Tree Rings and A Cosmic Mystery

Between AD 774 and 775 something weird happened. And that something weird resulted in a 12% increase of ¹⁴C (a radioactive form of carbon) in the atmosphere. How do we even know this? Tree rings.

Earlier work had enabled the dating of this phenomenon with the accuracy of a decade. Now, a new study improved this by investigating Japanese cedar trees, measuring the carbon-14 between AD 750 and 850. The one year resolution allowed the researchers to pinpoint the atmospheric change to the year mentioned above. A proposed explanation for this is a sudden and substantial increase in the intensity of the cosmic radiation hitting earth. But what caused this? Nobody really knows.

Atmospheric carbon-14 (AD 770 to 795). Measurements compared to simulations.
(Source: Miyake et al., 2012)

Two candidates are put forward, both of which are discarded by the authors.

‘Boom’
(Source: Wikimedia Common, author: NASA)

A supernova explosion: when stars go ‘boom’, gamma rays are produced. Upon hitting the atmosphere, these can convert nitrogen-14 into carbon-14. But, the authors argue, this isn’t what occurred during that year. By using data from other supernovae, they posit that if it was a supernova, it would have had to be close (in cosmological terms, at least. Specifically, 2 kiloparsecs or roughly 6.52 light years). Close enough to have been noticed. Yet, there are no historical records of such an event. And even if it did go by without reference in historical records, we should be able to see some remnants of it now, and we don’t.

‘Burp’
(Source: Wikimedia Commons, author: NASA)

A solar flare: when stars go ‘burp’, they emit, among other things, protons. By calculating the proton energy that would be required to explain the atmospheric change that was measured and dated with the help of the tree rings, the authors state that it must have been one serious solar flare. While such a ‘super flare’ might have been possible, again the lack of historical records suggests otherwise. The detrimental effect on the ozone layer of such an event would certainly have been mentioned somewhere in some form (for example, by recording mass extinctions or the occurrence of auroras in places where they normally aren’t seen).

Thus, the mystery remains.

With our present knowledge, we cannot specify the cause of this event. However, we can say that an extremely energetic event occurred around our space environment in AD 775. In the future, other high resolution records (such as ¹⁰Be and nitrate data), together with careful research of historical documentation around AD 775 and further surveys of undetected supernova remnants, may help us to clarify the cause.

Any guesses?

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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 DOI: 10.1038/nature11123

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