Chemical pretreatment and radial flow of 14C in tree rings

Abstract

THE prospect of very large increases in atmospheric CO2 concentrations by fossil fuel combustion during the next century has prompted an active interest in CO2 levels of the last century. One way of measuring these concentrations is by doing precise 14C determinations in wooden tissue of known age. The first question to be answered, then, is whether wood grown during a certain year truly reflects atmospheric 14C levels of that year. And if whole wood does not, what chemical treatment has to be given to it in order to isolate those substances that do reflect atmospheric 14C at the time of formation of the ring. Attempts to answer this question have been made but the only conclusion that investigators1–5 generally agree on is that all resin fractions are to be distrusted and that the amount of contamination by material from other years of growth depends on tree species. But in some cases resin extraction by organic solvents does not seem to be sufficient1,2. Having installed a new high precision 14C counting system, to be described elsewhere, we looked at this question again, in view of our further tree ring measurements and the chemical treatment to be applied to our samples. We found that in the tree studied, an oak, radial movement of 14C across ring boundaries is minor, even where the heartwood–sapwood transition takes place. If year to year variations are small, the effect is negligible. Furthermore, cellulose and wood, treated successively with hydrochloric acid, sodium hydroxide and again hydrochloric acid, both seem acceptable for monitoring past 14C levels, although they may not exactly refer to the same time of year.