Scientists digging into the dinosaur-killing asteroid crater answer all your questions

A team of researchers is investigating the Chicxulub crater, of the asteroid that wiped the dinosaurs (and many other creatures) some 65,5 million years ago. Now, they’ve set out to Reddit to answer all our questions. Sean Gulick from the University of Texas Institute for Geophysics (UTIG) and Joanna Morgan from the Imperial College London as well as postdoctoral fellow Chris Lowery have been drilling in the crater since April 14 and will spend approximately two months at sea retrieving rock cores before returning to their respective institutions to study the samples. These are some of their most interesting answers.

Sketch of the gravity anomaly map of the Chicxulub crater area. Red and yellow are gravity highs, green and blue are gravity lows, white indicates sinkholes, or “cenotes”, and the shaded area is the Yucatan Peninsula. Image via NASA.

Firstly, a comment:

Not a direct reply to anyone’s question, but I’m just going to leave this here:

http://joidesresolution.org/sites/default/files/Blast%20from%20the%20Past%20Poster.pdf

It’s a little old (you’ll notice that they have the boundary at 65 when we now have it at 66 million years ago), but it’s probably my favorite poster because it shows 1) how the microfossils that I study, which are called foraminifera, changed across the boundary (~90% of the ones that live in the upper water column went extinct), and 2) what the KPg Boundary looks like in deep sea sites, in this case from ODP Leg 171B from off the Atlantic coast of Florida.

(Chris)

Then the questions:

Q:After the impact, how quickly (or slowly) did it kill the dinosaurs?
How did it kill them (most interested about those that didn’t die quickly)?
What other animal and plant life was lost?
How big of a percentage of animal and plant life was lost?

A: Depends where they were. Within about 1500 km from Chicxulub dinosaurs would have been killed instantaneously by thermal radiation, as this impact behaved a bit like a very very large atomic bomb. On the other side of the Earth it could have taken months, following the destruction of much of the primary food source due to fires, as well as dark and cold conditions. 90 % of the near-surface plankton in the oceans, ammonites… about 75% of life in total (Jo)

Q: How sure are you this was their extinction event and what’s the science behind it?

We are very sure that this impact caused the KPg mass extinction, which killed ~75% of all species on the planet, including non-avian dinosaurs, marine reptiles like mosasaurs and plesiosaurs, and other unique marine groups like ammonites. As thattopicishot mentioned a little while ago, we know this because of the unique layer of clay found at the top of Cretaceous sediments worldwide, which contains iridium (very rare on earth; very common in asteroids) and minerals that are formed in impacts like shocked quartz and glass spherules that were blasted out of the crater. We know that this clay comes from the Chicxulub crater very simply because this layer gets thicker the closer you get to the crater. In sites very far away (Europe, Japan, Australia) it’s just a thin layer of clay with iridium; as you get closer (North Atlantic) it gets thicker, with a clear layer of spherules and shocked quartz below a clay layer with iridium; in the Gulf of Mexico this boundary layer can be hundreds of meters thick, and large chunks of rock that were blasted from the crater, meters-thick layers of sand deposited by tsunamis that washed back and forth across the basin, and thin a clay layer with iridium on top.
More generally, we are so confident that this impact was the cause of the mass extinction because we clearly see all these groups range right up to the boundary.
(Chris)

Reddit’s user didn’t waste the opportunity to add some memes to the discussion:

Q: Will you take full responsibility if you awaken an ancient godlike monstrosity from beyond the stars from its timeless slumber?
And on a more serious note, what are you hoping to discover from this impact site? Why is it unique from others?

A: Full responsibility taken. We want to know: how large craters are formed, is there a deep biosphere, how did life recover in the ocean above the crater? It is the most intact large crater on Earth, the only crater with a peak ring, the only impact linked to a mass extinction and the only crater with a global ejecta layer (Jo)

Drawing of the Chicxulub crater, by Blogue Dementia Cratera Chicxulub

About actually intercepting the K-Pg layers, they said:

We reached the K-Pg boundary at 620 m and are now at 840 m. We hope to get down to 1500 m. We are testing models for peak ring and large crater formation, we think peak rings are formed by rocks that were pushed down 20-30 km, then rebounded upwards above the Earth surface, and collapsed downwards and outwards about 40 km. We are also taking microbiological samples to see if there is a living a deep biosphere, and looking to see how life recovers after the impact in the ocean above the crater? (Jo)

Q: Do you expect to find fragments of the impactor, or are you mainly interested in an analysis of the crater and subsequent layers?

A: About 95% of the meteor was vaporized on impact, and ejected all around the Earth in the expanding vapor plume. That’s how we discovered the impact occurred in the first place – we found the meteoritic material in the K-Pg boundary layer all around the Earth. We are mainly interested in other things, how are large craters formed, is there a deep biosphere, how did life recover in the ocean above the crater? (Jo)

Some questions were on point:

  • What is the magnitude of seismicity associated with the impact?
  • Everyone is very interested in the impacts effects on life and the K-Pg mass extinction, but what about tectonics?, I am interested what effect an impact of this size who have on plate kinematics, and if the Impact caused plate motions to change, or far field changes in deformation styles along plate boundaries.
  • How deep is the Crater, and by using paired thermochronometers is it possible to reconstruct the thermal/cooling history of the impact site? or will they likely be reset by later burial?
  • How is the age of impact constrained? I assume mostly by stratigraphic dating of the Iridium layer found around the world, But do you have any plans to directly date the Impact? is it possible to use zircons quenched during impact? also would those zircons/other minerals produced during impact have a distinct geochemical/isotopic signature?

Great questions: -Magnitude 12 or even 13 have been estimated such that all the continental shelves around the Gulf of Mexico collapsed due to the seismic energy -So its not thought that the impact changed anything in terms of tectonics over the long term, but the amount of energy released might have induced some seismicity on tectonic boundaries. -The crater when it initially formed was 100 km wide and ~30 km deep but it rapidly (within minutes) rebounded and collapsed such that the final crater floor of the much larger final crater(~200 km) was only ~1-1.5 km deep. We are definitely looking to use thermochronometers to study the history of the thermal effects and cooling. -Age of the impact is constrained through magnetic stratigraphy, biostratigraphy, and most recently thermochronology. In fact it defines the boundary in the geologic timescale between Mesozoic and Cenozoic. -Jousting in June in Indiana and in July in Germany :-)

Sean

Q: since the crater has been there for 66 million years, what made you suddenly decide to probe it now?
Thanks and good luck!

Well, for the first 65,999,953 years we didn’t have a scientific ocean drilling program, so really we got here as quickly as we could. (Chris)

Q: How controversial is the Chicxulub impact theory in your field? Or is it generally uncontested and accepted as a fact?

Good question on our certainty! In the geological community the impact hypotheses is by far the most supported and with the best evidence amongst the theories. It explains the truly sudden nature of the event and direct evidence is present in terms of the iridium anomaly precisely at the extinction event. If I had to put a number on it greater than 95% of our community is convinced of the scientific evidence that the Chicxulub impact was the cause of the end Cretaceous mass extinction event. If you know scientists then that is saying something! We would like use the word Theory to now describe this conclusion. -Sean

I think it’s pretty widely accepted. There’s a (vocal) group that has been flying the flag for various alternate hypotheses for a while, but most of the community is on board with the impact theory. This paper does a pretty good job explaining the evidence and why competing theories are probably wrong. I think it’s written at a pretty generalist level, but I have a PhD in this and so I’m probably not the best judge.

http://eesc.columbia.edu/courses/w4937/Readings/Schulte.etal.2010.pdf

(Chris)

Lastly, Reddit’s users posed another interesting question about the availability of the resulting data.

Q: Will there be any open hole logging done in addition to core recovery, and will this data be made publicly available?

Yes we are open hole logging each interval that is exposed before we then case and move on to greater depths. All of our data collected out here will be publicly available through the International Ocean Discovery Program after the moratorium ends. – Sean

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