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 NW 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 retreat of a former ice sheet margin(s) across 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 grounded 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 extensive zones of iceberg ploughmarks distal to the outermost moraine. It is conceivable that this could have been driven by rising sea level. The data indicate a major reorganisation of the Irish Ice Sheet on the NW 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 NW margin suggests that ice sheet advance to the shelf edge likely occurred at about 29-27 ka BP, but that retreat from this shelf edge position did not take place until after 24 cal ka BP. 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.

New age constraints for the maximum extent of the last British–Irish Ice Sheet (NW sector)

“…It has been proposed that large North Atlantic wide iceberg-release events, such as Heinrich Event-2, at c. 25 ka BP, coupled with eustatic sea level rise may have led to a re-organization of the conjoined BIIS-FIS ice sheet along its Atlantic margin Clark et al, 2012;O'Cofaigh et al, 2012). We expect that the BIIS was highly dynamic during retreat, as shown by the complex pattern of seabed moraines and numerical modelling simulations (Hubbard et al, 2009); hence we stress that our data can only provide constraints for the retreat of the NW sector of the ice sheet.…”

Ice sheets viewed from the ocean: the contribution of marine science to understanding modern and past ice sheets

“…Impressive examples of these features have recently been imaged from the seas around Ireland and Britain where they have been used to reconstruct the maximum extent of past glaciation and the form and pattern of subsequent ice-sheet retreat [42,[58][59][60]. For example, offshore of northwest Ireland multi-beam swath bathymetric data during regional deglaciation (figure 5) [58,61]. The pattern of the moraines implies that the BIIS underwent major reorganization during shelf deglaciation from a 'line-source'-type margin that extended along the shelf edge to a more lobate form [42,61].…”

“…For example, offshore of northwest Ireland multi-beam swath bathymetric data during regional deglaciation (figure 5) [58,61]. The pattern of the moraines implies that the BIIS underwent major reorganization during shelf deglaciation from a 'line-source'-type margin that extended along the shelf edge to a more lobate form [42,61]. Recently, the large-scale distribution of glacigenic landform-sediment assemblages as mapped from marine geophysics and outlined above has allowed inferences to be made regarding the overall form and flow of palaeo-ice sheets during full glacial periods as well as the nature of their retreat.…”

Late Quaternary evolution and sea-level history of a glaciated marine embayment, Bantry Bay, SW Ireland

“…Firstly, despite improved understanding of the dimensions of the western portion of the BIIS (e.g., Ó Cofaigh et al, 2012b;Dunlop et al, 2010;Clark et al, 2012), there is still uncertainty about the extent, thickness and history of the ice-sheet. Secondly, there are large gaps in the RSL data around Ireland.…”