Stable carbon isotope chronologies from trees in the southwestern United States

Plant fixation of CO₂ and subsequent incorporation of this carbon into growth rings is the basis for attempts to use tree rings to reconstruct ¹³C/¹²C ratios (δ¹³C) of atmospheric carbon dioxide. Accurate δ¹³C chronologies of atmospheric CO₂ are important to distinguishing biospheric and fossil fuel CO₂ inputs to the atmosphere. We have sampled 14 sites in the American Southwest, pooling four cores from each of four trees at most of the sites. Cellulose from pooled 5‐year ring groups was analyzed to develop δ¹³C chronologies for each of the sites. The 10 pinyon (Pinus edulis, P. monophylla) chronologies, plus two additional pinyon chronologies formed from groups of sites where single trees had been analyzed, were examined in detail. Significant correlations of differences in successive values of ring width indices and δ¹³C (first differences) were used to “correct” each chronology for climate influences primarily responsible for high‐frequency fluctuation. The master corrected δ¹³C chronology reveals an overall decrease of 1.2–1.4‰ from 1700 to present, greater than the atmospheric change expected from fossil fuel CO₂ inputs alone and comparable to that measured in ice cores. The biospheric CO₂ input to the atmosphere implicated in this chronology is greatest is the nineteenth and early twentieth centuries. High‐frequency fluctuations in the chronologies appear dominantly related to climate (drought).