Getting to the core of climate

Plans are afoot to drill an ice core climate record, dating back more than 2000 years, in the heart of the Australian Antarctic Territory this summer.

Over six weeks the Aurora Basin North ice coring team will drill a 400m-long ice core at a site some 500km inland from Casey station. Chief Investigator, Dr Mark Curran, from the Australian Antarctic Division, said the ice core would fill a gap in the international science community’s climate knowledge.

‘According to the Intergovernmental Panel on Climate Change Fourth Assessment Report, our ability to reconstruct Southern Hemisphere climate is limited by the sparsity and quality of well resolved climate records, particularly over the last 2000 years,’ Dr Curran says.

‘This timeframe spans both the industrial era and a significant length of time before humans began influencing climate.

‘The Aurora Basin North site has a relatively high snow accumulation rate, equivalent to 13cm of water per year, which will allow us to discern annual climate records over this critical time period, for the first time in the region.’

Australian Antarctic scientists have climate records spanning the last 2000 years and beyond, from ice cores taken from the coast at Law Dome and inland at Dome C — albeit with limited measurements and different resolutions. However, differences in the snow accumulation rate at these two sites, and the low resolution of some of the records, make annual climate interpretation difficult. To address this problem the international ice core community has established the International Partnerships in Ice Coring Sciences (IPICS) to coordinate efforts to build an array of 2000 year old ice core records across Antarctica.

‘The Aurora Basin North project will provide one core for this array and it will also help us resolve uncertainties in the climate record between the Law Dome and Dome C sites,’ Dr Curran says.

The ice coring team plan to drill further back in time to about 3000 years, if all goes well, to get an understanding of natural climate variability, on global and regional scales.

‘Our previous ice core work has allowed us to establish a link between reduced rainfall in south-west Western Australia and increased snowfall in East Antarctica over the past 750 years,’ Dr Curran says.

‘We’ve also found a link between increased sea salt accumulation in Antarctic ice — associated with El Nino events and stronger winds in the Southern Ocean — and increased rainfall in Queensland and New South Wales over the past 1000 years.

‘We will use the detailed Aurora Basin record to continue to investigate regional climate linkages between the Antarctic and Australian climate.’

Climate chemistry

To reconstruct climate the Aurora Basin North team, which includes scientists from Australia, France, Denmark and the United States, will measure a number of chemical constituents in the ice core.

One of the first things they’ll measure in the field are oxygen isotopes (different nuclear forms of oxygen), which provide a temperature record. Ice formed under cooler conditions will contain more 16O, while ice formed under warmer conditions will contain more 18O.

‘We’ve purchased a laser spectrometer to measure oxygen isotopes, which has been tested and proven in the field by our Danish colleagues in Greenland,’ Dr Curran says.

‘So we’ll have a 2000 year temperature record as soon as we come out of the field, which will be a first for Australia. Previously we’d have to wait about two years to finalise our analysis using older, slower measurement techniques back in the laboratory.’

The team will also cut samples for later methanesulphonic acid (MSA) analysis. MSA is produced from the oxidation in the atmosphere of dimethylsulphide, which is itself produced by certain species of phytoplankton in the Southern Ocean. The amount of MSA in an ice core was found to be related to the maximum extent of sea ice in the region. In simple terms, this is because in years where there is more sea ice, there is more phytoplankton activity following sea ice decay and therefore more MSA production.

Back in their laboratories, scientists will also measure carbon dioxide (CO2), sulphates and beryllium-10 to assess the greenhouse gas ‘forcing’ (impact), volcanic forcing and solar forcing, respectively, on climate.

‘Measuring these elements will allow us to assess of the importance of these forcings for natural climate variations over time, and assess the extent of man-made greenhouse gas forcing,’ Dr Curran says.

Air for CO2 analysis will also be extracted in the field in the ‘firn’ ice at the top of the core. Firn ice is unconsolidated ice that contains a lot of air spaces — unlike consolidated ice where the air is trapped in bubbles. To collect the gas a core is drilled to the region of interest and a bladder is lowered into the resulting hole and pumped up to seal the hole. A tube inserted through the bladder is connected to a vacuum pump at the surface. Once in position, the pump is turned on to suck the air out of the surrounding firn. The recovered air will be stored in cylinders for analysis in Australia.

‘The firn air is “fresher” or more modern than the air trapped within the ice core. So to get the fullest record possible, it’s important that we capture the firn air and not contaminate it with today’s air,’ Dr Curran says.

Scientists from the Desert Research Institute in the United States will produce much of the fine detail climate record for the project using a unique continuous ice core melter coupled to a system that measures a range of chemical species and elements. These include dust tracers, such as magnesium and iron, ash from fires, seawater tracers such as sodium and bromine, and volcanic tracers such as copper and cadmium. These analyses will help date the ice core and provide information about natural aerosols and pollutions levels in the recent era.

Here’s the drill

The tool of choice to drill the 400m core is a Danish Hans Tausen drill. Dr Curran and Dr Andrew Moy, also from the Australian Antarctic Division, have both spent time in Greenland with a Danish ice coring team, learning to use the drill. They, along with Trevor Popp from the University of Copenhagen, will be the chief drillers. The trio will lead two three-person drill teams, operating in shifts across a 14 hour day.

The teams will also operate two narrower French and Australian drills. The Australian Antarctic Division’s Eclipse drill is capable of drilling to 400m but has not been tested to that depth. Both it and the French drill will drill cores side by side to 120m, providing more ice for large volume analyses, covering the last 800–1000 years. The French core will be used solely to measure sulphur isotopes, which require a large amount of ice for a single measurement.

Advanced planning

The Aurora Basin North ice coring project is an ambitious project but it has already achieved significant logistical success. Five shipping containers of equipment — including the drills, fuel and an entire field camp for 16 people — had to be shipped to the French station at Dumont D’Urville on the L’Astrolabe in March this year, for a traverse to the drill site in November.

‘We had to do all our field preparation a year in advance. So we had to set up the camp in Hobart to make sure everything worked, and then pack it in the order we wanted it unpacked in the field,’ Dr Curran says.

‘Then we had a nail-biting week where it looked like the ship wouldn’t get in to Dumont D’Urville because the ice was so thick. But they did get in and they pumped all the fuel for the traverse, got our gear off the ship, and departed the station; all within 24 hours.

‘It has been a huge team effort across the Australian Antarctic Division as well, with the science branch, store, and logistics sections working together through project manager Alan Elcheikh and project officer Meredith Nation, to achieve what we have so far.’

In November this year a French-led traverse team will depart Dumont D’Urville to deploy the fuel and equipment to the field site. This traverse will include an Australian mechanic, an air-ground support officer and ice core scientist Dr Tas van Ommen. The group will establish a ski-way for air support by a Basler and Twin Otter aircraft and set up initial camp infrastructure.

In December the first drill team, including Dr Curran, will fly to the site and begin drilling. A partial personnel changeover is planned mid-way through the season. Drilling is expected to finish in late January 2014.

One million year ice core

If all goes well the team will have demonstrated a capability to run an internationally collaborative project in an unexplored area of Antarctica. They will also have data they can use to help them locate a suitable site for an even more ambitious project — drilling a one million year old ice core.

‘The Australian Antarctic Territory has some of the thickest and likely the oldest ice in Antarctica,’ Dr Curran says.

‘Once we’ve extracted our shallower, 2000–3000 year old core, we’ll be able to combine our analyses from that core with aerial radar surveys of ice thickness and structure that have been conducted in the region recently through the ICECAP project [Australian Antarctic Magazine 19: 7, 2010]. This will allow us to identify a site most likely to contain one million year old ice.’

Wendy Pyper
Australian Antarctic Division

Aurora Basin North is a truly collaborative project with 12 international organisations involved: Australian Antarctic Division, Antarctic Climate and Ecosystems Cooperative Research Centre, Australian Nuclear Science and Technology Organisation, Curtain University, CSIRO, Desert Research Institute (United States), Institut Polair Francais (IPEV, France), Laboratory of Glaciology and Geophysical Environment (LGGE, France), Macquarie University, National Centre for Scientific Research (CNRS, France) University of Copenhagen and the University of New South Wales.