Published online by Cambridge University Press: 18 July 2016
When used with 14C records, an accurate reconstruction of the 13C/12C changes of atmospheric CO2 may be a key constraint to determining the historic activity of the biosphere as CO2 source or sink. We recently detailed the derivation of an atmospheric δ13C record for 1930–1979 from Arizona juniper tree rings (Leavitt and Long, 1983). Each tree was close to a weather station, allowing us to account for the effects of climate on the tree-ring δ13C records. The resulting atmospheric δ13C trend shows an overall decrease of ca 1.3‰, but instead of decreasing exponentially over the whole 50-year period, the best-fit curve decreases exponentially to ca 1960 and then flattens out. The drop from 1956 to 1978 was ca 0.55‰, still very similar to the measured atmospheric drop over that period of 0.65 ± 0.13‰ (Keeling, Mook, and Tans, 1979).