Carbon-14 as a marker of seismic activity

Abstract

The principle of carbon-14 dating is well known (1): the content of this radioisotope in a sample of an animal or a plant origin is assessed and the time elapsed from the formation of the organic material to the moment of assessment is calculated comparing the present content of carbon- 14 to that at the time the plant or animal was alive. This last is assumed at equilibrium with the atmospheric concentration of carbon-14, which, in turn, is assumed to have been constant through the ages. Knowing the decay constant of carbon-14, the time elapsed is deduced. Then this calculated age is entered in calibration diagrams that account for the actual variable atmospheric content through the years, to obtain the age of the sample, or more precisely, a time interval in which the age falls. Thus, the main idea behind the technique is that the atmospheric concentration of carbon-14 marked CO2 is essentially constant, or slowly variable, from year to year. To this, one word of caution needs be added: after WW2, and particularly from the 1950s, the concentration of carbon-14 in the atmosphere has become quite erratic due to nuclear weapon tests, and hence this technique is not used for dating samples from that time on.In the present work, the whole carbon-14 idea has been reused in a somewhat different context, and with a different purpose in mind.Afact to be kept in mind is thatCO2 contained in vast amounts within the Earth’s crust beneath the volcanic apparatus, the so-called fossil CO2, either degassed by the mantle or having been formed by metamorphic reactions in the crust, contains no trace of the carbon-14 isotope. Fossil CO2 release is often associated to seismic and volcanic activity: the question may then arise whether, on occasion of such major releases and in the presence of landscape conformation conducive to slow mixing (narrow valley bottoms, canyons, and the like), the carbon-14 contents of local vegetation may be affected by the presence of spent CO2. The Solfatara at Pozzuoli presented both the above-mentioned conditions: it has the required shape and it has endured large releases of fossil CO2 in the early 1980s. It presented itself as an ideal location to test this hypothesis. There are pine trees planted in the 1930s, as part of a reforestation plan: it was possible to select two recently dead trees, one in the Solfatara area and presumably as affected by the CO2 release as could be possible, and the other immediately outside and upwind of the area, constituting an ideal blank. Sections were taken from the two trees and analysed to determine the carbon-14 content of several rings corresponding to the years of interest. In the following sections, the method and the results will be presented and commented upon.