85,000 years of polar front, warm current variability, and ice rafting in contourites off southwest Ireland

Westgård, Adele1, Gallagher, Stephen J.1, Monteys, Xavier2, Foubert, Anneleen3, Rüggeberg, Andres3

1The University of Melbourne, Melbourne, Australia, 2Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland, 3Department of Geosciences – Geology, University of Fribourg, Chemin du Musée 6, CH – 1700 Fribourg, Switzerland

Integrated Ocean Drilling Program Expedition 307 Site U1318B cored a 90 m thick Quaternary contourite sequence in the Porcupine Seabight in 423 m water depth to understand paleoenvironmental conditions of cold-water coral carbonate mound growth initiation. Two warm contour currents flow northwards in the Porcupine Seabight: the Eastern North Atlantic Central Water (ENACW, ~600 m) and the Mediterranean Outflow Water (MOW, ~1000 m). However, while much work has been carried out on the carbonate mound province in the area the palaeoenvironmental context of the surrounding contourites has received scant attention. This work analysed planktonic foraminiferal assemblages and ice rafted debris (IRD) variability in the upper 30 m of Site U1318B to interpret oceanographic variability related to contourite sedimentation, and interpret the extent and timing of the British Irish Ice Sheet (BIIS) over the last 85,000 years.  

Samples from Site U1318B were processed for foraminiferal analysis. Planktonic foraminiferal assemblages (>150 µm fraction) were used to interpret sea surface temperatures (SST). The presence of the polar front (SST <8˚C) in the region was characterised by >80% Neogloboquadrina pachyderma (%NPS). IRD (>150 µm) abundance was counted and plotted by source area. An age model was developed using a 14C date, a nannofossil datum (Emiliana huxleyi/Gephyrocapsa caribbeanica) and regional correlation to MD01-2461 using %NPS and magnetic susceptibility data and to the Lisicki & Raymo (LR2004) stack.

The upper 30 m of Site U1318B is typified by alternations of dark brown-grey, occasionally laminated mud and medium to coarse sand facies with erosional bases. Muddy sand layers, sand lenses and dropstones are common. Planktic foraminiferal data suggests that in the last 85 ka the polar front was south of Site U1318B (>80% NPS) ~24 ka, ~28 ka, from 35 to 37 ka, 40 to 66 ka and ~68 ka. %NPS reached 70-79% on several times suggesting intermittent polar conditions. Subtropical taxa are rare when NPS are common. Maxima (<12%) of subtropical taxa occur several times in the last 35 ka suggesting intermittent ENACW influence. The IRD are predominantly sourced from the BIIS area. Increased IRD yield coincides with decreased %NPS. IRD maxima do not coincide with %NPS maxima since ice rafting was related to melting. IRD concentration gradually decreased after the last glacial maximum (LGM) (~20 ka).

Presence of subtropical taxa in contourite facies suggest increased current activity during warm periods. These currents likely slowed/shut down in association with the North Atlantic Meridional Overturning Circulation during cold periods. Periods of increased IRD conc­entration suggest BIIS extent reached the SW continental margin of Ireland from late MIS 5 (~80 ka) to MIS 2 (~20 ka). Significant decrease in IRD concentration suggests the BIIS started to retreat from the Porcupine Seabight following the LGM at around 20 ka.


Biography

Adele Westgård is an early career micropaleontologist and paleoceanographer. She graduates with a Master of Science from the University of Melbourne in December 2020 and will then move home to Norway to commence her PhD. Adele is particularly interested in climate and ocean variability at high latitudes during glacial periods.

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