Ice marginal dynamics of the last British-Irish Ice Sheet in the southern North Sea: Ice limits, timing and the influence of the Dogger Bank

Colm Ó Cofaigh,S Louise Callard,Derek Fabel,Tom Bradwell,Alicia Medialdea,Carol J Cotterill,Mark D Bateman,Richard C Chiverrell,Dayton Dove,David J.A Evans,Steven G Moreton,David H Roberts,Chris D Clark,Margot Saher

doi:10.1016/j.quascirev.2018.08.010

Abstract

The southern North Sea is a particularly important area for understanding the behaviour of the British-Irish Ice Sheet (BIIS) during the last glacial cycle. It preserves a record of the maximum extent of the eastern sector of the BIIS as well as evidence for multiple different ice flow phases and the dynamic re-organisation of the BIIS. However, to date, the known ice sheet history and geochronology of this region is predominantly derived from onshore geological evidence, and the offshore imprint and dynamic history of the last ice sheet remain largely unknown. Using new data collected by the BRITICE-CHRONO project this paper explores the origin and age of the Dogger Bank; re-assesses the extent and age of the glaciogenic deposits across the shallow areas of the North Sea between the Dogger Bank and the north Norfolk coast and; re-examines the dynamic behaviour of the BIIS in the southern North Sea between 31.6 and 21.5 ka.This paper shows the core of the Dogger Bank to be composed glaciolacustrine sediment deposited between 31.6 and 25.8 ka. Following its formation the western end of the Dogger lake was overridden with ice reaching ∼54°N where the ice margin is co-incident with the southerly extent of subglacial tills previously mapped as Bolders Bank Fm. This initial ice override and retreat northwards back across the Dogger lake was complete by 23.1 ka, but resulted in widespread compressive glaciotectonism of the lake sediments and the formation of thrust moraine complexes. Along the northern edge of the bank moraines are on-lapped by later phase glaciolacustrine and marine sediments but do not show evidence of subsequent ice override.The shallow seafloor to the west and southwest of the Dogger Bank records several later phases of ice advance and retreat as the North Sea Lobe flowed between the Dogger Bank and the Yorkshire/Lincolnshire coasts and reached North Norfolk. New optically stimulated luminescence (OSL) ages from Garrett Hill on outwash limit the arrival of the BIIS on the Norfolk coast to 22.8–21.5 ka. Multiple till sheets and chains of moraines on the seafloor north of Norfolk mark dynamic oscillation of the North Sea Lobe margin as it retreated northwards. This pattern of behaviour is broadly synchronous with the terrestrial record of deposition of subglacial, glaciofluvial and glaciolacustrine sediments along the Yorkshire coast which relate to post Dimlington Stadial ice marginal oscillations after 21.5 ka.With respect to forcing mechanisms it is likely that during the early phases of the last glacial maximum (∼30-23ka) the interaction between the southern margin of the BIIS and the Dogger Lake was critical in influencing flow instability and rapid ice advance and retreat. However, during the latter part of the last glacial maximum (22–21 ka) late-phase ice advance in the southern North Sea became restricted to the western side of the Dogger Bank which was a substantial topographic feature by this time. This topographic confinement, in addition to decoupling of the BIIS and the Fennoscandian Ice Sheet (FIS) further north, enabled ice to reach the north Norfolk coast, overprinting the seabed with late-phase tills of the Bolders Bank Fm.

Highlights

Investigating the external and internal forcing factors that control ice sheet behaviour is an important scientific and societal challenge if present and future changes to the cryosphere are to be understood and contextualised over decadal to millennial timescales (Sejrup et al, 2016; Bamber et al, 2009; DeConto and Pollard, 2016)
Ice sheet coupling forced ice flow north into the Atlantic to a marine-terminating margin at the Norwegian shelf break, whilst southerly directed flow terminated in the southern North Sea in a more stable terrestrial setting
Using new onshore and offshore geophysical, sedimentological and geochronological data collected by the BRITICE-Chrono project this paper aims to investigate the offshore glacial history of the southern North Sea to provide an integrated model for ice sheet advance and retreat in the region

Summary

Introduction

Investigating the external and internal forcing factors that control ice sheet behaviour is an important scientific and societal challenge if present and future changes to the cryosphere are to be understood and contextualised over decadal to millennial timescales (Sejrup et al, 2016; Bamber et al, 2009; DeConto and Pollard, 2016). The eastern sector of the last BIIS was important in influencing both the advance and retreat behaviour of the ice sheet (Carr et al, 2006; Davies et al, 2009; Graham et al, 2011). In the central and northern North Sea coalescence between the BIIS and the Fennoscandian Ice Sheet (FIS) radically changed ice sheet dynamics in the build-up to the LGM (~30e21 ka for BIIS; see Chiverrell and Thomas, 2010 for overview). As the last glacial cycle waned decoupling between the two ice sheets triggered ice divide migration in the northern and central sectors of the BIIS inducing rapid flow re-organisation in the North Sea (Livingstone et al, 2012; Clark et al, 2012), though the timing of this remains uncertain (Sejrup et al, 2016)

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