Location-dependent differences in the C-14 content of wood

The long C-14 chronologies currently used as calibration curves combine results from wood that grew in the western United States, the British Isles and Germany. Although these results show few significant differences in the C-14 content of contemporaneous wood when averaged over the length of the chronology (i.e, the means of overlapping sections of chronology are the same), closer examination shows considerable variability. Separating the sections of chronology according to the provenance of the wood used for calibration reveals patterns that suggest small but finite differences in the C-14 content of wood from different locations. We conclude that there is some evidence that German and American wood give dates older by between 20 and 40 yr from those of Irish oak for some periods. Additionally we suggest that the shift of the Belfast 1986 calibration data by ca. 18 yr toward older dates may not be valid and that the resultant offset between the Belfast 1986 and Seattle 1993 data shows a small but real difference in the C-14 content of contemporaneous American, German and Irish wood. Intralaboratory measurements made in Belfast on contemporaneous German and Irish oak, and bristlecone pine and Irish oak, give offsets of 39 and 41 yr, respectively, with the Irish oak dating younger. Previous studies, in which sample pairs of American and English and French wood were processed in the same laboratory, also showed American wood to be slightly depleted in C-14. None of the findings of this study would significantly alter calibrated C-14 dates.