BritIce Modelling Project Conclusions

In this paper we have applied a climatically coupled higher-order ice-sheet model to the British Isles down to 2.5-km resolution. By incorporating longitudinal stresses and new grounding-line physics, our model is able to capture the dynamics of the former British–Irish Ice Sheet palaeo-ice sheet and provide additional insight and chronological context to the work of Boulton and Hagdorn (2006). Specifically, the new modelling simulations presented here lend strong support to the dynamical, fast-flow paradigm that has emerged in the last decade. However, this needs to be qualified since we also find that these fast-flow streaming phases are part of a larger binge–purge cycle. In essence, this cycle is moderated by long phases of frigid, cold-based ice build up (‘binges’) that provide the ice sheet with its mass, thermal inertia and the required isostatic signature from which to launch streaming ‘purge’ phases. The trigger for such events appears to be a interplay between internal thermodynamic coupling and external climatic forcing that occur during rapid transition stages from cold to warm conditions, and is accompanied by extensive and dynamic ice streaming and interior draw-down. Longitudinal coupling plays a significant role in the mobilisation of extensive marine sectors of the ice sheet and these purge-advance phases are accompanied by large-scale calving events which release significant iceberg and freshwater runoff flux into the N Atlantic. The NGRIP isotope record, used to force the model, contains numerous stadial/interstadial cycles and the BIIS modelled in this study becomes phase-locked to these cycles, resonating and amplifying them. The wider implications for the offshore sediment record and an understanding of the broad forcing/response couplings between the global ocean – atmosphere – cryosphere systems are potentially profound and undoubtedly complex. Equally so, the onshore geomorphological imprint of such a dynamically fluctuating BIIS is correspondingly complex, but it is hoped that this work and further refined BIIS experiments may provide an initial synoptic framework from which individual sites can be assessed and interpreted. In this way, we hope that our model can be critiqued and improved in the future.

Specifically, with particular reference to our optimal subset of numerical experiments we find support for:

  1. Intense phases of cold-based and warm-based activity, in which ice streams play a defining role and draw-down large ice fluxes from the ice sheet interior.
  2. Intense persistent and ephemeral ice stream and outlet glacier activity, which displays switching and dynamical fluctuations on a century or less time-scale.
  3. The existence of a dynamic and extensive Irish Sea Ice Stream that can advance onto the Scilly Isles but only in conjunction with ice impinging on the SW coast of England.
  4. Complete glaciation of Ireland extending offshore across the west coast. This may be time-transgressive and is not necessarily coincident with the Last Glacial Maximum.
  5. Extensive glaciation across the North Sea Basin extending to the Atlantic shelf edge and encompassing an independent Shetland ice cap.
  6. Deglaciation in under 2 ka which decanted ca +2 m of global sea-level equivalent, under conditions analogous to those across the SW Greenland Ice Sheet margin today. This leads to a hypothesis that the North Sea and Irish Sea tunnel valley systems may have a supra-glacial melt-lake drainage origin.
  7. Significant widespread ‘inherited’ upland landscapes protected under persistent cold-based ice cover. We also find strong support for the re-interpretation that high-level trimlines mapped across NW Scotland actually represent an upper limit of glacial erosion, not glaciation.
  8. Independent, persistently cold-based ice caps over Scottish, Irish, Welsh and English upland areas with warm, surging outlet glaciers and lobes discharging on all fronts. More controversially, other cold-based ice caps existed over Dartmoor, Exmoor and the North Yorkshire Moors.