Trough geometry was a greater influence than climate-ocean forcing in regulating retreat of the marine-based Irish-Sea Ice Stream

Abstract

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 timeaveraged 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 cosmo genic 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.

Ice margin oscillations during deglaciation of the northern Irish Sea Basin

“…) shows a broad match between retreat and increasing summer insolation (Berger and Loutre, ). Following the short‐lived advance of the ISIS into the Celtic Sea, overall ice‐marginal retreat in the southern ISB was punctuated by still‐stands and oscillations, recorded from south to north along the coastlines of Ireland and Wales (Smedley et al ., ; Small et al ., ). Faster (e.g.…”

“…The faster net retreat rates (80–180 m a −1 ) were similar to the faster rates on the northern Llŷn Peninsula where the ice front was less constrained by trough geometry (84–139 m a −1 ) (Smedley et al ., ). The slower net retreat rates (13–20 m a −1 ) experienced in the NISB were similar to those reconstructed for the southern Llŷn Peninsula (8–41 m a −1 ) (Smedley et al ., ) and southern Irish coast (26 m a −1 ) (Small et al ., ). Slowing of net ice‐margin retreat appears to have occurred when the ice was pinned against the bedrock obstruction of the Isle of Man, and was characterized by a series of ice‐marginal oscillations within the interval 19.9 ± 0.7 and 18.3 ± 1.1 ka ; these are recorded in stratigraphical evidence for a series of standstill events and limited (<1 km) readvances of the ice margin (Thomas et al ., ).…”

“…Retreat of ice margins across the Llŷn Peninsula have been dated to between 23.9 AE 1.6 and 21.1 AE 0.6 ka (Smedley et al, 2017a). The Celtic Sea advance of the ISIS has been suggested to have been a rapid and short-lived event (Chiverrell et al, 2013) and was followed by rapid retreat (Smedley et al, 2017a;Small et al, 2018). Advance and rapid retreat of this nature is likely to have been accompanied by significant drawdown of the ice stream surface and was invoked to explain changes in the retreat of the western lateral margin of the ISIS (Small et al, 2018).…”

“…The Celtic Sea advance of the ISIS has been suggested to have been a rapid and short-lived event (Chiverrell et al, 2013) and was followed by rapid retreat (Smedley et al, 2017a;Small et al, 2018). Advance and rapid retreat of this nature is likely to have been accompanied by significant drawdown of the ice stream surface and was invoked to explain changes in the retreat of the western lateral margin of the ISIS (Small et al, 2018). The ages for ice thinning in the mountains of the Isle of Man and Cumbria are older than previously published ages in the range 18-16 ka for ice-free conditions in the Cumbrian Mountains (Ballantyne et al, 2009;Ballantyne, 2010;Wilson et al, 2013;Wilson and Lord, 2014), where a locally nourished ice field persisted after deglaciation of the NISB.…”

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

“…Following initial rapid retreat, the oscillating margin of the Donegal Bay Ice Lobe retreated slowly from mid‐shelf to the DBM (Ó Cofaigh et al , ), so that ice cover persisted over south Donegal until ∼17 ka, after which it retreated to the footslopes of the Blue Stack Mountains. This contrast in behaviour implies that different dynamics apply to extended marine‐based ice streams and lobes, which are sensitive to changes in sea level, confinement and bed slope (Smedley et al , ; Ó Cofaigh et al , ; Small et al , ) and land‐based ice masses in inter‐ice‐stream/lobe locations, which respond mainly to changes in climate inputs.…”