Transect 3 – Irish Sea East
R, Chiverrell, M Burke, F Thomas, D Roberts, D Evans, M Bateman, S Livingstone, G Duller, D Fabel, D Small, S Gibson, K van Landeghem, M Saher, A Medialdea
Fieldwork and activities:
The Eastern Irish Sea and Cheshire/Shropshire Ice Stream was sourced by ice flowing through the Solway Firth, from the Lake District (LD) and Pennines. The dominant ice-sources were: the East Irish Sea (EIS) draining southern Scotland, Solway lowlands and western LD; Eastern Lake District (ELD) draining the east and south LD; and Ribble Glacier (RG) fed by a weaker South Pennine Icecap. At the Last Glacial Maximum these coalesced and ran south across the floor of the Eastern Irish Sea, then split with lobes extending 1) W to SW merging with the West Irish Sea (WIS) Ice Stream between the Isle of Man (IoM) and Anglesey and 2) SSE through Cheshire to maximum limits near Wolverhampton.
Terrestrial and marine sectors of this transect are quite well served in terms of topographic and athymetric data. The terrestrial sequence of retreat has been comprehensively mapped in the English Midlands (Thomas and Chiverrell, unpublished; Thomas, 1995; Parkes et al. 2009), in Lancashire (Chiverrell et al., 2007), on the IoM (Thomas and Chiverrell, 2006) and the Solway lowlands (Evans et al., 2008).
The offshore record has received less attention with the exception of sectors mapped by Van Landeghem et al. (2009). The chronological control for the behaviour of this ice stream is poor and largely underpinned by dating key Quaternary sites (e.g. Four Ashes and Chelford), which constrain the advance to maximum limits to after ~26 kyrs (Morgan, 1973; Bateman, unpublished). Retreat stages are poorly constrained with ages restricted to the northern Irish Basin on the IoM ~16kyrs (Thrasher et al., 2009), Lancashire 19-17 kyrs (Telfer et al., 2009) and LD ~15-14 kyrs (Ballantyne et al., 2009), for the most part constraining the Heinrich 1 re-advance or late glacial limits. The remaining ages obtained for the region focus on organic-rich basal sediments of lakes and peat-bogs and as such only record the warming at the beginning of the late glacial interstadial. The successful OSL dating of outwash deposits with lithologies sourced from Permo-Triassic bedrock on IoM, in Cheshire and Lancashire (Thrasher et al., 2009) along with bedrock lithologies suitable for CN dating on IoM and in LD (Ballantyne et al., 2009) proves the considerable promise for a concerted effort at dating ice retreat.
Programme of work
Fig 1 shows a proposed pattern of ice retreat and tentative correlation between possible marginal positions on- and off-shore. The ice stream is divided from south to north into seven zones based on a chronological model developed to discern the rates of ice marginal retreat (Chiverrell et al., unpublished). Although a comprehensive bathymetric survey is unnecessary, new multibeam (MB) and seismic data are required for key sites in Zones 6 (yellow areas Fig. 1) to complement the existing coverage. After examination of existing MB data coring sites EIS1-10 are proposed at ice-marginal settings identified. Additional dating sites will be designated depending on what the seabed topography reveals and on additional commercial data becoming available. The spatial distribution of all the dating sites is envisaged to constrain the retreat of the ice margin through these zones:
- Zone 1: the maximum position dated by excavation at Four Ashes (14C) and OSL dating of sandar deposits in south Shropshire (Barnsley Lane Pit and 2x drill sites).
- Zone 2: Shrewsbury retreat positions date target sandur deposits at Condover and aggregate quarries in the north and south of the zone.
- Zone 3: Whitchurch moraine belt: OSL dating at Wood Lane Pit, basal lake deposits (Ellesmere), revisit Chelford sand extraction sites.
- Zone 4: Wrexham moraine belt: OSL dating sandur at Borras Quarry and other sandar (3x drill sites), offshore sites to record the separation of the West and East Irish Sea Ice during the retreat phase.
- Zone 5: Manchester embayment: gravel pits area Delamere – OSL dating of sandur deposits.
- Zone 6: (1) Bride – Kirkham moraine belt: OSL dating sandur fronting the Kirkham (Bradley’s Pit) and Bride (coastal) and (2) offshore surveying and sampling to (i) affirm and constrain the continuation of the Bride-Kirkham limit, (ii) determine the nature and temporal setting of possible sub-aerial ice-disintegration terrains in Liverpool Bay and (iii) constrain ice-flow directions and marginal positions from sub-glacial bedforms
- Zone 7: Terrestrial grounding limits: OSL Whitehaven deltas, sandar in the south LD, bottom sediments of Windermere/Coniston water, offshore cores in the Lune Deep, north of IoM and Solway Firth.
The EIS transect with its terrestrial continuation will provide crucially missing information on complex ice retreat mechanisms, documenting the comparatively well-defined deglaciation (outlined below). The EIS provides information on larger-scale ice retreat dynamics as the EIS is affected by drawdown of the WIS. These two ice streams are therefore best investigated as a pair.
- The EIS ice stream is unique in the BIIS in containing a S–N retreat pathway that passes from terrestrial over marine and back to terrestrial conditions.
- The ice faced the most pronounced adverse slope of any sector of the BIIS: rising >200m between Liverpool Bay and south Shropshire.
- The outlet width of the trough narrows markedly through Cheshire-Shropshire. During retreat, the ice stream thus moved from topographically constricted to unconstricted areas.
- Initial drawdown and retreat under macro-tidal conditions (Uehara et al., 2006).
- Great Britain’s west coast was exposed to ocean current warming during the marine retreat phase.
- Considerable potential to address thinning (i.e. CN dating on uplands in the IoM and in the LD).
- Early retreat, though marine influenced, is accomplished with a terrestrial margin and there are numerous gravel pits or potential borehole locations. Further north many of the known limits intercept the coast with good exposure. Offshore, the EIS is the only area in the Irish Sea where some the palaeo-glacial landscape has been preserved (Van landeghem et al., 2009), including ice-marginal moraines and depressions in between (existing commercial data, unpublished). There is also Lune Deep, a glacially scoured very deep depression, subsequently filled in part with muddy sediments and unique in its kind around the UK.
There is thus excellent potential for both on- and offshore dating with easy access to sediments.
- 10 offshore core sites, ~160 nm transit (totals 52 survey hours) and 20 terrestrial sample sites (most are gravel pits or coastal exposures)
- Offshore survey – multibeam and seismics (Yellow areas on Fig. 1). Assuming multibeam swath width of 4.5 times the water depth and 10% overlap, the delineated seafloor can be documented with ~430 nm of track lines, of which ~80 nm are surveyed with simultaneously deployed seismic equipment and half the usual surveying speed (totals 60 survey hours) Terrestrial (geophysics GPR, coring etc) – GPR and borehole investigations at 4 sites
- Estimated number of dating samples, 14C, CN and OSL
- Offshore 10 cores x 6 dates – 60 radiocarbon
- Onshore 30 OSL, 30 radiocarbon (10 organic and 15 shells)
- Onshore lacustrine (Ellesmere, Windermere and Coniston Water)
- Onshore 10 CN sites on the IoM and LD