Late Pleistocene Climate of Europe: A Review
The Pleistocene periglacial environment of Europe is believed to have been marked by intensive frost action and wind action and by the spread of arctic and subarctic plants and animals south to the Alps. The frost action is recorded by ice-wedge casts, involutions, solifluction deposits, block fields, and related deposits, some of which may record permafrost. The occurrence of extensive sand dunes and loess deposits may be related to the distribution of glaciofluvial plains, strong winds, and the absence of forest cover. The occurrence of a tundra zone in the basal levels of Late-glacial pollen sequences in central Europe indicates a frigid climate, and molluscan and mammalian faunas provide similar information. Poser and Budel have separately attempted to represent the Pleistocene climatic-vegetational zones of Europe on maps. Poser located the southern limit of permafrost according to the distribution of ice-wedge casts and involutions, and the northern limit of forest from pollen-analytical evidence summarized by Firbas. These two map lines were then used to help delineate several vegetational-climatic provinces. An additional study on the Late-glacial sand dunes led to the presentation of a map of mean summer air pressure for Europe. Budel located the Pleistocene frost limit on the basis of its presumed relation to the 1000-m snow line, which is identifiable in the mountains of central Europe. His representation differs from Poser9s chiefly for Hungary and France, where there is a dearth of pollen studies from which vegetational relations can be inferred. Budel distinguishes several vegetational zones throughout Europe on the basis of loess distribution and also from pollen diagrams where available. Budel believes that the ice sheet had only a minor effect on the climatic zonation of Europe during the Pleistocene. Although exception is taken to many details of the criteria used by Poser and Budel for vegetational zonation, the maps are considered useful as a basis for discussion of Pleistocene atmospheric circulation patterns. The sequence of climatic changes for the last cold period is inferred largely from the stratigraphy of periglacial loesses and intercalated soils, and from their correlation with the Alpine glacial sequence. The controversy concerning the classification of the Wurm period as single (following Penck) or as multiple (following Soergel) is reviewed; the validity of the Gottweig interstadial is questioned on the basis of recent field studies and radiocarbon dates in Austria and adjacent areas. The reduction of mean annual temperature during the late Pleistocene, as inferred from the snowline depression in the mountains, amounted to 5°–8°C under the general assumption of a 0.5°–0.7°C/100 m vertical temperature gradient (lapse rate) in the atmosphere. Calculations based on the occurrence of frost features and fossil plants in lowlands, however, suggest a temperature depression of 10°–12° C. The discrepancy may result from different lapse rates over mountains and over lowlands, according to Mortensen. Studies of modern lapse rates in arctic, subarctic, and subtropical regions are reviewed in order to evaluate this hypothesis. Maps of mean atmospheric pressure and circulation patterns are commonly based on extrapolation from those modern patterns that are related to snowy winters, cool summers, and other conditions favorable to glaciation, on the assumption that the Pleistocene climate was marked primarily by a different frequency of certain air-mass movements and related storm tracks compared to today. Possible causes for circulation changes are still highly speculative, although much recent interest is centered on variations in solar radiation such as are exhibited in sunspots.