We have used a new sedimentary record of geomagnetic field intensity, combined with published volcanic data, to examine the extent to which geomagnetic changes have affected 14C production in the past. The production record was then converted into an atmospheric Δ14C signal using a 4-box model of past oceanic circulation. The results indicate that changes in geomagnetic field intensity account for at least 80% of the Δ14C shift documented by published U-Th data, which are of the order of 500 per mil with respect to a constant production hypothesis. Model simulations show that changes in past oceanic circulation account for only ¿100 per mil of Δ14C variation, which is within the uncertainty of the geomagnetic correction. In terms of dating, these results show that the radiocarbon ages have to be shifted by 2 to 3.5 kyr towards older ages during the 20--40 ka interval. ¿ American Geophysical Union 1996