ENERGY, VOLATILE PRODUCTION, AND CLIMATIC EFFECTS OF THE CHICXULUB CRETACEOUS TERTIARY IMPACT/

A comprehensive analysis of volatiles in the Chicxulub impact strongly supports the hypothesis that impact-generated sulfate aerosols caused over a decade of global cooling, acid rain, and disruption of ocean c irculation, which contributed to the mass extinction at the Cretaceous /Tertiary (WT) boundary. The crater size, meteoritic content of the WT boundary clay, and impact models indicate that the Chicxulub crater w as formed by a short period comet or an asteroid impact that released 0.7-3.4 x 10(31) ergs of energy. Impact models and experiments combine d with estimates of volatiles in the projectile and target rocks predi ct that over 200 gigatons (Gt) each of SO2 and water vapor, and over 5 00 Gt of CO2, were globally distributed in the stratosphere by the imp act. Additional volatiles may have been produced on a global or region al scale that formed sulfate aerosols rapidly in cooler parts of the v apor plume, causing an early, intense pulse of sulfuric acid rain. Est imates of the conversion rate of stratospheric SO2 and water vapor to sulfate aerosol, based on volcanic production of sulfate aerosols, cou pled with calculations of diffusion, coagulation, and sedimentation, d emonstrate that the 200 Ct stratospheric SO2 and water vapor reservoir would produce sulfate aerosols for 12 years. These sulfate aerosols c aused a second pulse of acid rain that was global. Radiative transfer modeling of the aerosol clouds demonstrates (1) that if the initial ra pid pulse of sulfate aerosols was global, photosynthesis may have been shut down for 6 months and (2) that for the second prolonged aerosol cloud, solar transmission dropped 80% by the end of first year and rem ained 50% below normal for 9 years. As a result, global average surfac e temperatures probably dropped between 5 degrees and 31 degrees K, su ggesting that global near-freezing conditions may have been reached. I mpact-generated CO2 caused less than 1 degrees K greenhouse warming an d therefore was insignificant compared to the sulfate cooling. The mag nitude of sulfate cooling depends largely upon the rate of ocean mixin g as surface waters cool, sink, and are replaced by upwelling of deep ocean water. This upwelling apparently drastically altered ocean strat ification and circulation, which may explain the global collapse of th e delta C-13 gradient between surface and deep ocean waters at the WT boundary.