Abstract
ABSTRACTThe offshore sector around Shetland remains one of the least well‐studied parts of the former British–Irish Ice Sheet with several long‐standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice‐Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km2, an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice‐sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi‐proxy suite of new isotopic age assessments, including 32 cosmogenic‐nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector‐wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP, this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km2, accompanied by several ice‐margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re‐growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km2 to ca. 15 000 km2 between 19 and 18 ka BP, stabilizing at ca. 2000 km2 by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP, and may have involved one or more subsidiary ice centres on now‐submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine‐influenced Shetland Ice Cap.
Highlights
The last British–Irish Ice Sheet (BIIS) is thought to have covered ~850 000 km2 at its maximum extent, around 24 000–27 000 years ago, and contained enough ice to raise global sea levels by around 2.5 m when it melted (Clark et al, 2012)
Much pioneering work relating to the stratigraphy of Pleistocene glacial deposits in the northern North Sea Basin and West Shetland Shelf was carried out by the British Geological Survey (BGS) during the UK Continental Shelf mapping programme (1970–1990). This information sets out a broad stratigraphic framework of Late Quaternary events it is underpinned by very few absolute ages. Rather than review this stratigraphic and Quaternary geological framework – most of which is published within three BGS Offshore Regional Reports (Johnson et al, 1993; Stoker et al, 1993; Ritchie et al, 2011), and supplemented by a number of comprehensive reviews (e.g. Carr et al, 2006; Graham et al, 2011; Merritt et al, 2017) – we briefly summarize the existing Late Pleistocene reference dates within the northernmost sector (T1) of the former BIIS
Our new chronological dataset exceeds the total number of previously published deglacial ages for this important sector of the BIIS–Fennoscandian Ice Sheet (FIS)
Summary
The last British–Irish Ice Sheet (BIIS) is thought to have covered ~850 000 km at its maximum extent, around 24 000–27 000 years ago, and contained enough ice to raise global sea levels by around 2.5 m when it melted (Clark et al, 2012). The zone of interaction between the BIIS and the larger Fennoscandian Ice Sheet (FIS) remains the largest uncertainty in spatial and chronological reconstructions of the last BIIS In their recent, comprehensive, palaeoglaciological synthesis of the whole Eurasian ice sheet complex, which includes the BIIS and FIS, Hughes et al (2016) highlighted the North Sea Basin as one of two areas of major uncertainty, along with the eastern Barents Sea, where progress has been surprisingly limited over the past 50 years. The paucity of Late Pleistocene dating evidence across the vast majority of the North Sea Basin means that the timing of key events relating to the northernmost sector of the BIIS, including the growth and decay of a ‘Shetland ice cap’, FIS–BIIS ice‐ sheet interaction and separation, and subsequent ice‐mass deglaciation, are only weakly constrained or still uncertain.
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