Stunning fossil find records the last day of the dinosaurs

The Chicxulub impact was a massive, a global extinction, which extinguished upwards of 75% of life on Earth within a geologically short span of time. Climatic shifts ensued at the same time, and, much sooner after impact, water displacement by the ~10 km-wide impactor was recorded by tsunami (tidal wave) or seiche (tectonically activated) deposits around the Caribbean and what was then the ‘proto’ Gulf of Mexico. Aside from theoretical calculations, however, many of the immediate effects of the impact, especially within the proximal ~2100 miles close to the crater, are incompletely resolved. The complex interplay between the hot vapor plume, molten ejecta, and “wet” plume of vaporized carbonate target rock, for example, is incompletely understood.

Lead author, Robert Depalma said: “A tsunami would have taken at least 17 or more hours to reach the site from the crater, but seismic waves - and a subsequent surge - would have reached it in tens of minutes.”

An international team of scientists, including Prof. Phil Manning, has been exploring the uppermost Hell Creek Formation, a continental sequence in North America preserving the last 1.3 million years of the Cretaceous Period. The team discovered an anomalous, turbulently deposited layer of sediment at a new site they named ‘Tanis’. After many years work on the site and with the analysis of samples back at the lab, the team are sure that the site temporally coincides with the KPg boundary, the point at which the dinosaurs and other life became extinct.

The rapidly emplaced sediment layer was highly conducive to detailed preservation at momentary scale, but also contains ejecta tektites (melted Earth’s crust blasted into the upper atmosphere and returning as small ‘droplets’ of glass). The tektites and other impact-generated debris have been geochemically and geochronologically linked to the Chicxulub impact by the international team. The deposit is capped by a distinct KPg boundary clay, indicating it was deposited in a narrow window of time- after impact yet before settling of the finest impact debris

Perhaps most importantly, the sediment layer, which exhibits bidirectional flow and was likely emplaced by a massive hydro-disturbance such as a tsunami. This event was likely to have been triggered by the Chicxulub impact. Ballistic calculations of ejecta indicate that the ejecta tektites at Tanis would have arrived at approximately the same time as the seismic wave from the Chicxulub crater.

The team reports in the journal PNAS that the ultra-high-resolution temporal fidelity of the Tanis site provides the first sub-24-hour chronology of the KPg impact event. The site preserves unprecedented detail of one of the most pivotal moments in Earth’s geologic history, and provides an unmistakable northernmost record of the Chicxulub impact event.

The extraordinary preservation potentially afforded by the rapid emplacement of sediment provides a unique opportunity to examine exceptionally preserved terminal-Cretaceous organisms. The layers of sediment along with the remarkable fossils they contain, combined with the geochemical evidence, delivers unique insight into one of Earth’s most studied mass-extinction events.

Finally, Prof Manning added: “This is just the beginning of a long study on the Tanis site. The PNAS paper provides a geological framework from which broader paleontological work will take place in the future. We can definitely say that the Tanis site was not ‘consumed by the desert in a sandstorm that lasted a whole year’ (as in the Raiders of the Lost Ark), but it is certainly a rosette stone that might help us better read and understand events on the last day of the dinosaurs.”