Marine geophysical evidence for Late Pleistocene ice sheet extent and recession off northwest Ireland

Abstract

Multibeam swath bathymetry data collected through the Irish National Seabed Survey provides direct evidence for extensive glaciation of the continental shelf off northwest Ireland. Streamlined subglacial bedforms on the inner shelf record former offshore-directed ice flow. The major glacial features, however, consist of well developed, nested arcuate moraines which mark the position of former ice sheet margins on the shelf. Distal to these moraines, on the outermost shelf, prominent zones of iceberg ploughmarks give way into a well developed system of gullies and canyons which incise the continental slope. The large-scale, nested, arcuate moraines record the episodic retreat, probably punctuated by minor readvances or oscillations, of a lobate grounded ice sheet across this sector of the continental shelf during deglaciation. Initial retreat from the outer shelf was associated with an episode of ice sheet break-up and calving as recorded by extensive zones of iceberg ploughmarks distal to the outermost moraine. It is conceivable that this initial phase of retreat could have been driven by rising sea level. The data indicate a major reorganisation of the Irish Ice Sheet on the northwest shelf during deglaciation; an initial elongate ice sheet configuration extending along the shelf edge changed to a pronounced lobate form during retreat. Consideration of dated, marine stratigraphic records from the wider northwest margin suggests that ice sheet advance to the shelf edge likely occurred at about 29–27calkaBP, but that retreat from this shelf edge position did not take place until after 24calkaBP. Large-scale contrasts in continental margin morphology west of Ireland, from trough mouth fans in the north to gully/canyon systems further to south, reflects a combination of factors including spatial variations in sediment flux related to palaeo-glaciology.