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Friday, 1 July 2016

Ice in the Celtic Sea -- piedmont glacier or ice stream?

 These maps (which I published in 2011) show the alternative hypotheses for the glacier ice affecting the Celtic Sea.  There are inaccuracies on both maps, as recent publications have shown.

Below I reproduce the latest info from the BRITICE-CHRONO site, reporting on the ongoing programme of work.  Fascinating stuff!  As we can see, as the ice edge adjustments are being made,  there is a recognition that the ice that affected the Scilly Isles came from the NW and not from the NE (as would be required if an Irish Sea ice stream had affected the islands.  That accords with my observations on the islands, which I hope will be published before too long.

I have discussed this issue several time before, for example here: 

If the ice really did flow from Ireland as a piedmont lobe, one might expect to find southern Irish erratics in the glacial deposits left behind.  I begin to suspect that there is truth in both scenarios.  Maybe there was a piedmont lobe during the early part of each glaciation, followed by a dominant ice stream towards the glacial maximum?

The other thing that I still have difficulties with is the idea of that long lobe projecting soith-westwards, where there should have been a calving bay.  I have not seen anything convincing in the literature to show that the ice that pushed further south than the Isles of Scilly was genuinely grounded.  The glacial deposits in that area are more probably dumped from glacier ice floating free, beyond a calving ice edge.  In other words, they are glacio-marine deposits with dropstones, rather than deposits of true glacial till.  we look forward to seeing further info on this from the Britice-Chrono team.



Transect 4 – Irish Sea West

Team:  James Scourse (leader), Colm O'Cofaigh, Danny McCarroll, Geoff Duller, Dave Evans, Siwan Davies, L Yorke, Matt Burke, Katrien Van Landeghem, G Thomas

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.

Current understanding

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 landform/sediment 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.

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