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

Abstract

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.

Geological impacts on coastal wetland landscapes: some general effects of sediment autocompaction in the Holocene of northwest Europe

“…It seems undeniable from the experiments (e.g., Figures 8a, 9d and 1 Oa), as well as fr-om well-dated natural counterparts (e.g. Streif, 1972;Devoy, 1979;Vos and Van Heelingen, 1997;Haslett et al, 1-998), that autocompaction significantly and variably depressed such index points prior to sampling. The strongest influence will be felt in peat samples from transgressive overlaps, and in material from regressive overlaps above the lowermost organic bed.…”

Modelling the effects of sediment compaction on salt marsh reconstructions of recent sea-level rise

“…Compaction involves a combination of physical and biochemical processes that reduces the vertical thickness of the sediment column , distorting stratigraphic sequences (Allen, 3 1999;Bloom, 1964;Haslett et al, 1998;Jelgersma, 1961;Kaye and Barghoorn, 1964).…”

Compaction of lignite: a review of methods and results

“…Coal rank Source peat Bird et al, 2004;Van Asselen et al, 2009;Van Asselen, 2011density lignite Falini, 1965Volkov, 1965lignite Glockner, 1912Ting, 1977inclusions bituminous Teichmüller, 1955Stach et al, 1975;Zaritsky, 1997;DeMaris et al, 1983;Gayer andPe ek, 1992 lignite Piwocki, 1975;Stout and Spackman, 1989;Hurník, 1990;Kojima et al, 1998;Widera, 2013asubbituminous White, 1986 indirect petrographic bituminous Courel, 1978;Winston, 1986peat Bloom, 1964Haslett et al, 1998lignite Hurník, 1972Hager et al, 1981;Hager, 1993;Kasi ski, 1984Kasi ski, , 1985Widera, 2002;Widera et al, 2007 direct stratigraphic bituminous this paper …”

“…Most of the modern mires, for which the compaction has been extensively investigated, are younger than 10 ka (e.g. Bloom 1964;Haslett et al 1998;Allen 2000;Bird et al 2004;Long et al 2006;Van Asselen et al 2009;Van Asselen 2011). Thus, the plant matter in the mire beds is less decomposed than in the lignite seams.…”