HR: 1340h
AN: U23A-0863    [Abstracts]
TI: Deposition of Distal K/T Ejecta via Density Currents
AU: * Goldin, T J
EM: tgoldin@email.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721, United States
AU: Melosh, H J
EM: jmelosh@lpl.arizona.edu
AF: Lunar and Planetary Lab, University of Arizona, Tucson, AZ 85721, United States
AB: While the K/T boundary ejecta layer is well known, both the mechanics of deposition and the environmental effects of this deposition are less established. KFIX-LPL, a two-phase fluid flow code, allows us to model the interactions between the atmosphere and ejecta spherules. KFIX-LPL accommodates a range of flow regimes and includes a complete treatment of thermal radiation. We modeled a distal Chicxulub scenario (impact plume ejecta only) by injecting 250-μm spherules into the atmosphere at 8 km/s with an inflow density consistent with observed spherule volumes. The spherules fall through the thin upper atmosphere, compressing the atmosphere until the spherules decelerate due to drag and increasing atmospheric pressure. The particles accumulate in dense layers at ~50-km altitude. At intermediate distances from Chicxulub, such as North America where a dual-layer is observed at the K/T boundary, ejecta curtain material must also be considered. For these models we include an initial brief injection 500-μm terrestrial ejecta at 4.5 km/s in addition to the more prolonged flux of fireball material. The ejecta curtain material compresses the atmosphere to below 40 km in altitude. As this brief pulse ends, the atmosphere rebounds upwards and ejecta from the fireball pulse accumulates at a higher level and the two types of ejecta are deposited separately. In both the distal and North American models, the spherules initially settle through the atmosphere as individual particles, but as the ejecta near the ground, density currents form. The modeled instabilities are real density currents and not numerical artifacts, as confirmed by KFIX-LPL simulations of a series of tephra fall experiments in water (Carey 1997). We modeled these experiments by dropping spherical particles at various mass fluxes into water. Instability formation was evaluated using a criterion yielded by the ratio between turbulent instability growth rate and Stokes velocity of individual particles. Instabilities in our tephra fall models agree with both the instability criterion and the experimental results. Thus the modeled density currents are real and both the single ejecta layer observed in distal localities and the double layer observed in North American localities were deposited on a scale of hours rather than settling out slowly via Stokes flow. The K/T boundary layer truly represents a blink in geologic time.
DE: 0545 Modeling (4255)
DE: 3379 Turbulence (4490)
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 8136 Impact phenomena (5420, 6022)
SC: Union [U]
MN: 2007 Fall Meeting