Holocene 16 (1)

The paper examines the assumptions underpinning the use of radiocarbon dates to provide a chronology for the replacement of freshwater peats by marine/brackish clastic sediments in the stratigraphic column of many coastal lowland areas bordering the North Sea during the late Holocene. The results of investigations from thirteen sites in the Rye area of Romney Marsh are reported. Dates from apparently gradational contacts of a highly humified, laterally persistent, peat layer range from 3170--2840 cal years BP to 1290--1050 cal years BP. Multiple inundations or prolonged gradual inundation are nevertheless rejected, as discrete post-peat bodies of sediment are absent and because peat growth appears to have slowed down or ceased at many sites in advance of inundation. Additionally in the Rye area, sharp contacts are widespread and the pollen assemblages rarely indicate the occurrence of transitional plant communities. A review of the dating evidence from other coastal lowland regions reveals that multiple dating of the upper surface of peat beds invariably produces diachronous results. As a consequence time transgressive processes feature prominently as causal mechanisms underlying this shift. However, many of the dating difficulties recognized in the Rye area appear to apply to other regions. The authors conclude that radiocarbon dates from the upper surface of peat layers should in most instances only be regarded as limiting ages for the deposition of the overlying clastic sediments. New chronologies need to be built without a priori assumptions as to the underlying processes, ideally through the direct dating of the clastic sediments.