Was there a land bridge across the Irish Sea?
Another LifeThis month's proposition, from a scientist at the University of Ulster (UU), that there was never a land bridge between Ireland and Britain, revives a frustrating controversy.
If there never was such a land bridge, how did plants and animals arrive in Ireland in such an ecologically orderly way? Biologists and geologists have found their ideas on this totally at odds. One or more land bridges across the Irish Sea have seemed vital to explain the onward distribution of species from Europe via Britain - but seabed geology is against it. The Irish Sea floor has a great trough 100m deep running up the centre, shallowing only at the northern end. At an important conference on post-glacial colonization in 1983, UCC geographer Robert Devoy thought the most that might be granted was a "low, soggy, possibly shifting and partially discontinuous linkage" between the Scottish islands and Malin Head in Donegal.
This was similar to the scenario now on offer from Dr Andrew Cooper of UU Coleraine's Centre for Coastal and Marine Research, who has been doing geophysical research with colleagues from the University of Maine.
The team's seismic work on the strata of North Channel sediments seems to show that sea level never dropped far enough to allow a dry and durable land bridge to emerge. Globally, the sea fell by 130 metres as water was locked up in glaciers, but so heavy was the weight of ice on Scotland and Ireland, says Dr Cooper, that the fall in the channel was no more than about 30 metres. There might have been enough soggy, temporary islands to help the giant Irish "elk" make the crossing, holding up its antlers as it swam.
Until his death in 1997, the leading champion of a land bridge was Frank Mitchell, distinguished naturalist and Professor of Quaternary [Ice Age\] Studies in Trinity College Dublin. In successive editions of his masterly and popular Reading the Irish Landscape, he argued for an orderly migration of oakwoods, complete with forest animals "rather than to imagine the occasional acorn floating across the sea or being carried by a pigeon across the North Channel, while groups of pigs and deer were swimming across the tidal channels." His early choice for the location of the bridge was on a line from Wales to Wicklow, where a ridge runs east to west, across the central trough, south of the Isle of Man. But even allowing for erosion, the ridge seems to lie too deep ever to have been exposed in the post-glacial period.
Suggestions of moraine ridges of rocks across the Irish Sea, left behind by the glaciers and subsequently washed away, seemed to allow insufficient time for soil to form and trees to grow.
Frank Mitchell's convictions were finally encouraged, however, by the work of Robin Wingfield for the British Geological Society. In his theory, the slow northward retreat of the ice was followed at its boundary by a migrating "forebulge" as the earth's crust rebounded from the weight of the ice cap. This could have provided a land bridge to south-west Ireland around 11,000 years ago, leading Mitchell to picture "organised woods advancing up a dry coastal strip . . . along the shore of the Atlantic, and later a very remarkable type of automatic trackway across the Irish Sea." Recent work by a namesake, Trinity botanist Fraser Mitchell, on the northward migration of trees, suggests a direct route from Spain, rather than westwards from Britain. This and the timing (pine arrived in the south-west of Ireland 9,500 years ago) seem quite compatible with the creeping shift of the "forebulge".
Without a bridge of some sort, the real challenges for explanation are ecological: Ireland would have had to be "restocked" from an exposed (and now submerged) coastal margin with a wintry climate and very limited surviving species. In the traditional model of glaciation, the last big freeze left a margin of tundra in parts of Munster and the north-west. But new studies in the 1990s proposed three big ice domes that left no room for life on the island until the major withdrawal of ice began around 13,000 years ago.
Precisely how the giant deer reached us may turn out to be far less important than the fact that the post-glacial arrival of many lesser creatures - snails, for example - followed exactly the same sequence of species in Leinster, needing just the same changes in vegetation, as in western Britain. A passive and random arrival seems impossible.
Dr Cooper and his American colleagues not only worked with seismic profiling but took sediment cores last summer from Belfast Lough and along the northern coast of Antrim. They are now hoping to find peat or pollens to tell more about the vegetation history of the island's lost shoreline.