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The deposition of the K/T boundary layer: flaming forests, dead dinos, and some really bad weather

Wednesday, October 18, 2006
Goldin (Lunar and Planetary Lab and Department of Geosciences, University of Arizona)

The mechanics of impact ejecta deposition are not well understood, especially for planets with atmospheres where complex interactions occur between the ejected particles and the surrounding atmosphere. The K/T boundary ejecta layer is found world-wide and is thought to represent material from the vapor plume produced by the Chicxulub impact. We modeled a simplified Chicxulub scenario using KFIX-LPL, a two-phase fluid flow code which allows us to simulate deposition of the K/T boundary layer through the atmosphere. Air is modeled as a perfect gas and the spherules (condensed from the vapor plume), which are injected into the atmosphere, are modeled as a simple incompressible fluid with the properties of basaltic glass. The particles fall through the thin upper atmosphere, pushing the atmosphere downwards until the particles decelerate due to drag and increasing atmospheric pressure. The particles accumulate at ~50-km altitude and the deceleration heats the atmosphere around the particles (>700 K), causing expansion of the atmosphere and creating a sharp transition between hot dense atmosphere below the deceleration boundary and cool thin atmosphere above. Surface deposition of the global K/T boundary layer occurs on the scale of a few hours. Our models also shed light on the mysterious dual layer observed in several North American localities. Adding an initial injection of terrestrial ejecta (from the ejecta curtain) into our model atmosphere, as would occur at such intermediate distances from the impact, produces two distinct layers due to the alteration of the atmosphere's structure. Deposition of the lower terrestrial layer on the ground begins at ~80 minutes and that of the upper fireball layer begins at ~130 minutes. Our models show dramatic changes to the atmosphere, which have important environmental implications and provide the starting conditions and timeframes for chemical models examining the environmental consequences of Chicxulub. Our models, which include an accurate treatment of thermal radiation, also provide support for the delivery of significant thermal radiation to the Earth's surface.

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