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Tuesday, 12 May 2015

The Celtic Sea ice lobe -- again

There has been another paper on the Celtic Sea ice lobe -- ie the southernmost extension of the Irish Sea Glacier (otherwise referred to as the British and Irish Sea Ice Sheet or BIIS) during the Devensian maximum.  This one is by Mark Furze and others.  It contains detailed analysis of the sedimentary cores taken from the Celtic Sea Deep or Celtic Deep Basin, which we have discussed before in this blog. Neither the abstract nor the paper itself are easy to follow, but essentially the paper seeks to understand what happened when the glacier ice began to melt back towards the narrow constraints of St Georges's Channel at a time of rising sea level and isostatic recovery.  Was a temporary lake contained within the basin, or are the sediments best explained as glacio-marine, related to the gradual lifting of the ice margin as the sea flooded in from the south?  The evidence is equivocal, and to their credit the authors reserve judgment, and invite future researchers to come up with evidence that might prove conclusive.

What interests me rather more is the problem of ice limits.  The authors have chosen to stick with the lobe shape as proposed by Scourse and Furze in 2001 and as accepted by various authors since then.  I have never thought that the ice limit is very sensible, since the dashed red line on the map does not accord with the evidence for Devensian till on Lundy Island and Caldey Island and inside the mouth of Milford Haven, and neither does it fit well with what we know of the behaviour of large ice masses in unconstrained environments. 

That having been said, bit by bit the Devensian history of the Celtic Sea "theatre" becomes clearer.....


Furze, M. F. A., Scourse, J. D., Pien ́kowski, A. J., Marret, F., Hobbs, W. O., Carter, R. A. & Long, B. T. 2014 (January): Deglacial to postglacial palaeoenvironments of the Celtic Sea: lacustrine conditions versus a continuous marine sequence. Boreas, Vol. 43, pp. 149–174. 10.1111/bor.12028. ISSN 0300-9483.
Recent work on the last glaciation of the British Isles has led to an improved understanding of the nature and timing of the retreat of the British−Irish Ice Sheet (BIIS) from its southern maximum (Isles of Scilly), northwards into the Celtic and Irish seas. However, the nature of the deglacial environments across the Celtic Sea shelf, the extent of subaerial exposure and the existence (or otherwise) of a contiguous terrestrial linkage between Britain and Ireland following ice retreat remains ambiguous. Multiproxy research, based on analysis of 12 BGS vibrocores from the Celtic Deep Basin (CDB), seeks to address these issues. CDB cores exhibit a shell-rich upward fining sequence of Holocene marine sand above an erosional contact cut in laminated muds with infrequent lonestones. Molluscs, in situ Foraminifera and marine diatoms are absent from the basal muds, but rare damaged freshwater diatoms and foraminiferal linings occur. Dinoflagellate cysts and other non-pollen palynomorphs evidence diverse, environmentally incompatible floras with temperate, boreal and Arctic glaciomarine taxa co-occurring. Such multiproxy records can be interpreted as representing a retreating ice margin, with reworking of marine sediments into a lacustrine basin. Equally, the same record may be interpreted as recording similar conditions within a semi-enclosed marine embayment dominated by meltwater export and deposition of reworked microfossils. As assemblages from these cores contrast markedly with proven glaciomarine sequences from outside the CDB, a glaciolacustrine interpretation is favoured for the laminated sequence, truncated by a Late Weichselian transgressive sequence fining upwards into fully marine conditions. Reworked rare intertidal mol- luscs from immediately above the regional unconformity provide a minimum date c. 13.9 cal. ka BP for commencement of widespread marine erosion. Although suggestive of glaciolacustrine conditions, the exact nature and timing of laminated sediment deposition within the CDB, and the implications this has on (pen)insularity of Ireland following deglaciation, remain elusive.


Myris of Alexandria said...

What is a lonestone.

BRIAN JOHN said...

Lonestones -- they define them as "Small (typically <30 mm) subangular to rounded lithic clasts...." Any the wiser? I'm not......

BRIAN JOHN said...

Ah -- have read a bit further. They think at least some of them may be dropstones -- dropped into fine laminated silts and clays from floating ice that was debris-rich.