This is a story of the very big and the very small, but more importantly it is an interesting story of how scientists resolve different ideas with research and evidence. I have been following it for years and love to read about new updates.

The burning question is – what happened to the mammoths and other megafauna of North America about 13,000 years ago. Extinction itself is no big deal – most species that have ever lived are extinct, and all species go extinct eventually (at least so far). But this was somewhat of a mass extinction – a continent-wide extinction of large terrestrial mammals.

Theories as to the cause include overhunting by newly arrived human populations, climate change brought on by the transition from the recent glacial period to the current inter-glacial period, and (more recently) the impact of a large comet somewhere over Canada. It seems that overhunting likely was a factor, but not enough by itself to explain the loss of megafauna.

So at present the two main contenders are climate change vs impact. Recently the PBS program, NOVA, showed its new special on the conterversy before an audience of scientists. The program made the case for the impact theory, and revealed some new evidence. Scientists skeptical of the impact theory remain skeptical, however, indicating that this controversy is far from over.

Nanodiamonds

The new evidence revealed in the program is the discovery of nanodiamonds from a 12,900 year old layer of glacial ice in Greenland.  Nanodiamonds are exactly what they sound like – nanometer sized diamonds. They are formed under high temperature and pressure, such as would be present in a meteor or comet impact. They have been found in meteorites. Supporters of the impact theory point to nanodiamonds as a major source of evidence. Researchers have found nanodiamonds from multiple North American sites all dating from12,900 years ago. This North America-wide sediment layer of nanodiamonds is the same kind of evidence as the world-wide layer of iridium 65.5 million years ago is evidence for an impact leading to the mass extinction that claimed the dinosaurs.

Critics, however, charge that the nanodiamonds are not definitively of impact origin (where as the iridium layer at the K-T boundry is clearly an extraterrestrial isotope). They argue that the nanodiamonds could have been produced by earth-bound processes and simply been deposited in those sediment layers where they are found. Or, nandiamonds raining down in micrometeorites could have simply collected in the layers detected. They may be concentrated in a layer because the retreat of the glaciers caused large lake formations, or some similar cause.

This is where the new Greenland evidence come into play. Researchers found nanodiamonds in a 12,900 year old layer of glacial ice. This would be expected from the impact hypothesis, but is difficult to explain with the sedimenting hypothesis. That is, in fact, exactly why they looked there.

Critics

But the other side is not ready to give up yet. Mineralogist Bevan French of the Smithsonian National Museum of Natural History in Washington, D.C. is quoted as saying: “I still don’t think nanodiamonds by themselves are a signature for extraterrestrial material.”

There are also other lines of evidence that appear to contradict the impact theory. This is often how good science works – using multiple independent lines of evidence to triangulate to the most likely answer. In this case Dr Sandy Harrison from the University of Bristol and colleagues looked at evidence for large forest fires in North America from 15,000-10,000 years ago. What they found was evidence for scattered fires, but no continent wide forest fire event 12,900 years ago, as would be predicted by the impact theory.

Many researchers blame the Younger Dryas for the mass extinctions. This was a 1,300 year period of cooling following the warming that occured at the end of the most recent glacial period. The exact cause is unknown, but one theory is that fresh water from glacial melting dumped into the North Atlantic and shut down the salt-driven oceanic currents. But regardless of the cause, this was a period of rapid climate change – perhaps too rapid for wildlife to respond, hence the extinctions.

The Future

What we have are apparently contradictory lines of evidence – nanodiamonds suggest an impact, while the absence of continent-wide forest fires argues against an impact.  This controversy, however, will be resolved with evidence. There are specific questions that need to be answered – Can nanodiamonds be created on earth, or are their only origin meteorites and impacts? Will the nanodiamond layer evidence hold up under further investigation? Why isn’t there evidence for extensive forest fires at the same time?

Also, one bit of missing evidence is a large-enough impact crater in North America from 12,900 years ago. Impact theorists argue that a comet could have exploded above the ground (like with the Tunguska event of 1908), which is plausible – but this can also just be an excuse for the absence of evidence.

Researchers will continue to argue their respective cases, while they think about new predictions that flow from the various theories. They will then test those predictions, and over time one theory will slowly win out over the competitors.

Just like science is supposed to work.