Radioactive traces in tree rings reveal Earth’s history of unexplained ‘radiation storms’

In searching for planets and studying their stars, I have had the privilege of using some of the world’s great telescopes. However, our team recently turned to an even larger system to study the universe: Earth’s forests.

We analyzed the radioactive signatures left in tree rings around the world to study mysterious “radiation storms” that have swept the Earth half a dozen times over the past 10,000 years or so.

Our results, published today in Proceedings of the Royal Society Arule out “solar flares” as the culprit—but the real cause remains unknown.

A story written in tree rings

When high-energy radiation hits the upper atmosphere, it turns nitrogen atoms into radioactive carbon-14, or radioactive carbon. The radiocarbon then filters through the air and oceans, into sediments and swamps, into you and me, into animals and plants—including hardwoods with their annual tree rings.

For archaeologists, radiocarbon is a godsend. Once created, carbon-14 slowly and steadily decays into nitrogen—which means it can be used as a clock to measure the age of organic samples, in what’s called radiocarbon dating.

For astronomers, this is just as valuable. Tree rings give a year-by-year record of high-energy particles called “cosmic rays” dating back millennia.

The magnetic fields of the Earth and the sun shield us from the cosmic rays that shoot through the Milky Way. More cosmic rays reach Earth when these magnetic fields are weaker and fewer when the fields are stronger.

This means that the rise and fall of carbon-14 levels in tree rings encodes a history of the 11-year cycle of the solar dynamo (which creates the sun’s magnetic field) and Earth’s magnetic field reversals.

Miyake Events

But tree rings also record events that we cannot currently explain. In 2012, Japanese physicist Fusa Miyake discovered a spike in the radioactive carbon content of tree rings from 774 AD. It was so big that cosmic rays of several ordinary years must have arrived at the same time.

As more groups joined the investigation, tree-ring evidence of further “Miyake events” was revealed: from 993 AD. and 663 BC, and prehistoric events in 5259 BC, 5410 BC and in 7176 BC

These have already led to a revolution in archaeology. Finding one of these short, sharp spikes in an ancient specimen fixes its date to a single year, rather than the decades or centuries of uncertainty from conventional radiocarbon dating.

Among other things, our colleagues used the event of 993 AD. to reveal the exact year of the first European settlement in the Americas, the Viking village at L’Anse aux Meadows in Newfoundland: 1021 AD.

Could massive radiation pulses happen again?

In physics and astronomy, these Miyake facts remain a mystery.

How do you get such a huge pulse of radiation? A number of papers have blamed supernovae, gamma-ray bursts, explosions from magnetized neutron stars, and even comets.

However, the most widely accepted explanation is that the Miyake events are “solar superflares”. These hypothetical outbursts from the sun would be perhaps 50-100 times more energetic than the largest recorded in modern times, the 1859 Carrington Event.

If such an event were to occur today, it would destroy power grids, telecommunications and satellites. If these happen by chance, about once every thousand years, that’s a 1% chance per decade—a serious risk.

Noisy data

Our team at UQ set out to look at all available tree ring data and work out the intensity, timing and duration of the Miyake events.

To do this, we had to develop software to solve a system of equations that modeled how radiocarbon filters through the entire global carbon cycle, to calculate what fraction ends up in trees in which years, as opposed to the oceans , the swamps or you and me.

In collaboration with archaeologists, we have just published the first reproducible, systematic study of all 98 published Miyake event data trees. We have also released open source modeling software as a platform for future work.

Solar flare storms

Our results confirm that each event yields between one and four ordinary years of radiation in one go. Previous research showed that trees closer to the Earth’s poles recorded a larger spike – which we’d expect if solar flares were to blame – but our work, looking at a larger sample of trees, shows that’s not the case.

We also found that these events can occur at any point in the sun’s 11-year activity cycle. Solar flares, on the other hand, tend to occur around the top of the cycle.

Most puzzlingly, some of the spikes seem to last longer than can be explained by the slow creep of new radiocarbon through the carbon cycle. This suggests that either the events can sometimes last more than a year, which is not expected for a giant solar flare, or the tree growth periods are not as uniform as previously thought.

For my money, the sun is still the most likely culprit for Miyake’s events. However, our results suggest that we are seeing something more like a solar flare storm than a massive superflare.

To pinpoint exactly what is going on in these events, we will need more data to give us a better picture of what we already know. To get that data, we’ll need more tree rings—and also other sources, like ice cores from the Arctic and Antarctic.

This is truly interdisciplinary science. Normally I think of wonderfully clear, precise telescopes: it’s much harder to understand the complex, interconnected Earth.

This article is republished from The Conversation under a Creative Commons license. Read the original article.