Map of gravity anomalies in the Chixculub crater. The circular portion is ~180 km across. The white line is the superimposed coast of the Yucatán Peninsula. Image from the Geological Society of Canada.
There are plenty of scientists who disagree with the Chicxulub-extinction link. One of the most prominent is Gerta Keller from Princeton, whose work has been in the media in recent years (see Science Daily, for example). In 2004, the Geological Society of London hosted a written debate between Keller and some pro-bolidists (found here) – and she has advanced her case since then. Keller disputes the causation based largely on sedimentary deposits in the crater itself, in northern Mexico and Texas, and elsewhere. Keller and her coauthors report locales where the interpreted Chicxulub impact ejecta layer significantly underlies the last occurrence of Cretaceous microfossils; thus, they interpret as much as 300,000 years between the impact and the mass extinction. Strictly speaking, Keller’s work, especially pre-2009, supports either multiple impacts or the flood basalt volcanism of the Deccan Traps as likely culprits, but in her recent work she more directly invokes the volcanism. Schulte and coauthors cite a number of sedimentological and paleontological studies that disagree with Keller’s interpretations: for example, Bralower, one of the 41 authors, leads a new Geology paper providing suggesting that the Keller work grossly underestimates sedimentation rate in the deposits proximal to Chicxulub; Schulte et alia also cite studies that interpret some of Keller’s deposits to be reworked and therefore not representative of primary depositional relationships.
Unsurprisingly, opponents of the Chicxulub-extinction link, like Norman Macleod (quoted here), suggest that the authors don’t address key counter evidence even while the authors claim they’ve cinched the case. I’m not in the best position to evaluate the sedimentological arguments, and there are other lines of arguments I won’t address here, but rest assured they are plentiful. My reading of the literature suggests that most scientists now agree that there was a large impact near the time of the extinction, and the disputes tend to be over the exact timing and the effects, and whether the effects of volcanism superseded or added to the impact’s effects. Likewise, most scientists who agree with Schulte and others probably recognize that extinction events are complicated – for instance, that dinosaurs were fading out for millions of years before the end of the Cretaceous. As the media is prone to do, they’ve exaggerated the finality of this paper’s assertions, and we'll hear much more about the topic. In the meantime, I find Schulte study’s conclusions to be convincing, namely that while there are environmental models that need work:
…alternative multi-impact or volcanic hypotheses fail to explain the geographic and stratigraphic distribution of ejecta and its composition, the timing of the mass extinction, and the scale of environmental changes required to cause it.
Bryan at In Terra Veritas discusses the Schulte study in the first relevant post to appear on Research Blogging. While he’s open to the Chicxulub hypothesis, he objects to parts of the paper and to the media’s coverage of the debate based on [1] the fact that there’s nothing new in the paper, [2] statistical issues with the fossil record, and [3] the problem of assigning causation to correlation. (He makes other lucid points and I encourage you to read his post and not rely only on my simplified rendition. To his credit, he also includes some evidence that would convince him of the bolide-extinction link). His point [1] is trivially correct because it’s a review paper, but I don’t think that diminishes its value as there is probably enough work in this field to support a yearly review. On Bryan’s points [2] and [3], I think he may be correct in the strict sense but I differ on the practicalities. There are sampling problems with the fossil record such as the Signor-Lipps effect Bryan invokes, but I don’t accept that this means we can’t build a case for an extinction cause that is valid to a reasonable degree of scientific certainty (especially if the model invokes a single cause finishing the event and allows for earlier effects to set the stage). And it’s true that [3] correlation doesn’t prove causation – that’s something scientists remind each other frequently. Moreover, we could go back to Karl Popper to remember that, in terms of the rigid philosophy of science, we can only disprove causations and never prove them. It all depends how rigorously we use the word “prove”. The equivalent of “proof” in our everyday scientific parlance is the accumulation of evidence that supports a theoretical framework and the falsification of other causes. Part of that framework, of course, has to include mechanisms that tie a cause to an effect. These frameworks are always subject to re-evaluation, but strong and multiple correlations with plausible linking mechanisms are how we demonstrate causality in geology. We may not be there yet with Chicxulub, but it looks to me like the burden of evidence shifted towards the skeptics some time back.
Articles cited:
Alvarez, L., Alvarez, W., Asaro, F., & Michel, H. (1980). Extraterrestrial Cause for the Cretaceous-Tertiary Extinction Science, 208 (4448), 1095-1108 DOI: 10.1126/science.208.4448.1095
Bralower, T., Eccles, L., Kutz, J., Yancey, T., Schueth, J., Arthur, M., & Bice, D. (2010). Grain size of Cretaceous-Paleogene boundary sediments from Chicxulub to the open ocean: Implications for interpretation of the mass extinction event Geology, 38 (3), 199-202 DOI: 10.1130/G30513.1
Hildebrand, A. R., Penfield, G. T., Kring, D. A., Pilkington, M., Camargo Zanoguera, A., Jacobsen, S. B., and Boynton, W. V. (1991). Chicxulub Crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology, 19 (9), 867-871.
Keller, G., Adatte, T., Juez, A., & Lopez-Oliva, J. (2009). New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico Journal of the Geological Society, 166 (3), 393-411 DOI: 10.1144/0016-76492008-116
Article reviewed:
Schulte, P., Alegret, L., Arenillas, I., Arz, J., Barton, P., Bown, P., Bralower, T., Christeson, G., Claeys, P., Cockell, C., Collins, G., Deutsch, A., Goldin, T., Goto, K., Grajales-Nishimura, J., Grieve, R., Gulick, S., Johnson, K., Kiessling, W., Koeberl, C., Kring, D., MacLeod, K., Matsui, T., Melosh, J., Montanari, A., Morgan, J., Neal, C., Nichols, D., Norris, R., Pierazzo, E., Ravizza, G., Rebolledo-Vieyra, M., Reimold, W., Robin, E., Salge, T., Speijer, R., Sweet, A., Urrutia-Fucugauchi, J., Vajda, V., Whalen, M., & Willumsen, P. (2010). The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary Science, 327 (5970), 1214-1218 DOI: 10.1126/science.1177265
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