Prolonged production of 14C during the ~660 BCE solar proton event from Japanese tree rings

Hirohisa Sakurai,Kimiaki Masuda,Toru Moriya,Hiroko Miyahara,Minoru Sakamoto,Kazuho Horiuchi,Fusa Miyake,Takumi Mitsutani,Motonari Ohyama,Fuyuki Tokanai

doi:10.1038/s41598-019-57273-2

Abstract

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth’s atmosphere. Several rapid 14C increases in the past, such as the 775 CE and 994CE 14C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid 14C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of 10Be and 36Cl in ice cores, was presumed similar to the 775 CE event; however, as the 14C increase of approximately 10‰ in 660 BCE had taken a rather longer rise time of 3–4 years as compared to that of the 775 CE event, the occurrence could not be simply associated to an extreme SPE. In this study, to elucidate the rapid increase in 14C concentrations in tree rings around 660 BCE, we have precisely measured the 14C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669–633 BCE. Based on the feature of 14C production rate calculated from the fine measured profile of the 14C concentrations, we found that the 14C rapid increase occurred within 665–663.5 BCE, and that duration of 14C production describing the event is distributed from one month to 41 months. The possibility of occurrence of consecutive SPEs over up to three years is offered.

Highlights

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth’s atmosphere
Since the phenomenon of rapid increase in CR intensity in 775 CE was solved1–3, 14C analysis in annual rings has played a major role in searching for another rapid 14C increase event[4,5]. While such events have been suggested to originate from an extreme SPE6–10, the other type with a rather long period of 14C increase has recently been reported in German oak tree rings[11]
The increases of 14C coincide with those of the ice core-based radionuclides, exceeding the variations of 11-year cycle in the range of 4‰13. Both SN and gamma-ray burst (GRB) were inappropriate for the ~660 BCE event, as the large amount of 10Be were detected in the ice cores

Summary

Introduction

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth’s atmosphere. Employing the 11-box model by Güttler et al.[7], but with several conditions, i.e. stratosphere-troposphere exchange times of 1.5-year and 2.0-year[16] and 14C production share rates between the stratosphere and troposphere (strat:trop = 70%:30%, 80%:20%, and 90%:10%), we examined Δ14C response to a square pulsed 14C input (single-pulsed event).

Results

Conclusion