Late Devensian and Holocene shorelines of the British Isles and North Sea from models of glacio-hydro-isostatic rebound

Abstract

Sea-level change around the British Isles since the time of the last glacial maximum is largely due to of the crustal rebound from the glacial unloading of northern Britain and the concomitant melt-water loading of the adjacent seas and Atlantic Ocean. Minor, but not insignificant, contributions also result from the rebound caused by the unloading of the distant ice sheets, including Fennoscandia and North America. Observations of sea-level change for this period constrain the glacio-hydro-isostatic rebound model parameters describing the effective lithospheric thickness or rigidity and the effective mantle viscosity, as well as certain ice sheet characteristics such as the ice thickness at the time of the last glacial maximum. The models permit palaeobathymetry and palaeoshorelines to be predicted for the British Isles region, including the North Sea. The resulting evolution of the coastlines exhibits a complex behaviour through time, one that is quite different from the usual models in which sea-level change is assumed to be a function of time only. In part this is because of the delayed response of the mantle to the spatially variable and time-dependent ice and water loads, and in part because the unloading history of the British ice sheet is different from those of the major global ice sheets. Thus, maximum emergence of the North Sea occurred after deglaciation had started and lasted for an extended period from about 15000 to 12000 (radiocarbon) years BP. During this relative sea-level still-stand shoreline features could have formed, for example, along the western edge of the Norwegian Trough when access to the firths of eastern Scotland would have been via a long and shallow marine inlet. Shoreline retreat across the North Sea became relatively rapid after about 10000 years. The model predictions for the Irish and Celtic Seas also suggest a complex behaviour, with the formation of a wide land bridge between about 20000 and 13 OOO years ago. The model also suggests that as long as the Scottish ice extended across the northern Irish Sea, until about 14 OOO years ago, there would have been a large freshwater periglacial lake located further south. Both the predicted sea-level height-age relations and the shoreline positions are consistent with a large body of observational evidence but some discrepancies occur, particularly in northern Scotland and Ireland where the ice heights may have been somewhat greater than assumed in the model.