Sedimentology and chronology of the advance and retreat of the last British-Irish ice sheet on the continental shelf west of Ireland

Peters, J. L., Benetti, S., Dunlop, P., Ó Cofaigh, C., Moreton, S. C. , Wheeler, A. J. and Clark, C. D. (2016) Sedimentology and chronology of the advance and retreat of the last British-Irish ice sheet on the continental shelf west of Ireland. Quaternary Science Reviews, 140, pp. 101-124. (doi: 10.1016/j.quascirev.2016.03.012)

Abstract

The last British-Irish Ice Sheet (BIIS) had extensive marine-terminating margins and was drained by multiple large ice streams and is thus a useful analogue for marine-based areas of modern ice sheets. However, despite recent advances from investigating the offshore record of the BIIS, the dynamic history of its marine margins, which would have been sensitive to external forcing(s), remain inadequately understood. This study is the first reconstruction of the retreat dynamics and chronology of the western, marine-terminating, margin of the last (Late Midlandian) BIIS. Analyses of shelf geomorphology and core sedimentology and chronology enable a reconstruction of the Late Midlandian history of the BIIS west of Ireland, from initial advance to final retreat onshore. Five AMS radiocarbon dates from marine cores constrain the timing of retreat and associated readvances during deglaciation. The BIIS advanced without streaming or surging, depositing a bed of highly consolidated subglacial traction till, and reached to within ∼20 km of the shelf break by ∼24,000 Cal BP. Ice margin retreat was likely preceded by thinning, grounding zone retreat and ice shelf formation on the outer shelf by ∼22,000 Cal BP. This ice shelf persisted for ≤2500 years, while retreating at a minimum rate of ∼24 m/yr and buttressing a >150-km long, 20-km wide, bathymetrically-controlled grounding zone. A large (∼150 km long), arcuate, flat-topped grounding-zone wedge, termed here the Galway Lobe Grounding-Zone Wedge (GLGZW), was deposited below this ice shelf and records a significant stillstand in BIIS retreat. Geomorphic relationships indicate that the BIIS experienced continued thinning during its retreat across the shelf, which led to increased topographic influence on its flow dynamics following ice shelf break up and grounding zone retreat past the GLGZW. At this stage of retreat the western BIIS was comprised of several discrete, asynchronous lobes that underwent several readvances. Sedimentary evidence of dilatant till deposition suggests that the readvances may have been rapid and possibly associated with ice streaming or surging. The largest lobe extended offshore from Galway Bay and deposited the Galway Lobe Readvance Moraine by <18,500 Cal BP. Further to the north, an ice lobe readvanced at least 50 km offshore from Killary Harbour, possibly by ≤15,100 Cal BP. The existing chronology currently does not allow us to determine conclusively whether these readvances were a glaciodynamic (internally-driven) response of the ice sheet during deglaciation or were climatically-driven. Following the <18,500 Cal BP readvance, the Galway Lobe experienced accelerated eastward retreat at an estimated rate of ∼113 m/yr.

• Item Type: Articles
• Additional Information: JLP acknowledges a University of Ulster PhD studentship and Vice-Chancellor's Scholarship from 2013 to 2016. This work was supported by the NERC Radiocarbon Facility NRCF010001 (allocation number 1722.0613), NERC consortium grant NE/J009768/1, for radiocarbon dating. Further radiocarbon dating support was provided by the International Association of Sedimentologists (Postgraduate Research Grant
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Moreton, Dr Steven
• Authors: Peters, J. L., Benetti, S., Dunlop, P., Ó Cofaigh, C., Moreton, S. C., Wheeler, A. J., and Clark, C. D.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Quaternary Science Reviews
• Publisher: Elsevier
• ISSN: 0277-3791
• ISSN (Online): 1873-457X
• Published Online: 04 April 2016
• Copyright Holders: Copyright © 2016 Elsevier
• First Published: First published in Quaternary Science Reviews 140:101-124
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher