The oldest ice
Scientists have used ice cores to give amazing record of the earth’s atmosphere and climate over the past 800,000 years. Drill cores through the ice caps in Antarctica and Greenland give ice that is literally layered, with each layer representing a year’s snowfall. The chemistry of the snow can be used as a record of the temperature at the time it formed, and the gaps in-between ice crystals contain tiny slices of the last atmosphere the ice exchanged with.
The last atmosphere sampled in an ice core comes a little time after the snow falls, because it takes a lot of weight on top of snow to turn it to ice. That little bit of gas trapped in most ice makes the ice look white – you can even test that one in your freezer at home.
So far, scientists have had limits in the oldest ice they can recover. Once the pile of ice in a glacier gets thick enough, it spreads out and flows away. Ice at the very bottom of an ice cap also has a small extra heat source – heat flowing out of the planet Earth isn’t enough to melt much of the glacier, but it can cause the ice to recrystallize. Once these reactions happen, some of the information contained in the ice is lost, such as the yearly banding and the gas stored in the ice.
Ice that has been altered and had most of its gas removed turns blue, as seen in this beautiful image. Areas of blue ice show up in Antarctica where deep, old ice comes back to the surface. So far, scientists have mostly avoided drilling cores in these areas because they don’t have the kinds of annual bands that can tell scientists how old the ice is – ice cores aren’t useful if you don’t know how old the ice is.
However, a new technique was developed a few years ago to recognize how old ice is even if it doesn’t have annual layers. An isotope of argon gas, argon-40, builds up slowly in the Earth’s atmosphere due to radioactivity in the crust. By measuring the amount of argon-40 in blue ice, a team from Princeton was able to locate ice that was 1-million years old in 2014 (http://bit.ly/2uR5npi).
That same team, with some new grad students, traveled back to Antarctica and took 2 more drill cores from the same area. These next 2 cores were even older – 1.5 and 2.7 million years old, making them the oldest ice humans have recovered from Antarctica.
For the last 800,000 years, the planet Earth has been locked in cycles of glacial expanse and contraction. These glaciations were triggered once CO2 got low enough to allow ice sheets to expand – a drawdown in CO2 probably triggered by chemical reactions and weathering happening associated with the growth of the Himalayan mountains.
From records in the ocean, it is thought that the glaciations extend much farther back in time, several million years earlier, but we don't have records for those as precisely as we do for the ones found in ice cores. These new ice cores contain tiny bits of gas from the time they were formed, and they show similar concentrations of CO2 and other greenhouse gases to those found in the glaciations we have better records of. However, they don’t show the swings we see in the modern glaciations. This could make it more complicated for geoscientists to understand exactly what triggered the growth and collapse pattern of more recent ice sheets.
This ice also confirms that the current CO2 contents in the atmosphere of over 400 ppm are far above those that have occurred on Earth any time in the last 2.7 million years.
Antarctica has likely been glaciated for tens of millions of years, ever since CO2 levels started dropping about 30 million years ago. The team is hoping they can find ice that is more than 5 million years old somewhere at this site – that ice would predate the drop in CO2 and growth of ice sheets recorded in ocean sediments at that time. If they could find it, that ice could tell us how much the atmosphere really had to change to trigger the first ice sheets on continents other than Antarctica, which 5 million years ago had not yet begun to form.
-JBB
Image credit: Christopher Michel https://flic.kr/p/dKA9up
Press report and conference abstract: http://bit.ly/2w2Gtan http://bit.ly/2wgUBMC