By James Scourse
A wonderful place…….
The first of the BRITICE-CHRONO marine transects (transect 4, Celtic Sea) was completed late on Saturday evening. It has been hugely successful – the result of unbelievably excellent weather and sea state, detailed planning and effective delivery by a great team. This has been a controversial and enigmatic part of the British-Irish Ice Sheet for decades with generations of Quaternary geologists attempting to reconstruct glacial events from meagre and sporadic sequences. It was the focus of my PhD back in the early 80’s. A lot of this was spent onshore on the Scillies where the evidence suggested that the Late Devensian maximum advance straddled the northern islands – a conclusion that caused me not inconsiderable grief at the time because large and influential parts of the UK Quaternary community could not accept that the last ice sheet reached this far south. Subsequent work with colleagues using new techniques has supported this original interpretation. I also analysed a series of 12 or so BGS vibrocore samples recovered in the 70’s from the central and southwestern Celtic Sea containing “glacigenic” facies. A northern suite resembling the Scilly Till I interpreted as basal till facies, whereas a southern group – containing spectacular microfossil assemblages – appeared to be glacimarine. On the basis of this available evidence I suggested a mid-shelf grounding line and marine terminus to the Irish Sea Ice Stream. I was unable to explain the origin of some apparently “basal” type diamictons very close to the shelf break; they might possibly be iceberg turbates. More recently I suggested – with additional information from palaeotidal simulations – that the huge Celtic Sea linear ridge bedforms are tidal features reworking the sediments of the terminal ice stream and the Channel River.
Then, starting in the late 2000’s, I became aware that Daniel Praeg from Italy and Steve McCarron from Ireland had become interested in these ridges and were suggesting in conference presentations (e.g. INQUA 2011) that the ridges might actually be subglacial “ giant eskerine” bedforms which, if it were true, would mean that the ice sheet reached right to the shelf break. In Daniel’s model the shelf break diamictons are just that – evidence for shelf edge glaciation. One of the original BGS cores – site 44 – recovered till from the flank of a sand ridge which might suggest that the ridges at least partly pre-dated the glacial event; Daniel, following Pantin & Evans (1984) suggested that the ridges have a carapace of glacigenic sediment and were therefore overridden by ice. But, alternatively, do the glacigenic sediments dive through and under the ridges? A major unanswered question was/is the stratigraphic relationship of the glacigenic sediments to the ridges. There was something faintly ironic in all this: I’d had a lot of grief having argued for an advanced southerly position for the ice sheet, and now here was another team arguing for an even more spectacularly extended southerly limit.
Daniel, with great persistence and motivation, has organised a series of geophysical and coring campaigns with Italian, Irish and BGS colleagues – the last in February-March this year – to attempt to resolve the two models. Spectacularly their last cruise recovered overconsolidated diamicton and normally consolidated glacimarine sediments close to the shelf edge at the southern end of Cockburn Bank (for further details). I won’t steal their thunder because their work is being prepared for publication, but it is fascinating and has injected energy into our researches in this area. Daniel and Steve and colleagues Dayton Dove and my former research student Gill Scott, are now working alongside BRITICE-CHRONO colleagues to help address these questions. Having Daniel as a participant on this James Cook cruise has been a delight and the two hypotheses have been constructively batted to and fro, day and night, with lots of jocular repartee on the nature of things emerging on the sub-bottom profiler; “that’s clearly a buried drumlin”, “no, it’s a proto tidal sand ridge” etc etc.! Were that all scientific controversies were discussed in such a friendly, stimulating and constructive way.
So, what have we found? Searching for glacigenic sediments in this area is like looking for a needle in a haystick, so aggressive was the subsequent transgressive episode. Much of the sedimentary evidence has simply been eroded (incorporated into sand ridges??!) or buried. The BGS only found glacigenic sediments in 12 cores of the hundreds that were taken. Well, about a third of all our coring deployments recovered glacial or glacimarine sediments, from sites extending from the shelf edge to the Celtic Deep, a total of 14 vibrocores and 5 piston cores. This success is a testament to painstaking preparation – including a reconnaissance geophysical cruise – led by Katrien Van Landeghem, Sara Benetti, Lou Callard and colleagues – so that our targets were well defined. Excellent onboard sub-bottom data has also been crucial, pored over night and day by Daniel, Katrien, Colm, Richard and myself, and the expertise of the BGS and NOC coring teams. There is no doubt that these samples and their contextual geophysical data will transform our understanding of the LGM in the Celtic Sea, a topic that continues to fascinate, bemuse and, occasionally, infuriate. One of our key targets, site 44, stubbornly refused to yield anything but sand – dubbed the “sands of woe” by Lou Callard – that left Daniel, head in hands, muttering “Oh bloody, bloody, hell”!
What about the two hypotheses…well, I already have some modified interpretations emerging – new working hypotheses if you like – but I’m not going to be pushed on these until we have the data analysed from the cores. Having said that, I think Daniel might be partly right and partly wrong, and that I, too, might have been partly right and partly wrong. Such is science!
Britice-Chrono NERC Consortium 