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FURZE, MARK F.A.  University of Alberta.

The pattern of deglaciation and sea-level change across the now submerged continental shelf of the Celtic and Irish Seas has been a controversial topic in British Quaternary studies and continues to raise both important glaciological and sea-level questions.

Evidence is presented here for a large freshwater lacustrine system on the southern margin of the retreating Irish Sea Ice-Stream (ISIS) occupying the Celtic Deep Basin (CDB) existing from deglaciation until well into the Late Glacial Interstadial. This freshwater body is evidenced by laminated basal muds containing sparse reworked marine floras. Marine ingress into the lake, c.12 ka BP, by the breaching of its southern sill is evidenced by iron-oxide stained gravels representing a lag deposit of winnowed lacustrine dropstones. Iron-oxide precipitation and staining is envisaged to have occurred due to the resuspension of anoxic lake bottom sediments by oxygenated marine waters during lag formation. A marine breaching of a southern sill is further supported by massive mollusc-rich 14C-dated marine sands. These deposits, occurring above the laminated facies at the southern end of the CDB, are interpreted as wash-over sediments recording initial marine ingress into the basin. Interdigitating lacustrine, marine and estuarine sediments at the northern margin of the basin along with the modern height of the southern sill enable estimates of the former lake level and area. The stratigraphy within the CDB can be contrasted with that occurring outside it, characterised by massive basal muds containing rich in situ boreo-arctic assemblages and lag deposits attesting to marine inundation, but lacking the iron staining found within the basin.

The existence of such a lake body in the Late Pleistocene Celtic Deep has marked implications for models of deglacial sea-levels in the region. The controversial reconstructions of Eyles & McCabe (1989) suggest deglaciation occurred in a marine setting with rapid calving back up the Irish Sea. However, their interpretation of many of the classic coastal exposures bordering the region as being of glacimarine origin has received considerable criticism. In the light of recent micropalaeontological, sedimentological and geomorphological work (e.g. McCarroll, 2001; Scourse & Furze, 2001; Cofaigh & Evans, 2001) at these sites the high deglacial sea-level hypothesis is generally considered to be incorrect. The occurrence of a freshwater lake system during deglaciation lends its weight to the low deglacial sea-level hypothesis as well as extending the argument into the critically important offshore zone. Additionally, the nature and timing of lake formation and marine ingress fits in well with recent evidence suggesting that the southern limit of the Last Glacial Maximum ISIS did not, as is generally considered, lie across the southern entrance to St George’s Channel. Rather a lobe continued down the Celtic Sea to impinge on the northern Isles of Scilly and terminate in a tidewater setting near the shelf edge (Scourse, 1991; Scourse & Furze, 2001; Hiemstraa et al., in prep).

The occurrence of such a lake also has considerable biogeographical implications. Implicit in its existence is the presence of a large, though perhaps low-lying, isthmus of land between Britain and Ireland separating the lake from the advancing Atlantic to the south. By establishing the approximate timing of marine ingress into the lake the lifespan of this land-bridge can be estimated with its consequent implications for the arrival (or non-arrival) of terrestrial fauna in Ireland.

Eyles, N. & McCabe, A.M., 1989, The Late Devensian (less-than 22,000 BP) Irish Sea Basin - the sedimentary record of a collapsed ice-sheet margin: Quaternary Science Reviews, v. 8(4), p. 307-351

Hiemstraa, J.F., Evansa, D.J.A., Scourse, J.D., Furze, M.F.A., McCarroll, D. & Rhodes, E., in prep, The Late Quaternary Glacial Landforms and Sediments of the Isles of Scilly: Quaternary Science Reviews.

McCarroll, D., 2001, Deglaciation of the Irish Sea Basin: a critique of the glaciomarine hypothesis: Journal of Quaternary Science, v. 16(5), p. 393-404

Cofaigh, C. & Evans, D.J.A., 2001, Sedimentary evidence for deforming bed conditions associated with a grounded Irish Sea glacier, southern Ireland: Journal of Quaternary Science, v. 16(5), p. 435-454

Scourse, J.D., 1991, Late Pleistocene Stratigraphy and Paleobotany of the Isles of Scilly: Philosophical Transactions of the Royal Society of London Series B-Biological Sciences, v. 334(1271), p. 405-448

Scourse, J.D. & Furze, M.F.A., 2001, A critical review of the glaciomarine model for Irish sea deglaciation: evidence from southern Britain, the Celtic shelf and adjacent continental slope: Journal of Quaternary Science, v. 16(5), p. 419-434

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