Evaluating the Chicxulub Impact Dinosaur Extinction Hypothesis

Science Magazine claimed the Mexican crater named Chicxulub is the smoking gun of the dinosaurs’ demise, and the media fell in line.  Are there reasons to doubt the story?

The media responses to Science’s two evolution-promoting papers (Chicxulub date, and earliest mammal ancestor) were swift and uncritical (see 2/07/13 entry).  Yet a look at the original papers reveals problems, and some thought recalls old questions:  Why would an impact (or volcanism for that matter) eliminate all the dinosaurs of all sizes, but not many other more delicate species that survived unharmed?

The new evidence is mostly circumstantial, primarily the date of the impact at 66.038 million years, very close to when evolutionists believe dinosaurs went extinct, corresponding to the the Cretaceous-Tertiary (K-T) boundary, dated at 66.048 million years ago.  To achieve the alleged five-significant-figure resolution, some uncooperative argon-argon dates had to be brought into agreement with uranium-lead (U-Pb) dates.  “Recent studies have shown why the argon isotope method always yielded slightly younger ages than the U/Pb method,” Heiko Pälike wrote for a Perspective piece in Science about the main paper by Paul Renne et al.  Pälike used this alleged agreement (whether forced or not) to calibrate another dating method, orbital eccentricity changes.  How this could be linked to a dinosaur extinction event, or to a meteor’s choice of when to hit the earth, was not explained.

Pälike left the question of the cause of the dinosaur extinction still unanswered:

Does this study finally close the debate about the cause of the demise of the dinosaurs? Not quite yet. Renne et al. suggest that brief cold snaps in the late Cretaceous were stressful to an ecology adapted to the long-lived preceding hothouse climate, and that the Chicxulub impact delivered a final coup de grâce to ecosystems, shifting the planet permanently into a new state. The next task will be to use the improved dating methodologies to precisely date the largest individual magmatic events during Deccan flood basalt formation. These dates will help to evaluate the role volcanism played in the initial onset of environmental and biotic change prior to the K-Pg boundary.

At the outset, it does seem a bit strange to invoke disasters to advance evolution, but that’s what Renne’s team claimed:  “Mass extinctions manifest in Earth’s geologic record were turning points in biotic evolution.”  The other Science paper claims that mammalian diversity exploded after this catastrophe.

Renne et al., however, only dated the impact to a claimed resolution of plus or minus 32,000 years, still leaving some room for dispute about the cause.  Science Daily and the BBC News quoted Renne claiming the two events (impact and extinction) are “within a gnat’s eyebrow” of being contemporaneous.  The main paper is mostly a story built on the claimed high-resolution date of the Chicxulub impact:

Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress. [Abstract]….

We suggest that the brief cold snaps in the latest Cretaceous, though not necessarily of extraordinary magnitude, were particularly stressful to a global ecosystem that was well adapted to the long-lived preceding Cretaceous hothouse climate.  [Conclusion]

It would seem, though, that the stress would have applied just as much to other ecosystems as to those of dinosaurs.  Moreover, Renne et al. seem to forget that some dinosaurs lived in the polar regions; they occupied a wide range of ecosystems from deserts to the poles (see 1/10/2012 about titanosaurs in the antarctic).  Despite the claimed high-resolution dates, all the other criticisms of the impact hypothesis still apply (e.g., 1/21/2008 #9, 1/29/2009 #1, 3/29/2006).

On Live Science, the son of impact-theory-founder Luis Alvarez turned his father’s apparent vindication into a rebuke of Lyellian geology and a stubborn scientific community unwilling to think outside the paradigm:

“It flew in the face of the position that geologists and paleontologists at the time had for gradual explanations for everything that happened in the Earth’s past, a position that went by the name of uniformitarianism,” said Walter Alvarez. “The notion that this mass extinction was caused by an impact, or even the notion that there was a sudden mass extinction, raised a lot of dispute at the time, and people strongly challenged the idea.“

Renne himself admitted to being a skeptic at first.  It remains to be seen what skeptic Gerta Keller will say, since she had gone on record claiming the impact was too late after the extinction by 300,000 years (10/12/2006, 4/29/2009).  And Alvarez himself admitted that the impact may not have been the sole cause of the extinction, Live Science noted.  He believes, though, that uniformitarianism is done; “we now have a completely different view of how the Earth works in terms of gradual changes versus catastrophic changes.”  That fact would seem to have a wider “impact” on science than a meteor on earth.  National Geographic liked the impact theory, but reserved a larger role for the Indian volcanoes (the Deccan Traps) than other articles.

In short, then, the whole impact-extinction scenario is based on alleged higher-resolution radiometric dates of two events: the Chicxulub impact and the KT boundary.  “Considering the statistical errors in the two analyses, the impact and the dino die-offs may have occurred at the same time, or they may have occurred no more than 32,000 years apart, Science NOW said.  With that in mind, it may be premature to have headlined the article, “Big Smash, Dead Dinos.”

More weight is being put on this claim than it can bear.  Just because scientists, by force of motivation, can bring one flawed, assumption-laden dating method into conformity with another flawed, assumption-laden dating method does not mean an impact actually killed the dinosaurs.  It is just as valid to assert (using their own assumptions and error bars) that the dinosaurs died out 32,000 years before the impact, or vice versa.

Philosophically, it is a poor argument to claim that an impact killed the dinosaurs, “although other factors may have played an important role.”  OK, what other factors?  What roles did they play–major roles, cameo appearances or bit parts?  Did the combination of other roles overpower the impactor’s role?

None of these confident storytellers is explaining why an impact selectively killed off all dinosaurs but not mammals, birds, and insects, much less how catastrophes generated elephants, giraffes and whales out of shrews.  It may be a shrew’d myth, but not a fact of science (since no scientist was there to watch).  Is this a new tale of the taming of the shrew, or the shaming of the true?