Supernova Signals in Tree Rings
By: Gunnar De Winter
Tags: Technology, History, Science, Physics, Space

Supernova Signals in Tree Rings

Can the impact of nearby supernovae be studied by looking at radiocarbon in tree rings?

Remnant of Kepler’s Supernova SN 1604 (Wikimedia commons, NASA/ESA/JHU/R.Sankrit & W.Blair)

Supernova in the sky

The stars might seem eternal, but they too have a limited lifespan. Near the end of their lives, stars go through runaway nuclear fusion. They go ‘boom’. This explosion is known as a supernova.

Supernovae can be as bright in the night sky as an entire galaxy. Big boom.

Following the supernova a star collapses into a neutron star or black hole, or is entirely destroyed — the star’s eventual fate depends upon its mass.

As a consequence of the powerful explosion, a supernova blasts away many of the material that once was a star. Supernovae are an important source of many elements in the space between the stars.

Stellar fireworks that imbue the universe with many useful elements, what’s not to like?

The massive amount of radiation might be a bummer, though.

A supernova’s blinding brightness is merely a side-effect of the incredible amount of energy that is released. Much of that energy lies beyond the visible spectrum, including X-rays and gamma rays.

If a supernova would occur relatively close to earth, we would be in trouble. One of the main initial effects it would have is that the ozone layer would be (partially) stripped away, which would bathe earth in unfiltered solar radiation. Bad idea.

Even supernovae farther away could leave traces.

Supernova in the trees

(Wikimedia commons, Albert Briggs)

Maybe we can spot these traces in tree rings. At least, that is what a new study proposes.

Supernova traces could come in the form of radiocarbon (or carbon-14). This is a type of carbon that is very rare on earth and only occurs in trace amounts. It is delivered by cosmic radiation in a fairly steady flow.

In old tree rings, however, we sometimes see a spike in radiocarbon.

It has been though that these spike represent periods with a lot of solar flares. But this study proposes a new hypothesis: supernovae.

The scientist behind the study was able to align the eight closest supernovae from the last 40,000 years (which we can date based on their remnants, i.e. how far the boom and its material have traveled) with large radiocarbon spikes in ancient tree rings.

Four of these supernovae (Vela, at 12 740 years BP; S165 at 7431 BP; Vela Jr. at 2765 BP; and HB9 at 5340 BP) appear to coincide particularly well with tree ring spikes and changes in temperature and other ecological effects.

This does not mean that solar flares are not a possible explanation, but it does position supernovae as a viable hypothesis that requires further testing, something the author acknowledges in the conclusion:

In conclusion, the presented data and analysis do not rule out solar flare or solar activity hypotheses for rapid-onset 14C increases. However, SNe [supernova event] causation instead is viable for many of them… If SNe instead caused some or all of these events, then part of the solar system radiation hazard to life is from hard photons from nearby SNe, and some events may have produced recorded paleoecological changes.

From exploding stars to tree rings, it’s a wild universe out there.