Cosmogenic radionuclides reveal an extreme solar particle storm near a solar minimum 9125 years BP.

During solar storms, the Sun expels large amounts of energetic particles (SEP) that can react with the Earth's atmospheric constituents and produce cosmogenic radionuclides such as C, Be and Cl. Here we present Be and Cl data measured in ice cores from Greenland and Antarctica. The data consistently show one of the largest Be and Cl production peaks detected so far, most likely produced by an extreme SEP event that hit Earth 9125 years BP (before present, i.e., before 1950 CE), i.e., 7176 BCE. Using the Cl/Be ratio, we demonstrate that this event was characterized by a very hard energy spectrum and was possibly up to two orders of magnitude larger than any SEP event during the instrumental period. Furthermore, we provide Be-based evidence that, contrary to expectations, the SEP event occurred near a solar minimum.