Trough geometry was a greater influence than climate-ocean forcing in regulating retreat of the marine-based Irish-Sea Ice Stream (2018)
by David Small, Rachel K. Smedley, Richard C. Chiverrell, James D. Scourse, Colm Ó Cofaigh, Geoff A.T. Duller, Stephen McCarron, Matthew J. Burke, David J.A. Evans, Derek Fabel, Delia M. Gheorghiu, Geoff S.P. Thomas, Sheng Xu, Chris D. Clark
Marine terminating ice streams are a major component of contemporary ice sheets and are likely to have a fundamental influence on their future evolution and concomitant contribution to sea-level rise. To accurately predict this evolution requires that modern day observations can be placed into a longer-term context and that numerical ice sheet models used for making predictions are validated against known evolution of former ice masses. New geochronological data document a stepped retreat of the paleo−Irish Sea Ice Stream from its Last Glacial Maximum limits, constraining changes in the time-averaged retreat rates between well-defined ice marginal positions. The timing and pace of this retreat is compatible with the sediment-landform record and suggests that ice marginal retreat was primarily conditioned by trough geometry and that its pacing was independent of ocean-climate forcing. We present and integrate new luminescence and cosmogenic exposure ages in a spatial Bayesian sequence model for a north-south (173km) transect of the largest marine-terminating ice stream draining the last British−Irish Ice Sheet. From the south and east coasts of Ireland, initial rates of ice margin retreat were as high as 300−600 m a−1, but retreat slowed to 26 m a−1 as the ice stream became topographically constricted within St George’s Channel, a sea channel between Ireland to the west and Great Britain to the east, and then stabilized (retreating at only 3 m a−1) at the narrowest point of the trough during the climatic warming of Greenland Interstadial 2 (GI-2: 23.3−22.9 ka). Later retreat across a normal bed-slope during the cooler conditions of Greenland Stadial 2 was unexpectedly rapid (152 m a−1). We demonstrate that trough geometry had a profound influence on ice margin retreat and suggest that the final rapid retreat was conditioned by ice sheet drawdown (dynamic thinning) during stabilization at the trough constriction, which was exacerbated by increased calving due to warmer ocean waters during GI-2.
The geochronological data presented here allow us to test a conceptual model of ISIS deglaciation in south and east Ireland inferred from the sediment-landform assemblage record. Integration of new geochronological data using Bayesian age modeling produces a conformable age model that supports the conceptual model of deglaciation, with extremely rapid (300–600 m a–1) retreat from maximum extent, a slowing of retreat (26 m a–1) during the period 25.9–24.2 ka, ice margin stabilization (3 m a–1; 24.2–22.1 ka), rapid retreat (152 m a–1; 22.1–21.6 ka), and finally a return to slower retreat rates (21 m a–1; 21.6–19.5 ka).
This timescale strongly suggests that aspects of ISIS behavior during deglaciation displayed a complex relationship to external climatic forcing. Extremely rapid advance of the ISIS to its maximum extent, and its subsequent retreat at 26–25 ka is not directly correlated with distinct climate forcing in the North Atlantic region. Such behavior may be explained as a dynamic instability in response to overextension of the ice stream to the maximum limit that rendered it vulnerable to rapid retreat. Similarly, the stabilization of the ice margin at the Screen Hills spans a time of distinct warming (GI-2) with the subsequent rapid retreat occurring during colder conditions of GS-2. The stabilization at the Screen Hills is the most distinct change in pace of ISIS retreat evidenced by the data presented here and it occurs where there is a step-change in the con ning trough geometry highlighting the important role that this plays in condition- ing ice margin retreat. However, contrary to this, the ISIS subsequently underwent rapid retreat without major changes in trough geometry. We speculate that this represents a delayed response of the ice margin to the climate forcing of GI-2. Overall, changing trough geometry and internal feedbacks related to the overextension, retreat, and stabilization of the ISIS appear to obscure the role of external drivers such as climatic forcing.
The conceptual model and geochronological data presented here provide evidence for specific ice margin behavior during overall deglaciation that provides a testing ground for numerical models that likely require high resolution representations of grounding line dynamics. As contemporary ice streams in Greenland and Antarctica evolve in response to anthropogenic climate change they will undergo retreat that is conditioned both by climatic forcing and their internal dynamics. Our data highlight the potential for the evolution of rapid ice margin retreat to be highly nonlinear and conditioned strongly by trough geometry.