Multiradionuclide evidence for an extreme solar proton event around 2,610 B.P. (∼660 BC)

Significance This study provides evidence of an enormous solar storm around 2,610 B.P. It is only the third such event reliably documented and is comparable with the strongest event detected at AD 774/775. The event of 2,610 years B.P. stands out because of its particular signature in the radionuclide data [i.e., carbon-14 ( 14 C) data alone does not allow for an unequivocal detection of the event]. It illustrates that present efforts to find such events based solely on 14 C data likely lead to an underestimated number of such potentially devastating events for our society. In addition to 14 C data, high-resolution records of beryllium-10 and chlorine-36 are crucial for reliable estimates of the occurrence rate and the properties of past solar proton events.