Early deglaciation of the British-Irish Ice Sheet on the Atlantic shelf northwest of Ireland driven by glacioisostatic depression and high relative sea level

Deglaciation chronology of the Donegal Ice Centre, north‐west Ireland

“…Two 10 Be exposure ages from bedrock and a glacially transported boulder on Aran Island, 20 km south‐west of Bloody Foreland, have yielded an uncertainty‐weighted mean age of 21.7 ± 0.8 ka (21.5 ± 1.8 ka) (Cullen, ). The consistency of these two mean ages provides strong support for retreat of the ice‐sheet margin across the Irish sector of the Malin Shelf between ∼26–25 and ∼22–21 ka (Clark et al , ; Ó Cofaigh et al , , ), decoupling of Malin Shelf ice from Donegal Bay ice at ∼22–21 ka and the beginning of ice retreat at that time from the present coast towards the mountains. At Glencolumbkille in south‐west Donegal, 10 Be exposure ages of 17.8 ± 0.6 ka (17.9 ± 1.5 ka) and 19.6 ± 0.7 ka (19.8 ± 1.7 ka) from vein quartz in, respectively, a glacially transported boulder and a roche moutonnée were reported by Ballantyne et al ().…”

“…) represents the limit of a readvance of ice fed from Donegal Bay. Radiocarbon ages of 20.24 ± 0.24 and 17.92 ± 0.16 cal ka BP obtained for foraminifera retrieved, respectively, from the distal and proximal sides of the moraine constrain its age (Ó Cofaigh et al , ). This broad interval encompasses the timing of both the readvance at Corvish in northern Donegal and that of the CHR in north‐east Ireland (McCabe and Clark, ; McCabe et al , ; Clark et al , ; Ballantyne and Ó Cofaigh, ), but the resolution of the dating evidence is inadequate to establish contemporaneity.…”

“…The Errigal col and Poisoned Glen sites lie, respectively, 15 and 17 km south‐east of the Bloody Foreland site; if the mean deglaciation ages for these sites are representative, then the net ice‐margin retreat rate from Bloody Foreland to both sites was ∼4 m a −1 ; taking the associated uncertainties into account suggests that net retreat rate is unlikely to have exceeded 5 m a −1 , and may have been as low as 3 m a −1 . By contrast, assuming that the ice margin began to retreat from the shelf edge within the interval 26.3–24.8 ka (Ó Cofaigh et al , ), the implied net rate of offshore ice‐margin retreat from the shelf break to Bloody Foreland falls within the range ∼19.2–33.3 m a −1 . Ó Cofaigh et al () inferred that ice‐sheet retreat from the shelf edge was initiated by calving associated with high sea levels induced by glacio‐isostatic depression rather than changing climate, and the marked slowing of retreat after the ice margin had become land‐based in northern Donegal appears consistent with this interpretation: the inferred slow net retreat rates of Donegal ice in this sector over the period ∼21–∼17 ka suggest that the retreating ice was close to equilibrium with prevailing climate, and experienced only a slight net negative mass balance during this period.…”

“…More recent work utilizing geophysical techniques to image seabed topography has demonstrated that a concentric sequence of nested moraine ridges indicative of deposition by a grounded ice mass extends westwards to the shelf break 90–100 km from the west coast of Donegal (Sejrup et al ., ; Benetti et al ., ; Dunlop et al ., ; Ó Cofaigh et al ., ). Radiocarbon dates obtained for marine microfauna in cores retrieved from sediments on the Atlantic shelf north‐west of Ireland confirm that these moraines were deposited at the margin of the last ice sheet, and indicate that ice nourished in Donegal began to retreat from the shelf edge in the interval between 26.3 and 24.8 ka cal BP (Ó Cofaigh et al , ).…”

“…Radiocarbon ages for mixed benthic foraminifera within glacimarine sediments in 76–99 m water depth on either side of the moraine (Fig. ) constrain moraine formation to between 20.24 ± 0.24 and 17.92 ± 0.16 cal ka BP (Ó Cofaigh et al , ), and moraine formation at 20–19 ka was inferred by Ó Cofaigh et al . ().…”