Late Devensian and Holocene relative sea-level change in North Wales, UK

Deglacial sea-level index points defining relative sea-level (RSL) change are critical for testing glacial isostatic adjustment (GIA) model output. Only a few observations are available from North Wales and until recently these provided a poor fit to GIA model output for the British-Irish Ice Sheet. We present results of an integrated offshore geophysical (seismic reflection), coring (drilling rig), sedimentological, micropalaeontological (foraminifera), biostratigraphical (palynology) and geochronological (AMS 14C) investigation into a sequence of multiple peat/organic sediment horizons interbedded within a thick estuarine–marine sequence of minerogenic clay-silts to silty sands from the NE Menai Strait, North Wales. Ten new sea-level index points and nine new limiting dates from the Devensian Late-glacial and early Holocene are integrated with twelve pre-existing Holocene sea-level index points and one limiting point from North Wales to generate a regional RSL record. This record is similar to the most recent GIA predictions for North Wales RSL change, supporting either greater ice load and later deglaciation than in the GIA predictions generated before 2004, or a modified eustatic function. There is no evidence for a mid-Holocene highstand. Tidally corrected RSL data indicate initial breaching of the Menai Strait between 8.8 and 8.4 ka BP to form a tidal causeway, with final submergence between 5.8 and 4.6 ka BP. Final breaching converted the NE Menai Strait from a flood-dominated estuary into a high energy ebb tidal delta with extensive tidal scouring of pre-existing Late-glacial and Holocene sequences. The study confirms the value of utilising offshore drilling/coring technology to recover sea-level records which relate to intervals when rates of both eustatic and isostatic change were at their greatest, and therefore of most value for constraining GIA models. Copyright © 2011 John Wiley & Sons, Ltd.