Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30t.530k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 530-550
Other
1
Craters, Chicxulub, Kara, Popigai, Cretaceous- Tertiary, Impacts, Atmosphere, Mass Extinctions
Scientific paper
Studies of the Chicxulub impact event suggest that large volumes of evaporites and carbonates in the target may have been vaporized, enhancing concentrations of S-rich aerosols and CO2 in the atmosphere, which may have, in turn, been partly responsible for the mass extinction that occurred at the Cretaceous-Tertiary boundary. We note that in this and other impact events the projectile is also vaporized and propose that it may be an additional source of climatically-active elements in the atmosphere. In particular, we suggest asteroids and comets that are vaporized during hypervelocity impact events can inject large masses of S into the stratosphere where it can then affect the radiation budget of the Earth, alter the chemistry of the ozone layer, and eventually be converted to sulfuric acid rain. We also suggest that these vaporized objects can inject large masses of Cl and Br into the stratosphere where they, in addition to previously described NOx [e.g., 1], can destroy ozone and, in the case of Cl, also acidify rainwater. To quantify these types of perturbations, we calculated the masses of S, Cl, and Br that can be injected into the stratosphere following the impact of several classes of asteroids and model comets. The calculations indicate, for example, that relatively small carbonaceous asteroids, 0.3 km in diameter, contain 5 times more S than the entire modern stratosphere, or approximately the same as that produced by the 1982 eruption of El Chichon. Using current cratering rates [2], we infer that these types of perturbations occur at an average rate of 1 per 10,000 years. Larger impact events, capable of injecting 10^15 g of S into the stratosphere and possibly causing mean surface temperatures to decrease by 2 degrees C for 3 years or longer [3], occur at an average rate of 1 per 1 million years. These results indicate that significant S can be added to the stratosphere even in those cases where the target (like that at Chicxulub) does not contain vast evaporite deposits. Specific calculations of 17 past impact events on Earth indicate that stratospheric S was enhanced up to 10^5 times the current value; it seems likely that some of the larger events, like the 35 +/- 5 Ma Popigai impact event and the 73 +/- 3 Ma Kara and Ust-Kara impact events, produced enough S-rich aerosols to perturb the climate. In addition to S, asteroids with 0.1 to 10 km diameters inject 3 x 10^8 to 9 x 10^14 g of Cl and 2 x 10^5 to 5 x 10^12 g of Br into the stratosphere, not including any Cl and Br produced from target lithologies (like seawater), nor any Cl and Br produced from biomass burning that may have occurred after the impact. These calculations indicate that impacting asteroids with diameters of a few tenths of a kilometer or larger can inject enough Cl and Br into the stratosphere to destroy a significant portion of the stratospheric ozone [4,5]. Unfortunately, it is difficult to determine the exact magnitude and duration of these climatic perturbations because they depend in large part on the still uncertain chemistry and microphysics that occur in the vapor plume as it rises and cools, and in the stratosphere once the ejecta is deposited there. Nonetheless, it seems clear that asteroid and comet impacts are an important source of climatically-active material and that relatively small impact events (and, thus, those that occur frequently) can potentially affect the global environment. References: [1] Zahnle K. J. (1990) GSA Spec. Pap. 247, 271-288. [2] Shoemaker E. M. (1983) Annu. Rev. Earth Planet. Sci., 11, 461-494. [3] Sigurdsson H. (1990) Palaeogeo., Palaeoclimat., Palaeoec. (Global and Planetary Change Section), 89, 277-289. [4] Mankin W. G. and Coffey M. T. (1984) Science, 226, 170-172. [5] McElroy M. B. et al. (1986) Nature, 321, 759-762.
Hunten Don M.
Kring David A.
Melosh Henry Jay
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