Carbon-12 is the main isotope in the carbon absorbed by the grapevines, and is very stable, while only tiny amounts of carbon-14, a radioactive isotope, are found in the plant.
The amount of carbon-14 has varied over the years, too, which makes it a useful tool for judging the true age of a wine.
"Until the late 1940s, all carbon-14 in the Earth's biosphere was produced by the interaction between cosmic rays and nitrogen in the upper atmosphere," said Graham Jones of the University of Adelaide.
"This changed in the late 1940s up to 1963 when atmospheric atomic explosions significantly increased the amount of carbon-14 in the atmosphere," said Jones, who led the study and presented its findings Sunday at the annual meeting of the American Chemical Society, held in California.
More and more fossil fuel has been burnt since the bomb tests stopped in the 1960s and this has had the effect of diluting the radioactive carbon-14 in the atmosphere.
That is turn changes the ratio of carbon-14 to carbon-12 in plants, like grapevines.