A 2500-year multi-proxy reconstruction of climate change and human activities in northern Spain: The Lake Arreo record

Lake Arreo sequence (western Ebro Basin, Spain) illustrates the century-scale climatic variability and human interactions in the landscape during the last 2.5. kyr in the low lands of northern Spain. Two sediment cores from shallow-water and deep-water environments were analyzed using sedimentological, geochemical, mineralogical, biological - diatoms, pollen and charcoal content - and radiometric techniques for absolute dating. The shallow-water sequence indicates a rapid evolution from an alluvial-influenced wetland prior to 7th century BC to a wetland during the Ibero-Roman Humid Period (BC 630-AD 465) and a deeper, carbonate producing lake during the Dark Ages Cold Period (AD 465-890). The deep-water core shows the transition from a more saline lake during the arid Medieval Climate Anomaly (MCA, AD 890-1300) to less saline, meromictic conditions, particularly since the onset of the Little Ice Age (LIA, AD 1300-1870). During the last 2.5. kyr, arid conditions occurred prior to 1st century AD, during the MCA and late 19th-mid 20th century while colder temperatures and relatively more humid conditions were more frequent during the Dark Ages, particularly the 7th century AD and the LIA. The evolution of the lake also reflects changes in grazing and agricultural practices since the Roman Period associated to the exploitation of nearby salt mining. Periods of intense human pressure on the lake watershed occurred during the High Middle Ages (AD 890-1180) and the Modern Period (AD 1600-1830).