Colm O Cofaigh

Marine geophysical data and sediment cores were collected from the continental shelf and slope offshore of SE Greenland during cruise SD041 of the UK research vessel the RRS Sir David Attenborough in 2024. The cruise collected a range of geological, geophysical, oceanographic and biological data. The cruise was part of the ‘Kang-Glac’ project, the aim of which is to investigate the response of the Greenland Ice Sheet to ocean warming during the last 11,700 years. Marine geophysical data and radiocarbon-dated sediment cores provide a clear record of an extensive Greenland Ice Sheet which expanded and retreated across the continental shelf offshore of SE Greenland during, and following, the last glacial maximum. An ancestral Kangerlussuaq Glacier flowed along Kangerlussuaq Trough, a cross shelf bathymetric trough which extends from the mouth of Kangerlussuaq Fiord to the edge of the continental shelf, where it terminates in a trough-mouth fan. Streamlined subglacial bedforms record convergent ice flow into the trough. Sediment cores from along the trough recovered subglacial tills recording a grounded ice sheet. The tills are overlain by a range of deglacial, glacimarine facies recording ice sheet retreat by melting and iceberg calving. A suite of new radiocarbon dates were obtained on foraminifera and shells from the deglacial facies in a transect of cores extending from the trough-mouth fan to the inner shelf. The dates constrain the timing of initial retreat from the outer shelf and allow the position of the grounding-line to be tracked during retreat. The new radiocarbon dates significantly improve the offshore temporal constraints on the post-LGM deglaciation for this sector of the Greenland Ice Sheet and, allied with core sedimentology and foraminferal assemblage data, allow assessment of the role of ocean warming in driving retreat of the ancestral Kangerlussuaq Glacier.

Cruise SD041 of the UK research vessel the RRS Sir David Attenborough to the continental shelf offshore of SE Greenland took place in July-August 2024. The cruise was part of the UK NERC-funded ‘Kang-Glac’ project, a large multi-disciplinary, international, research project jointly led by British Antarctic Survey and Durham University, UK. The cruise collected a range of geological, geophysical, oceanographic and biological data from the continental shelf offshore of Kangerlussuaq Fjord, SE Greenland, and in several adjoining fjords. The aim of the Kang-Glac project is to investigate the response of the Greenland Ice Sheet to ocean warming during the last 11,700 years of the Holocene. During the cruise marine geophysical data in the form of multibeam swath bathymetric imagery of seafloor landforms and sub-bottom profiler data of shallow acoustic stratigraphy were collected, in addition to a suite of sediment cores. Data collection targeted a large cross-shelf bathymetric trough (‘Kang-Trough’) which extended from the mouth of Kangerlussuaq Fiord to the edge of the continental shelf, as well as a series of smaller fjords to the northeast. These marine geophysical data and sediment cores provide a clear record of an extensive Greenland Ice Sheet (GrIS) which expanded and retreated across the shelf via Kang-Trough. Landforms comprise well developed streamlined subglacial bedforms which show convergent GrIS flow into the trough, as well as occasional transverse moraines recording episodic retreat. Sediment cores recovered subglacial tills recording a grounded ice sheet in the cross-shelf trough overlain by a range of deglacial glacimarine facies recording retreat by melting and iceberg calving. Cores from the adjacent trough mouth fan on the continental slope targeted glacigenic debris flows which likely were deposited when the GrIS was grounded at the shelf edge and delivered glacigenic debris onto the slope. Collectively the data provide new insights into past GrIS extent, dynamics, and the nature of associated glacigenic sediment delivery from the LGM through the Holocene in the SE sector of the Greenland continental margin.

Marine geophysical data combined with radiocarbon dated sediment cores provide a record of the advance and retreat of the ancestral Northeast Greenland Ice Stream (NEGIS) across the continental shelf offshore of NE Greenland during the last glaciation. Today, NEGIS is the largest ice stream to drain the Greenland Ice Sheet (GrIS), holding a sea-level equivalent of 1.1-1.4 m. However, the longer-term history of the ice stream, especially on the adjoining outer continental shelf has, to date, been poorly constrained. Streamlined subglacial landforms record grounded ice flow in the outer shelf section in cross shelf bathymetric troughs, with mega-scale glacial lineations recording former streaming flow towards the shelf edge. Flow transverse landforms in the form of downlow-tapering, sediment wedges occur at the shelf edge and on the outer-mid shelf of the bathymetric troughs. These landforms differ in their morphology from the classic ‘ramp-step’ form of typical grounding wedges but are similarly interpreted as a form of grounding-zone wedge in which sediment prograded and thinned away from the grounding-zone. The wedges record a shelf-edge terminating, grounded ancestral NEGIS, as well as the subsequent episodic retreat of the ice stream inshore during deglaciation. Beyond the shelf edge, glacigenic debris flows imaged on acoustic stratigraphic profiles and recovered in sediment cores document sediment delivery onto the slope; such deposits are typical of submarine slopes offshore of shelf-edge terminating palaeo-ice streams. On the outer shelf subglacial tills and grounding-zone proximal sediments overlain by deglacial stratified glacimarine sediments record ice stream advance and retreat in the troughs. Radiocarbon dates from glacimarine sediments in these cores indicate early deglaciation from the shelf edge but with relatively slow rates of subsequent ice-stream retreat across the outer shelf.

Understanding the pace and drivers of marine‐based ice‐sheet retreat relies upon the integration of numerical ice‐sheet models with observations from contemporary polar ice sheets and well‐constrained palaeo‐glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine‐geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice‐marginal landforms, to reconstruct the timing and rate of ice‐sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine‐ to a terrestrially‐based ice‐sheet margin. Seafloor bathymetric data in the form of moraines and grounding‐zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid‐shelf where it formed a large grounding‐zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice‐free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice‐sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio‐isostatically induced, high relative sea level. Net rates of ice‐sheet retreat across the shelf were slow (62–19 m a⁻¹) and reduced (8 m a⁻¹) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine‐based sectors of ice sheets.

This is the author accepted manuscript. The final version is available from Geological Society of London via https://doi.org/10.1144/M46.8

Over the last two decades, marine science, aided by technological advances in sediment coring, geophysical imaging and remotely operated submersibles, has played a major role in the investigation of contemporary and former ice sheets. Notable advances have been achieved with respect to reconstructing the extent and flow dynamics of the large polar ice sheets and their mid-latitude counterparts during the Quaternary from marine geophysical and geological records of landforms and sediments on glacier-influenced continental margins. Investigations of the deep-sea ice-rafted debris record have demonstrated that catastrophic collapse of large (10 ⁵ –10 ⁶ km ² ) ice-sheet drainage basins occurred on millennial and shorter time scales and had a major influence on oceanography. In the last few years, increasing emphasis has been placed on understanding physical processes at the ice–ocean interface, particularly at the grounding line, and on determining how these processes affect ice-sheet stability. This remains a major challenge, however, owing to the logistical constraints imposed by working in ice-infested polar waters and ice-shelf cavities. Furthermore, despite advances in reconstructing the Quaternary history of mid- and high-latitude ice sheets, major unanswered questions remain regarding West Antarctic ice-sheet stability, and the long-term offshore history of the East Antarctic and Greenland ice sheets remains poorly constrained. While these are major research frontiers in glaciology, and ones in which marine science has a pivotal role to play, realizing such future advances will require an integrated collaborative approach between oceanographers, glaciologists, marine geologists and numerical modellers.