May 16 2017

The Impact That Killed the Dinosaurs

asteroid impactI have loved science as long as I can remember, partly because scientists have the best stories to tell. The stuff that actually happened is usually far more interesting than any fantasy, and has the added bonus of being real. Reality is complex, dramatic, and interesting. Reality is also endlessly surprising.

Lucas wanted to evolve Star Wars. I always thought that was an interesting idea, adding a new layer to the film medium. It would love to see a great director do that well, not only updating special effects, but evolving the story as the culture evolves. Unfortunately, most of the changes Lucas made were crap, watering down character arcs and adding blandness. Some of the extra CG was OK.

Science stories, however, seem to always just get more interesting. That is because the universe has already written the entire script from the beginning, and we are just peeling back the narrative layers one by one. As the story gets deeper, it all makes sense because it has to. Plot twists are never contrived, because they were baked in from the beginning. 

The K-Pg Extinction Event

Think about our understanding of the history of life on Earth and what an interesting story that is. Let’s focus on the dinosaurs – the first description of a dinosaur fossil was in 1824, by William Buckland, who described a megalosaurus (giant lizard). People had probably discovered dinosaur bones previously, including a 1676 discovery of a giant thigh bone, but in the 19th century scientists starting putting together that there was a previous age when giant reptiles ruled the Earth.

These giant reptiles are not around today, so something must have happened to them. Over the next century scientists floated many ideas, all speculative, and none backed by solid evidence. Then, in the 1970s, Luis and Walter Alvarez discovered a thin layer of iridium at what was then known as the K-T boundary (now referred to as the K-Pg boundary) about 66 million years ago. This layer separates the Cretaceous period from the Paleogene period (and also the Mesozoic from the Cenozoic era).

The iridium in this layer is of an isotope that is mainly found in meteorites, not in the earth, so the suggestion here is that a massive meteor containing iridium hit the Earth right at the K-Pg boundary.

That is also the point in geological time when there was a mass extinction, killing 75% of species on Earth including all of the dinosaurs (by which I mean the non-avian dinosaurs, for you cladists). The implication is hard to avoid – the asteroid impact killed the dinosaurs. Scientists estimate that the asteroid was 10 km in diameter. Its impact likely threw up a massive cloud of dust that blocked the sun for months at least, breaking the first link in the food chain and causing ecosystems to collapse. Being large was a particular disadvantage during this time, and few tetrapods over 25 kg survived (only some turtles and crocodilians).

The impact hypothesis was further confirmed when the crater was discovered in the Yucatan peninsula. This not only confirmed the impact, but also provides further analysis of the extinction event. Paleontologists now know where ground zero was, and this has an effect on the fossil layers. Close to the impact, the layers were jumbled by the direct force of the impact. On the opposite side of the planet the dust layer was very thin and therefore provides low resolution for dating fossils relative to each other and the impact.

However, there is a sweet spot in between, where the layers are preserved and thick enough to see clearly where fossils are relative to the impact. When you examine the K-Pg boundary in this sweet spot distance from the impact it is clear – there are dinosaurs right up to the impact, and not beyond. The K-Pg boundary is the extinction event.

But wait, there’s more

The story of the impact and the death of the giant lizards is a good one. It captures the imagination. Further, there is solid evidence to support it. It therefore may be a little surprising to learn that there is a real and enduring controversy over whether or not the dinosaurs were really killed off by the impact.

Here is the problem – the dinosaurs were already going extinct before the asteroid hit. Surveys of the number of dinosaur species and genera show a steady decline leading up to the K-Pg boundary. There is still some controversy about this conclusion as well, with some paleontologists thinking that the decrease is an artifact of the record. The probability of finding a specimen right up to the boundary layer decreases with the rarity of the species, so species should statistically drop off as you get closer to the boundary. This is certainly part of the picture, but it does also seem that dinosaurs were in decline.

However, this doesn’t mean they were going extinct. You can’t just extrapolate trends like that. They may have rebounded in species numbers going forward if the asteroid didn’t hit.

There is also evidence that the  climate was changing, partly due to volcanic activity at the Deccan traps. This is a massive volcano complex in modern day India. They spewed sulphur and dust into the atmosphere and would have made the dinosaurs very unhappy. Some scientist claim that the Deccan traps were killing off the dinosaurs anyway, and the asteroid, at most, was the coup de grace.

But now it seems that the activity at the Deccan traps were not only occurring at the same time as the asteroid impact, the impact itself dramatically increased volcanic activity. It now seems that the asteroid impact and the volcanic activity at the Deccan traps were a “one-two punch” that caused the mass extinction.

I personally disagree with scientists who downplay the role of the impact in the mass extinction. Clearly life was on hard times because of increased volcanic activity, but it also seems clear that the asteroid impact was more than a punctuation mark – it had a major role to play in the mass extinction. Further, it is entirely possible and even probable that the non-avian dinosaurs would have survived even until today without the impact.

Now for a plot twist

There is a new wrinkle in the story. The BBC is reporting (mainly to promote their upcoming series on the topic, but whatever) that scientists have recently discovered that the location of the impact at the K-Pg boundary may have been critical to its deadly effects.

Drilling into the impact crater, geologists have found that it contains a lot of the mineral gypsum (calcium sulfate dihydrate), which contains a large amount of sulphur. When the asteroid hit, therefore, it threw up a large amount of sulphur which was important to the deadliness and density of the resulting dust layer. If that dust layer were not as dense, then light would have returned more quickly, and more species may have survived until the food chain was reestablished.

Gypsum forms by evaporation of sedimentary rock, and so occurs where there is shallow water or where there has been water in the past.

What all this may mean, therefore, is that the precise location of the impact, in the shallow seas of the Yucatan peninsula, may have been critical to its deadliness. If the asteroid struck the deep ocean, it would have had less of an impact on the underlying rocks and threw up less dust. If it struck the middle of a continent, there would have been less gypsum and less sulphur.

A shallow sea was therefore perhaps the deadliest place to strike – too little water to cushion the impact, but enough to allow for high levels of gypsum and therefore sulphur. This further means that if the asteroid struck shortly before or after it actually did the rotation of the Earth would have changed the location of the impact.

This is all a reminder of the contingent nature of history and how fantastically improbable any particular outcome is. Of course, something had to happen, and whatever did happen would be incredibly improbable. Anyone looking back at events would find their own existence less likely than winning the lottery. In a way we have all won the cosmic lottery.

That does not mean that a supernatural force was at work or that we exist by design. Someone had to win. We are just the lucky few who get to exist out of all the possible alternatives that don’t.

Somewhere in the nonexistent probabilities of alternate realities there is a dinosaur happily munching trees because an asteroid was an hour late to the Earth 66 million years ago (maybe).


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