Tree Rings Reveal Mysterious Solar Activity 7,000 Years Ago

Bristlecone pine trees like this were used in the study. Prof. A.J.T. Jull

By looking at tree rings, scientists have discovered evidence of a powerful and mysterious solar event that happened in 5480 BCE.

An international collaboration measured the level of a carbon isotope, known as Carbon-14 (14C), in tree rings, which is produced by cosmic rays hitting the highest layers of the atmosphere. In a paper published in the Proceedings of the National Academy of Sciences, the researchers discovered a spike in 14C about 7,000 years ago.

Carbon-14 has the same chemical properties as carbon, but it’s slightly heavier because it has two more neutrons than the standard form. It enters the carbon cycle through trees and plants that absorb it from the atmosphere, and it is often used to date archeological findings.

“We measured the 14C levels in the pine sample at three different laboratories in Japan, the US, and Switzerland, to ensure the reliability of our results,” A. J. Timothy Jull of the University of Arizona said in a statement. “We found a change in 14C that was more abrupt than any found previously, except for cosmic ray events in AD 775 and AD 994, and our use of annual data rather than data for each decade allowed us to pinpoint exactly when this occurred.”

It is not easy to work out what the cause is behind the spike, although we know it is related to the strength of the Sun's activity.

"We think that a change in the magnetic activity of the Sun along with a series of strong solar bursts, or a very weak Sun, may have caused the unusual tree ring data," lead author Fusa Miyake of Nagoya University added.

When the Sun is very active, it might generate stronger geomagnetic storms, with a deluge of high-energy particles reaching Earth. A very weak Sun also affects Earth – our magnetosphere shrinks and cosmic rays can reach the atmosphere more easily.

"Although this newly discovered event is more dramatic than others found to date, comparisons of the 14C data among them can help us to work out what happened to the Sun at this time," Miyake said.

There are no detailed descriptions of what the Sun was doing seven millennia ago, so the team hopes that astronomical observations might provide clues to better understand the cause of the spike. Sun-like stars elsewhere might be going through similar cycles.

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