“20,000 years of climate history compressed into just one meter of ice”

“20,000 years of climate history compressed into just one meter of ice”

Wearing two layers of gloves, Florian Krauss slides a cube of ice into a gold-plated cylinder that glows red in the light of a targeting laser. He steps back and looks at the machine, strewn with cables and measuring devices, that can turn polar ice into important climate data. If this were a real piece of precious Antarctic ice millions of years old, rather than a test cube, the next step would be to use an extraction vessel that creates a vacuum seal. Then the 150 megawatt main laser is switched on. It sublimates the ice slowly and directly into gas. Krauss, a doctoral student in the Department of Climate and Environmental Physics at the University of Bern, could then find out what the concentration of greenhouse gases was at a point in time far in the past.

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To better understand the role of atmospheric carbon dioxide in Earth’s climate cycles, scientists have long turned to ice cores collected in Antarctica. There, the layers of snow accumulated and thickened over the course of hundreds of thousands of years. The air conditions prevailing at the time are enclosed in a grid of small bubbles that serve as tiny time capsules. By analyzing these gas bubbles and the other components of the ice – such as dust and water isotopes – the scientists were able to establish a connection between the concentration of greenhouse gases and temperatures 800,000 years ago. Now it goes back even further.

1.5 million years back

The European initiative “Beyond EPICA”, an acronym for “European Project for Ice Coring in Antarctica”, has been around for three years. Her next goal is to recover the oldest ice core ever found: it is said to go back 1.5 million years. That would extend the climate record to the transition to the middle Pleistocene, an enigmatic period that brought about a major change in the frequency of Earth’s climate variability – cycles of recurrent glacial and interglacial periods.

However, successfully extracting an ice core this old, which takes years to calculate, is just the easy part. Next, the scientists have to painstakingly free the trapped air from the ice. Krauss and his colleagues developed an innovative new method for this. “We’re not interested in the ice itself, only the air samples it contains, so we had to find a new way to extract air from the ice,” he says.

How do you get the air out of the ice?

Simply melting it is not an option as carbon dioxide dissolves easily in water. So far, the scientists have therefore used mechanical extraction methods in which they crushed samples of individual ice layers to release the air. But with the “Beyond EPICA” ice core, which is already stored in the freezer at the University of Bern at 50 degrees Celsius below zero, crushing it would be too tedious.

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The oldest ice at the very bottom of the core is likely to be so compressed and the individual annual layers so thin that no air bubbles can be seen – they are pressed into the lattice of the ice crystals and form clathrate inclusion compounds. “At the bottom we expect 20,000 years of climate history compressed into just one meter of ice,” says Hubertus Fischer, head of the research group in Bern that takes care of the ice cores. That’s one-hundredth the thickness of all existing ice core records.

The new method that Krauss and Fischer have now developed is called deepSLice. It consists of two parts. The laser-induced sublimation extraction device (LISE) fills half a room in the team’s lab. LISE continuously shines a near-infrared laser on a 10-centimeter piece of ice core, causing it to go straight from solid to gas under extremely low pressure and temperatures. The sublimated gas then freezes in six chilled metal dip tubes, each containing the air from a centimeter of ice core. They are cooled down to -258 degrees Celsius.

Finally, the samples are placed in a custom-built absorption spectrometer based on quantum cascade laser technology, which shoots photons through the gas sample to measure the concentrations of carbon dioxide, methane and nitrous oxide simultaneously. Another big benefit of this system is that it requires a lot less ice (and labor) than the old method of analysis, which used scientists to measure methane by melting ice — which doesn’t dissolve in water — and carbon dioxide by crushing ice.

Unique Abilities

DeepSLice offers “a unique capability that nobody else has,” says Christo Buizert, an ice core scientist at the University of Oregon and head of ice analysis for COLDEX, the US equivalent of “Beyond EPICA,” which is currently engaged in “friendly competition ” with the Europeans to find a continuous core of ice up to 1.5 million years old.

“Ice sublimation is one of the most difficult methods to extract gases from ice,” says Buizert. “You’ve been trying for a long time.” It’s a very promising way because you can get 100 percent of the gases out. “But it’s very difficult to do, so the fact that they’ve done it is very impressive.” However, the researchers estimate that Krauss and Fischer still have about three years until they have the necessary sample in their hands.

Until then, there are still some problems to be solved. This includes the question of how the samples can be used for additional analyzes after they have been passed through the spectrometer.

The Bern team believe their technology will be ready when the ancient ice core finally makes it to Europe in freezer containers on a ship from Antarctica via Italy. “Our latest results have shown us that we are on the right track. We actually achieved the precision that we wanted,” says Krauss. “I’m sure the system will be ready.”

(jl)

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