干旱区生态环境知识资源中心

Holocene climatic changes along coastal regions from south-east France to south-east Spain were studied using pollen ratios. Comparing modern pollen rain, vegetation and climate along selected transects from the Atlantic Ocean to the Mediterranean, we obtained threshold values of two different ratios corresponding to the different climatic conditions along the transects. These pollen ratios and threshold values were employed to characterize the Holocene climatic changes from nine Mediterranean coastal sites. The results were compared with data from marine and continental pollen sequences distributed in the western Mediterranean basin, and with additional regional data independent of human activity: lake-level fluctuations, alpine glacier advance and retreat chronology, C-14 anomaly and cooling phases in Eastern France and Central Europe. The role of anthropogenic activities and climate on the changes in vegetation is discussed. Six major changes in vegetation cover were identified. They correspond to aridification phases that occurred around 9500-9000 yr BP (10 900-9700 cal BP), 7500-7000 yr BP (8400-7600 cal BP), 4500-4000 yr BP (5300-4200 cal BP), 3700-3300 yr BP (4300-3400 cal BP), 2600-1900 yr BP (2850-1730 cal BP) and 1300-1000 yr BP (1300-750 cal BP). These arid episodes were regional responses to more global climatic changes and determined the changes in the vegetation cover. Humans undoubtedly enhanced the vegetation changes, but none the less had to adapt to these new climatic conditions. (C) 2000 Elsevier Science B.V. All rights reserved.