New Study of the K-Pg Extinction Event

Everybody’s favorite extinction event is the end of the age of the dinosaurs.  Like so many popular mystery stories, there are too many clues.   (“In the Library with the Candlestick” or maybe “in the Parlor with the Lead Pipe”.)  An asteroid (or comet), the Deccan volcanoes, and plenty of less spectacular factors may have played a role.

In recent years, more detailed evidence is accumulating about the Chicxulub impact, which was certainly a very bad day for Dinosaurs and everyone else. (this, this, this, this, this)   But massive vulcanism was happening in India at the time, and the climate certainly changed rapidly, which probably involved many interacting factors and feedback loops.

So what happened to the Dinosaurs at the end, at the Cretaceous-Paleogene boundary?

This year an international team of researchers report on detailed modelling of global paleoclimate and the fossil record for this period, in an effort to untangle the multiple events [1].

“That both volcanism and the impact event occurred within several hundred thousand years of the K/Pg extinctions is beyond reasonable doubt.” ([1], p. 1)

As in any good mystery, the solution (or at least our best estimate) depends on timing.

The researchers conclude that the massive Deccan Traps vulcanism started before the K-Pg boundary, and continued throughout and after the impact.  However, they find little evidence of extinction or exceptional climate change before the impact.

On the other hand, the impact event created a huge, short lived “nuclear winter”, during which many species died out. This extinction dramatically altered the biology of the Earth and oceans, producing “biologically amplified carbon cycle change “.  The effects of the continuing volcanism would have contributed to long recovery from the disaster.

Interestingly, these two catastrophes may have worked against each other, moderating the effects over the next million years.  I.e., the impact caused massive global cooling, while the ongoing vulcanism was causing massive global warming.

“Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from post-extinction volcanism.”  ([1], p.1)

It goes to show that it’s not necessarily easy to wipe out a whole planet, or expunge all the life on an ocean world like ours.

Obviously, these findings depend on complex models of the Earth, as well as still limited data. This looks pretty solid, but we’ll have to see if additional data or alternative models support these conclusions in the future. The estimates of global temperature and biology are known from a relative few samples, so understanding may change with more data.

My own non-expert caveat is that this sort of modelling can be very sensitive to artifacts such as the granularity of time and space. In other words, if and when we can redo this work at 100 or 1000 times finer grain, the could be surprises (such as, this [3]).

Assuming these results hold up, it looks like the Chicxulub event killed the Dinosaurs, which is surely the most satisfying way conceivable to tap out old T. rex and company.

And, as I have said before, our species, indeed our whole extended family, truly are Children of Chicxulub.

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1. Pincelli M. Hull, André Bornemann, Donald E. Penman, Michael J. Henehan, Richard D. Norris, Paul A. Wilson, Peter Blum, Laia Alegret, Sietske J. Batenburg, Paul R. Bown, Timothy J. Bralower, Cecile Cournede, Alexander Deutsch, Barbara Donner, Oliver Friedrich, Sofie Jehle, Hojung Kim, Dick Kroon, Peter C. Lippert, Dominik Loroch, Iris Moebius, Kazuyoshi Moriya, Daniel J. Peppe, Gregory E. Ravizza, Ursula Röhl, Jonathan D. Schueth, Julio Sepúlveda, Philip F. Sexton, Elizabeth C. Sibert, Kasia K. Śliwińska, Roger E. Summons, Ellen Thomas, Thomas Westerhold, Jessica H. Whiteside, Tatsuhiko Yamaguchi, and James C. Zachos, On impact and volcanism across the Cretaceous-Paleogene boundary. Science, 367 (6475):266, 2020. http://science.sciencemag.org/content/367/6475/266.abstract
2. Lucas Joel, Meteorite or Volcano? New Clues to the Dinosaurs’ Demise, in New York times. 2020. https://www.nytimes.com/2020/01/16/science/dinosaurs-extinction-meteorite-volcano.html
3. Jun-xuan Fan, Shu-zhong Shen, Douglas H. Erwin, Peter M. Sadler, Norman MacLeod, Qiu-ming Cheng, Xu-dong Hou, Jiao Yang, Xiang-dong Wang, Yue Wang, Hua Zhang, Xu Chen, Guo-xiang Li, Yi-chun Zhang, Yu-kun Shi, Dong-xun Yuan, Qing Chen, Lin-na Zhang, Chao Li, and Ying-ying Zhao, A high-resolution summary of Cambrian to Early Triassic marine invertebrate biodiversity. Science, 367 (6475):272,  2020. http://science.sciencemag.org/content/367/6475/272.abstract

More Chicxulub Results

In the last four decades, we have learned about the Chicxulub impact which seems to have caused a mass extinction and the end of the dinosaurs.  While it is easy to see that being hit by a mile-wide chunk of rock is bad, it has taken some work to figure out just how it could wipe out so many species all around the world, land, sea, and air.

This year, several studies report detailed models of the impact.  A sample core from the Chicxulub crater indicates that the sediments impacted included very large amounts of Sulfur, which would have ejected into the atmosphere and even the stratosphere and above.  In addition to the short term effects, the Sulfur would have been suspended as particulates, with world wide effects on climate akin to a nuclear winter scenario.

This fall, an international team of researchers report a study of fossil records of marine species [1]. These species are highly sensitive to pH, and thus mark the overall acidity of the ocean.  The study shows that populations were stable for 100,000 years (despite the Deccan vulcanism), but plunged dramatically right at the time of the  Chicxulub impact.  This suggests that the massive ejections of Sulfur into the atmosphere led to instant acidification of the ocean from fallout and acid rain.

These findings indicate that there was a massive die off of plankton, which would account for die off of many marine species which depend on them.  It also dramatically changed the Carbon cycle.

The study indicates that surface waters rapidly acidified and rapidly bounced back.  The latter would reflect the lack of plankton which absorb Carbon and Calcium from the water.  Modelling these effects at depths, the researchers hypothesize that the impact caused a substantial reduction, but not a complete collapse of plankton.  The oceans did not recover for 100,000 years or more.

This research does not support hypothesized role of outgassing from the Deccan traps, or any other increase in CO2.  These effects are “overwhelmed by the biogeochemical effect of extinction” ([1], p 3)

These results certainly make sense, especially in light of other recent research.  This is a nice, careful study.  Of course, this is based on limited data, and I can’t evaluate the models in detail.

If these hypotheses hold up, we begin so see that the Chicxulub impact did, indeed, kill off the dinosaurs and many other species.  At least part of the disastrous effect was due to the luck of where the object hit, in shallow water with Sulfur rich sediments.  The research suggests that such an impact would cause massive damage, around the world, and the fallout would cause catastrophic changes to the atmosphere and oceans which cause the extinction of many plankton and plants. Larger animals would quickly starve.

Further, these changes lasted for hundreds of thousands of years.  The Earth did, indeed, recover from this sudden perturbation, but that process took millennia.

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1. Michael J. Henehan, Andy Ridgwell, Ellen Thomas, Shuang Zhang, Laia Alegret, Daniela N. Schmidt, James W. B. Rae, James D. Witts, Neil H. Landman, Sarah E. Greene, Brian T. Huber, James R. Super, Noah J. Planavsky, and Pincelli M. Hull, Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact. Proceedings of the National Academy of Sciences:201905989, 2019. http://www.pnas.org/content/early/2019/10/15/1905989116.abstract
2. Lucas Joel, The Dinosaur-Killing Asteroid Acidified the Ocean in a Flash, in New York Times. 2019: New York. https://www.nytimes.com/2019/10/21/science/chicxulub-asteroid-ocean-acid.html