collisionBy Evan

Several of the regular contributors to this blog deal with radiocarbon dating on a regular basis. The concentration of radiocarbon in the atmosphere is not constant – something that proves to be a major headache when trying to precisely find the age of your sample. For example, events that happened during the cold period known as the Younger Dryas (which happened between 12,900 and 11,500 years ago) cannot be precisely dated using radiocarbon because the concentration in the atmosphere increased rapidly. Potential causes of changes in atmospheric radiocarbon concentration include increases in carbon dioxide due to vegetation changes and ocean-atmosphere interactions, changes in the production rate (radiocarbon is produced by the bombardment of nitrogen gas in the upper atmosphere by cosmic rays), changes in the earth’s magnetic field (the magnetic field protects the Earth from cosmic rays), and more recently, nuclear weapons testing and fossil fuel burning. Changes in the concentration of radiocarbon for the past 12,400 years or so is fairly well known from measuring it in trees preserved in swamps. This allows for matching the concentration of radiocarbon through time with tree rings, an absolute chronology.

Apparent radiocarbon age versus real age (top plot), and the atmospheric concentration of radiocarbon during the Younger Dryas.
Apparent radiocarbon age versus real age (top plot), and the concentration of radiocarbon (bottom) during the Younger Dryas (from the INTCAL09 calibration curve). Note that a sample that grew any time between 12,200 and 12,600 years ago would give an apparent radiocarbon age of about 10,400 years.

A big science news story that came out this week was that a galactic collision maybe have been responsible for a large increase in radiocarbon concentrations at 774-775 AD. The production rate of radiocarbon at that time increased tenfold. The study by Hambaryan and Neuhäuser investigated several possible causes of the spike in radiocarbon. They ruled out solar activity, suggesting it was not possible for the sun to output enough high energy particles to cause such a large spike in radiocarbon (not to mention such an event would have an impact on life on Earth). They ruled out supernovas, as there is no historic record of a supernova at that time, nor are there any known remnants of a supernova that could explain the spike in gamma rays at that time. In addition, nearby supernovas in 1006 AD and 1054 AD did not produce an observed spike in radiocarbon. They also investigated flares from a special class of neutron star, known as a magnetar. Outbursts from these kinds of stars are quite devastating, but the authors suggested that it would need to be within 39 parsecs (about 127 light years) away from Earth. A neutron start that close would have been detected by now (the closest known neutron star is 770 light years away).

Sudden jump in radiocarbon concentration at 774-775 AD.
Sudden jump in radiocarbon concentration at 774-775 AD. From Miyake et al (2012)

The remaining possibility was the collision of two neutron stars, or a neutron star with a black hole. The outburst would have only been observable for maybe one day (while the light from supernovas usually last several months to years), which is an explanation for why there are no historical accounts of the event. Such a collision would have had to happen between 1000 and 4000 parsecs (3200-13000 light years), well within our galaxy. If the event happened any closer than 1000 parsecs, it would have had devastating consequences for life on Earth. The authors also calculated the probability of such events happening on a regular basis, as spikes in atmospheric radiocarbon happen once every 3000 years or so. They calculated that a collision of neutron starts could happen once every 5000 years, though the error on that number is substantial. Because the frequency is not high enough, they suggest that we might not have a good estimate on the amount of neutron stars in our galaxy, or that the collision of white dwarfs might produce enough energy to create massive gamma ray bursts.

What could be concluded from this? Well one thing is that events that happen outside of our solar system could potentially impact life on Earth. These sort of outburst events might happen relatively frequently on a geological time scale. Could a gamma ray burst be the cause of minor extinctions? As mentioned earlier in the article, the Younger Dryas was a period where radiocarbon concentrations increased. It is also a time when megafauna started to go extinct. This is a pretty provocative idea, I’ll admit, and there is little evidence that the increase in radiocarbon was a transient event. The results do give one more idea on the cause of fluctuations of radiocarbon, which is useful to those who want to use it as a proxy for various geological studies.