IMPACT WINTER AND THE CRETACEOUS-TERTIARY EXTINCTIONS - RESULTS OF A CHICXULUB ASTEROID IMPACT MODEL

The Chicxulub impact crater in Mexico is the site of the impact purpor ted to have caused mass extinctions at the Cretaceous/Tertiary (K/T) b oundary. 2-D hydrocode modeling of the impact, coupled with studies of the impact site geology, indicate that between 0.4 and 7.0 x 10(17) g of sulfur were vaporized by the impact into anhydrite target rocks. A small portion of the sulfur was released as SO3 or SO4, which convert ed rapidly into H2SO4 aerosol and fell as acid rain. A radiative trans fer model, coupled with a model of coagulation indicates that the aero sol prolonged the initial blackout period caused by impact dust only i f the aerosol contained impurities. A larger portion of sulfur was rel eased as SO2, which converted to aerosol slowly, due to the rate-limit ing oxidation of SO2. Our radiative transfer calculations, combined wi th rates of acid production, coagulation, and diffusion indicate that solar transmission was reduced to 10-20% of normal for a period of 8-1 3 yr. This reduction produced a climate forcing (cooling) of -300 Wm(- 2) which far exceeded the +8 Wm(-2) greenhouse warming, caused by the CO2 released through the vaporization of carbonates, and therefore pro duced a decade of freezing and near-freezing temperatures. Several dec ades of moderate warming followed the decade of severe cooling due to the long residence time of CO2. The prolonged impact winter may have b een a major cause of the K/T extinctions.