Radiocarbon from nuclear weapons tests

Abstract

The distribution in time and space of C14 from the 1961–1962 nuclear weapons tests of the U.S. and the USSR is used as a tracer for atmospheric mixing phenomena and exchange of carbon dioxide between the atmosphere and the sea. The mean residence time of C14 injected into the stratosphere by the tests or produced by cosmic rays is about 1.5 years before entering the troposphere. The C14 enters the troposphere predominantly at midlatitudes in the late spring. Dispersion of the C14 injected into the troposphere is well accounted for by eddy diffusion, with the meridional eddy diffusion coefficients being proportional to the variance of the meridional component of the wind speed. Exchange of carbon dioxide between the atmosphere and the sea (and therefore uptake of excess C14 by the sea) is taken to be proportional to the square of the average wind speed over the sea surface. This leads to the conclusion that the bulk of the C14 will eventually be taken up by the oceans of the southern hemisphere. A mean residence time of 2.5 years for a carbon dioxide molecule in the troposphere before uptake by the sea is deduced from the model, giving a mean residence time of 4 years for a C14 atom in the atmosphere before uptake by the sea.