774–775 carbon-14 spike
The 774–775 Carbon-14 Spike was an increase of 1.2% in the carbon-14 content of tree rings during the years AD 774 or 775, which was about 20 times higher than the normal rate of variation. It was found during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology. A surge in a specific isotope of beryllium, detected in ice cores of Antarctica, has been associated with the 774-75 event. Evidence was also found in German oak.
The team that discovered it (Miyake et al.) considered various possible origins for the event, in the end hypothesizing that it was the result of a cosmic ray event, or radiation burst. Another group suggested that a short gamma-ray burst, which is not associated with a supernova, may have been responsible. Carbon-14 could also be formed as a result of gamma rays from supernovas or proton storms from solar flares; however, analysis initially appeared to rule out these possibilities.
No supernova was recorded in that time frame, and even an unobserved supernova would have been expected to produce a supernova remnant that would be observable today. However, the Anglo-Saxon Chronicle mentions a "red crucifix" seen in the skies of Britain in the year 774 AD, and it has been suggested that this might be evidence of an unrecognized supernova.
The initial analysis by Miyake et al. concluded that an implausibly large solar flare would have been required to account for the event, which would have produced remarkable auroras, of which there is no record, or drastically affected the ozone layer, with major biological or ecological consequences, for which there is no evidence. However, two other groups separately concluded that the initial calculations were incorrect and it would have required a very strong but not inexplicably strong solar particle event, and that a local solar origin was therefore the most likely explanation.
- Becker, K. (15 June 2012). "Mystery cosmic event left its mark in 774 and 775 AD". Boulder Daily Camera. Retrieved 2012-06-24.
- Miyake, F.; Nagaya, K.; Masuda, K.; Nakamura, T. (2012). "A signature of cosmic-ray increase in AD 774–775 from tree rings in Japan". Nature 486 (7402): 240–242. Bibcode:2012Natur.486..240M. doi:10.1038/nature11123.
- Timmer, J. (22 January 2013). "Why was Earth bombarded with high-energy particles in the year 774?". Ars Technica.
- Usoskin, I. G. et al. (2013). "The AD775 cosmic event revisited: The Sun is to blame". Astronomy & Astrophysics 552 (1): L3. arXiv:1302.6897. Bibcode:2013A&A...552L...3U. doi:10.1051/0004-6361/201321080.
- Hambaryan, V. V.; Neuhauser, R. (2013). "A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5". Monthly Notices of the Royal Astronomical Society 430 (1): 32–36. arXiv:1211.2584. Bibcode:2013MNRAS.430...32H. doi:10.1093/mnras/sts378.
- Lovett, R. A. (2012). "Ancient text gives clue to mysterious radiation spike". Nature News. doi:10.1038/nature.2012.10898.
- Lovett, R. A. (2012). "Mysterious radiation burst recorded in tree rings". Nature News. doi:10.1038/nature.2012.10768.
- Thomas, B. C.; Melott, A. L.; Arkenberg, K. R.; Snyder, B. R. (2013). "Terrestrial effects of possible astrophysical sources of an AD 774-775 increase in 14C production". Geophysical Research Letters 40 (6): 1237. arXiv:1302.1501. Bibcode:2013GeoRL..40.1237T. doi:10.1002/grl.50222.
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