Scientist 1: The mass extinction was driven by prolonged volcanism. Vast eruptions released carbon dioxide and sulfur over roughly 300,000 years, acidifying the oceans and warming the climate gradually. On this model, extinctions should be smeared across many rock layers rather than concentrated in one, and no global iridium spike is expected because volcanoes emit little iridium. Volcanic gases should, however, leave a distinctive enrichment of mercury at the boundary.

Scientist 2: The extinction was caused by a single asteroid impact. The collision scattered iridium-rich dust worldwide in one thin layer and drove extinctions within a single narrow horizon. On this model the boundary should also contain shocked quartz and glassy impact spherules. There is no particular reason for the impact to elevate mercury, which is not a characteristic product of a collision.