Cosmogenic exposure age constraints on deglaciation and flow behaviour of a marine-based ice stream in western Scotland, 21–16 ka

David Small,Sara Benetti,Dayton Dove,Colin K Ballantyne,Derek Fabel,Chris D Clark,Delia M Gheorghiu,Jennifer Newall,Sheng Xu

doi:10.1016/j.quascirev.2017.04.021

Abstract

Understanding how marine-based ice streams operated during episodes of deglaciation requires geochronological data that constrain both timing of deglaciation and changes in their flow behaviour, such as that from unconstrained ice streaming to topographically restricted flow. We present seventeen new 10Be exposure ages from glacial boulders and bedrock at sites in western Scotland within the area drained by the Hebrides Ice Stream, a marine-based ice stream that drained a large proportion of the former British-Irish Ice Sheet. Exposure ages from Tiree constrain deglaciation of a topographic high within the central zone of the ice stream, from which convergent flowsets were produced during ice streaming. These ages thus constrain thinning of the Hebrides Ice Stream, which, on the basis of supporting information, we infer to represent cessation of ice streaming at 20.6 ± 1.2 ka, 3–4 ka earlier than previously inferred. A period of more topographically restricted flow produced flow indicators superimposed on those relating to full ice stream conditions, and exposure ages from up-stream of these constrain deglaciation to 17.5 ± 1.0 ka. Complete deglaciation of the marine sector of the Hebrides Ice Stream occurred by 17–16 ka at which time the ice margin was located near the present coastline. Exposure ages from the southernmost Outer Hebrides (Mingulay and Barra) indicate deglaciation at 18.9 ± 1.0 and 17.1 ± 1.0 ka respectively, demonstrating that an independent ice cap persisted on the southern Outer Hebrides for 3–4 ka after initial ice stream deglaciation. This suggests that deglaciation of the Hebrides Ice Stream was focused along major submarine troughs. Collectively, our data constrain initial deglaciation and changes in flow regime of the Hebrides Ice Stream, final deglaciation of its marine sector, and deglaciation of the southern portion of the independent Outer Hebrides Ice Cap, providing chronological constraints on future numerical reconstructions of this key sector of the former British-Irish Ice Sheet.

Highlights

Ice streams are faster flowing corridors of ice that exert a major control on the mass balance of ice sheets both past and present (Stokes and Clark, 2001; Bennett, 2003; Stokes et al, 2016)
In this paper we aim to constrain the timing of deglaciation of a major marine-based ice stream, the Hebrides Ice Stream (HIS), which drained the western part of the former British-Irish Ice Sheet (BIIS)
We tentatively suggest that these moraines are related to the later period of ice streaming inferred by Dove et al (2015), after the margin of the HIS retreated from the shelf edge

Summary

Introduction

Ice streams are faster flowing corridors of ice that exert a major control on the mass balance of ice sheets both past and present (Stokes and Clark, 2001; Bennett, 2003; Stokes et al, 2016). They extend into the accumulation zones of ice sheets and, as well as. Terrestrial ice streams have been reconstructed from air photos and satellite imagery (e.g. Dyke and Morris, 1988; Stokes and Clark, 2001; De Angelis and Kleman, 2007; Stokes et al, 2009) and, in the marine environment, the increasing availability of high resolution geophysical data, multibeam swath bathymetry, has allowed many former marine-based ice streams to be identified and reconstructed (e.g. Sejrup et al, 2003; O ’Cofaigh et al, 2005; Ottesen et al, 2005; Andreassen et al, 2008 Bradwell and Stoker, 2015)

Objectives

Methods

Results

Discussion

Conclusion