The current state of knowledge about the end-Cretaceous extinctions was highlighted in two-day symposium titled “What Killed the Dinosaurs: A Fresh Look at One of Earth’s Greatest Mysteries.” On April 29th and 30th, 2017, more than 150 people from diverse backgrounds gathered at UC Berkeley’s Faculty Club to listen to presentations by invited experts and participate in spirited discussions. Foremost goals of the symposium were to build consensus about what we know regarding this catastrophic event and to identify fruitful areas of future research to clarify what we don’t know. The symposium was open to the general public and the presentations were intended to engage experts and non-experts alike. Broad participation in discussions indicated that this was successful.
The featured speakers and their titles were:
- David Evans (Royal Ontario Museum and University of Toronto) “Dinosaur diversity and ecological dynamics in the latest Cretaceous”
- Pincelli Hull (Yale University) “The K-Pg boundary from a marine invertebrate and geochemical perspective”
- Daniel Peppe (Baylor University) “Plants and the K-Pg extinction event”
- Gregory Wilson (University of Washington and Burke Museum) “Mammals inherit the Earth: How the K/Pg mass extinction killed off dinosaurs and opened the way for mammals”
- Courtney Sprain (UC Berkeley) “The timeline of terrestrial ecosystem change across the K-Pg boundary in the Hell Creek region, Montana”
- Walter Alvarez (UC Berkeley) “The K-Pg impact-extinction discovery in a broad context”
- Jan Smit (Free University of Amsterdam) “Bio- and lithostratigraphy bearing on the K-Pg extinction”
- Paul Renne (Berkeley Geochronology Center and UC Berkeley) “It’s about time: Chronology of the Deccan Traps and the Chicxulub impact”
- Mark Richards (UC Berkeley) “Did the Chicxulub impact trigger accelerated Deccan volcanism?”
- Stephen Self (UC Berkeley) “Deccan volcanism and potential atmospheric effects”
- Thomas Tobin (University of Alabama) “The relevance of climatic changes to the K-Pg extinction”
Presentation of new 40Ar/39Ar geochronologic data from the Deccan Traps, funded by this project’s NSF grant, was one highlight of the symposium. The new data provide the best constraints yet for the stratigraphic location of the Cretaceous-Paleogene boundary within the sequence of lava flows comprising the Deccan Traps. The boundary appears to coincide with a major transition in the chemistry, volume, and frequency of lava eruptions, which further supports the hypothesis of Richards et al. (2015) that the Chicxulub impact induced fundamental changes in the existing Deccan magma system.
A common theme in many of the lectures and subsequent discussion was the possible role of climate change in the mass extinctions. Broad consensus emerged that there was a significant global warming event beginning several hundred thousand years before the Chicxulub impact, manifest in both marine and terrestrial ecosystems, indicated by diverse paleontological and geochemical data. The onset of this warming event coincided closely with the initiation of major volcanic activity in the Deccan Traps, suggesting that greenhouse gases resulting from the latter were the cause.
Likewise, ample data presented and reviewed at the symposium indicate that there was an abrupt cooling event, of the same magnitude as the earlier warming event, essentially synchronous with the sudden extinction of marine invertebrates and the final extinction of non-avian dinosaurs. This cooling event could be due to either effects of the Chicxulub impact or Deccan volcanism, or both, via reduced insolation caused by atmospheric dust and albedo-enhancing sulfate aerosols. Only the latter is plausibly linked to Deccan volcanism, but if the volcanism was a major source of sulfate aerosols it remains puzzling why the emission of these gases would be delayed by hundreds of thousand of years after the initial release of greenhouse gases (mainly carbon dioxide and methane) implicated in the earlier warming event. There was consensus that further research is needed to understand the degree to which gases produced either directly from Deccan magma, or liberated from heated sources in the magma ascent path, are injected into the atmosphere during or prior to eruption. This may clarify why climate effects possibly attributable to Deccan-derived gases were asynchronous.
Exciting new discoveries from North America were presented, including a mass mortality assemblage of late Cretaceous fishes co-deposited with impact ejecta. This deposit yields insights into the energy released by the Chicxulub impact as well as the detailed timeline of events following the impact on the scale of hours to minutes. Such discoveries highlight the fact that the fossil record is not static, and ongoing field exploration is likely to reveal additional details about what happened at the end of the Cretaceous.
The event was enabled by generous funding from UC Berkeley’s Institute for International Studies, and was jointly sponsored by the Berkeley Geochronology Center and UC’s Museum of Paleontology and Department of Earth and Planetary Science.