There have been five mass extinctions since the divergent evolution of early animals 600 -450 million years ago (Figure 1). The cause of the third and fourth was volcanic activity, while an asteroid impact led to the fifth. But triggers of the first and second mass extinctions had, until now, been unknown.
The first mass extinction occurred at the end of the Ordovician. This age is between the divergence of the Ordovician and land invasion of vascular land plant and animals. Animals in the Ordovician-Silurian comprised marine animals like corals, trilobites, sea scorpion, orthoceras, brachiopods, graptolite, crinoid and jawless fish. Approximately 80% of species disappeared at the end of the Ordovician.
A team led by Dr. David S. Jones of Amherst College and Professor Kunio Kaiho of Tohoku University, looked into possible triggers of the first mass extinction. They took sedimentary rock samples from two places - North America and southern China - and analyzed the mercury (Hg) in them. They found Hg enrichments coinciding with the mass extinction in both areas.
This, they believe, is the product of large volcanic eruptions because Hg anomaly was also observed in other large igneous province volcanisms.
Huge volcanic eruptions can produce sulfate aerosols in the stratosphere. Sulfate aerosols are strong, light-reflecting aerosols, and cause global cooling. This rapid climate change is believed to be behind the loss of marine creatures.
Kaiho's team is now studying the second mass extinction in the hopes of further understanding the cause and processes behind it.
Department of Earth Science, Faculty of Science
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