May 9. A Devonian cratering event?

In the History of the Earth book, written back in 1994, I had a page for a possible Devonian cratering event when the earth-moon system might have undergone a greater-than-usual bombardment from meteorites. In 1994, Copernicus and other young craters on the moon were thought to have formed as recently as 350 to 400 million years ago, the Devonian. Now age dates put the formation of Copernicus at more like 800 million years ago, or even as old as a billion years ago – still young, as the moon goes, but back in the Precambrian in terms of earth’s time scales. 

Copernicus (NASA photo)

So was there an impact period during the Devonian on earth? It doesn’t look like it. There are a dozen or more known large features that represent Devonian impacts. Some, such as the Siljan Ring in Sweden, are pretty big – at 52 km or 32 miles across, it’s the largest known crater in Europe. Even after much erosion, the edge of the crater is still evident in the topography – a lake fills part of the crater rim. It formed about 377 million years ago, during the Late Devonian.

Some of the Devonian craters have fairly reliable age dates, ranging from about 396 million years ago to about 360 million years ago. Nine well-dated events in 36 million years. Given the error bars in the dating, three of the impacts, including the Siljan Ring, were about 380 million years ago, plus or minus five million years or so. So I don’t think we can call this anything like a “cratering event,” and we probably really can’t say the Devonian has a greater incidence of impacts than any other time in earth history.

We talked about one likely global impact event, 480 million years ago, during the Ordovician. That one was based on multiple fossil meteorites in many locations around the world and it seems reasonable to call that a specific event. 

There’s plenty of research going on to investigate the possibility that there may be some predictable periodicity to impacts on earth – obviously we have a vested interest in knowing that, even if it is on scales of many thousands of years or more. Check out a 2005 paper on this topic. This is challenging work in part because there’s a bias toward information about younger craters, simply because younger craters and associated information are better preserved and often better exposed than the old ones.

For now, I’m going to say that there was no Devonian cratering event, nor even any noteworthy increase in impacts then. But you have to realize two things – first, I’m not an expert on this, and even though I reviewed a lot of papers before making that statement, I could have missed something significant. The other thing to realize – to always realize – is the problem of sampling – the total number of impact craters known on earth is only a couple hundred. That’s really not enough to make any confident, far-reaching conclusions about periodicity or increased occurrence. And remember that the older you get, the more likely that all the evidence of an impact might be eroded away or subducted into the earth.

—Richard I. Gibson

Image credit: NASA

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.

If you have questions about this or any topic, or suggestions for how we can improve the podcasts, please let me know! You can post a review with ideas on the iTunes page for History of the Earth, or add a comment on the blog.

—Richard I. Gibson

Simon Wellings’ blog - primary resource for this post
Article by Schmitz et al.
More info  

Image is a drawing of the 1833 Leonid meteor shower