Atmospheric Interactions during Global Deposition of Chicxulub Impact Ejecta

Abstract

Atmospheric interactions affected both the mechanics of impact ejecta deposition and the environmental effects from the catastrophic Chicxulub impact at the Cretaceous-Paleogene (K-Pg) boundary. Hypervelocity reentry and subsequent sedimentation of Chicxulub impact spherules through the Earth's atmosphere was modeled using the KFIX-LPL two-phase flow code, which includes thermal radiation and operates at the necessary range of flow regimes and velocities. Spherules were injected into a model mesh approximating a two-dimensional slice of atmosphere at rates based on ballistic models of impact plume expansion. The spherules decelerate due to drag, compressing the upper atmosphere and reaching terminal velocity at ~70 km in altitude. A band of spherules accumulates at this altitude, below which is compressed cool air and above which is hot (>3000 K) relatively-empty atmosphere.Eventually the spherule-laden air becomes unstable and density currents form, transporting the spherules through the lower atmosphere collectively as plumes rather than individually at terminal velocity. This has implications for the depositional style and sedimentation rate of the global K-Pg boundary layer. Vertical density current formation in both incompressible (water) and compressible (air) fluids is evaluated numerically via KFIX-LPL simulations and analytically using new instability criteria. Models of density current formation due to particulate loading of water are compared to tephra fall experiments in order to validate the model instabilities.The impact spherules themselves obtain peak temperatures of 1300-1600 K and efficiently radiate that heat as thermal radiation. However, the downward thermal radiation emitted from decelerating spherules is increasingly blocked by previously-entered spherules settling lower in the atmosphere. This self-shielding effect strengthens with time as the settling spherule cloud thickens and becomes increasingly opaque, limiting both the magnitude and duration of the thermal pulse at the ground. For a nominal Chicxulub reentry model, the surface irradiance peaks at 6 kW/m 2 and is above normal solar fluxes for ~25 minutes. Although biologic effects are still likely, self-shielding by spherules may have prevented the global wildfires previously postulated. However, submicron dust may act as a hot opaque cap in the upper atmosphere, potentially increasing the thermal pulse beyond the threshold for forest ignition.