After a supernova explosion, for at least tens of thousands of years, its remnants are visible – a spreading gas cloud. By studying the motion of gas, temperature, and other properties, astronomers can also determine the timing of a supernova explosion.
It is known that in the last 40 thousand years, at least 18 supernovae have exploded less than 1,500 parsecs away from Earth. Their gamma radiation, reaching the Earth’s atmosphere, could decompose some nitrogen atoms and convert their nuclei into radioactive carbon.
Carbon – both conventional and radioactive – is incorporated by living organisms, including trees, so that changes in the abundance of radioactive carbon are reflected in their grooves. Several sudden changes in the abundance of radioactive carbon in the Earth’s atmosphere have been known in the past, but no attempt has been made to link them to the timing of specific supernova explosions.
This is exactly what the new study did. Of the 18 supernova explosions, eight left traces of radioactive carbon in the recognizable grooves, and four traces were really significant, exceeding the percentage. The largest peak, even a three percent increase, occurred 12,800 years ago.
Interestingly, that was the beginning of global warming, called Junior Dryas. Whether it was caused by a comet explosion that left the rest of Velha – we can not say for sure, but this study shows that such a hypothesis is at least plausible.
The results of the study are published in the International Journal of Astrobiology; a publicly available (as if a little earlier) version of the article can be found at arXiv.