GEOMAGNETIC-FIELD CONTROL OF C-14 PRODUCTION OVER THE LAST 80 KY - IMPLICATIONS FOR THE RADIOCARBON TIME-SCALE

We have used a recent estimate of the geomagnetic dipole field strength over the last 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale. The magnetic record was converted into a C-14 production signal which was then used to produce a synthetic atmospheric DELTA-C-14 record using a two box model for the carbon cycle. This calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic C-14 atoms over the period of time considered, and that DELTA-C-14 changes due to other causes, such as climatic changes, have been of lesser importance. The results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18-40 ky b.p., in good agreement with recent U-Th and C-14 age comparisons. Moreover the results predict that a negligible difference between true and C-14 ages should be observed around 45-50 ky.