March 4. Great Ordovician Meteor Shower

Sometime in the early Ordovician, around 470 million years ago, it appears that there was a massive collision between some objects out in interplanetary space. The result on earth was a rain of meteorites, and we know about it because some of the meteorites are preserved. They fell into Ordovician sediments, so we can date the impacts quite well; in Sweden, at one locality, there are about 90 such meteorite fragments, adding up to about 8 kilograms of material.

Wait, you say, how can 90 little chunks and 8 kilograms make for a “Great” meteor shower? Well, such things are incredibly rare, because they represent such instantaneous events. These little pieces didn’t make craters – although there is a probable Late Ordovician crater in Sweden, the Lockne Crater. But there are little grains of the mineral chromite, iron-chromium oxide, that are known to come from extraterrestrial sources. They and a few other fossil meteorites have been found in Ordovician rocks from Scotland and Argentina as well as the big find in Sweden. It probably does represent a global event.

So what? Interesting, but what’s the big deal? Some scientists have linked the meteorite impact event with a couple of other global events. The Buttermere formation in England’s Lake District contains evidence for a massive landslide along what was then a continental margin. John Parnell, at Aberdeen University, suggests that it and 13 similar deposits of similar Ordovician age around the world could have been a response to a rain of meteorites over a relatively short period of time. It may well be possible, but there are many other possible causes of massive landslides in earth history – falling sea level, tectonic activity, and volcanism, to name a few. So the jury is still out on that one.

More intriguingly, Birger Schmitz at the University of Lund, Sweden, and colleagues identified a close correlation in time between the meteorite falls and the onset of the Great Ordovician Biodiversity Event, a surge in life comparable in many ways to the Cambrian Explosion. Why would a disaster, a global-scale set of meteorite impacts, stimulate the diversification of life? The idea is that there was a quick, short, intense period of extinctions, and maybe “sterilization” of habitats, which was followed by a very opportunistic expansion by the survivors into those empty niches, by proliferating and adapting types of animals. There’s nothing like a disaster, at least up to a point, and room to grow to stimulate life into experiments and diversification.

The close timing is interesting, but really doesn’t prove anything. There are many other potential factors in the Ordovician Biodiversity Event, which we’ll talk about tomorrow.

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—Richard I. Gibson

Simon Wellings’ blog - primary resource for this post
Article by Schmitz et al.
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Image is a drawing of the 1833 Leonid meteor shower