Tree-Rings Reveal Two Strong Solar Proton Events in 7176 and 5259 BCE

The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events have been directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as 14C, 10Be and 36Cl have been found. Analyzing annual 14C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric 14C occurring in 7176 and 5259 BCE. The ~2% increases of atmospheric 14C recorded for both events exceed all previously known 14C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which may threaten modern infrastructure. © 2022, The Author(s).The Laboratory of Ion Beam Physics is partially funded by its consortium partners PSI, EAWAG, and EMPA. N.B. is funded by the Swiss National Science Foundation (SNSF grant #SNF 197137). The establishment of the BRAMS Facility was jointly funded by the NERC, BBSRC and the University of Bristol and the measurements in this work were partly funded by an ERC Proof of Concept grant awarded to R.P.E. and financing E.C. postdoctoral contract (LipDat H2020 ERC-2018-PoC/812917). We thank Bisserka Gaydarska for sub-sampling the inter-laboratory replicates from M49, M234, Q2729 and Q2750, Cathy Tyers for reviewing the dating of the Irish and German samples, and Alexander Land for assistance in dating sample M49. P.F. received funding from the SNF Sinergia project CALDERA (no. 183571). R.H. is funded by Russian Science Foundation (grant № 21-14-00330). I.U. acknowledges the support from the Academy of Finland (grant 321882 ESPERA). C.L.P.’s and M.W.S.’s work on bristlecone pine was funded by the M.H. Wiener Foundation (ICCP Project). K.N. acknowledges the support provided by the Austrian Science Fund FWF (grant I-1183-N19)