Cretaceous Extinctions: Evidence Overlooked

TitleCretaceous Extinctions: Evidence Overlooked
Publication TypeJournal Article
Year of Publication2010
AuthorsKeller, G, Adatte, T, Pardo, A, Bajpai, S, Khosla, A, Samant, B
Date Publishedmay

In their Review “The Chicxulub Asteroid impact and mass extinction at the Cretaceous-Paleogene boundary” (5 March, p. 1214), P. Schulte et al. analyzed the 30-year-old controversy over the cause of the end-Cretaceous mass extinction and concluded that the original theory of 1980 was right: A large asteroid impact on Yucatan was the sole cause for this catastrophe. To arrive at this conclusion, the authors used a selective review of data and interpretations by proponents of this viewpoint. They ignored the vast body of evidence inconsistent with their conclusion—evidence accumulated by scientists across disciplines (paleontology, stratigraphy, sedimentology, geochemistry, geophysics, and volcanology) that documents a complex long-term scenario involving a combination of impacts, volcanism, and climate change. Here, we point out some of the key evidence that Schulte et al. overlooked.

The underlying basis for Schulte et al.'s claim that the Chicxulub impact is the sole cause for the Cretaceous-Paleogene (K-Pg) mass extinction is the assumption that the iridium (Ir) anomaly at the K-Pg boundary and Chicxulub are the same age. There is no evidence to support this assertion. No Ir anomaly has ever been identified in association with undisputed Chicxulub impact ejecta (impact glass spherules), and no impact spherules have ever been identified in the Ir-enriched K-Pg boundary clay in Mexico or elsewhere (12). In rare deep-sea sites where the Ir anomaly is just above impact spherules, it is due to condensed sedimentation and/or nondeposition.

A Chicxulub impact–generated tsunami is another basic assumption of Schulte et al. to account for the impact spherules in late Maastrichtian sediments (including a sandstone complex) in Mexico and Texas. Multiple lines of evidence contradict this assumption and demonstrate long-term deposition before the K-Pg, including burrowed horizons, multiple impact spherule layers separated by limestone, and spherule-rich clasts that indicate the original deposition predates the K-Pg and excludes tsunami deposition (14).

Evidence of the pre–K-Pg age of the Chicxulub impact can also be found in sediments above the sandstone complex in Texas and northeastern Mexico and above the impact breccia in the Chicxulub crater. Evidence shows that the K-Pg boundary is not linked to the sandstone complex and impact spherules (1247).

Evidence that supports the pre–K-Pg age of the Chicxulub impact is also found in the presence of a spherule layer in late Maastrichtian sediments below the sandstone complex in northeastern Mexico and Texas (248).

Deccan volcanism is dismissed by Schulte et al. as much older and of no consequence in the K-Pg mass extinction. Recent Deccan volcanism studies show the contrary (911). These studies link the mass extinction with the main phase of Deccan eruptions.

When this evidence is taken into account, it is clear that the massive Chicxulub and Deccan database indicates a long-term multicausal scenario and is inconsistent with the model proposed by Schulte et al.   PDF