CHICXULUB EJECTA: INSIGHT FROM NUMERICAL MODELS (Invited Presentation)

Impacts are extremely hazardous on both a local and global scale and can occasionally change the course of evolution. The Chicxulub impact caused a global catastrophe and, although the principal drivers for the K-Pg mass extinction remain a matter of some debate, the ejection of dust, soot and climate-active gases (carbon dioxide, sulfur oxides, water vapor, nitric oxide) into the atmosphere are likely to have played a significant role. In my talk I discuss recent numerical results on Chicxulub ejecta – from their origin within the growing impact crater to their global distribution. Numerical results are based on the usage of impact hydrocodes such as iSALE, CTH, and SOVA and should be verified by comparison with geological records.

A standard scenario for the emplacement of the K-Pg boundary ejecta layer assumes a combination of two mechanisms: 1) ballistic emplacement of mainly molten target material that travels within an ejecta curtain up to ~4000 km; 2) precipitation of projectile/target condensates from a global (much larger than the Earth’s radius) vapor plume. It appears, however, that this scenario is inconsistent with observational data (for example, the presence of shocked quartz in the K-Pg layer which would be annealed within the plume).

Recent numerical models revealed that: 1) the impact plume is of minor importance as it contains little material, mainly the projectile and sediments; 2) the total amount of high-velocity (> 8 km/s) ejecta is not large enough to form the K-Pg global layer. Instead, ballistic ejecta (usually expanding as the ejecta curtain) interact with the atmosphere and the non-negligible part of these ejecta is transformed into a dust cloud propagating quickly at an altitude of ~100km from the impact site. The cloud is fast enough to cover the Earth within the first few hours after the impact and dense enough to form the global layer. Although the cloud is optically thick, even micron-sized particles precipitate quickly and global darkness cannot last long enough. In contrast, aerosol particles (also part of the cloud) stay in the atmosphere forever and are the main reason of the mass extinction.

Surprisingly, the suggested mechanism of ejecta emplacement is to some extent similar to the initial ideas of Alvarez’s revolutionary paper.