Aug 30 2010

Did One or Two Impacts Kill the Dinosaurs?

Note: Late post today. I am covering the in-patient service and more time constrained than usual.

By now most people know that the dinosaurs (now clarified as non-avian dinosaurs), along with 85% of species alive at the time, became extinct 65.5 million years ago as a result of a massive meteor impact. This is almost certainly the impact crater at Chicxulub, which dates to the correct time. In addition, examination of fossils and geological layers centers this extinction event at Chicxulub.

This is referred to at the K-T extinction, referring to the end of the Cretaceous and beginning of the Tertiary periods. However, use of the designation “Tertiary” is being phased out, and the K-T extinction is now being referred to as the K-Pg extinction – for Cretaceous-Paleogene.

While the single impact theory is the current consensus, there are two significant if minority competing theories. One is the Deccan Traps flood basalts – a 200 thousand year long event spanning the K-Pg boundary that involved massive volcanic eruptions, which could have causes extinctions through release of dust and sulfuric aerosols into the atmosphere. While not dead, this hypothesis has not fared well under recent evidence and is supported by only a small minority of paleontologists.

Another theory is that there were multiple impacts, two or more, all around the time of the K-Pg extinction. This is based largely on the presence of other craters that date to the same time period. There is also the Boltysh impact crater in the Ukraine, the Silverpit crater in the North Sea, and the Shiva crater. The thinking is that an asteroid or comet may have broken into multiple pieces which showered the earth over a period of time.

A new study published in Geology provides evidence that suggests that the Boltysh crater occurred 2-5 thousand years before the Chicxulub impact. The study team looked at spikes in fern spores in geological layers in the Boltysh crater. Ferns recover quickly after an impact and quickly colonize the devastated area. Therefore a spike in fern spores is a marker for an impact. What they found is that there is a fern spike in the sediment layer that likely resulted from the Boltysh impact itself. And then, 2-5 thousand years later, this layer was itself devastated resulting in another fern spore spike. They believe this second devastation was due to the Chicxulub impact. (Incidentally, scientists who study pollen and spores are called palynologists.)

This is interesting, if indirect, evidence. It still leaves us with the bulk of the evidence showing that the Chicxulub impact was the major cause of the K-Pg extinction, and probably enough to explain it by itself. The complete extinction of non-avian dinosaurs appears to have occurred right at that time. But it is possible that the ecosystem was being stressed by the Deccan Traps eruptions. It is also possible that one or more smaller impacts also contributed to the extinction event.

This is an interesting refinement to the impact theory, and I am mostly interested in how paleontologists make inferences about what happened in the past. The spore spike is a cool line of evidence, one I did not know about before.

9 responses so far

9 thoughts on “Did One or Two Impacts Kill the Dinosaurs?”

  1. clgood says:

    So are these paleopalynologists?

  2. botgrad says:

    Love your blog! But I’ve got to nit-pick.
    Brief intro to plant life cycles:
    Ferns do not have pollen but spores (the result of meiosis) which germinate and grow into gametophytes, the multicellular life stage which produces gametes (via mitosis). In angiosperms and other seed plants (eg conifers) pollen represents the much reduced (but still generally multicellular) gametophyte encased within the old spore cell wall. When gametes fuse they grow into the sporophyte which produces spores (not gametes). The dominant multicellular stage for most land plants is the sporophyte. This life cycle is known as alternation of generations.

  3. CivilUnrest says:

    It seems inevitable that, if humanity is around for any length of time, we’ll have to cope with a major impact of some sort. I wonder how we’ll fair.

  4. bachfiend says:

    Hi Steve,

    I take it that it’s a typographical error when it’s said that the Deccan Traps basalt occurred over a period of 200 million years? I’ve read that it was actually from 68 to 60 MYA, and the majority spanned perhaps 300,000 years.

    I have read two recent popular books; “Dinosaur Odyssey: Fossil Threads in the Web of Life” by Scott Sampson and “Vanished Ocean: How Tethys Reshaped the World” by Dorrik Stow (both of which I strongly recommend). Sampson summarises the theories as either the Silver Bullet (one event) or Blitzkrieg (several events) hypotheses leading to the K-T extinction, favouring one event probably the asteroid theory. Stow favours the volcanic theory, putting out that iridium is also released in volcanic eruptions and shocked quartz can also be produced. The major query would be how explosive the volcanic eruptions were, whether the lava just gradually flowed over the Indian subcontinent or whether it was highly explosive throwing a lot of debris high into the atmosphere?

    The question is; how quickly did the mass extinction occur? 1% of the dinosaurs were around at the time of the K-T boundary, so 99% of dinosaurs had already gone extinct (which isn’t saying much; all species eventually go extinct or evolve into something else). Were the 1% in gradual decline or did they disappear suddenly (with the exception of the avian dinosaurs, which are still doing very well)?

  5. CJKlok says:

    Hi Steve

    Just a minor nitpicking. The Wiewiora paper refers to a change in the ratio of fern spores to angiosperm pollen. Ferns do not produce pollen at all. Angiosperms (conifers, cycads etc.) and flower plants do.

    Some minor editing of the above piece will set the record straight and minimize confusion.

  6. Thanks – I corrected the errors in the original post. Can you tell I was rushed today? 🙂

  7. SpicyCupcake says:

    @Rikki-Tikki-Tavion In the future you can just refer them to Episode one of Phil Plait’s new series Bad Universe! It’s first air was last Sunday at 9pm central/10 Eastern on Discovery Channel (Phil, where are my promotion dollars on this thing? =) ). He beat the stoping the metereo with a nuke subject pretty well.

  8. SpicyCupcake says:

    A note to myself; never post without proof reading.

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