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T5
BRITICE-CHRONO Transect 8: constraining the timing and style of British-Irish Ice
Sheet retreat in NW Scotland
T. Bradwell 1 *, D. Fabel 2 , D. Small 2 , R.C. Chiverrell 3 , M.J. Burke 3 and C.D. Clark 4
1 British Geological Survey, Edinburgh
2 School of Geographical and Earth Sciences, University of Glasgow, Glasgow
3 School of Environmental Sciences, University of Liverpool, Liverpool
4 Department of Geography, University of Sheffield, Sheffield
The aim of Transect 8 (T8) of the NERC-funded consortium BRITICE-CHRONO is to
establish the timing and style of retreat of the British-Irish Ice Sheet (BIIS) from the
continental shelf N and W of Lewis, into The Minch and back to the Highlands of NW
Scotland. The transect encompasses the flowpath and catchment of the former Minch ice
stream – a large dynamic fast flow zone of the BIIS (Bradwell et al., 2007). The research
involves collecting new geochronological data from the terrestrial and marine realms within
a ~30,000 km 2
area (T8). A marine geological cruise on the RRS James Cook is
scheduled for 2015; however this poster focuses on the results of the terrestrial campaign
so far (year 1). T8 covers a large area and includes the rugged seaboard, lochs and
mountains of NW Scotland; the Isle of Lewis; the islands of Skye, Raasay and Rona; and
numerous small outlying islands. Uncertainty currently surrounds the extent and timing of
ice sheet deglaciation on the Hebrides Shelf and the Isle of Lewis, with few absolute dates
existing to bracket ice sheet retreat (Stoker et al., 1993; Bradwell et al., 2007). During
LGM, the BIIS reached onto the mid-shelf and overwhelmed the small island of North
Rona depositing boulders there at c. 25 ka BP (Everest et al., 2013). It remains uncertain
how rapidly and by what mechanism ice sheet retreat took place in this sector, although
existing chronological constraints place the retreating ice-sheet margin in the Summer
Isles region at c. 15 ka BP (Stoker et al., 2009). It is speculated that the Minch Ice Stream
may have broken up rapidly, possibly by flotation, as it retreated shoreward into deeper
water. Two field sampling campaigns were undertaken in 2013: one along the NW
seaboard of mainland Scotland; the other on Lewis. In total 85 rock samples were
collected for terrestrial cosmogenic-nuclide (TCN) analysis, most from glacially transported
(erratic) boulders on glacially streamlined bedrock and ice sheet moraines. The samples
are currently being processed at the University of Glasgow. In addition, field sampling in
northern Lewis of glaciofluvial and ice-marginal deltaic deposits for optically stimulated
luminescence (OSL) dating is planned for early 2014. Crucially, the suite of new TCN and
OSL age estimates will help to constrain the retreat rate and ice volume loss in this
dynamic ice stream sector as it transitioned from a marine-influenced to a terrestrial ice
sheet.
Keywords: BRITICE-CHRONO; British-Irish Ice Sheet; deglaciation; NW Scotland, The
Minch; Outer Hebrides
Bradwell, T., Stoker, M., Larter, R. 2007. Geomorphological signature and flow dynamics of The Minch
palaeo-ice stream, NW Scotland. Journal of Quaternary Science, 22: 609-617.
Stoker, M.S., Bradwell, T., Howe, J.A. Wilkinson, I.M., McIntyre, K. 2009. Lateglacial ice-cap dynamics
in NW Scotland: evidence from the fjords of the Summer Isles region. Quaternary Science Reviews, 28:
3161-3184.
Stoker, M.S. Hitchen, K., Graham, C.C., 1993. The geology of the Hebrides and West Shetland shelves
and adjacent deep-water areas. UK Offshore Regional Report. British Geological Survey, HMSO.
150pp.
Everest, J.D., Bradwell, T., Stoker, M.S., Dewey, S. 2013. New age constraints for the maximum extent
of the last British-Irish Ice Sheet (NW Sector). Journal of Quaternary Science, 28: 2-7.