Scientists unravel the difficult complexities of radiocarbon in ice cores

ANSTO scientists Dr. Andrew Smith, Dr. Quan Hua and Dr. Bing Yan investigated how in situ cosmogenic radiocarbon (14C) is produced in the top layer of compacted snow (the ‘burn layer’). I contributed to a paper that sheds light on what is retained and what is lost. “) and the underlying shallow ice in the Greenland ice deposits.

The findings, published in The Cryosphere by a large international team led by the University of Rochester (USA), have implications for all measurements of 14°C within ice sheets.

‘Atom Hunter’ Dr Smith has been collaborating with Professor Vasily Petrenko and his team at the University of Rochester for many years, primarily to improve our understanding of past global atmospheric methane budgets, and to research ice sheets in the Arctic and Antarctic. Trace atmospheric gases are extracted from the core.

This study investigates the effects of carbon-containing gases such as CO (carbon monoxide), carbon dioxide (CO2), and methane (CH4) extracted from past air trapped deep within ice sheets and bubbles in ice cores. Affects the interpretation of isotopic measurements.

“The radiocarbon components of these gases, 14CO, 14CO2, and 14CH4, provide valuable insight into the movement of carbon in the carbon cycle. This gas is currently responsible for approximately 23% of the global warming we are experiencing. This is particularly important for methane, as it contributes to

“Because methane has a relatively short lifetime in the atmosphere of about nine years, reducing methane emissions will have a much faster impact on climate change than carbon dioxide. They are removed from the atmosphere by hydroxyls, which are highly reactive compounds. Radical, yeah,” he added.

Measuring 14CO allows us to understand how this extremely short-lived “atmospheric detergent” has changed globally in the past. The team is currently doing similar work in a contemporary setting under the umbrella of the FETCH4 project.

The amount of carbon that can be extracted from these gases is small, and they are also extracted from the air in the firns, or ice core bubbles. Even with the relatively large amounts of samples the team extracts in the field, sample sizes are on the order of tens of micrograms of carbon or less.

“The Accelerator Science Center’s micro-14C capabilities are critical to the success of this very challenging and important research. The technology and trust our research teams have developed over decades are also critical,” said Dr. Smith.

He first began measuring 14 degrees CO2 in sheep and ice air in the late 1990s as part of a National Greenhouse Advisory Committee project. When the firn is compressed, air is trapped in the ice as bubbles. Until then, the air is still in contact with the atmosphere, following a more tortuous path with increasing depth.

It gradually closes in the transition region. For this reason, air is always younger than ice, which contains bubbles, and different age spreads exist for different gases due to differences in diffusion coefficients. We need to understand this process and “run backwards” through the model to interpret the recording of air trapped inside the bubbles.

“It turns out that ‘radiocarbon bomb pulses’ produced by ground-based nuclear tests in the 1950s and 1960s provided sharp, precisely measured pulses of 14CO2 in the atmosphere. This helped us refine our numerical modeling to describe the air-trapping process, and we have since used this same technique in many fields. ” said Dr. Smith.

But at that point, it became clear that to get the full picture, we needed to understand the formation of 14C in the ice itself through the interaction of neutrons and muons with the oxygen atoms in H2O.

“Various air extraction techniques such as ice melting, grating, crushing, and sublimation result in different ratios of 14C in the atmosphere and in situ, resulting in location choices between accumulating and melting ice sheets. , which until now eluded researchers, yielded different results.

“Disentangling the trapped atmospheric and in situ cosmogenic components requires a detailed understanding of the production, retention, and loss of in situ cosmogenic 14C in ice,” Dr. Smith explained. did.

“Interestingly, we are now using this new knowledge to embark on a very ambitious project in a remote part of Antarctica, three kilometers above sea level on the Antarctic Plateau, more than 1,000 kilometers from the coast.” said.

“A team of six people will travel to Dome Concordia to drill ice cores at this specially selected location between November 2024 and February 2025, melting the ice and discovering the content contained therein. We plan to release the air. Due to the very low snow rate at this time, the local 14C signal will predominate over the atmospheric signal.

“Measurements of 14CO at the center in late 2025 will allow us to reconstruct the flux of high-energy cosmic rays over the past 7,000 years, knowledge that has so far been inferred from meteorite studies. We hope our research will significantly improve this.”The team needs to complete their work before winter sets in, as temperatures drop below -70 degrees Celsius. ”

In-farm gas diffusion and in situ 14C production

Hardened snow (fern) is porous and gradually compresses into ice, trapping air and creating bubbles that move downward with the ice. Trapped air contains 14C from cosmic production in the air as 14CO2, 14CO, and 14CH4. However, 14C is also generated in situ by neutron (n) fragmentation of O atoms in H2O (ice), capture of slow muons (𝜇-), and interaction with fast muons (𝜇f, > 10GeV) .

Neutron production occurs only at the top, at a depth of about 20 m, which corresponds to the ice, while muon production occurs at much deeper depths. A “hot” 14C atom in situ produces 14CO2, 14CO, and 14CH4. Most of the in situ14C leaks out of the skin layer, but remains below the skin layer.