Past atmospheric CO₂ levels and the ¹³C/¹²C ratios in tree rings

Intra-ring differences of the ¹³C/¹²C ratio in tree rings are found to be very large in four Dutch trees. In some cases differences were measured over 4” ° δ¹³C. Even the ¹³C/¹²C ratio averaged over entire rings sometimes still exhibits abrupt jumps of over 1” ° in δ¹³C from year to year. Significant correlations of δ¹³C in tree rings with yearly average temperature and precipitation are found. Responses of the atmosphere to a ¹³C labelled input of CO2 are calculated in 3½ box perturbation model, consisting of an atmosphere, a biosphere, an oceanic mixed layer and a leak of the perturbation to the deep seas. The average ¹³C record obtained from the four trees is interpreted in terms of past biospheric growth and shrinkage by using the 3½ box model.