NUMERICAL MODELING OF THE CHICXULUB EJECTA

The K/Pg boundary marks the latest mass extinction on Earth and is widely recognized as a global ejecta layer from the Chicxulub impact crater. However, some properties of this layer (thickness, shocked quartz distribution, chemical composition) are inconsistent with the idea that ejecta was transported around the globe purely on a ballistic path.

The Chicxulub impact is modeled with the 3D hydrocode SOVA complemented by the ANEOS equation of state for geological materials. In three separate stages the following processes are modelled: 1) the impact and initial ejection of materials; 2) the ballistic flight of ejecta on a spherical earth; and 3) ejecta re-entry into the atmosphere (at an altitude of 200 km re-entering tracers are replaced by real particles with a size-frequency distribution in accordance with their maximum shock pressure; the interaction of these particles with the atmosphere is modeled using a multi-phase approximation).

Up to distances of 1000-1500 km from the crater, massive ballistic ejecta are deposited rapidly. At larger distances, the atmosphere/ejecta interaction becomes significant. Re-entering ejecta heat the upper atmosphere and create strong winds. These winds disperse small fragments (molten spherules and shocked quartz grains of < 1 mm in diameter) preferentially downrange. For two-three hours after the impact, these dispersed ejecta travel up to a few thousand km from their re-entry site (final deposition through the dense lower atmosphere or ocean may take days or weeks). This mechanism is much more intense than observed for volcanic aerosols in stable atmospheric flows. The results (ejecta thickness and composition) are compared with available geological data.

We also estimate an amount of ejected climatically active gases (carbon dioxide, sulfur oxides, water vapor, methane), and discuss their influence on the Earth’s climate.