Tree-rings reveal two strong solar proton events in 7176 and 5259 BCE

Abstract

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.

Extreme solar events

“…69 on 20 January (* 1.2) (Asvestari et al 2017a, b;Cliver et al 2020a, b;Usoskin et al 2020a, b; International GLE Database (https://gle.oulu.fi)). The recently reported 7176 BC event has also been confirmed to be a hard spectrum event (Brehm et al 2021(Brehm et al , 2022Paleari et al 2022). These hard spectra imply that while the high-energy SEP fluence of the 774 AD event was extreme, the fluence in the 10-30 MeV range was not enhanced commensurately.…”

“…The SEP event scenario for the 774-775 AD event is preferred over the main competing galactic GRB hypothesis for three reasons: (1) the latitudinal gradient and inferred global symmetry of the cosmogenic signal (Sukhodolov et al 2017;Uusitalo et al 2018;Bu ¨ntgen et al 2018) (if a GRB source were located near to Earth's equatorial plane, the expected latitudinal gradient would be opposite to the observed polar enhancement of the cosmogenic signal; Uusitalo et al 2018), 17 (2) the significant response in 10 Be (e.g., Mekhaldi et al 2015;Sukhodolov et al 2017) which is not expected for a GRB (Pavlov et al 2013a), and (3) the identification of four additional cosmogenic nuclide events in 993-994 AD (Miyake et al 2013(Miyake et al , 2014Fogtmann-Schulz et al 2017;O'Hare et al 2019), * 660 BC (Park et al 2017;O'Hare et al 2019), 7176 BC (Paleari et al 2022Brehm et al 2022), and 5259 BC (Brehm et al 2022). In addition to these five confirmed SEP events, three other candidates are known presently that need to be independently confirmed; 1052 AD and 1279 AD (Brehm et al 2021) as well as 5410 BC (Miyake et al 2021).…”

“…• The fact that no events with F 30 [ 10 11 (cm 2 year) -1 have been found, and are unlikely to be found, over the Holocene, poses an upper 90% confidence interval limit P([ 10 11 (cm 2 year) -1 ) B 1.9 9 10 -4 year -1 . • Five historical events (7176 BC, 5259 BC, 660 BC, 774 AD, 993 AD) with F 30 [ 5 9 10 10 (cm 2 year) -1 (Mekhaldi et al 2015;O'Hare et al 2019;Paleari et al 2022;Brehm et al 2022) have been so far discovered and confirmed during the Holocene, but additional similar events may be found as the search continues. Accordingly, a lower limit of P([ 5 9 10 10 (cm 2 year) -1 )-C 1.2 9 10 -4 year -1 has been set.…”

“…18Fig. 51 Ratio of the modelled annual radiocarbon production Q of known (blue) and candidate (light blue) historical SEP events (according to Bu ¨ntgen et al 2017; O'Hare et al 2019; Sakurai et al 2020;Brehm et al 2021;Miyake et al 2021;Paleari et al 2022;Brehm et al 2022) relative to that of GLE No. 5(Usoskin et al 2020b).…”

Regional Differences in Carbon-14 Data of the 993 CE Cosmic Ray Event

“…So far, several candidates of SEP-driven 14 C spikes have been reported: 774 CE, 993 CE, ~660 BCE, 7176 BCE, 1052CE, 1279CE, 5259 BCE, and 5411 BCE (Miyake et al, 2012Park et al, 2017;Sakurai et al, 2020;Brehm et al, 2021;Brehm et al, 2022;Miyahara et al, 2022). The first four events have also been detected in 10 Be and 36 Cl data in ice cores, and the consistency of the origin of large-scale SEP events (Mekhaldi et al, 2015;Miyake et al, 2015, Miyake et al, 2019bO'Hare et al, 2019;Paleari et al, 2022), whose scale has been estimated to be 30-100 times larger than the largest GLE (Usoskin and Kovaltsov 2021), has been confirmed.…”