The years 774 and 775 were eventful times in human history. Charlemagne conquered the Lombards and began his campaigns into Westphalia. Heizei, a future emperor of Japan, was born, while Byzantine Emperor Constantine V died and was succeeded by his son Leo IV.
It also appears that, sometime in 774 or 775, planet Earth was hit by a short but strong burst of gamma rays, one of the most powerful forms of energy known. The evidence comes in the form radioactive isotopes formed in the atmosphere as a consequence of the gamma ray burst. They include a radioactive form of beryllium (10Be, if you’re curious) found at higher concentrations in Antarctica, as well as carbon-14 (14C, the same isotope of carbon that makes carbon-dating possible) in tree rings across Europe, North America, and Japan. It’s the 14C from the Japanese tree rings that permitted precise dating of the radiation accumulation to 774-775. After this sudden radiation spike, which was 10-20 times greater than our normal levels of background radiation, radiation levels quickly returned to normal globally. While elevated, these levels of radiation were still too low to have a detrimental effect on life. But, ever since this event was first reported fifteen years ago (and definitively confirmed last year), scientists have wondered what could have caused it. Given the global distribution of the radiation, and the fact that the radiation had to be formed when high-energy particles reacted with our atmosphere, they looked to outer space for answers.
A group of astrophysicists from the Friedrich Schiller University of Jena proposed a gamma ray burst as the culprit after they and other researchers discounted several other celestial possibilities. A supernova, a massive stellar explosion, in our galactic neighborhood is one possibility. However, the remnants of a supernova of that size and magnitude would be detectable today from telescopes, observatories, and spacecraft. In addition, such an event would surely have been visible to the naked eye in 774 or 775. Yet, there is no record anywhere of such an explosion from ancient skywatchers, though more distant supernova events in 1006 and 1054 (the latter producing today’s gnarled Crab Nebula) were recorded with great interest by Chinese, Arabic, Indian, and Japanese astronomers at the time. A massive solar flare from our own sun could also be responsible for the 774-775 radiation spike. However, some (including the Jena researchers) believe that our sun’s typical “massive” solar flares are still insufficiently destructive to account for the levels of radioactivity detected in Japan, Antarctica, and Europe.
The researchers from Jena assert that a short burst of gamma rays striking Earth’s atmosphere at some brief point during 774 or 775 is sufficient to account for the radiation levels detected globally during that time period. These short bursts are thought to occur during specific and highly destructive stellar events, such as the merger neutron stars, black holes, or white dwarf stars. These are also thankfully rare events, occurring every 10,000 to 1 million years in each galaxy. If confirmed, this short gamma ray burst would be the first of this type of event known in our galaxy.
If this radiation spike was due to a short gamma ray burst, Earth was very lucky. The 774-775 burst was short enough and distant enough (some 3,000-12,000 light years away) to avoid any sort of mass extinction. If it occurred, it was probably too far away for humans to even see it. A closer gamma ray burst lasting as little as ten seconds, could have obliterated half of Earth’s protective ozone layer, exposing surface and near-surface life to lethal levels of radiation from our sun. In fact, one group of researchers at the University of Kansas believe that such a close gamma ray burst initiated a mass extinction at the end of Earth’s Ordovician period. Back then (440-450 million years ago), most of Earth’s complex life consisted of phytoplankton and marine invertebrates. Up to 60% of marine invertebrates perished in this extinction. While most scientists believe the end-Ordovician extinction event was triggered by a massive ice age (when the ancient continent Gondwana moved across the south pole), this group from the University of Kansas and Washburn University has argued that a short but massive gamma ray burst from a “hypernova” (a supermassive supernova) just 6,000 light years away could have triggered a brief ozone-depleting burst of energy. Phytoplankton, a base of Earth’s food chain, would have been devastated by the lethal levels of radioactivity seeping in through the planet’s crippled ozone layer. Though ozone levels would have recovered in subsequent decades, the damage to the base of the food chain would have triggered subsequent die-offs of other marine life.
This 450 million year old gamma ray burst is just a hypothesis, and one that is not widely accepted. However, it is important to consider this possibility since, while rare, these types of cosmic events can have potentially catastrophic consequences for our biosphere. What we know about short gamma ray bursts today comes largely from NASA projects such as the Swift spacecraft, which was launched nearly a decade ago to learn more about these rare and high-energy events across the universe. Detecting them is difficult and controversial. The same Kansas-based research group that promotes a short gamma ray burst’s role in the end-Ordovician extinction event actually opposes the theory that the 774-775 radiation spike is due to a smaller and more distant short gamma ray burst. They instead believe a massive solar flare is responsible. It will take more data, more sifting through archeological and paleontological evidence, to clarify what happened 1,200 years ago and 450 million years ago. But, gamma ray bursts are out there, and at least we’re aware of them.
- Hambaryan VV, Neuhäuser R. 2013. “A Galactic short gamma-ray burst as a cause for the 14C peak in AD 774/5.” Monthly Notes of the Royal Astronomical Society doi: 10.1093/mnras/sts378. [Friedrich Schiller University of Jena research group, advocating the short gamma ray burst hypothesis for 774-775]
- Melott AL, Thomas B. 2012. “Causes of an AD 774-775 14C increase.” Nature 491: E1-E2. [Kansas-based research group, advocating here for the solar flare hypothesis for 774-775, though in other papers they argue that a short gamma ray burst may have initiated the end-Ordovician extinction event.]