Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE‐CHRONO reconstruction

Abstract

The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling.

Reconstruction of the palaeo‐sea level of Britain and Ireland arising from empirical constraints of ice extent: implications for regional sea level forecasts and North American ice sheet volume

“…To address this spatial mismatch, we created the hybrid reconstruction adopting the revised margin history from Clark et al (2022a) for the advance and retreat across the North Sea (very close to the middle reconstruction) but with an altered central dome configuration. As the transect in Fig.…”

“…Figure 4 provides an overview of the modelling procedure used to generate this reconstruction. The hybrid reconstruction used the optimum reconstruction from Clark et al (2022a) as a margin input, and most closely resembles the ‘Middle’ reconstruction for the North Sea. Though our aim was to keep the margin input and shear stress independent from the RSL comparison, our results showed a discrepancy across Scotland resulting from the spatial pattern of the minimum/maximum estimated shear stress values (see ‘How thick was the British‐Irish Ice Sheet?'…”

“…Subsequently, through the BRITICE‐CHRONO project (Clark et al, 2021), a high density of geochronological data constraining the retreat of the BIIS has been collected (Bradwell et al, 2021a, b; Evans et al, 2021; Chiverrell et al, 2021; Scourse et al, 2021; Ó Cofaigh et al, 2021; Benetti et al, 2021). This has culminated in an empirical reconstruction of ice margin history from 32 to 15 ka bp at 1‐ka time steps (Clark et al, 2022a). These ice margin positions supersede those used in previous GIA models (Bradley et al, 2011).…”

“…These ice margin positions supersede those used in previous GIA models (Bradley et al, 2011). Despite the increase in available data, uncertainty in the ice margin position through time remains, especially in comparatively data‐poor regions such as the North Sea (Clark et al, 2022a). Furthermore, the thickness of the ice sheet remains poorly constrained, especially given the reinterpretation of trimlines as possible thermal boundaries rather than ice elevation limits (Ballantyne & Stone, 2015).…”

“…Exploration of regional and temporal differences in the mismatch between the original six input reconstructions and the RSL data led to the development of an improved reconstruction by tuning aspects of the reconstruction and map of basal shear stress. These experiments were important for deciding the optimum ice sheet reconstruction over the North Sea that underpinned the recent BRITICE‐CHRONO ice sheet reconstruction (Clark et al, 2022a) and for the production of palaeotopographies used in that work, and which are made available for others in a data publication (Clark et al, 2022b). Using this new constrained ice sheet reconstruction enables us to demonstrate the utility of the UK and Irish sea level database for studying the North American and Antarctic ice sheets.…”