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The carbon 14 content of about 150 wood samples, dated by dendrochronology, has been measured. The wood samples of known age used in this investigation cover the past 2000 years. Some were supplied by Dr. Huber of the dendrochronological laboratory at Munich, and some by the Arizona Tree-Ring Laboratory. Wood from European oak, American fir, Hitchcock, and sequoia was used for the measurements. The measurements show that the C14 activity of atmospheric CO2 has not been entirely constant in the past but has varied by several percent. Two types of variations can be recognized: (1) those that occurred on a time scale of the order of 100 years; (2) those with a time constant of more than 1000 years. Type 1 changes occur only in the atmospheric C14 reservoir; they show correlations with average sunspot numbers; they are caused by changes in the cosmic-ray intensity, which determines the production rate of C14. Type 2 changes involve the total C14 inventory, including the oceans; their causes cannot yet be established conclusively. The empirical relationship between conventional radiocarbon dates and the true ages of wood samples is graphically shown for the past 2000 years.
Hans E. Sueß (Wed,) studied this question.