Shock-induced vaporization of anhydrite and global cooling from the K/T impact

Discovery of abundant anhydrite (CaSO4) and gypsum (CaSO4. 2H(2)O) in the o therwise carbonate sediment comprising the upper 3 km thick layer of the ta rget rock at the K/T impact site has prompted research on these minerals. E valuation of the severity of the proposed extinction mechanism involving in jection of impact-generated SO2 and SO3 into the stratosphere entails deter mination of criteria for shock-induced vaporization of these minerals. In t he present work we present new data on the vaporization criteria of anhydri te. These are based on the reanalysis of the shock wave experiments of Yang and Ahrens [Earth Planet. Sci. Lett. 156 (1998) 125-140], conducted on mat erial with 30% porosity, in which the shock- (fully or partially) vaporized sample interacts with an overlying LiF window. The velocity histories, mon itored using a velocity interferometer, are compared with numerical simulat ions employing an improved equation of state for porous anhydrite and its v aporization products. We also employ the 'entropy criterion for vaporizatio n of material under shock compression. The values of the entropies of incip ient and complete vaporization for anhydrite are determined to be 1.65+/-0. 12 and 3.17+/-0.12 kJ (kg K)(-1), respectively, and the corresponding press ures for incipient and the complete vaporization along the Hugoniot for the solid material are 32.5+/-2.5 and 122+/-13 Cpa, respectively as compared w ith 81+/-7 and 155+/-13 GPa previously reported by Yang and Ahrens. Along w ith these criteria, the use of the recent estimate of diameter (100 km) for the Chicxulub transient crater [O'Keefe and Ahrens. J. Geophys. Res. 104 ( E11)(1999) 27091-27104: Morgan et al., Nature 390(1997) 472-476] that is sm aller than previously assumed, along with Ivanov et al.'s [Geol. Soc. Am. S pec. Pap. 307 (1996) 125-142] 2-D hydrodynamic simulation to determine the shock attenuation and Pope et al,'s [J. Geophys. Res., 102 (E9) (1997) 2164 5-21664] radiative transfer model, yields the maximum decrease in the avera ge global surface temperature of 12-19 K for 9.0-9.5 years at the K/T bound ary. Thus, the global cooling is inferred to have been less severe than tha t indicated by the upper limit of the range of 5-31 K decrease lasting for similar to 12 years calculated by Pope et al. Because ambient global surfac e temperatures at K/T time were similar to 18-20 degreesC warmer than prese nt values, this cooling event produced cold, but not freezing conditions at the Earth's surface. (C) 2001 Elsevier Science B.V. All rights reserved.