Extinction!

Patterns of extinction

Generally, it’s difficult link any particular catastrophic event to a subsequent mass extinction. Instead palaeontologists comparing the pattern of mass extinctions as a whole with the patterns of each of the three possible mechanisms. Figure 2 compares the record of mass extinctions with geological records of sea-level falls, flood-basalt eruptions, and impacts of bolides (asteroids or comets).

Click the image to see a larger version Figure 2. The record of extinctions (bottom), compared with - from top - major sea-level changes, giant eruptions of basalt, and impact craters.

References: Courtillot, V, Jaeger, J-J, Yang, Z, Féraud, G, Hofmann, C. 1996. The influence of continental flood basalts on mass extinctions: where do we stand? In Ryder, G, Fastovsky, D, and Gartner, S, eds. The Cretaceous-Tertiary event and other catastrophes in earth history: The Geological Society of America, Special Paper 307, 513-525. Grieve, R, Rupert, J, Smith, J, Therriault, A. 1996. The record of terrestrial impact cratering. GSA Today 5: 193-195 Hallam, A. 1992. Phanerozoic sea-level changes. New York; Columbia University Press.

Looking at Figure 2, it is clear that impacts show the worst record of association with mass extinction events. Sizeable impacts are as likely to have occurred during low-extinction stages as during high-extinction stages. In fact, there’s only one compelling example of an association between a large impact and a major extinction event. The Chicxulub event of 65 million years ago was the largest impact of the last 600 million years, and third largest in the known history of our planet. It blasted out a large crater in Mexico. At the same time, we find the Maastrichtian mass extinction event, also known as the K/T mass extinction. This was the disaster in which the dinosaurs perished, and it is ranked as the third to fifth largest extinction event of the last 600 million years.

Moving on to the second possible mechanism, abrupt falls in sea level show a rather better level of association with extinctions. Each of the three largest mass extinctions during the last 250 million years (the time interval for which we have the most accurate time resolution) corresponds to a major sea-level change. This supports the idea that rapid falls in sea level have a detrimental effect on the diversity of marine invertebrates.

Giant eruptions

But the strongest association - by far - comes with the third prospective extinction mechanism, as seen in the record of continental flood-basalt eruptions. These are vast outpourings of basaltic lava, similar to the eruptions responsible for the Hawaiian Island chain, but taking place entirely on land.