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Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved December 21, 2016 (received for review August 17, 2016)
Significance
Carbon-14 contents in tree rings tell us information of the past cosmic ray intensities because cosmic rays produce 14C in the atmosphere. We found a signature of a quite large increase of incoming cosmic ray intensity in the mid-Holocene (the 5480 BC event) from the measurement of 14C content in North American tree rings. The cause of this event is supposed to be an extremely weak sun, or a combination of successive strong solar bursts and variation of a solar magnetic activity. In any case, 14C variation of the 5480 BC event is extraordinary in the Holocene, and this event indicates the abnormal solar activity compared with other periods.
Abstract
Radiocarbon content in tree rings can be an excellent proxy of the past incoming cosmic ray intensities to Earth. Although such past cosmic ray variations have been studied by measurements of 14C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual 14C data. There is a little understanding about annual 14C variations in the past, with the exception of a few periods including the AD 774−775 14C excursion where annual measurements have been performed. Here, we report the result of 14C measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1- to 2-y resolution, and a finding of an extraordinarily large 14C increase (20‰) from 5481 BC to 5471 BC (the 5480 BC event). The 14C increase rate of this event is much larger than that of the normal grand solar minima. We propose the possible causes of this event are an unknown phase of grand solar minimum, or a combination of successive solar proton events and a normal grand solar minimum.
Footnotes
- ↵1To whom correspondence should be addressed. Email: fmiyake{at}isee.nagoya-u.ac.jp.
Author contributions: F.M., A.J.T.J., and I.P.P. designed research; F.M., I.P.P., L.W., M.S., C.H.B., T.L., R.C., K.M., and T.N. performed research; F.M. analyzed data; and F.M., A.J.T.J., and I.P.P. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1613144114/-/DCSupplemental.
Large 14C excursion in 5480 BC
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