Transect 6 – Donegal Bay
S Benetti, C O’Cofaigh, C Purcell, K Weilbach, P Wilson, C Ballantyne
Fieldwork and activities:
High-resolution bathymetric data collected on the NW Irish shelf by the Irish National Seabed Survey has allowed detailed mapping of glacial features in this area. A series of well developed nested arcuate moraines (in pale blue in figure) is found across the shelf offshore of the mouth of Donegal Bay. These moraines define retreat of a former lobate ice margin that extended to the shelf edge. Extensive zones of iceberg scours offshore of the outermost moraine suggest an initial retreat characterized by ice sheet break-up and calving followed by grounded, episodic retreat. A further lobe of Scottish sourced ice is indicated by the set of prominent moraines (in green) located north of Donegal Bay in the Malin Sea. A smaller set of moraines (in magenta) is superimposed on the Donegal Bay moraines. They record a late-stage, northwards readvance and subsequent retreat, of an ice lobe emanating from Killala Bay (Ó Cofaigh et al., in press; Dunlop et al., 2010). These data provide evidence that this sector of the ice sheet supported a major outlet draining through Donegal Bay to the shelf edge. Indirectly, dated marine stratigraphic records from deep water sites neighbouring the NW Irish shelf suggest that the ice sheet extended to the shelf edge about 29-27 cal ka BP with subsequent retreat from this position after 24 cal ka BP (Ó Cofaigh et al., in press). However, other authors have proposed an older maximum extent (e.g. Bowen et al., 2002; McCabe et al., 2007).
This area was extensively sampled by vibrocoring during two cruises in 2008 and 2010, with the specific aim of coring the glacial features observed on the shelf. Most of the cores have been split and described. This initial work shows a typical succession (from top to bottom) across the cores of reworked sands (possibly Holocene in age), fining upward gravel with abundant shells and shell fragments, and finally diamicton. Some of the facies contacts are sharp or erosional. Shells and encrusting bryozoans on pebbles within the diamicton may provide datable material. Petrographic analysis on thin sections is been carried out by GSNI on pebbles from selected cores from the Malin Sea and the Donegal Bay area. Initial results on one of the Malin Sea cores (27) show a provenance from Isla in Scotland, thus providing the first hard evidence for an incursion of Scottish ice near the shelf edge on the Irish shelf.
This area on the western margin of the BIIS is at the transition between a northern BIIS drained by ice streams reaching the shelf edge and building TMFs and a southern BIIS characterized by an ice sheet flow in which subglacial drainage networks supplied meltwater and sediment directly to the shelf edge. Millennial scale fluctuations of the BIIS in this region were postulated based on deep sea cores and related to a sensitivity of the ice margin to oceanographic processes. Constraining the chronology of ice retreat along this margin will provide key information on the (a)synchronicity of BIIS retreat on its western margin, even locally by comparing Donegal Bay and Malin Sea data. Extensive core coverage in this area will also allow a more detailed facies interpretation, which may help defining the glacial processes happening during retreat.
- No ship time required (Min) 20 marine cores for three transects: Malin Sea (cores 26-28; 48-49) Donegal Bay (cores 1-19; 25; 30-33); Killala Bay (cores CE32-34).
- 6 x AMS analysis of marine fauna (minimum) per core to determine age of glacial and glacially derived facies boundaries to constrain timing of ice sheet retreat (+readvances) along three flow lines.
- C-14 (marine fauna)
- 30 radiocarbon/20 cosmogenic/20 OSL (locations and exact number of dates TBC)