Environmental Sciences Research Institute

Reconstruction the extent and dynamics of the British Irish Ice Sheet on the Irish continental shelf

Glacial Landforms such as drumlins, ribbed moraines and end moraines provide a record of the nature, scale and timing of ice-sheet oscillations. However, despite a century of investigating these landforms around the British Isles there is still great uncertainty concerning the dimensions, dynamics and history of the former British Irish Ice Sheet (BIIS). This is mainly because it extended offshore onto the continental shelf around Ireland and Britain during the last glacial cycle meaning significant portions of the glacial record lie concealed beneath the sea. Unravelling this record is a major challenge but will provide critical information on both the extent of glaciation in this region and on the deglacial history of the BIIS. Recent advances in marine remote sensing now provide an opportunity to solve these longstanding scientific problems. Multibeam sonar surveys can be used to produce high resolution three dimensional images of the seafloor and can be combined with seismic data to investigate the internal structure of glacial features. In 1999 the Irish Government funded a €32 million survey of the entire Irish continental shelf known as the Integrated Mapping For the Sustainable Development of Ireland’s Marine Resource/Irish National Seabed Survey (INFOMAR/INSS). This multibeam sonar survey provides high resolution Digital Elevation Models (DEMs), seismic data and sediment samples of the seabed. The team have used these datasets for the first time to map the submerged glacial record on the Irish continental shelf. The project has established that the entire NW shelf was extensively glaciated during the ice age by two dynamic ice lobes that came offshore across the shelf from both Ireland and Scotland (Fig. 1). Large-scale nested arcuate end moraines record the former slow retreat of an ice sheet lobe across this sector of the continental shelf during regional deglaciation. Initial retreat from the outer shelf was associated with an episode of ice sheet break-up and calving as recorded by the extensive zones of iceberg ploughmarks distal to the outermost end moraine 90 km off the coast of Donegal at the shelf break . It is conceivable that this could have been driven by rising sea level. The data indicate that the BIIS underwent a major reorganisation during deglaciation of the NW Irish shelf changing from a ‘line-source’ type margin which extended along the shelf edge to a more lobate form. Collectively these data imply an extensive ice sheet margin that extended offshore onto the shelf and may have reached the shelf edge at the LGM.