This record will help solve a long-standing mystery about the timing of past ice ages – why, almost one million years ago, the cycle of ice ages shifted from a regular 41,000 year cycle to an ice age every 100,000 years.
Witness to the past
The Antarctic ice cap is formed by layers of snowfall, which are compressed at depth into ice. Trace chemicals and particles trapped in the ice layers provide data on how the climate and atmosphere has changed over time: past temperature, the frequency and intensity of volcanic eruptions, sea ice extent, dustiness and wind patterns are all recorded.
Air pockets in the snow become trapped as bubbles in the ice. Each bubble is a pristine sample of the atmosphere, from the time the snow was transformed to ice. Carbon dioxide, methane, oxygen and other gasses can be extracted from the bubbles to reveal their past atmospheric concentrations.
The 41,000 year ice age cycle matches a known periodicity in the tilt of Earth’s axis. The 100,000 year cycle matches a periodicity in the shape of the Earth’s orbit. Why the ice age cycles shifted from being paced by one orbital parameter to the other is unresolved. The Antarctic ice sheet witnessed these changes and the answer to the puzzle may lie in the ancient ice.
A leading theory is that declining atmospheric CO2 levels were the cause of the longer, colder ice ages. The million year ice core record will provide the essential CO2 record to test this theory. Alternately, progressive erosion of material beneath the ice sheets may have made the ice sheets more stable and longer lasting. We can test this theory too, because the ice records gasses emitted by bedrock.
There are also other big questions to be addressed. The changes in energy the Earth receives from the sun as a result of orbital variations is small. Yet the climate response is profound. During the depths of the ice ages, kilometre-thick ice sheets advanced over the northern American and Eurasian continents. Sea levels fell by over 100 metres due to the huge volume of water transferred to these ice sheets, and global temperatures fell by 5 to 10 degrees. The million year ice core will provide crucial data to better understand how and why the climate system responds so strongly to these relatively small changes in energy input.
By greatly extending the detailed record of Earth’s climate history, the million year ice core will also place current changes in climate and greenhouse gas concentrations into a deeper context.
Australia’s contribution to international geophysical surveys has helped narrow the location of the ice that is likely to contain a one million year climate record. A site high on the Antarctic plateau, called Little Dome C, will be targeted. Surveys indicate that this location has approximately 2.8 kilometre-thick ice, with ice layers that are undisturbed and continuously preserved. Importantly, the site also appears free from melting at the base by geothermal heat.
Australia and a European team will lead two drill operations near Little Dome C. Each team is building ice core drills to suit the conditions they will face and their operational methods and experience.
Australia has also assembled a fleet of tractors, snow groomers and sleds, to tow a mobile station and drill camp infrastructure and equipment the 1200 kilometres from Casey research station to the drill site.
The first tractor traverse in support of the Australian component of the project is expected in 2022–23, with delivery of much of the mobile station and camp infrastructure. Options to begin the drilling in 2021–22, with air support, are currently being explored.
Once drilling starts in earnest, seven or eight people will work two 8–9 hour shifts on the drill.
The team expect to drill about 150 metres per week, so that over a field season of 6–8 weeks, they could drill between 900 and 1200 metres of ice core. This will produce between 6.5 and 8.5 tonnes of ice core a year.
Four to five years of drilling will be required to drill to the base of the ice sheet. Ice exceeding one million years old, ideally reaching up to 1.5 million years, is expected in the few hundred metres above the bedrock.
The cores will be transported by traverse to the ship at Casey research station, for the journey back to Tasmania. Many years of careful laboratory work at the Australian Antarctic Division, and with national and international partners, will be undertaken to measure the gasses and trace chemicals in the ice cores. Competing theories on the cause of the ice age cycles will then be tested by the cold hard data!