Another rapid event in the carbon-14 content of tree rings

Previously, we have observed that the atmospheric C-14 content measured in tree rings showed a strong increase from AD 774 to 775. Although the cause of this event can be explained by a large solar proton event or a short gamma-ray burst, a more detailed discussion of the cause is difficult because the rate of occurrence of such rapid C-14 events remains unknown. Here we report new C-14 measurements from AD 822 to 1020, and the discovery of a second rapid increase of C-14 content from AD 992 to 993. The Be-10 flux in the Antarctic ice core shows peaks corresponding to these two C-14 events. The proportions of flux increase (C-14/Be-10) of the two events are consistent with each other. Therefore, it is highly possible that these events have the same origin. Considering the occurrence rate of C-14 increase events, solar activity is a plausible cause of the C-14 increase events.