A theoretical exercise in the modeling of ground-level ozone resulting from the K–T asteroid impact: Its possible link with the extinction selectivity of terrestrial vertebrates

The extinction pattern of the Maastrichtian indicates that long-term and short-term events contributed to the Cretaceous–Tertiary (K–T) mass extinction at 65 Ma. However, it is not clear how the impact events are linked with the extinction selectivity; e.g. non-avian dinosaurs became extinct, whereas birds survived. The post-impact air quality is discussed, and attention is focused on the then land vertebrates. Although ground-level (tropospheric) O3 is a powerful irritant on the order of 0.1 ppm toxicity, the presence of ground-level O3 has hardly been considered since the K–T impact theory was reported about 30 years ago. Under the post-impact conditions reconstructed by simulating the carbon cycle (including isotope balance) and impact chemistry, a trajectory model suggests that the then photochemical reactions formed ground-level O3 whose concentration was apparently low at ∼ 1.0 ppm, but it is much greater than the current level of ∼ 0.04 ppm: that is, an O3 concentration above the health-threatening level persisted on the ground after the K–T impact. All land vertebrates must have suffered from respiratory O3 irritation at the time. However, analysis suggests that variables of O3 characteristics – hourly variation, short half-life in water and decomposition due to catalytic effects in soil – were randomly combined with variables of lifestyle features such as habitat, torpor, etc. to form new variables (i.e. survival rates): a high survival probability for amphibians; middle/high probabilities for semi-aquatic reptiles, mammals and birds; low/middle probabilities for marsupials and terrestrial reptiles; and a zero probability for non-avian dinosaurs.