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

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.