The Island of Ireland: Drowning the Myth of an Irish Land-Bridge?

Abstract

At the Last Glacial Maximum (LGM) c. 26 000 calendar years ago, global sea levels were around 120 m lower than present due to the storage of water on land in the form of large, high-latitude ice sheets. This lowering of sea level exposed portions of the modern seafloor surrounding north-west Europe, forming `land-bridges that joined Britain and Ireland to the rest of the continent. Sometime later, these land-bridges were drowned by rising sea levels as the ice sheets melted in response to a warming climate. Precisely when these land connections were severed has been a subject of debate for several decades, driven in part by the desire to understand the postglacial recolonisation of Ireland by plants and animals. The level of the sea relative to the land surface (relative sea-level) results from the interplay between vertical changes in both land and sea level. These processes can be simulated by computer models that describe the response of the solid Earth to the loading and unloading of glacial ice (glacial rebound models). In addition to simulating relative sea-levels, the output from these models can, when combined with bathymetric and topographic data, be used to produce first-order palaeogeographic reconstructions. This paper uses palaeogeographic reconstructions of this kind to investigate the location and duration of possible land-bridges joining Ireland to Britain. These reconstructions are derived from a recently developed glacial rebound model for Ireland that incorporates an updated British-Irish Ice Sheet component and is trained by geological sea-level indicators from around the Irish coast. The resulting reconstructions suggest that Ireland was separated from Britain by c. 16 000 calendar years ago, at which time climate was still cold and local ice caps persisted in parts of the country. No support is found for the idea that a Holocene land-bridge was instrumental in the migration of temperate flora and fauna into Ireland.