Because of their catastrophic outcomes, it’s tempting to think of mass extinctions as sudden apocalypses — an asteroid strikes, the Earth is engulfed in flames, and when the smoke clears, half of all life is dead. However, evidence suggests that mass extinctions are more complicated than this:
- Mass extinctions are complex and may have multiple causes. For example, the end-Cretaceous mass extinction does seem to have been triggered by an asteroid impact, but continental-scale volcanic activity, climate change, and changes in atmospheric and oceanic chemistry likely also played a part.
- The intensity of mass extinctions can be amplified by the interplay among the atmosphere, oceans, and geologic activity. For example, large scale volcanic activity at the end of the Permian may have killed some organisms directly through eruption events and through the poisonous gases released — but many more extinctions probably resulted from the effect of the eruptions on other Earth systems. Greenhouse gases released by the volcanic activity would have caused global warming, and this climate change would have caused shifts in ocean chemistry and circulation. The changes in ocean chemistry could have prevented reefs from being formed, and changes in circulation could have caused huge areas of oxygen-depleted waters, which could, in turn, imbalance ecosystems the world over resulting extinction on a global scale.
- Some organisms may be more vulnerable during mass extinctions than others. For example, during the end-Permian mass extinction, marine invertebrates with shells made of calcium carbonate — like clams — were more likely to go extinct than animals that don’t depend on this compound — such as certain kinds of sponges.<sup”>4 That’s because the end-Permian mass extinction likely involved ocean acidification, which makes less calcium carbonate available for organisms that need it to build their shells.
Although mass extinctions occur in geologically short periods of time, they are not instantaneous. Mass extinctions involve complex processes that may play out over many thousands of years. Figuring out what processes occurred during a mass extinction is a challenge for scientists.
4Clapham, M.E., and J.L. Payne. 2011. Acidification, anoxia, and extinction: a multiple logistic regression analysis of extinction selectivity during the Middle and Late Permian. Geology 39:1059-1062.
Giant clam photo by H. Vannoy Davis © 2001 California Academy of Sciences; Staurocalyptus photo courtesy of NOAA/Monterey Bay Aquarium Research Institute