Pattern, style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf

Abstract

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast‐flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice‐stream‐dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km2 of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic‐nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector‐wide ice‐sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice‐sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice‐sheet sector undergoing large size variations in response to sub‐millennial‐scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine‐terminating ice‐sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice‐sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice‐sheet build‐up in NW Scotland, with the Minch ice‐stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding‐zone wedges and moraines on the mid‐ and inner continental shelf attest to significant stabilizations of the ice‐sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present‐day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea‐level c. 26 kabp, followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice‐shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp, as the grounding‐line migrated into shoreward deepening water, co...

Exploring controls of the early and stepped deglaciation on the western margin of the British Irish Ice Sheet

“…For the former MSIS, the geomorphological evidence shows the presence of a compound ridge close to the shelf edge comprising a series of moraines and grounding‐zone wedges (GZWs) mapped from 55°30′N to 56°30′N (Dunlop et al ., 2010; Callard et al ., 2018). Further to the north, a series of morainal banks with a similar N–S orientation have been broadly mapped from seismic data down to 150 m water depth and are likely to be the continuation of the same ice margin and to be related to the extension of the Outer Hebrides Ice Cap on the Scottish continental shelf (Bradwell et al ., 2021). Moraines of different orientations are observed at the boundary between the DBIS (T6) and MSIS (T7) on the Malin Shelf [trending, respectively NW–SE and NE–SW; Figs.…”

Spatial Relationships between Pockmarks and Sub-Seabed Gas in Fjordic Settings: Evidence from Loch Linnhe, West Scotland

“…Deposition of organic matter in contemporary nearshore glacial settings has been found within the fjords of western Svalbard [37] where gas-charged sediment is observed resulting from organic matter associated with marine productivity distal to the glacial margin. Further examples of biogenic gas accumulations within Late Pleistocene (Weichselian) glaciomarine muds are found in the North Sea Basin [38] and in one or two nearshore basins off northwest Scotland [39][40][41]. Within Loch Linnhe, total carbon content has been estimated at 92.28 Mt [23], but with very low C/N ratios recorded within the uppermost 4 m of sediment [28].…”

Collapse of the Last Eurasian Ice Sheet in the North Sea Modulated by Combined Processes of Ice Flow, Surface Melt, and Marine Ice Sheet Instabilities

“…Geochronological dates with a quality control rating of Green or Amber by Small et al. (2017) were used, along with additional offshore dates from (Bateman et al., 2018; Bradwell et al., 2021; Callard et al., 2018; Evans et al., 2018, 2019, 2021; Roberts et al., 2018, 2019), resulting in 131 geochronological dates for comparison with modeled deglaciation ages. ATAT calculates the wRMSE between modeled deglaciation ages and geochronological data, accounting for the uneven spatial distribution of dates.…”

“…The Automated Timing Accordance Tool (ATAT) (Ely et al, 2019a(Ely et al, , 2019b) is used to identify matches between modeled deglaciation ages and geochronological data (in calibrated years before present). Geochronological dates with a quality control rating of Green or Amber by Small et al (2017) were used, along with additional offshore dates from (Bateman et al, 2018;Bradwell et al, 2021;Callard et al, 2018;Evans et al, 2018Evans et al, , 2019Evans et al, , 2021Roberts et al, 2018Roberts et al, , 2019, resulting in 131 geochronological dates for comparison with modeled deglaciation ages. ATAT calculates the wRMSE between modeled deglaciation ages and geochronological data, accounting for the uneven spatial distribution of dates.…”