A comparative study of the retreat of marine-terminating ice sheets across the Northern Atlantic

: the southern sectors of the British-Irish ice sheet and the Newfoundland ice sheet

Abstract

Palaeo ice-sheet reconstructions are considered a key approach to increase our understanding of past climate change and how this impacts on the cryosphere. These reconstructions have shown that ice sheets can have a relatively fast response to climate and ocean forcing mechanisms. This has raised concerns about the future stability of ice sheets in a warming world, especially those that are marine-based or marine-terminating, such as the Greenland and Antarctic Ice Sheets. However, predictions of ice sheets stability are complex and their long-term accuracy remains a major weakness in climate change science. Palaeoglaciological reconstructions offer one critical approach to improve our understanding of how ice sheets respond to climatic drivers over full glacial cycles or through dynamic periods of ice sheet history. As such, palaeo-ice sheets act as useful analogues for helping to determine the important drivers that can influence ice sheets in the future. This study examines the southern margins of two former ice sheets: the British-Irish ice Sheet (BIIS) and the Newfoundland Ice Sheet (NIS). These are located on opposite sides of the North Atlantic Ocean but at similar latitudes. Both ice sheets were grounded below sea level, were drained by ice streams and had extensive marine margins potentially exposed to changes in large-scale ocean and atmospheric
circulation. Therefore, they represent good analogues for modern marine-terminating ice sheets.

New marine geophysical and sediment data were analysed across the Celtic Sea shelf, between Ireland and the UK, which was occupied by the Irish Sea Ice Stream (ISIS), the largest outlet of the BIIS. Geomorphological mapping shows a large meltwater drainage system, including tunnel valleys, beneath the central axis of the former ISIS. This evidence implies significant and erosive meltwater release, potentially influencing the rapid retreat across the shelf. At this stage (~25 cal BP), the ISIS was also retreating close to the southern Irish coastline. Some 30 km off the coast, a relatively large grounding-zone wedge was formed, together with a sequence of morainic ridges, which are capped by glacimarine laminated and massive muds. These features show a stepped retreat of the ISIS margin towards to the coastline. The difference in behaviour of the retreating ice sheet near the present-day coast compared to that in the central axis was probably governed by topographical and geological controls, including variations in water depth and the presence of bedrock outcrops acting as pinning points.

On the other side of the Atlantic Ocean, new data on the southern shelf of Newfoundland were analysed. Here, fjords served as outlets for sediment-laden meltwater draining the former NIS. Intense and widespread calving occurred across the NIS marine margin following its extension to the shelf edge. When the ice sheet reached the present-day coastline, it stabilised at the mouth of the fjords and formed a series of moraines that record a still-stand of the ice margin. The combination of new and extant data suggest that the still-stand occurred between 16.3 ka cal BP and 15 ka cal BP. Stratified glacimarine sediments accumulated at the mouth of the fjords during a period of prevailing cold-water conditions when relative sea level was ~30 m higher than today.

Comparison between the two study areas shows different topographic settings and asynchronous ice-sheet behaviour during the last deglaciation. The onset of retreat between the two ice sheets is around 10 ka apart. A comparison of the results against existing proxy data from the North Atlantic Ocean highlighted that deglaciation of both shelves was initiated in the absence of ocean warming, when eustatic sea level was at a minimum. Internal glaciological factors were therefore most likely responsible for the demise of both marine sectors. This demonstrates that marine-terminating ice margins can internally trigger their own demise in very different glaciological settings and within overall cold conditions. Such information provides additional data for ice sheet numerical models that investigate links between rate and pattern of retreat and the drivers of ice sheet variability.

Date of Award	Mar 2022
Original language	English
Supervisor	Paul Dunlop (Supervisor), Ruth Plets (Supervisor) & Sara Benetti (Supervisor)