THE TWO-EXTINCTION MODEL FOR THE K-PG EXTINCTIONS

The K-Pg extinction consisted of two separate extinction events, terrestrial and marine, that were separated in time by a matter of months to years. The infrared heat pulse and subsequent fires caused by reentrant ejecta within a few hours of the Chicxulub impact explain the extinction patterns in the terrestrial environment, patterns that resulted from interspecific variations in habitat features: Species that lived either underground or under water would have been least vulnerable to heat and fire. This hypothesis is supported by new field data and numerical modeling. But because water provided natural shelter from heat and fire, a second mechanism is required to explain effects of the impact in the marine environment where a majority of species went extinct, and at the same time explain why freshwater ecosystems experienced very little extinction. Impact winter involving freezing temperatures and a cessation of photosynthesis caused by dust from the impact can explain both the marine extinctions and the contrast with freshwater environments. Higher resilience to stress, greater access to refugia, and abundance of detrital foods explain the lower extinction rates for the freshwater biota in response to the impact winter. A failure to separate all of these distinct geographic and temporal domains has often caused confusion in the past through attempts to find a single mechanism (e.g. sea level variations, climate change, volcanism, diseases, and so forth) that applies to all of the domains. The K-Pg extinctions indeed had a single cause (the Chicxulub impact), but two different mechanisms (an impact winter in the marine environment, and largely a heat pulse and subsequent fires in the terrestrial environment, although an impact winter would also affect the terrestrial environment), and three separate spatial domains (terrestrial, marine, and freshwater). Only when all these different domains and mechanisms are properly separated does the pattern of two separate and distinct K-Pg extinction events, terrestrial and marine, and minimal extinction in freshwater ecosystems, become clear.