They are coming thick and fast. Yet another paper relating to the deglacial phase of the Devensian Irish Sea Ice Stream. This is a very technical paper, somewhat difficult to penetrate, but again there is very useful information in it. Underpinning the conclusions of the authors are more luminescence and cosmogenic age determinations, particularly from the SE coast of Ireland. I remember visiting those sites almost 60 years ago in the company of Francis Synge and Frank Mitchell..........
The new work is very relevant to West Wales because whatever went on here was mirrored pretty closely on the other side of St George's Channel. So the sequence of glaciation and deglaciation over there will give us more than a few guidelines......
The model now looks like this: Maximum extent into the Celtic Sea around 27,000n years ago. Then extremely rapid (300–600 m per year) retreat from maximum extent, a slowing of retreat (26 m per year) during the period 25.9–24.2 ka, ice margin stabilization (3 m per year; 24.2–22.1 ka), rapid retreat (152 m per year; 22.1–21.6 ka), and finally a return to slower retreat rates (21 m per year; 21.6–19.5 ka). The right-hand diagram reproduced above shows the assumed ice edge positions looping across St George's Channel towards Pembrokeshire in phases 1-7, each one tied to a specific Irish location. Quote: "Our Bayesian age model indicates that initial ice marginal retreat onto the southern Irish coast occurred at 25.9 ± 1.4 ka (Boundary 2). Retreat of the ISIS from the southern coast of Ireland is constrained by the modeled age (Boundary 3) of 25.1 ± 1.2 ka. Deglaciation to the Wexford coast, and associated deposition of the Screen Hills complex, occurred between 24.2 ± 1.2 ka and 22.1 ± 0.7 ka (Boundaries 4–7)." So this must have been the critical period for the wastage of ice from western Pembrokeshire. Let's hope that dating work is planned for some of the key Pembrokeshire sites...........
Now I'm going to have another gripe about the flowlines on the left-hand and middle maps. The easternmost flowlines in the area under scrutiny cannot be correct; and it is ironic that in a paper devoted to topographic controls and trough geometry the authors are showing ice flowing from NNE towards SSW where there appears to be no topographic control whatsoever, and where the laws of ice physics say that flow must have been from NW towards SE,or perpendicular to the ice edge. I will keep on banging on about this until somebody shows me some evidence that I am wrong. Heyho -- all good fun!
On balance, another great contribution from members of the BRITICE-CHRONO research team.
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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
GSA Bulletin (2018) May 28, 2018
https://doi.org/10.1130/B31852.1
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 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.
CONCLUSIONS
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.
https://doi.org/10.1130/B31852.1
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 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.
CONCLUSIONS
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.
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