Chicxulub

In the 1990s the Chicxulub impact was linked to the K–T boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact-generated tsunami deposit. Since that time a preponderance of evidence has failed to support this interpretation, revealing long-term deposition of the sandstone complex, the K–T boundary above it and the primary impact spherule ejecta interbedded in Late
Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub impact predates the K–T boundary. Impact-tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic disturbance, although no such evidence has been presented. The decades-old controversy over the cause of the K–T mass extinction will never achieve consensus, but careful documentation of results that are reproducible
and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important step in that direction by showing (1) that the stratigraphically older spherule layer from El Pen˜on, NE Mexico, represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that this impact caused no species extinctions.

Non-avian dinosaurs went extinct 66 million years ago, geologically coincident with the impact of a large bolide
(comet or asteroid) during an interval of massive volcanic eruptions and changes in temperature and sea level.
There has long been fervent debate about how these events affected dinosaurs. We review a wealth of new data
accumulated over the past two decades, provide updated and novel analyses of long-term dinosaur diversity
trends during the latest Cretaceous, and discuss an emerging consensus on the extinction’s tempo and causes.
Little support exists for a global, long-term decline across non-avian dinosaur diversity prior to their extinction
at the end of the Cretaceous. However, restructuring of latest Cretaceous dinosaur faunas in North America led
to reduced diversity of large-bodied herbivores, perhaps making communities more susceptible to cascading
extinctions. The abruptness of the dinosaur extinction suggests a key role for the bolide

New constraints on the timing of the Cretaceous-Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental flood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1–1.0 J/m3 throughout the upper ~200 km of Earth’s mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume “head.” It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava flows, which together may account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/Cretaceous-Paleogene time, a huge pulse of mantle plume–derived magma passed through the crust with little interaction and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which in turn might help to determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the Cretaceous-Paleogene extinction.