X-rays of Scotland's seabed reveal how Ice Age ended

Vibrocore drill being deployed from ship

Vibrocorer drill being deployed from ship

26 October 2017 by Sarah McDaid

Scientists have X-rayed sediment cores taken from the seabed around the coast of North West Scotland and Shetland to solve the mystery of what happened to Scotland's ice sheet at the end of the Ice Age.

The cores of seabed sediment were collected during the 30-day BRITICE-CHRONO expedition in 2015 and are now stored at 3°C at the British Geological Survey's new cold store facility in the Lyell Centre, Edinburgh.

The £3·7 million, five-year NERC-funded BRITICE-CHRONO project has collected and dated onshore and offshore material to help pinpoint precisely when and how the last British-Irish ice sheet disappeared.

Dr Tom Bradwell, a lecturer at the University of Stirling, led the sampling around the Scottish coast during BRITICE-CHRONO, selecting locations and overseeing the collection of cores from the seabed. Dr Bradwell and colleagues collected over 200m of cores from the seafloor in the Minch and around Shetland and, since their return in 2015, have been analysing the cores using a range of techniques.

Vibrocorer infographic

Vibrocorer infographic text

RD1 is a multi-purpose seabed sampling tool. It is a superb tool for geologists to investigate what lies just beneath the seafloor.

Height: 7.7 metres, width: 3.4 metres, weight: 5,000kg, max depth: 2,000 metres

If the sediment is soft, it is used as a vibrocorer. A vibrocore drill uses vibrations to 'burrow' into the sea floor. The vibrocorer can extract core samples to a depth of six metres.

For harder sediments and rock it uses a drill bit that rotates like a cork-screw. Using this method, RD1 can extract core samples to a depth of five metres.

Parts of the tool are annotated:

  • Vibrating pot: A one-tonne weight that contains a shaker motor. This creates the vibrations and the downward force that drives the drill head into the seabed.
  • Core barrel: A long tube attached to the drill head that collects sediment core samples.
  • Control computer
  • Hydraulic motor
  • Removable legs
  • Retraction winch: Pulls the core barrel from the seabed when drilling is complete.

X-ray technology has thrown up the clearest clues as to when and why the ice receded around Scotland. High-resolution X-ray CT scans of each core reveal what the naked eye cannot see: micro-fine layers of mud, silt and sand that have settled out of meltwater from the front of an enormous, partly floating glacier that once filled the Minch.

The zebra-like downcore stripes are occasionally interrupted by large isolated pebbles or thick gravelly layers, dropped by huge icebergs breaking off the front of the ice sheet.

Dr Tom Bradwell said:

This is the first time that we've taken continuous high-quality sediment cores from beneath a former ice stream, in some of the deepest waters around the British Isles, some of which are still uncharted. Radiocarbon dates from shells found in these cores place the ice sheet margin in the Minch 20,000 years ago. By 16,000 years ago the ice was back on land; and by 14,000 years ago virtually all the ice in Scotland had melted.

It's vital that we understand how the last British ice sheet grew and the style and rate at which it receded. At the moment, we see massive ice shelves breaking up around West Antarctica, but we don't know how long this will go on for.

Working on former ice sheets like the one that covered Scotland allows us to view the whole ice-sheet growth and decay cycle, rather than just get a 'snapshot' in time. The similarities between what we see happening in West Antarctica today and what we see on the seabed around Scotland are striking. We're pretty sure that these new cores will give us the information we need to accurately say, for the first time, when and why it happened.

The scope of the BRITICE-CHRONO project means the British-Irish ice sheet is rapidly becoming the ice sheet anywhere in the world and a benchmark against which ice sheet models can be tested and improved.

The cores were collected using the British Geological Survey's vibrocorer. At each site, the vibrocorer was lowered from the research ship onto the seabed, where it was kept upright by its legs.

Dr Carol Cotterill, a marine geoscientist with the British Geological Survey, said:

The Vibrocorer basically vibrates its way into the seafloor and, once we have penetrated the full length of the core barrel, it is retracted and lifted back onto the ship. It takes around 30-40 minutes to collect each core once the vibrocorer has been lowered onto the seabed, or up to an hour if it's particularly sticky sediment.

During the BRITICE-CHRONO expedition, the team took a winch with over one kilometre of cable on it so that they could sample some of the deepest parts of the sea around Scotland.

Before the vibrocorer is deployed, surveys are taken of sample sites to check conditions on the seafloor. The vibrocorer is almost 5·5m wide, yet only three pads are ever in contact with the seafloor, minimising its footprint.


UnEarthed

The British Geological Survey's vibrocorer which collected the seabed samples from the BRITICE-CHRONO expedition will soon be taking part in another mission. The vibrocorer will be stationed outside Dynamic Earth from 17-19 November 2017 during UnEarthed, NERC's free public event that will bring the world of environmental science to the Scottish public. Visitors will be able to see the vibrocorer up close and quiz the scientists about its travels to the deepest parts of the sea. For more information and to sign up to email updates about the event, visit the UnEarthed website - external link.