Review of radiocarbon data from atmospheric and tree ring samples for the period 1945-1997 AD

A summary of C-14 data from atmospheric sampling and measurements on wood f rom annual tree rings for the period 1945-1997 AD is presented and evaluate d. Atmospheric records are characterized by different distributions of bomb -test C-14 between the Northern and Southern Hemispheres, latitude dependen ce, and seasonal fluctuations. Radiocarbon data from tree rings are summari sed and plotted against atmospheric records from similar latitudes. In some cases, discrepancies are found. Possible reasons for this include: 1) the use of stored carbohydrate from the previous year, 2) different C-14 levels in the air around subcanopy trees due to respiration of CO2, 3) regional a nd local effects of anthropogenic CO2 and C-14 sources, 4) Sampling of wood material too close to ring boundaries, and 5) insufficient pretreatment of tree ring samples for dating. But in cases where trees were carefully sele cted and the samples adequately pretreated, radiocarbon data from tree ring s show excellent agreement with direct atmospheric sampling records.