Ireland's ice age uncovered


Scientists are revealing clues about climate change by looking at the movement of Ireland’s glaciers during the last ice age, writes DICK AHLSTROM

MAPS OF THE seabed, ancient plankton and particles from outer space are all helping to explain how the glaciers that blanketed Ireland waxed and waned during the last ice age. This in turn helps paeleoglaciologists learn more about the climate change that drives ice ages.

Dr Paul Dunlop is head of the Quaternary Environmental Change Research Group at the University of Ulster, Coleraine. The Quaternary Period covers the past 1.5 million years, but his group is particularly interested the past 120,000 years, when the last ice age came and went.

“The real goal for the team here is to study climate change,” he says. They do this by finding ways to track changes over time in the ice sheet that covered Ireland in a kilometre-deep blanket of ice.

“The Quaternary is characterised by climate change, and this caused changes in the ice sheets,” says Dunlop. These come and go in a fairly regular way with a period of 100,000 years of cold typically followed by an interglacial period, perhaps 10,000 years long. The changes can set in quickly with a rapid slide into a new ice age, he says.

“The problem with the ice sheets is they are so destructive they destroy all the evidence of past glaciations. It is very difficult to get information on previous ice sheets. The deeper you go back in geological time the more vague it becomes.” Even so his group is finding ways to retrieve this information.

He was at NUI Galway when Ireland’s comprehensive seabed survey was underway, an ambitious mapping programme that included all of Ireland’s extensive continental shelf out to the edge of the deep Atlantic waters and beyond.

As a glaciologist he was aware of studies of ice-sheet grounding points off Antarctica. These occur where the heavy ice resting on the ground reaches its furthest limits and the floating ice shelf begins. While the shelf ice leaves no mark, the sheet ice presses into the seabed and deposits a continual supply of rock and overbear, scraped off by the slowly moving ice flow.

Dunlop had moved up to Coleraine at this stage and began studying some of the seabed mapping data for Donegal and over the Malin continental shelf. It didn’t take long before he started to find underwater gravel ridges, formed in a succession of long arcs.

“The big unknown question was: how far out did the ice sheet extend?” he says. The research team found it extended for many kilometres, in come cases out to the edge of the continental shelf. He concentrated on the Malin shelf region. “The most striking features we see are the ridges, 10km long. We have definitely mapped the western extent of the ice sheet,” Dunlop says.

One of the biggest ridges found was 14m high, 125km long and 11km wide. “There are also features gouged into the seabed, icebergs ploughing up the sea floor.”

The ridges are seen closer and closer to the coastline, retreating as the climate warmed and the ice began to melt. “This gives us a nice high-resolution data set on climate change. It shows you what the footprint was as it retreated and how rapidly it retreated.”

The timing of the retreat, and also the temperatures that might have caused it, can be estimated if the researchers take core samples from the ridges, looking for the remains of foraminifera plankton.

The ocean waters advanced as the ice retreated, and these cold-water species quickly occupied the space, growing and dying at the edge of the ice. Their 1mm long shells got mixed up in the gravel left by the ice, and when dug out can be radiocarbon dated, giving Dunlop a date for the furthest extent of the ice sheet. This method provides a date of between 29,000 and 27,000 years ago.

Cosmogenic radionuclide dating can also be used to track the land-based retreat of the ice. The ice would have sheared off a layer of older rock, leaving “fresh” material exposed once the ice was gone. Incoming cosmic rays from deep space kick off a cascade of new particles that eventually strike the exposed stone. They alter it, producing different elements or isotopes, and scientists have found a way to estimate the time when the rock first lay bare.

This is invaluable information for researchers trying to understand the climate change that drives ice ages, says Dr Robert Meehan, a consultant geologist and expert on the impact of the ice on Ireland’s topography.

The current “interglacial” or warm period has already lasted for at least 10,000 years, so the next ice age could be just around the corner. “This is where the arguments about climate change become interesting,” he says.

Climate change deniers claim the climatic changes we are seeing today are part of a natural cycle and are not driven by human activity. But a closer examination of the data being provided by these studies suggests this is not the case for the changes we see today.

The weight of thousands of cubic kilometres of ice pressing down on the surface where Ireland is today was enough to distort the earth’s crust.

Prof Marshall McCabe from the University of Ulster conducted research into this “isostatic depression”, showing Ireland was pushed down by more than 150m during the last ice age. This is more than the height of the spire on Dublin’s O’Connell Street, which measures about 121m.

The ice left the south of the island of Ireland before the north, possibly because of large ice domes that had formed in what is now Co Armagh. This uneven ice clearance in turn has caused the island to “tilt” with the south slightly higher today than the north as the rebounding process continues.

McCabe’s work also showed Ireland’s ice sheet was sensitive to changes in the Atlantic Ocean, with the sheet growing and decaying in response.

What lies beneath

Ireland as we know it today was literally made by glaciers. Successive ice ages sent wave after wave of glaciers that scored and scoured the surface, leaving behind drumlins and the Burren, not to mention trillions of tonnes of gravelly waste.

Dr Robert Meehan has spent 25 years studying how the ice ages shaped our topography and has developed comprehensive maps of the glacial sediments the ice left behind.

“I am currently compiling the first digital glacial sediment and land form map for the Geological Survey of Ireland,” he says.

The last ice age set in about 120,000 years ago and was at its peak by about 29,000 years ago. Such was the extent of ice – up to 1km – that Ireland would have seemed to be multiples of its size today, occupying space out as far as the end of the continental shelf.

This would have extended well out to sea for 90km or more, says paeleoglaciolgist Paul Dunlop. This is the point where the gently sloping shelf abruptly drops off on to the deep Atlantic seabed.

With such an amount of water locked up in ice, the sea level would have been an estimated 90m lower than it is today, Dunlop says. This means the ice extending beyond what are today’s coastlines would have been grounded firmly on “land” and not floating on the ocean’s surface like the ice shelves that surround Antarctica.

Meehan’s studies have shown that only 107sq km of Ireland’s 85,000sq km land mass remained above the ice sheet that blanketed us. The island and all points north “were completely smothered”, he says.

“Once the depth of ice reaches 50m it starts to flow. All of the ice was flowing, it was scouring the landscape.” The results are visible today in places such as Glendalough in Co Wicklow, he says.

The flat midland boglands were sculpted by the ice. “There was a glacial lake where the peat bogs are now.”

The glaciers steered around mountainous areas, but “most of Ireland is really smooth”, he says.

So why doesn’t the whole island look like the Burren, raw rock left bare by the scouring ice? Only 10 per cent of Ireland is rock outcrop, Meehan says. “The rest is buried by deep glacial sediments.”

When he says buried, he means buried. The present day Curragh in Co Kildare sits atop 110m of sediment, gravel and sand before reaching bedrock. Some of the deepest sedimentary deposits up to 125m thick overlay Offaly, Laois and south Kildare, he says. “There are trillions of tonnes of sediment.”

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