RAPID FALL OF THE K/T SULFURIC ACID AEROSOLS AND OCEANIC pH REDUCTION

Introduction: Geologic evidence indicates that the K/T mass extinction occurred on 65 million years ago is caused by an asteroid or comet impact [e. g., 1]. However, the actual mechanism that caused the mass extinction is still very controversial. The impact generated sulfuric oxides are thought to cause an environmental perturbation and it is one of the most plausible hypotheses of the K/T killing mechanism [e. g., 2, 3]. Laboratory experiments by Ohno et al. [4] suggest that the degassed sulfur have been dominated by SO 3 , not SO 2 , and that the sulfuric oxide should have quickly formed sulfuric acid aerosols. The mechanism of the environmental perturbation due to the sulfuric acid aerosol depends strongly on its residence time. If the residence time was longer than several years, blockage of the sunlight would last several years, resulting in significant decrease in the oceanic temperature [e. g., 5, 6]. If the residence time was short, the sulfuric acid could also cause a mass extinction by strong sulfuric acid rain fall [e. g., 2, 7]. Injection of ~10 16 mol of sulfuric acid, which is an upper estimate for the K/T event, to the upper 100m of the world ocean would not lead to a significant change in oceanic pH if the timescale of the fall of sulfuric acid aerosol was slow enough to achieve dissolution equilibrium between the ocean and the atmosphere [8]. The residence time of sulfuric acid aerosols is estimated as 6-9 months [5] and several years [6], which is comparable or longer than the timescale to achieve equilibrium. However, previous theoretical models for aerosol fall out [5, 6] underestimate the fall out rate significantly because they calculate coagulation and sedimentation of the sulfuric acid aerosols assuming that only sulfuric acid aerosols exist in the atmosphere. The mass of sili-cate dust particles in the atmosphere immediately after the K/T impact is estimated to be much larger than the mass of the sulfuric acid aerosols, ~10 14 kg [e. g., 9]. Thus, in this study, we carried out a theoretical model calculation of coagulation and sedimentation of atmospheric aerosols when sulfuric acid aerosols coexist with silicate condensate particles in order to estimate accurately the residence time of the K/T sulfuric acid aerosols under more realistic conditions. Theoretical model: In our model, the time evolution of the distributions of the sulfuric acid and the silicate condensate particles were …