Transect 4 – Irish Sea West
R Chiverrell, K van Landeghem, L Yorke, M Burke, G Thomas, C O’Cofaigh, D Evans, D McCarroll, S Davies, G Duller
Fieldwork and activities:
The Irish Sea Ice Stream (ISIS), the largest drainage conduit for the MIS2 British-Irish Ice Sheet (BIIS; Fig. 1), was fed by ice flowing southwards from southern Scotland, northern Ireland and the English Lake District. It divided around the uplands of North Wales into two discrete sectors, the EIS flowing into the Cheshire-Shropshire lowlands and the western ISIS (here denoted as WIS) flowing
southwards through the central Irish Sea and into the Celtic Sea. Glaciers emanating radially from the Welsh Ice Cap contributed to WIS north and west of of Wales, and from the Wicklow Mountains on the eastern side of the Irish Sea. On entering the Celtic Sea through the topographic constraint of St George’s Channel, WIS ice extended westwards along the south coast of Ireland west of Cork and southwards as far as the Isles of Scilly. WIS was grounded 100 km SW of Scilly where a palaeo-grounding line marks an LGM transition to a marine margin. Glacimarine sediments are found on the continental shelf south and west of this position.
The WIS is the best dated sector of the BIIS, but this database is populated entirely from 14C, OSL and cosmogenic data from onshore samples. The age and maximum southern limit of the WIS has been established in a series of papers, both onshore in the Isles of Scilly (Scourse, 1991; Hiemstra et al., 2006; McCarroll et al., 2010) and offshore (Scourse et al., 1990; Scourse & Furze, 2001) and correlated with the deep-sea IRD record (Scourse et al., 2000, 2009a; Haapaniemi et al., 2010). Timing and extent of WIS advance along the south coast of Ireland has been established by O’Cofaigh & Evans (2007). Reworking of proximal glacial and glacimarine sediments by strong tidal currents along the ice margin into tidal sand ridges has been established (Scourse et al., 2009b); this reworking has had the effect of removing or masking much of the glacial sedimentary record across the Celtic shelf. The BGS has an extensive archive of core material and seismic data from across the shelf but much of this remains uninvestigated, particularly in the northern Celtic Sea including the Celtic Deep Basin. There are significant uncertainties in our understanding of the dynamics of the WIS. Whilst ice streaming has been assumed (e.g. Stokes & Clark, 2001; Roberts et al., 2007; Chiverrell et al. in prep.) an alternative hypothesis is that a piedmont lobe extended south of Ireland, driving ice southeastwards towards Scilly. Ice-flow indicators on Scilly and Lundy (Rolfe et al., 2011) suggest ice flow from the NW. The eastern margin of the LGM WIS south of Wales is very uncertain. Lundy was overridden by LGM ice (Rolfe et al., 2011) and it is possible that the Fremington Till of North Devon represents a glacilacustrine sequence of LGM age deposited in a lake body dammed by offshore ice. There is no convincing evidence that the WIS impinged on the north coast of Devon or Cornwall anywhere between Fremington and Scilly. Recent compilation and Bayesian analysis of the WIS geochronological database constrains the advance and retreat phases of WIS (Chiverrell et al. in prep.), demonstrating rapid advance and collapse of this sector of the ice sheet (Fig. 2). This analysis, the first to be applied to glacial landsystems, identifies a prior sequence based on landformsediment relationships to reduce uncertainty in the probability distribution for individual dates. This can be used as a model for the analysis of other sectors of the BIIS.
Programme of work
Since the chronology of the WIS is relatively well-known, the programme of work required amounts to refining this chronology and testing the competing ice stream vs. piedmont lobe hypotheses. Fig. 1 shows a proposed pattern of ice retreat and possible correlation between on- and offshore ice-marginal positions, divided into zones that coincide with the EIS zones based on the Chiverrell et al. (in prep.) reconstruction. Work is proposed as follows:
- Zone 1: Offshore transect (Scilly-Fastnet) to test the piedmont lobe hypothesis; this sector is very poorly known and will require full seismic survey with coring at identified key target ice-marginal locations. Investigation of the Celtic Deep sequence containing multiple till units via re-evaluation of the BGS archive; BGS holds extensive but uninvestigated seismic and core data from this basin so no new offshore work proposed. Onshore on Scilly OSL dating of the Tregarthen Gravel (ice-proximal sandar) on Tresco, cosmogenic dating of the Shipman Head Boulder Moraine on Bryher and 14C dating of pollen extracts from sub-till organic sequences.
- Zone 2: Cardigan Bay: OSL dating at Banc-y-Waren and coastal sites. Screen Hills: more OSL ages south of Kilkeel to strengthen existing database.
- Zone 3: Dublin to Screen Hills: OSL ages from coastal sections. Northern Llŷn: OSL ages from coastal sections to strengthen existing database.
- Zone 4: North of Dublin: OSL ages from coastal sections
- Zone 6: Jurby Head, Isle of Man: OSL ages to strengthen existing database.
The WIS transect will be investigated as a pair with the EIS. Taken together these two sectors that define the ISIS provide the most well-constrained sector of the BIIS and the Bayesian analysis already undertaken 1. demonstrates the feasibility of the entire proposal and 2. provides a model for the approach to be adopted in other sectors. The objectives in the WIS sector are therefore to refine the geochronological model already constructed and to test the piedmont lobe vs. ice stream hypotheses for the LGM advance to the Scillies
- The WIS is geographically the largest conduit draining the BIIS.
- The WIS is unique amongst the BIIS ice streams in that it terminated mid-shelf and not at the shelf break and is therefore not linked via a cross-shelf trough to a trough mouth fan on the slope.
- The WIS is characterized by upflow changes in bed gradient, including a pronounced adverse slope into the southern Celtic Deep Basin, and lateral topographic constraint in St Georges Channel.
- The retreating marine margin of the WIS was fully exposed to meridional migrations of the North Atlantic Polar Front during deglaciation.
- The retreating margin of the WIS is tightly coupled to deep marine records via fingerprinting of IRD enabling independent age controls via tuning of deep ocean records to the Greenland ice cores. Deep sea sequences also enable freshwater pulses emanating from the WIS to be detected in stable isotopic records.
- Initial drawdown of the retreating margin under strongly contrasting tidal conditions: megatidal to the SE, microtidal to the NW (Irish sector), enabling analysis of the impact of tidal regime on retreat.
- Scilly-Fastnet transect: seismic survey (sub-bottom profiling) ~286 nm (two-way) (total 95 survey hours). 20 offshore coring sites, ~320 nm (total 104 survey hours). Terrestrial: GPR and borehole investigations at 4 sites.
- Estimated number of dating samples, 14C, CN and OSL: offshore cores 20 cores x 3 dates – 60 14C; onshore 24 OSL, 20 14C; onshore 5 CN for Scilly. Total 80 14C, 24 OSL, 5 CN