Science Spin May 2009

Glacial past can help predict future climate

By Sean Duke

The reconstruction of the extent of the British-Irish ice sheet that covered much of the two islands during the last Ice Age about 20,000 years ago might initially seem like an obscure academic topic holding little relevance to the lives of most of us. However, such work, which is being undertaken by researchers based at the University of Ulster, could ultimately serve to provide specific answers to some of the most important questions facing us all today. For example, if the Earth warms by 3 degrees C what will that mean for the world's ice sheets? How much melting will occur, how fast will it be, and where?

Background

Dr Sara Benetti, UU Coleraine, is one of the scientists working on charting the British-Irish ice sheet. The better understanding of the extent, the thickness, and the speed of melting that took place for this ice sheet, she said, can provide clues to the behaviour of important modern ice sheets, like in Greenland and Antarctica. The advantage of studying an ancient ice sheet, she said, is that there is a sediment record laid down over thousands of years, whereas for a modern ice sheet there is no such sedimentary record available.

Scientists interested in the British-Irish ice sheet, such Dr Benetti and her colleague at UU Coleraine Dr Paul Dunlop, have long suspected that glacial features on the Irish landmass that could be traced to the water's edge, did not simply end there, but continued into the offshore area. However, there was no way of proving this theory.

This picture changed in 1999, with the launch of the well funded Irish National Seabed Survey (INSS). The INSS surveyed a huge part of the massive Irish offshore, which is roughly 10 times as large as the Republic's landmass (70,000 square km as against 652,000 square km) between 2002 and 2005. This was followed up by another major survey, the Integrated Mapping for the Sustainable Development of Ireland's Marine Resource (INFOMAR) project. INFOMAR, in contrast to the INSS surveyed bays near to shore. It began in 2007, and is funded under the National Development Plan up to 2013.

The data sets that emerged from the INSS and INFOMAR were of huge interest to Dr Benetti and Dr Dunlop, as they provided the first strong evidence that glacial features on land, did, as they suspected continue into the offshore. However, good as the marine survey data was, the only way to confirm this was to go to sea, and take some samples.

Celtic Explorer

In the summer of 2008, Dr Benetti, Dr Dunlop, along with scientists from the Geological Survey of Ireland (GSI), and a group of eager students, went to sea for six days on the Celtic Explorer research vessel. The primary goal was to take marine core samples in multiple locations to confirm the presence of glacial sediments on the continental shelf.

The group was lucky in that the weather was good, and they worked round the clock until weather forced them to return to Galway after six days. At the end of that time, they had managed to take 52 marine cores in suspected glacial sediment, while another 2 cores were recovered in areas that were of interest to the GSI.

There were two methods for taking sediment cores on the cruise. On the continental shelf a machine called a Vibracore, which was supplied by the GSI, was used. This machine takes sediments by vibrating a core barrel into the ocean floor. Then in deeper waters, between 2,000 and 2,500 depth, a piston core machine was used. This latter machine is capable of taking quite long cores in deep waters, without disturbing the sediments.

The core sites were chosen based on the interpretation of 'multi-beam' data that had emerged from the INSS and INFOMAR. Multi-beam technology is essentially an echo-sounding technology where acoustic signals are fired from the bottom of a boat, or from beneath a plane, into the water, where they travel to the bottom, and then bounce back to a detector. The time that it takes to bounce back provides an accurate indication of depth.

Samples

The samples that were recovered varied in size from half a metre up to six metres in length, and the size was dependent on the nature of the sediments being cored. The scientists decided to open some of these 'cores' onboard, splitting them into two halves. This confirmed to everyone's delight that the features that were suspected as being glacial, were glacial. The interpretation of the survey data was being shown to be correct.

The majority of the cores were not, however, opened onboard. These were kept in storage at 4 degrees Celsius to preserve all the chemical properties of the sediment, and brought ashore in Galway, and then transferred to Coleraine. In Coleraine there are cold storage facilities that can preserve the sediments indefinitely. Dr Benetti said they have applied for more funding to take the project on to the next stage of analyzing the cores in detail.

The primary goal will be to date the glacial sediments recovered and the 'post' glacial sediments. From that, it will be possible to determine how long it took the ice to melt from the continental shelf (which was on land at the time). From that, the next aim will be to work out how much water was released from the melting of the British-Irish ice sheet into the North Atlantic, and how fast that water was released into the ocean. This data could be used to determine what is happening to the Antarctic ice at the moment.

Another research goal from the cores is to work out the volume of the British-Irish ice sheet - how thick it was. At the moment it is impossible to say whether it was 100 metres thick or 1,000 metres thick. This precision can only be determined from looking at all the deposits, on land and in the offshore area, and seeing what size the glacial deposits were.

"Also, we are going to look at physical properties to try and work out what type of ice was on top of the sediment when it was deposited?," said Dr Benetti. "How fast it was moving? If it was there for a long time, or a short time?"

The UU researchers want to be able to have the kind of data on the British-Irish ice sheet, that will enable them to say with confidence what is likely to happen to modern ice sheets if the world's temperature increases by say, 2 degrees C. "That is the way we want to go," said Dr Benetti. "You need to know what is happening to the past ice sheet to tell what is happening to the modern ice sheet."

The information from the cores collected on the continental shelf and slope could also be useful for geo-technical reasons. For instance, if an exploration company wanted to put an oil and gas rig somewhere in the Irish offshore, they will need to know how stable the sediment is in any particular area.

Future

The research team of Dr Benetti, Dr Dunlop and Professor Colm O'Cofaigh, at Durham University don't expect to do all the future core work on their own. They would like to bring in other scientific experts. For example, the work determining the volume of the British-Irish ice sheet will require input from scientists with very specific expertise. "The area that we are studying received ice from the northwest of Ireland, but was also possibly receiving ice from Scotland, so we also looking to bring in people who are more experienced in the Scottish ice sheet, and see if we can work together," said Dr Benetti.

The longer term goal, said Dr Benetti, would be to take more cores, in deeper waters. On the cruise last summer the cores were collected in waters of up to 2,500 metres depth, as the length of the cable onboard the Celtic Explorer was 2,600 metres. The sediments in deeper waters are finer than nearer to shore, she said, and that means there is potential to collect longer cores. This would help to provide the remaining data that could enable a full glacial map of Ireland - comprising both the onshore and offshore - to be drawn up.