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 prog ressive, followed by retrogressive, vegetation succession through time . Problems arise, however, when we try to match pollen records obtaine d from terrestrial peat sequences with those obtained from lacustrine deposits. This is especially the case at around about 12,000 BP: condi tions seem to have varied significantly depending on the geomorphologi cal position of individual site. Terrestrial sites are more prone to h iatuses 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 develo pment, coincides with changes in the local vegetation that indicate mo re intense freeze-thaw conditions. More continental conditions are ass umed, implying more severe winters by comparison with the preceding ti me-interval. At ca. 10,850 BP, both vegetation and coleopteran evidenc e indicate a distinct drop in the average July temperature, from 18-15 -degrees-C to 11-10-degrees-C. Consequently, the mean annual temperatu re was also lowered (estimated to between -2 and -5-degrees-C) and con ditions approximated those of permafrost environments. Simultaneously, effective precipitation increased considerably resulting in a pronoun ced rise in lake-levels arid large-scale floodings of rivers. Soon aft er, at ca. 10,500 BP, summer temperatures appear to have risen while e ffective precipitation declined considerably resulting in a fall in la ke-levels and periodic exposure of river beds, which provided sources for the accumulation of large inland dunes.