Research into a substance created in the atmospheric nuclear bomb tests of the Cold War may provide some of the answers to big questions about the greenhouse effect. The work by researchers at ANSTO and Sydney University, may also help scientists solve Cold War murder mysteries and authenticate vintage wines by extending the reach of radiocarbon dating.
The substance - carbon-14 - can be used to trace the complex pattern of atmospheric circulation across the globe, according to Quan Hua, of ANSTO, and Dr Mike Barbetti, of Sydney University. They say the isotope can give hints on how greenhouse gases are distributed, and help in the modelling of climate systems.
Carbon-14 is the material central to radiocarbon dating. It forms naturally in trace amounts when cosmic rays - energetic particles from space - interact with nitrogen in the atmosphere. But carbon-14, or radiocarbon, was also formed artificially in the nuclear weapons tests of the 1950s and 1960s. Nuclear particles neutrons generated in the blasts interacted with atmospheric nitroge around the fireballs of the explosions. The radiocarbon reacted with oxygen to form carbon dioxide, and was ferried southward by atmospheric circulation.
"The bomb tests were effectively giant, uncontrolled tracer injections of radiocarbon into the atmosphere," said Dr Barbetti, Director of the Universitys NWG Macintosh Centre for Quaternary Dating.
"By looking at the distribution of radiocarbon, scientists can get insights into atmospheric circulation patterns, and the exchange of carbon dioxide between the atmosphere and the oceans."
Mr Hua, Dr Barbetti and colleagues have used trees to get a fix on when the isotope reached the south after weapons tests in the northern hemisphere. Mr Hua, of ANSTOs Physics Division, said the concentration of radiocarbon in the north almost doubled just before the Nuclear Test Ban Treaty of 1963.
"Although radiocarbon levels were measured in various parts of the northern hemisphere during the Cold War, few readings were taken in the south," he added. "We wanted to find out how quickly it got to our part of the world, and in what quantities."
The team turned to the natural records - trees - which absorb carbon dioxide from the atmosphere and use it to build new wood. Working on Huon pines from Tasmania and three-leaf pines from Thailand, the group was able to determine atmospheric radiocarbon levels in tropical regions and the southern hemisphere from 1952 to 1975.
The scientists, some of them world-renowned experts on tree ring dating, took tiny samples of wood formed in consecutive years. They measured the concentration of radiocarbon in each tree ring using ANSTOs ANTARES tandem accelerator, an instrument so sensitive it can measure concentrations as low as one atom in a thousand million million. The scientists found that radiocarbon took one to two years to reach the southern hemisphere after its formation.
"Our results should help atmospheric scientists fill in gaps in their data and improve their models," Dr Barbetti said.
The work, to be published in the Journal of the International Association of Wood Anatomists, could also have important implications for forensic science. Until now the Cold War era has been in a radiocarbon twilight zone, at least in the southern hemisphere.
Radiocarbon dating works accurately only if the atmospheric levels of radiocarbon are known.
Radiocarbon dating works accurately only if the atmospheric levels of radiocarbon are known.
"We can now date organic material like human hair formed in that period," Dr Barbetti said, adding that the technique was accurate to about a year for such young samples. "Another application could be in the verification of the age of wines and spirits, and wooden furniture."
Published: 30/11/1998