LOCATION-DEPENDENT DIFFERENCES IN THE C-14 CONTENT OF WOOD

The long C-14 chronologies currently used as calibration curves combin e results from wood that grew in the western United States, the Britis h Isles and Germany. Although these results show few significant diffe rences in the C-14 content of contemporaneous wood when averaged over the length of the chronology (i.e, the means of overlapping sections o f chronology are the same), closer examination shows considerable vari ability. Separating the sections of chronology according to the proven ance of the wood used for calibration reveals patterns that suggest sm all but finite differences in the C-14 content of wood from different locations. We conclude that there is some evidence that German and Ame rican wood give dates older by between 20 and 40 yr from those of Iris h 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 Se attle 1993 data shows a small but real difference in the C-14 content of contemporaneous American, German and Irish wood. Intralaboratory me asurements made in Belfast on contemporaneous German and Irish oak, an d bristlecone pine and Irish oak, give offsets of 39 and 41 yr, respec tively, 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 deplete d in C-14. None of the findings of this study would significantly alte r calibrated C-14 dates.