Fluctuations of atmospheric carbon-14 concentrations during the past century

Abstract

Variations in atmospheric carbon-14 concentrations during the past century have been studied through the analyses of wines, spirits and plant seeds. The results reveal that short-term fluctuations of carbon-14 concentrations have occurred which are negatively correlated with solar activity. Maximum correlation occurs with carbon-14 concentration minima preceding sunspot maxima by one year. The overall magnitude of the fluctuations, 3 % of the natural carbon-14 concentration, appears too large to be attributed to variations in the isotope production rate by solar modulation of the galactic cosmic ray flux. It is proposed that the fluctuations are the result of solar-sensitive mixing of stratospheric and tropospheric air masses through variations of the incident ultraviolet and corpuscular radiation over each solar cycle. Theoretical considerations require that stratospheric carbon-14 levels fluctuate in direct correlation with solar activity. These variations of natural carbon-14 activities in the troposphere represent a significant deviation from the basic assumption of the radiocarbon dating method and may introduce further uncertainty in the dating of ‘young’ materials. In addition, they endorse the universal use of the N. B. S. modern standard in carbon -14 assay. A long-term decrease in tropospheric carbon-14 concentrations was also observed and is attributed to a reduction in the mean production rate of carbon-14 through enhanced solar activity.