ANSTO scientists Dr Andrew Smith, Dr Quan Hua and Dr Bin Yang contributed to a paper which explains how cosmogenic radiocarbon in situ (14C) is produced, retained, and lost in the top layer of compacted snow (the “fir layer”) and the shallow ice below at an ice accumulation site in Greenland.
The results published by a large international team led by the University of Rochester (United States) in The cryosphere have implications for all measures involving 14C in ice caps.
An ‘atom hunter’, Dr Smith has been a long-time collaborator with Assistant Professor Vasilii Petrenko and the University of Rochester team, extracting traces of atmospheric gases from ice cores of the Arctic and the ‘Antarctica, primarily to improve our understanding of the past global atmospheric methane budget.
This research has implications for the interpretation of isotopic measurements of carbon-containing gases such as CO (carbon monoxide), carbon dioxide (CO2) and methane (CH4) extracted from past air trapped in the firn layer or deeper in the ice core bubbles.
“The radiocarbon component of these gases, 14CO, 14CO2 And 14CH4 provides invaluable information about the movement of carbon in the carbon cycle. This is particularly important for methane, as this gas currently contributes to around 23% of the global warming we experience,” explained Dr Smith.
“As methane has a relatively short lifespan of around nine years in the atmosphere, mitigating our methane emissions will have a much faster impact on climate change than for carbon dioxide. Methane is mainly removed from the atmosphere by a very reactive compound, the hydroxyl radical, ‘OH,” he added.
Measurement of 14CO allows us to understand how this very ephemeral “atmospheric detergent” evolved on a global scale in the past. The team is now carrying out similar work in a contemporary atmosphere, under the aegis of FETCH4 project.
The amount of carbon that can be extracted from these gases, which are in turn extracted from firn air or ice core bubbles, is tiny. Even with the relatively massive samples the team extracts in the field, sample sizes are on the order of tens of micrograms of carbon, or even less.
“The microphone14The C capacity of the Center for Accelerator Science is essential to the success of this very difficult and important work. The techniques and confidence developed by the research team over decades are also crucial,” Dr Smith said.
He first began to measure 14C of CO2 in fir air and ice in the late 1990s as part of a National Greenhouse Advisory Committee project. The compacting tree traps air in the form of bubbles in the ice. Until then, the air is still in contact with the atmosphere, via paths that become more and more tortuous as the depth increases.
The closure is gradual in the transition region. For this reason, the air is always younger than the ice that contains the bubbles and there is an age gap, different for each gas due to different diffusion coefficients. This process must be understood and “stepped back” with models to interpret the record of air trapped in the bubbles.
“It turns out that the ‘radiocarbon bomb,’ produced by surface nuclear testing in the 1950s and 1960s, provided a strong and well-measured pulse of 14CO2 in the atmosphere. Measuring this in firn air and ice core bubbles proved useful in refining numerical modeling that describes the air trapping process, and we have since used this same technique at many sites ” said Dr. Smith.
However, it became clear at the time that the production of 14C in the ice itself, through the interaction of neutrons and muons with the oxygen atoms of H2O, had to be understood to develop a complete picture.
“This has eluded researchers until now because different air extraction techniques, melting, grating, grinding and sublimation of ice have resulted in different ratios of air and in situ pollution.14C and the choice of site, whether an accumulating ice sheet or an ablation ice sheet, also gave different results.
“A detailed understanding of in situ cosmogeny 14The production, retention and loss of C in ice are necessary to disentangle the trapped in situ atmospheric and cosmogenic components,” explained Dr Smith.
“Interestingly, we are about to use this new knowledge to embark on a very ambitious project, in a remote location in Antarctica, more than 1,000 km from the coast and 3 km from “altitude on the Antarctic plateau,” he said.
“A team of six people is set to travel to Concordia Dome where they will drill ice cores at this specially selected site between November 2024 and February 2025 and melt the ice to release the air it contains. due to the very low rate of snow accumulation at this location, site, the in situ 14The C signal will dominate the atmospheric signal.
“The measurements of 14CO at the Center later in 2025 will allow us to reconstruct the flow of high-energy cosmic rays over the past 7,000 years, knowledge that has been inferred so far from meteorite studies. We hope that our work will improve considerably on this point. The team must complete the work before winter as temperatures drop to -70°C or lower. »
Gas diffusion in fir and in situ14Production C
Compacted snow (firn) is porous and is gradually compressed into ice, occluding air into bubbles which then move downward with the ice. The trapped air contains 14C from cosmogenic production in the air as 14CO2, 14CO and 14CH4. However, 14C is also produced in situ by neutron spallation (n) of O atoms in H2O (ice), slow capture of muons (𝜇-) and by interactions with fast muons (𝜇f, > 10GeV).
Neutron production only occurs in the upper 20 m ice-equivalent depth, whereas muon production occurs at much greater depths. “Hot” in situ14C atoms produce 14CO2, 14CO and 14CH4. Most in situ14C escapes from the fir layer but is retained below.
More information:
Benjamin Hmiel et al, Characterization of cosmogenic in situ 14CO production, retention and loss in firn and shallow ice at Summit, Greenland, The cryosphere (2024). DOI: 10.5194/tc-18-3363-2024
Provided by the Australian Nuclear Science and Technology Organization (ANSTO)
Quote: Scientists unravel the difficult complexities of radiocarbon in ice cores (October 18, 2024) retrieved October 18, 2024 from
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