Quantifying ice sheet thinning using cosmogenic nuclide ages

By Richard Selwyn Jones, from the Antarctic Research Centre at Victoria University of Wellington. In this article, Richard describes his research and how he uses cosmogenic nuclide ages to estimate rates of ice sheet thinning in Antarctica.

All images are copyright Richard Selwyn Jones.

An Antarctic expedition to estimate ice sheet thinning

Antarctica might seem like a desolate and inhabitable continent with little to offer other than perhaps the sight of penguins, but it holds the answers to many scientific questions. Simply put, Antarctica is covered by kilometers-thick ice, frozen water which will melt when sufficiently warmed. Sea level around the world is therefore dependent on how fast Antarctica’s ice sheets melt.

My research looks at past changes of ice sheet thickness. As a glacier melts and thins, rocks on its flanks become exposed to the atmosphere (specifically cosmic radiation). A technique called surface-exposure-dating is used to estimate when rocks became exposed at the glacier surface, creating a timeline of glacier thinning as the ice melts.

Ice-moulded bedrock near Mackay Glacier, East Antarctica

Ice-moulded bedrock near Mackay Glacier, East Antarctica

Mackay Glacier, East Antarctica

Mackay Glacier in East Antarctica

Mackay Glacier in East Antarctica. Over the Landsat Image Mosaic of Antarctica

On this expedition I headed to Mackay Glacier, an outlet of the East Antarctic Ice Sheet that feeds into the Ross Sea. The plan was to survey and ‘minesweep’ for suitable samples at several locations along the length of the glacier, and would require two camp moves. Following deployment by helicopter from Scott Base 100 km away, we set up our camp. We had our main Polar Pyramid tent (for communal morning and evening cooking and warmth), three small mountain tents (each with an en-suite pee bottle and a sanitiser wipes ‘shower’), a fuel cache and a toilet.

Each day would start with a reluctant unzipping of the sleeping bag and then a rather hasty wriggle into our salopettes, tops, jackets and then boots, hats and gloves. Only after melting enough snow for water, a cup of coffee and tea, a breakfast of bacon and hashbrowns, an assessment of the weather and a radio-transmitted daily action plan, could we set off for fieldwork.

Antarctic field camps

Antarctic field camps

You can investigate Mackay Glacier yourself, using the Google Maps satellite image below.


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Fieldwork in East Antarctica

Fieldwork at each site would always start with a survey of the area, mapping landforms and interpreting past ice-flow. Only after this would we start to look at suitable samples to collect. Such samples would need to show signs that they’d been transported by ice but had then remained in place after melting out of the glacier. Before a rock sample could be chucked in the bag we had to spend an hour taking notes and measurements.

Sampling bedrock around Mackay Glacier to determine the age at which it became ice-free.

Sampling bedrock around Mackay Glacier to determine the age at which it became ice-free.

Coping with extreme cold

Days were fairly long with five minutes to half an hour break for lunch, depending on the weather and morning’s progress. Lunch consisted of crackers and cheese, warm soup, a muesli bar, some dried fruit and some squares of chocolate. Constant daylight meant the working day often dragged on. In the evening our meal usually involved frozen meat (thawed in the roof of the tent or sleeping bag if required), frozen veg and a form of carbohydrate mixed with some spices. The evenings then progressed to gin & tonics, whisky and/or Kahlua, to keep our insides warm.

Fieldwork in a snow storm

Fieldwork in a snow storm

Simple tasks in Antarctica are often made harder by moisture freezing around fingers and wind blowing into faces. Some mornings I definitely wanted to stay in the sleeping bag rather than get up for another 12 hours of walking over boulders, through snow or around crevasses with 20 kg of equipment and rock on my back. Having said that, even slightly demoralising moments are far outweighed by the experience of Antarctic fieldwork, and at least the rest of the year would be in a (relatively) warmer geochemistry lab.

Surface-exposure age dating of rocks

Dating the rock samples will provide an estimate of how much the ice sheet has melted and thinned in this area, and also how fast this happened at different periods in the past. Numerical computer modelling of the glacier for these different time periods will help us understand whether this part of the ice sheet is susceptible to rising sea level, warming oceans or increased atmospheric temperatures.

Office of Rock and Ice

Below is a video of the expedition, complete with time-lapse photography, scenic pans and some rock sawing.

About the author

 

Richard Jones from Victoria University of Wellington

Richard Jones from Victoria University of Wellington

Richard Selwyn Jones is a PhD researcher at Victoria University of Wellington, New Zealand, and is co-founder of a new climate science-public interaction website Climatica. His research investigates the glacial history and behaviour of outlet glaciers in the Ross Sea region of Antarctica and how they respond to potential forcing mechanisms.

Follow him on Twitter: @selwynox and as @Climatica.

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