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

Previously, we have observed that the atmospheric ¹⁴C 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 ¹⁴C events remains unknown. Here we report new ¹⁴C measurements from AD 822 to 1020, and the discovery of a second rapid increase of ¹⁴C content from AD 992 to 993. The ¹⁰Be flux in the Antarctic ice core shows peaks corresponding to these two ¹⁴C events. The proportions of flux increase (¹⁴C/¹⁰Be) 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 ¹⁴C increase events, solar activity is a plausible cause of the ¹⁴C increase events.