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Evaluating late-Holocene relative sea-level change in the Somerset Levels, southwest Britain

P. Davies

R. H. F. Curr

C. F. C. Davies

Quaternary Research Unit, Faculty of Applied Sciences, Bath Spa University College, Newton Park, Newton St Loe, Bath BA2 9BN, UK

K. Kennington

Port Erin Marine Laboratory, School of Biological Sciences, University of Liverpool, Port Erin, Isle of Man IM9 6JA, UK

C. P. King

A. J. Margetts

Quaternary Research Unit, Faculty of Applied Sciences, Bath Spa University College, Newton Park, Newton St Loe, Bath BA2 9BN, UK

Holocene relative sea-level (RSL) change is investigated at Nyland Hill (Somerset Levels). The lithostratigraphy comprises turfa peat underlying marine clay, both onlapping a sloping Upper Palaeozoic basement. The altitude of the peat-clay contact ranges from 2.42 to 4.52 m OD, becoming lower down basement slope, yet ¹⁴C dates of three samples taken laterally along the contact at different altitudes yield similar ages (3640-3330, 3715-3460, 3725-3465 cal. yrs BP), suggesting either very rapid (‘instantaneous’) RSL rise or sediment compaction. Biostratigraphic data indicate a gradual transition to marine conditions across the peat-clay contact. Therefore, the height difference is interpreted as evidence for significant compaction. We consider 4.64 m OD the minimum pre-compaction altitude, indicating maximum observed compaction of 2.22 m. The clay surface, reclaimed during Roman occupation, represents an anthropogenically induced negative tendency and is dated chemostratigraphically to 1776 ± 46, using a datum related to the onset of local Roman lead mining AD 43-49. Sedimentation rates of the marine clay are established: 1.58-1.92 mm yr^-1 and 0.8-0.96 mm yr^-1 at sites of maximum and less severe compaction respectively. RSL continued to rise throughout the deposition of the clay unit at a rate of 0.41-0.82 mm yr^-1, a view that disagrees with previous models that imply stabilised RSL by 3000 BP. These earlier studies underestimate compaction with implications for subsequent studies employing these sea-level data, particularly in crustal studies where the apparent trend of subsidence is overestimated/increased.

Key Words: Holocene • sea-level change • compaction • sedimentation • tectonics • Somerset Levels

The Holocene, Vol. 8, No. 2, 197-207 (1998)
DOI: 10.1191/095968398669499299


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