LATE-GLACIAL ENVIRONMENTAL-CHANGES IN THE NETHERLANDS - SPATIAL AND TEMPORAL PATTERNS

The history of vegetational changes during the Weichselian Lateglacial in the Netherlands is based primarily on pollen records derived from lacustrine sediments. They provide a fairly consistent picture of progressive, followed by retrogressive, vegetation succession through time. Problems arise, however, when we try to match pollen records obtained from terrestrial peat sequences with those obtained from lacustrine deposits. This is especially the case at around about 12,000 BP: conditions seem to have varied significantly depending on the geomorphological position of individual site. Terrestrial sites are more prone to hiatuses in response to episodes of pronounced reductions in effective precipitation. The spread of Pinus from ca. 11,300 BP, which has been interpreted traditionally as a successive phase in vegetational development, coincides with changes in the local vegetation that indicate more intense freeze-thaw conditions. More continental conditions are assumed, implying more severe winters by comparison with the preceding time-interval. At ca. 10,850 BP, both vegetation and coleopteran evidence indicate a distinct drop in the average July temperature, from 18-15-degrees-C to 11-10-degrees-C. Consequently, the mean annual temperature was also lowered (estimated to between -2 and -5-degrees-C) and conditions approximated those of permafrost environments. Simultaneously, effective precipitation increased considerably resulting in a pronounced rise in lake-levels arid large-scale floodings of rivers. Soon after, at ca. 10,500 BP, summer temperatures appear to have risen while effective precipitation declined considerably resulting in a fall in lake-levels and periodic exposure of river beds, which provided sources for the accumulation of large inland dunes.