High-Precision 14C Measurement of Irish Oaks to Show the Natural Atmospheric 14C Variations of the AD Time Period

Abstract

The high-precision 14C measurement of bi-decade and decade samples of Irish Oak for the time period AD 50 to 1830 is presented. The samples were taken from dendrochronologically dated Irish Oak providing an absolute chronology for this period. While the natural atmospheric 14C concentration shows cyclic deviations from a constant value, the amplitudes of such deviations vary considerably. Repeated measurement and interlaboratory checks justify a claim to accuracy in the data given. Comparison of a large number of data sets between Belfast and Seattle show that the different techniques of scintillation counting of 14C6H6 and gas counting of 14CO2 derived from different wood species grown in different continents give no significant bias. Thus, it is justifiable to use this data set as a high-precision calibration curve for the AD period. Over most of this period bi-decade samples were measured giving a slightly reduced resolution of short-term variation when compared to the measurement of decade intervals; however, any difference is not apparent when such curves are compared. It is of much greater importance that a comparison of data sets is without bias if a combined calibration curve is to have sufficient integrity for general use. Averaging the data from Belfast and Seattle would improve the validity of such an AD 14C calibration curve since it would tend to smooth out slight local variations and become internationally more acceptable.