Concerns as to the stability of present-day ice sheets and their future contribution to sea-level rise have prompted the investigation of palaeo ice sheets in order to study ice dynamics and evolution through deglaciation. Such observations of ice sheet behaviour are essential in the process of constraining and validating numerical ice sheet models in order to forecast the future response of Greenland and Antarctic ice sheets to warming and sea level rise which can rapidly destabilise marine-terminating margins and contribute to global sea-level rise. Therefore, a palaeo ice sheet of similar characteristics to Greenland and Antarctica is required in order to accurately capture the occurrence of marine ice sheet instability. A suitable analogue is the last British-Irish Ice sheet, which was drained by numerous marine-terminating ice streams. The largest of these was the Irish Sea Ice Steam (ISIS), which terminated on the continental shelf of the Celtic Sea. New modern geophysical and sediment core data collected during numerous scientific cruises of the Celtic Sea provide a unique opportunity to determine the stratigraphic position of glacial sediments and the origin of the ridges from which these sediments were recovered, and to investigate the imprint of glaciation and thus reconstruct deglacial dynamics.

Seismic data of the mid- and outer-shelf show that the largest shelf ridges in the west comprise two vertically stacked units which are draped by superficial deposits. The upper sandy bulk of these megaridges is correlative to the Melville Formation (MFm), comprising truncated and dipping bedding planes, which overlies mounds of the Upper Little Sole Formation (ULSFm) comprising Last Glacial Maximum glacigenic sediments that are exposed on the lower megaridge flanks. Stiff and deformed glacigenic sediments comprising the ULSFm record their overriding by the ice margin during its oscillatory retreat across the shelf before being partially eroded during marine transgression by megatidal conditions which fuelled the development of overlying sand ridges comprising the MFm. This updated stratigraphy suggests the presence of a glacigenic core under the MFm on the western shelf. In the east, channels of the ULSFm contain undisturbed seismic facies of unknown lithology, with implications for the ice margin extent in the French sector. Multi-beam bathymetric data reveal a large assemblage of seafloor glacial features preserved on the inner-shelf, including terminal and ribbed moraines, streamlined bedrock, iceberg grounding pits, meltwater channels and tunnel valleys. These features record the southwest advance of a grounded warm-based ice lobe and its northeast marine deglaciation in response to rising relative sea level into St. George’s Channel and towards the southern coastline of Ireland.

These findings provide new palaeo observations of ice stream instability, whereby the ISIS rapidly broke up through calving in response to fracturing and thinning due to instability induced by ice margin flotation and deformable proglacial sediments in a rising relative sea level environment. Such factors need to be considered in the stability of present-day marine-terminating ice sheets. The maximum extent and subsequent northwest retreat of the ice margin across the mid- and outer-shelf, followed by more-complex retreat patterns on the inner-shelf, provide a framework against which numerical whole ice sheet models can be tested. Combined, these observations have implications for the study of ice stream stability and the validation of numerical ice sheet models.