Timing, forcing and onshore-offshore correlations on the western margin of the British-Irish Ice Sheet

Abstract

This research investigates the impact and timing of glaciations on the western Irish shelf and their sedimentary record. A multiproxy approach including seismic interpretation, marine sediment core analysis, radiocarbon dating, geomorphological analysis and Cosmogenic Nuclide (CN) exposure dating of landforms and erratics onshore is used to assess the glacial dynamics on the western margin of the British Irish Ice Sheet (BIIS) during the last glacial period. The combination of the methodologies and materials used and subsequent correlation with proxy climate records allows for an interpretation of the forcing mechanisms associated with the waxing and waning of the BIIS in this region. Analysis of seismic profiles from the western Irish shelf reveals for the first time the internal stratigraphy of the large morainic complex, situated between ~52.5°N and ~54°N, which has been deposited by a grounded ice sheet on the mid-shelf. An early Quaternary shift in the style of sediment deposition at the outer Donegal Bay margin is associated with the onset of glaciations on the western Irish shelf. Lithofacies interpretations and radiocarbon dating of marine sediment cores suggest an asynchronous retreat pattern of the last BIIS from north to south on the western Irish shelf. This study bridges the knowledge gap between previous investigations of the region and establishes a shelf-wide correlation of grounded ice sheet retreat on the western Irish shelf based on sediment core analysis and radiocarbon datings for the first time. The palaeoenvironmental changes during ice sheet retreat are assessed by an increased percentage of benthic foraminifera assemblages associated with temperate environments from a core at the outer shelf. The synthesis of CN exposure ages of erratics onshore, radiocarbon dated sediments from offshore and mapped glacial features allows for chronological constraint of a glacial readvance into Donegal Bay that flowed offshore from north county Mayo after 17.6 cal ka BP after the main-Last Glacial Maximum (LGM) ice sheet had retreated eastward towards the Sligo coast. The forcing mechanism for this ice margin readvancing into Donegal Bay is therefore interpreted as a glacidynamic response associated with debuttressing during general ice retreat, i.e. the retreat of an ice lobe eastward towards the Sligo coast that acted as a buttress to the ice lobe with a perpendicular ice flow pattern advancing from northern county Mayo into Donegal Bay; thus resulting in the ice margin readvance of the debuttressed ice lobe from northern county Mayo. Finally, dated deglacial offshore and onshore moraines and erratics available to this study are used to build a time-constrained reconstruction of the dynamic behaviour of the western margin of the BIIS from the LGM to its deglaciation during the Woodgrange Interstadial (Irish terminology/equivalent to the Bølling-Allerød Interstadial).

Date of Award	Mar 2017
Original language	English
Supervisor	Sara Benetti (Supervisor) & Paul Dunlop (Supervisor)