Holocene sea-level changes along the Channel coast of South-west England

Abstract

A Holocene relative sea-level (RSL) history for the south Devon coast is reconstructed through an analysis of twelve cores extracted from the coastal back-barrier environments of Bantham Sands, North Sands, Slapton Sands and Blackpool Sands. Foraminifera preserved in the sediments are assigned an indicative meaning based on the vertical distribution of their modem counterparts from contemporary salt marsh and mudflat environments in the Erme and Salcombe-Kingsbridge estuaries. The contemporary data provide the first inter-tidal (-2.6 to +2.6 m MTL) foraminifera-based transfer function for south-west England from which sea-level changes can be predicted with good precision (RMSEP = ±0.29 m). Sea-level index points (SLIPs) from basal facies provide the longterm (103 yr) RSL history. SLIPs derived from non-basal minerogenic and peat sequences are subject to post-depositional consolidation and a vertical correction, using the Paul and Baffas (1998) method, was therefore applied to these index points. The vertical displacement of fine minerogenic sediments ranges from <0.1 m above basal facies to >1 m at minerogenic-peat contacts, increasing to >2 m in organic peat facies. The age of each SLIP is obtained by AMS 14C dating of bulk sediment or plant material. The electrical resistivity method of geophysical survey provides additional subsurface mapping information of the back-barrier sediments, and the lithostratigraphic results support the view that a complex barrier-lagoon system existed along the south Devon coastline during the early Holocene. Almost 13 m of RSL rise occurred along the south Devon coastline ca. 9000 to 4400 cal years BP. The rate of early Holocene RSL rise is -5.4 ± 2.1 m/ka and -1.1 ± 0.2 m/ka during the mid- to late Holocene. The pattern and rate of RSL rise is similar to other sea-level curves produced for the region. Lambeck's (1993a, b, 1995) geophysical model predictions fit the data well during the early Holocene but Peltier's (1998) model is the best overall fit of the robust data used to reconstruct the early to mid- Holocene RSL history. Rates of middle to late Holocene sea-level rise are generally faster than the models predict. The area is still undergoing glacio-isostatic adjustment (GIA) equating to a coastal land subsidence of -1.16 m/ka since ca. 4400 cal years BP. The dataset increases the current Holocene SLIP database for the south-west peninsula and southern Britain by 23% and for south Devon by almost fivefold.