Observed increase concentration of carbon-14 in tree rings dated 774 or 775
The774–775 carbon-14 spike is an observed increase of around 1.2% in the concentration of the radioactivecarbon-14 isotope intree rings dated to 774 or 775CE, which is about 20 times higher than the normal year-to-year variation of radiocarbon in the atmosphere. It was discovered during a study ofJapanese cedar tree-rings, with the year of occurrence determined throughdendrochronology.[1] A surge inberyllium-10 (10Be), detected in Antarctic ice cores, has also been associated with the 774–775 event.[2] The 774–775 CE carbon-14 spike is one of severalMiyake events and it produced the largest and most rapid rise in carbon-14 ever recorded.[3][4]
Thecarbon-14 spike around 774. Colored dots are measurements in Japanese (M12) and German (oak) trees; black lines are the modeled profile corresponding to the instant production of carbon-14.[2]
The signal exhibits a sharp increase of around 1.2% followed by a slow decline, which is consistent with an instant production of carbon-14 in the atmosphere,[2] indicating that the event was short in duration. The globally averaged production ofcarbon-14 for this event is(1.3 ± 0.2) × 108 atoms/cm2.[2][7][8]
Several possible causes of the event have been considered.
TheAnglo-Saxon Chronicle recorded "a red crucifix, after sunset", which has been variously hypothesised to have been asupernova[9] or theaurora borealis.[2][10]
Annus Domini (the year of the Lord) 774. This year theNorthumbrians banished their king,Alred, fromYork at Easter-tide; and choseEthelred, the son of Mull, for their lord, who reigned four winters. This year also appeared in the heavens a red crucifix, after sunset; theMercians and the men ofKent fought atOtford; and wonderful serpents were seen in theland of the South-Saxons.
In China, there is only one clear reference to an aurora in the mid-770s, on 12 January 776.[11][12] However, an anomalous "thunderstorm" was recorded for 775.[13]
As established byIlya G. Usoskin and colleagues,[14] the current scientific paradigm[15] is that the event was caused by asolar particle event (SPE) from a very strongsolar flare, perhaps the strongest known.[16]Another proposed origin, involving agamma-ray burst,[8][17] is regarded as unlikely, because the event was also observed in isotopes10Be and36Cl – a gamma-ray burst would not have produced significant10Be, and cosmogenic radionuclides are concentrated near the poles, suggesting a flux of charged particles.[16]
The event of 774 is the strongest spike over the last 11,000 years in the record of cosmogenic isotopes,[18] but several other events of the same kind (Miyake events) have occurred during theHolocene epoch.[18] The993–994 carbon-14 spike was about 60% as strong;[19] another event occurred inc. 660 BCE.[20][21] In 2023 the strongest event yet discovered was reported, which occurred in 12,350-12,349 BC.[22]
The event of 774 had no significant consequences for life on Earth,[23][24] but had it happened in modern times, it might have produced catastrophic damage to modern technology, particularly to communication and space-borne navigation systems. Also, a solar flare able to produce the observed isotopic effect, would pose considerable risk to astronauts.[25]
14C variations are poorly understood, because annual-resolution measurements are available for only a few periods (such as 774–775).[26] In a 2017 study, a14C increase of (2.0%) was associated with a 5480 BC event, but it is not associated with a solar event because of its long duration, but rather to an unusually fastgrand minimum of solar activity.[26]
^Güttler, D.; Beer, J.; Bleicher, N. (2013). "The 774/775 AD event in the southern hemisphere".ETH-Zurich: Laboratory of Ion Beam Physics: Annual Report 2013. LIBRUM. p. 33.ISBN978-3-9524038-4-6.OCLC887695262.
^Stephenson, F.R. (2015). "Astronomical evidence relating to the observed14C increases in A.D. 774–5 and 993–4 as determined from tree rings".Advances in Space Research.55 (6):1537–45.Bibcode:2015AdSpR..55.1537S.doi:10.1016/j.asr.2014.12.014.
^Ya-Ting Chai & Yuan-Chuan Zou (2015). "Searching for events in Chinese ancient records to explain the increase in14C from 774–775 CE and 993–994 AD".Research in Astronomy and Astrophysics.15 (9): 1504.arXiv:1406.7306.doi:10.1088/1674-4527/15/9/007.S2CID124499827.
^Usoskin, I.G.; Kromer, B.; Ludlow, F.; Beer, J.; Friedrich, M.; Kovaltsov, G.A.; Solanki, S.K.; Wacker, L. (2013). "The AD775 cosmic event revisited: the Sun is to blame".Astronomy and Astrophysics Letters.552: L3.arXiv:1302.6897.doi:10.1051/0004-6361/201321080.
^Townsend, L. W.; Porter, J. A.; deWet, W. C; Smith, W. J.; McGirl, N. A.; Heilbronn, L. H.; Moussa, H. M. (2016-06-01). "Extreme solar event of AD775: Potential radiation exposure to crews in deep space".Acta Astronautica. Special Section: Selected Papers from the International Workshop on Satellite Constellations and Formation Flying 2015.123:116–120.Bibcode:2016AcAau.123..116T.doi:10.1016/j.actaastro.2016.03.002.
