Submarine sediment and landform record of a palaeo‐ice stream within the British−Irish Ice Sheet

Abstract

This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe the Pleistocene sediment and landform record of a large ice‐stream system that drained ∼3% of the entire British−Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we describe: the ice‐stream terminus environment and depocentre on the outer shelf and continental slope; sediment architecture and subglacial landforms on the mid‐shelf and in a large marine embayment (the Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft‐bed (sediment) and hard‐bed (bedrock) subglacial landform assemblages in the central and inner parts of the Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast‐flowing ice‐sheet corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice‐bed coupling in a zone of flow convergence associated with ice‐stream onset; and a downstream bedform evolution (short drumlins to km‐scale glacial lineations) suggesting an ice‐flow velocity transition associated with a bed substrate and roughness change in the ice‐stream trunk. Chronology is still lacking for the timing of ice‐stream demise; however, the seismic stratigraphy, absence of moraines or grounding‐line features, and presence of well‐preserved subglacial bedforms and iceberg scours, combined with the landward deepening bathymetry, all suggest that frontal retreat in the Minch was probably rapid, via widespread calving, before stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long‐lived, ice‐sheet margin position only 5–15 km offshore strongly support this model. Reconstructed ice‐discharge values for the Minch ice stream (12–20 Gt a−1) are comparable to high mass‐flux ice streams today, underlining it as an excellent palaeo‐analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice Sheets.

Middle to Late Devensian glaciation of north‐east Scotland: implications for the north‐eastern quadrant of the last British–Irish ice sheet

“…In particular, the dataset reveals multiple ridges of distinct lobate moraines around Shetland and including the Bosies Bank, in the outer Moray Firth (Figure 1). The pattern of ice-marginal sea-bed features show unambiguously that a significant re-expansion of ice occurred over Orkney and Shetland Clark et al, 2012;Bradwell and Stoker, 2015a) (Figure 1), when Scandinavian and Scottish ice possibly coalesced again in the northern North Sea Basin and adjusted to new quasistable marginal positions. The BIIS had readvanced off western Scotland by 19k cal a BP (Knutz et al, 2001) and the re-expansion peaked between c. 18.4 and 15.4k cal a BP (Brendryen et al, 2015).…”

“…However, doubts surround the published geochronometry and it is difficult to explain why early deglaciation of the central NSB occurred during a period including the global LGM when other sectors of the BIIS were expanding Sejrup et al, 2016). It is generally agreed that the northern sector of the last BIIS built up towards the end of MIS 3, after c. 32k a BP, when ice extended to the continental shelf margin (Stoker and Holmes, 1991;Whittington and Hall, 2002;Wilson et al, 2002;Bradwell et al, 2008;Jacobi et al, 2009;Chiverrell and Thomas, 2010;Gibbard and Clark, 2011;Graham et al, 2011;Clark et al, 2012;Everest et al, 2013;Hughes et al, 2014Hughes et al, , 2016Hiemstra et al, 2015;Bradwell and Stoker, 2015a). It is unlikely that ice-free areas existed anywhere in northern Scotland during the global LGM, when the BIIS was thick (Ballantyne, 2010;Fabel et al, 2012) (Figure 2).…”

“…It is unlikely that ice-free areas existed anywhere in northern Scotland during the global LGM, when the BIIS was thick (Ballantyne, 2010;Fabel et al, 2012) (Figure 2). Major readjustments occurred during later stages when ice streams became established in the Moray Firth (Merritt et al, 1995), the Minch (Bradwell and Stoker, 2015a) and the Firth of Tay (Golledge and Stoker, 2006), possibly as a result of switches in glacial behavoir.…”

“…The northern part of the last BIIS built up towards the end of MIS 3, after c. 32k a BP (Whittington and Hall, 2002;Jacobi et al, 2009) at a time when Central Scotland remained largely ice free, judging from dates of 34.5k to 28.1k cal a BP obtained on glacially over-ridden organic deposits at Sourlie (Bos et al, 2004) and Balglass (Brown, 2007) (Figure 1). Ice expanded westwards to the shelf break shortly after 30k cal a BP (Wilson et al, 2002;Scourse et al, 2009;Bradwell and Stoker, 2015a;Bradwell and Stoker, 2015b et al), it had reached at least as far as North Rona at 25-27 ka and probably over-ran St Kilda, north-west of the Western Isles (Figure 1) at this time (Hiemstra et al, 2015). The Fennoscandian Ice Sheet (FIS) first expanded southwards into Denmark through the Kattegat, between 29 and 26k cal a (Houmark-Nielsen and Kjaer, 2003).…”

Seabed geomorphology of the Porcupine Bank, West of Ireland

“…The aim was to characterise the acoustic signature of the material forming or cut by the feature and infer its potential nature and/or internal structure (Bradwell & Stoker, 2015;Mitchum, Vail, & Sangree, 1977). An average estimated acoustic velocity of 1650 m/s was used to calculate the thickness of the sedimentary units as calculated in the nearby vicinity of the Porcupine Bank western slopes (Haughton et al, 2005;Øvrebø, Haughton, & Shannon, 2006).…”