Late Quaternary reconstruction of British-Irish Ice Sheet variability through the analysis of deep-water sediments from the Donegal Barra Fan and the Rockall Trough, North Atlantic.

Abstract

This research focusses on the evolution of the British-Irish Ice Sheet (BIIS) through the last glacial period and highlights its dynamic behaviour in terms of ice sheet advance and retreat, across the Irish and UK continental shelves. A multidisciplinary approach was applied to sediment cores retrieved from two different locations in the Rockall Trough, North East Atlantic Ocean, between 1000 and 3000 m water depth: from the glaciogenic Donegal-Barra Fan and the basin floor in the south-eastern portion of the trough. The methodologies used in this study include the interpretation of physical properties (in particular magnetic susceptibility and lightness), x-radiographs, grain size and stable isotopes analyses, radiocarbon dating, the determination of biofacies in planktonic foraminifera and the calculation of Neogloboquadrina pachyderma (sinistral) abundances, ice-rafted debris (IRD) concentrations and IRD fluxes. This research provides an overall interpretation on the sedimentary processes active along the North East Atlantic margin during the last 130 ka, in particular in relation to glacial and glaciomarine sedimentation linked to the BIIS. The presence of IRD of local origin in the deep water demonstrates how, during the last glacial period, the BIIS first extended offshore during MIS5b and later during MIS4 with marine-based calving margins. The BIIS maximum extent was reached before 20 ka BP, with a marine-terminating ice sheet that reached the shelf edge in most places. At this time, the maximum calculated value in sedimentation rates (13.8 cm/ka) suggests a significant input of sediment through meltwater plume deposition into the trough. Between 20 and 18 ka BP, the BIIS along the western Irish margin, north of the Porcupine Bank, was retreating through calving, shown by peaks in IRD concentrations and fluxes. The final stages of BIIS deglaciation are revealed by the cores from the Donegal-Barra Fan. Here glaciomarine sediments were delivered to the deep waters until around 16 ka BP. This is inferred to be the time when
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the ice sheet had retreated entirely to an inshore or terrestrial position and therefore glacial sediment was no longer delivered to the shelf edge and outer shelf. Two distinct IRD-rich layers are related to two major BIIS calving events at 17.8 ka BP and 16.9 ka BP, which suggest that here the BIIS finally deglaciated by massive calving rather than a slow retreat across the shelf. An additional third IRD-rich layer is observed higher in the cores and dated at the Younger Dryas (ca. 12.7 ka BP). Icebergs responsible for the delivery of this last IRD pulse to the region are inferred to be sourced from a re-advancement and marine extension of the Scandinavian Ice Sheet at that time. Overall, the sediment record analysed in this study reveals how deep water sediments can provide a long and continuous record of glacial processes for large, marine-terminating ice sheets and can be used effectively for palaeoglaciological reconstructions. The results of this research can inform the current modelling of past ice sheet dynamics, which in turn will provide data for the prediction of possible future scenarios for present day marine-based ice sheets.

Date of Award	Sept 2018
Original language	English
Supervisor	Sara Benetti (Supervisor) & Paul Dunlop (Supervisor)