Abstract
ABSTRACTNew optically stimulated luminescence dating and Bayesian models integrating all legacy and BRITICE‐CHRONO geochronology facilitated exploration of the controls on the deglaciation of two former sectors of the British–Irish Ice Sheet, the Donegal Bay (DBIS) and Malin Sea ice‐streams (MSIS). Shelf‐edge glaciation occurred ~27 ka, before the global Last Glacial Maximum, and shelf‐wide retreat began 26–26.5 ka at a rate of ~18.7–20.7 m a–1. MSIS grounding zone wedges and DBIS recessional moraines show episodic retreat punctuated by prolonged still‐stands. By ~23–22 ka the outer shelf (~25 000 km2) was free of grounded ice. After this time, MSIS retreat was faster (~20 m a–1 vs. ~2–6 m a–1 of DBIS). Separation of Irish and Scottish ice sources occurred ~20–19.5 ka, leaving an autonomous Donegal ice dome. Inner Malin shelf deglaciation followed the submarine troughs reaching the Hebridean coast ~19 ka. DBIS retreat formed the extensive complex of moraines in outer Donegal Bay at 20.5–19 ka. DBIS retreated on land by ~17–16 ka. Isolated ice caps in Scotland and Ireland persisted until ~14.5 ka. Early retreat of this marine‐terminating margin is best explained by local ice loading increasing water depths and promoting calving ice losses rather than by changes in global temperatures. Topographical controls governed the differences between the ice‐stream retreat from mid‐shelf to the coast.
Highlights
The assessment of the rate and style of ice sheet retreat closely relates to many globally important scientific and socio‐economic questions (IPCC, 2013)
We summarize 71 radiocarbon ages from offshore glacial and glaciomarine sediments previously presented in Schiele (2017), Callard et al (2018) and Ó Cofaigh et al (2019); and 41 terrestrial cosmogenic nuclide (TCN) ages already included in Schiele (2017), Small et al (2017a) and Wilson et al (2019); and the legacy ages published previously (Small et al, 2017b) that have been included in the Bayesian age modelling
The Malin Deep and the trough of the Sea of Hebrides, are separated by the Stanton Bank, a bedrock high at the centre of the inner Malin Shelf (Dobson and Whittington, 1992). For the former Donegal Bay Ice Stream (DBIS), the shelf offshore NW Ireland in the southern part of the Malin Sea has a smoother profile with a gentle gradient from the mouth of Donegal Bay to the shelf edge, with Donegal Bay having the characteristics of an over‐deepened basin like those further to the north (Fig. 1)
Summary
The assessment of the rate and style of ice sheet retreat closely relates to many globally important scientific and socio‐economic questions (IPCC, 2013). Constraining the pace of ice‐sheet retreat for both past and present ice sheets can improve our understanding of how large ice masses respond to local and global, internal and external forcing, such as glaciological, climatic and oceanographic changes. Once insights gained from such knowledge are incorporated into ice sheet models, they can improve the predictions on how modern ice sheets will evolve with the current changing climate, ocean temperature and sea level (Rignot et al, 2010; Joughin et al, 2014). Ice streams react more readily than other parts of the ice margin to any perturbation in ocean circulation, atmospheric temperature and sea‐ice distribution because of both thermal (melting) and mechanical (floatation and calving) stressors that occur along the margins of marine‐terminating ice sheets (e.g. Hulbe et al, 2004; Payne et al, 2004; Scambos et al, 2004; Shepherd et al, 2004; Joughin et al, 2012)
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