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Interpretation of radiocarbon dates from the upper surface of late-Holocene peat layers in coastal lowlands

Centre for Earth and Environmental Science Research, School of Earth Sciences and Geography, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, UK; M.Waller{at}kingston.ac.uk

A. J. Long

Department of Geography, Durham University, Science Site, South Road, Durham DH1 3LE, UK

J. E. Schofield

Department of Geography and Environment, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF, UK

Marine/brackish clastic sediments replace freshwater peats in the stratigraphic column of many coastal lowland areas bordering the North Sea during the late Holocene. Radiocarbon dates are routinely used to provide a chronology for this shift. We examine the assumptions underpinning this approach. The results of investigations from 13 sites in the Rye area of Romney Marsh, southeast England, are reported. Dates from apparently gradational contacts of a highly humified, laterally persistent, peat layer range from 3170-2840 cal. yr BP to 1290-1050 cal. yr 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. We 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.

Key Words: Late Holocene • coastal change • radiocarbon dating • peat accumulation • coastal lowlands • Romney Marsh • England

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