Seabed shape and strength can hasten collapse of ice sheets

Professor Geoff Duller, whose state of the art luminescence dating method made it possible to date the retreat of the ice sheet.

Scientists have shed new light on the collapse of glaciers in Antarctica and Greenland by studying the history of the last British-Irish Ice Sheet.

A team involving researchers at the Department of Geography and Earth Sciences at Aberystwyth University reconstructed the behaviour and studied the physical properties of an ice stream – a huge fast-flowing glacier – which melted more than 15,000 years ago.

Led by the University of Stirling, they carried out the work by analysing the sediments and landforms the ice stream left behind, both on land and underwater.

The researchers focused on the former Minch Ice Stream, to the northwest of Scotland, and found that the shape (width and depth) and strength (geology) of its underwater bed were major drivers in hastening ice sheet retreat and ice loss to the ocean.

Using state of the art luminescence dating methods, Professor Geoff Duller and his team from Aberystwyth University were able to date the retreat of the ice sheet.

This work involved measurement of the light emitted by grains of quartz in sediments deposited by the glacier as it retreated.

The luminescence dating technique pioneered by Prof Duller involves measuring the light emitted from individual grains of sand using a laser beam.

Many thousands of individual grains are measured and the data sifted to identify those giving accurate ages.

Studies have previously modelled the impact of trough shape in comparable settings in Antarctica; however, this is the first time that geology has been identified as a factor in expediting the process of ice stream retreat.

Experts believe the findings will help to better predict the fate of the Earth’s ice sheets – in Antarctica and Greenland – and the magnitude of future sea level rise in a warmer world.

Lead author Dr Tom Bradwell, a lecturer in Stirling’s Faculty of Natural Sciences, said: “We used the former Minch Ice Stream – which has a similar geometry and underlying geology to ice streams in Greenland and West Antarctica – as a test-bed for what happens before, during and after ice-shelf collapse.

“Ice shelves support or regulate ice flow from ice streams to the ocean. Even though ice shelves may appear stable, subtle changes in the speed of ice streams – driven by a change in underlying geology or an increase in water depth – can result in ice shelves becoming unstable and lead to rapid break up. This new research documents evidence of this process from the past, something which is currently underway in parts of Greenland and Antarctica.

“Our research indicates that the seabed landscape over which ice streams flow – its different shape and strength, from the relatively soft continental shelf to the hard rocky landmass – can drive step changes in retreat rate, greatly speeding up ice loss to the ocean.

"We recommend that ice stream bed width, depth and strength are all considered in future modelling ice sheet experiments seeking to predict ice sheet retreat and future sea-level rise in a warmer world.”

Professor Duller from Aberystwyth University said: “This is a key example of how laboratory analyses of the single grains of sand can tell us about the behaviour of large ice sheets in the past, and help us to predict future changes that may affect humanity.”

The paper published on Wednesday 24 April 2019 in Science Advances forms part of the wider BRITICE-CHRONO project – led by Professor Chris Clark, at the University of Sheffield – which used a range of onshore and offshore dating techniques and digital mapping methods to reconstruct the complete deglaciation history of the entire British-Irish Ice Sheet from 30,000 to 15,000 years ago.

Ice streams are ‘flow corridors’ within an ice sheet that move significantly faster – up to 1,000 metres a year – than the surrounding ice. They are to 10 to 100 km wide, and hundreds of kilometres in length. They transport vast amounts of ice from inland to the oceans and account for most of the ice leaving the ice sheet.

Dr Bradwell’s team identified a “major step change” in ice stream behaviour around 18,500 to 16,000 years ago – with the collapse of ice shelves in NW Scotland followed by rapid ‘runaway’ ice-front retreat.

“This behavioural change was out of tune with climate forcing,” Dr Bradwell explained. “Instead, we attribute it to variability in the seabed landscape – most importantly the width and depth of the ice stream trough and changes in the geology, or strength, of the underlying bed. In a glacial climate, these factors would have controlled ice loss to the ocean as the ice sheet retreated – at certain times causing it to accelerate via ice-shelf collapse and rapid iceberg calving.

“The impact of these underlying factors on ice-sheet retreat are almost certain to be enhanced under global warming scenarios.”

The study included a 30-day science cruise and two onshore fieldwork campaigns. The research team combines marine and terrestrial geologists, geochronologists, glaciologists and ice-sheet modellers from a number of institutions: The universities of Stirling, Sheffield, Durham, Liverpool, Bangor, Aberystwyth, and Glasgow, as well as the Scottish Universities Environment Research Centre, and the British Geological Survey.

The BRITICE-CHRONO project was conducted between 2013 and 2018 and is funded by the Natural Environment Research Council (NERC), part of UKRI (UK Research and Innovation).

The paper Ice-stream demise dynamically conditioned by trough shape and bed strengthwas published inSciences Advances on Wednesday 24 April 2019.

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