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Carbon-14 Spiked Worldwide Over 1200 Years Ago, And The Sun Is To Blame

by | Apr 9, 2020 | New, News

Do not let making a living prevent you from making a life.

— John Wooden

Despite violent events like flares, coronal mass ejections, sunspots, and other complex physics occurring in the outer layers, the Sun’s interior is relatively steady: producing fusion at a rate defined by its interior temperatures and densities at every internal layer. However, these surface dynamics can have enormous effects on a star’s planets, including here on Earth. (NASA/SOLAR DYNAMICS OBSERVATORY (SDO) VIA GETTY IMAGES)

In 774/775, tree rings show a spike in carbon-14 unlike anything else. At last, scientists think they know why.

Every once in a while, science gives us a mystery that comes as a complete surprise. Typically, when we slice open a tree and examine its rings, we discover three different forms of carbon in each ring: carbon-12, carbon-13, and carbon-14. While the ratios of carbon-12 and carbon-13 don’t appear to change with time, carbon-14 is a different story. Its abundance slowly decays with a half-life of a little over 5,000 years, with a typical variation of about 0.06% from year-to-year in the rings.

But in 2012, a team of Japanese researchers were analyzing tree rings dating to the years 774/775, when they noticed an enormous surprise. Instead of the typical variations they were used to, they saw a spike that was 20 times larger than normal. After years of analysis, the unlikely culprit has finally been revealed: the Sun. Here’s the scientific story of how we know.

An illustration of a protoplanetary disk, where planets and planetesimals form first, creating ‘gaps’ in the disk when they do. As soon as the central proto-star gets hot enough, it begins blowing off the lightest elements from the surrounding protoplantary systems. The pre-solar nebula likely consisted of all sorts of radioactive isotopes, but ones with short half-lives, like carbon-14, are all gone by the present day. (NAOJ)

Long ago, our Solar System formed from a molecular cloud of gas. Embedded amidst the hydrogen and helium left over from the Big Bang were the full suite of heavy elements making up the remainder of the periodic table, returned to the interstellar medium from the corpses of previous generations of stars. Prominent among those elements was carbon, the fourth most common element in the entire Universe.

Most of the carbon that exists on Earth, formed from that long-ago event, is carbon-12, consisting of six protons and six neutrons in its nucleus. A tiny fraction of our carbon, about 1.1%, is in the form of carbon-13, with one extra neutron compared to its more common carbon-12 counterpart. But there’s another form of carbon that’s not only rare but unstable, carbon-14 (with two extra neutrons over carbon-12), that holds the key to unlocking this mystery.

All carbon atoms consist of 6 protons in their atomic nucleus, but there are three main varieties that exist in nature. Carbon-12, with 6 neutrons, makes up the most common form of stable carbon; carbon-13 has 7 neutrons and makes up the remaining 1.1% of stable carbon; carbon-14 is unstable, with a half-life of a little more than 5,000 years, but is constantly being formed in Earth’s atmosphere. (PUBLIC DOMAIN IMAGE)

Unlike carbon-12 and carbon-13, carbon-14, with six protons but eight neutrons in its nucleus, is inherently unstable. With a half-life of a little more than 5,000 years, carbon-14 atoms will decay into nitrogen-14, emitting an electron and an anti-electron neutrino when the decay occurs. Any carbon-14 atoms that were created prior to the formation of Earth would have all decayed away long ago, leaving none of them behind.

But here on Earth, we do have carbon-14. Approximately 1 out of every one trillion carbon atoms have eight neutrons in them, indicating that there must be some way for those unstable isotopes to be produced on Earth. For a long time, we knew that carbon-14 existed, but we didn’t understand its origin. In the 20th century, though, we finally figured it out: carbon-14 comes from high-energy cosmic particles that collide with our world.

Cosmic rays, which are ultra-high energy particles originating from all over the Universe, strike protons in the upper atmosphere and produce showers of new particles. The fast-moving charged particles also emit light due to Cherenkov radiation as they move faster than the speed of light in Earth’s atmosphere, and produce secondary particles that can be detected here on Earth. (SIMON SWORDY (U. CHICAGO), NASA)

From sources such as the Sun, the stars, stellar corpses, black holes, and even galaxies outside the Milky Way, space is flooded with these high-energy particles known as cosmic rays. Most of them are simple protons, but some are heavier atomic nuclei, others are electrons, and a few are even positrons: the antimatter counterpart of electrons.

Regardless of their composition, the first thing a cosmic ray will collide with when it encounters the Earth is our atmosphere, which leads to a cascading chain reaction of interactions. A variety of new particles will be produced, including photons, electrons, positrons, unstable light particles like mesons and muons, and more familiar particles like protons and neutrons. In particular, the neutrons are incredibly important for the production of carbon-14.

Cosmic ray shower and some of the possible interactions. Note that if a charged pion (left) strikes a nucleus before it decays, it produces a shower, but if it decays first (right), it produces a muon that will reach the surface. Many of the ‘daughter’ particles produced by cosmic rays include neutrons, which can convert nitrogen-14 into carbon-14. (KONRAD BERNLÖHR OF THE MAX-PLANCK-INSTITUTE AT HEIDELBERG)

Most of Earth’s atmosphere — about 78% — is made up of nitrogen gas, which itself is a diatomic molecule made of two nitrogen atoms. Each time a neutron collides with a nitrogen nucleus, which consists of 7 protons and 7 neutrons, there’s a finite probability that it will react with that nucleus, replacing one of the protons. As a result, a nitrogen-14 atom (and a neutron) transforms into a carbon-14 atom (and a proton).

Once you produce that carbon-14, it behaves just like any other atom of carbon. It readily forms carbon dioxide in our atmosphere, and mixes throughout the atmosphere and the oceans. It gets incorporated into plants, consumed by animals, and readily makes its way into living organisms until it reaches equilibrium concentrations. When an organism dies (or a tree ring is fully formed), no new carbon-14 enters it, and so all the existing carbon-14 slowly but steadily decays away.

If one knows how carbon-14 decays and can measure how much carbon-14 (relative to carbon-12) is present today, it’s straightforward to learn how much carbon-14 was present when a specific event occurred in a ‘fossilized’ relic from the past. (EXETERPAUL / WIKIMEDIA COMMONS)

When you hear the term “carbon dating,” this is what scientists are referring to: measuring the carbon-14 to carbon-12 ratio. If we know what the initial carbon-14 to carbon-12 ratio was when an organism was alive (because it only varies by ~0.06% from year-to-year, typically), and we measure what the carbon-14 to carbon-12 ratio is today (where some of it has decayed away due to its unstable, radioactive nature), we can infer how long it’s been since that organism stopped uptaking carbon-14.

As far as we can tell, the levels of carbon-14 remained roughly constant throughout the world throughout the past few millennia. The only known fluctuation in this pattern, at least as of the early 2010s, was from the detonation of nuclear weapons in the open air. And yet, in 2012, we got a scientific shock: in approximately the year 774/775, two independent cedar trees in Japan were analyzed for carbon-14 in their rings, and saw a tremendous spike that was some 20 times larger than natural variations could explain.

The colored dots with error bars show the C-14 data measured in Japanese (M12) and German (Oak) trees along with the typical profile for the instant production of C-14 (the black curve). Note how large the ‘spike’ is in 774/5 compared to the previous years and the uncertainties. (ISOSIK / WIKIMEDIA COMMONS)

The only natural explanation that makes any sense is if, right around that time, Earth experienced an excessive bombardment of these cosmic rays, creating a spike in the amount of carbon-14 that gets created. Even though it’s a small excess in absolute terms — just 1.2% more carbon-14 than normal — it’s far above any natural variation that we’ve ever seen.

Moreover, it’s a spike that’s subsequently been confirmed to exist in tree rings around the world, from Germany to Russia to New Zealand to the United States. The results agree across countries, and could be explained by anything from increased solar activity to a cosmic flare to a direct hit from a distant gamma-ray burst. But the carbon-14 evidence has subsequently been joined by a few other historical and scientific peculiarities, and with the latter enabling us to solve the mystery.

The northern lights (aurora borealis) from the Arctic Circle on March 14, 2016. The rare purple color can sometimes be created in aurorae near the poles, as a combination of blue and red emission lines from atoms can create this uncommon sight along with the more typical green. Red aurorae on their own, while unusual, do occur as well, and could be reasonably described as a ‘crucifix’ under the right conditions. (OLIVIER MORIN/AFP/GETTY IMAGES)

Historically, a “red crucifix in the heavens” was recorded in the Anglo-Saxon Chronicle of 774, which could correspond to either a supernova (no remnant has ever been found) or to an auroral event. In China, an anomalous “thunderstorm” was recorded in 775, so notable that it was the only such event recorded.

But scientifically, the tree-ring data has been joined by ice core data from Antarctica. While the tree-rings show a spike in carbon-14 in 774/775, the ice core data shows a corresponding spike in radioactive beryllium-10 and chlorine-36, which suggest an association with a strong, energetic event of solar particles. An event such as this would have perhaps been on par with the now-famous Carrington event of 1859, which is the largest recorded solar storm in recent history, with the historical data remaining consistent with this explanation as well.

The carbon-14 data (center) along with the associated spikes in the ice core data of beryllium-10 (top) and chlorine-36 (bottom) are all consistent with a proton-rich solar flare event for the origin of this excess in 774/775. (FLORIAN MEKHALDI ET AL., NATURE COMMUNICATIONS 6, 8611 (2015))

Two other events have subsequently been unearthed that could show similar spikes in these isotopes: a slightly weaker outburst in 993/4 and an even earlier one dating back to ~660 BCE. The combined data from all three events point to a common origin that necessarily involves a large flux of protons in a specific energy range.

This is consistent with a relatively common event seen in the Sun: the ejection of solar protons. However, it’s not consistent with the gamma-ray burst scenario, which cannot produce the necessary proton flux to explain the beryllium-10 simultaneously. The same Japanese team that initially suggested the gamma-ray burst explanation for the 774/5 tree-ring data, upon conducting their own measurements of the 993/4 event, concluded:

it is highly possible that these events have the same origin. Considering the occurrence rate of [carbon-14] increase events, solar activity is a plausible cause of [these] events.

A solar flare from our Sun, which ejects matter out away from our parent star and into the Solar System, is a relatively typical event. However, a large-magnitude, proton-rich flare could indeed cause the spikes we’ve seen in carbon-14 and other isotopes in the past, and do a whole lot of damage to our infrastructure in the process. (NASA’S SOLAR DYNAMICS OBSERVATORY / GSFC)

Every so often, the Sun does eject energetic particles right in the direction of Earth. Sometimes the Earth’s magnetic field deflects them away, other times it funnels these particles down into our atmosphere. When they arrive, they can create aurorae, disrupt our local magnetic fields, and — if we’re technologically advanced — they can induce all sorts of current in our electrical grids and devices, potentially causing trillions of dollars worth of infrastructure damage.

We now know there are a variety of solar events that impact Earth, and that the largest-magnitude events we’ve experienced occur more than once-per-millennium. We cannot predict when the next one will arrive, but it’s certain that the consequences for human society will be greater than they’ve ever been when it comes. Carbon-14 levels will certainly spike again in the future, but when that happens, a lot more than tree rings and ice cores will be affected. It’s up to us, collectively, to decide how we’ll prepare.

Starts With A Bang is now on Forbes, and republished on Medium on a 7-day delay. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Carbon-14 Spiked Worldwide Over 1200 Years Ago, And The Sun Is To Blame was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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