditions prevailed from 3,600 to 700 cal yr BP; an extremely low sedimentation rate and scarcity of data from this period do not allow us to make a coherent interpretation. Fluctuating conditions were recorded during the last ~700 cal yr BP, with wetter conditions prevailing during the first part of the interval (with C/N rate below 20) up to 350 years ago. In general, a gradual trend toward more arid conditions occurred since ~6,900 cal yr BP, with a further increase in aridity since ~5,100 cal yr BP. This evidence is consistent with other contemporaneous peri-Mediterranean records.