At the end of the Permian era 252 million years ago, life almost disappeared from the oceans, an event known as The Great Dying. Now scientists have made their most detailed reconstruction yet of the way events unfolded using the shells of brachiopods (bivalves similar to modern clams or mussels), firmly pinning the blame on gases from volcanic eruptions.
By some estimates, 96 percent of all marine species and two-thirds of land vertebrates died out at the end of the Permian era. No other mass extinction came close.
Most scientists who have studied the event blame an immense outbreak of volcanic activity in Siberia, which pumped enough gases into the atmosphere to change both the climate and the chemistry of the oceans. A few renegades hold to alternatives, such as an asteroid impact or microbes. To learn more, Dr Hana Jurikova of GEOMAR Helmholtz Centre for Ocean Research used brachiopod shells to track the era's marine acidity.
"These are clam-like organisms that have existed on Earth for more than 500 million years,” Jurikova said in a statement. Her sample included well-preserved brachiopods from the (then oceanic) southern Alps from before and during the first phase of the extinction, with confirming shells from China. Boron isotopes in the shells indicate the acidity of the oceans at the time, a method that has recently started to provide abundant insights into past climatic conditions.
The mood of a creature without a central nervous system is difficult to assess, but we can safely say these bivalves were not as “happy as a clam”. Plunging pH levels made life extremely difficult for shell-building organisms. Since the increased acidity was almost certainly a result of dissolved carbon dioxide, which would have reflected changes to the atmosphere, Jurikova and colleagues were able to estimate the climate impacts as well.
In Nature Geoscience, they calculate that atmospheric carbon dioxide peaked at 4,400 parts per million (more than 10 times modern levels), warming the planet by almost 10ºC (18ºF).
In addition to hotter conditions wiping out most terrestrial plants and animals, the changed atmosphere sped up weathering, washing more nutrients to the oceans. This produced world-wide versions of fertilizer-driven giant algal blooms, leading to a drastic fall in oxygen. "This domino-like collapse of the inter-connected life-sustaining cycles and processes ultimately led to the observed catastrophic extent of mass extinction at the Permian-Triassic boundary," Jurikova said.
The dating of vast Siberian lava flows to 252 million years ago, coinciding with the Great Dying, had long aroused suspicion the events were related, but the paper reveals the nature of the connection. The authors note the quantity of carbon released into the atmosphere at the time far exceeded the burning of all fossil fuel reservoirs. However, it took place over tens of thousands of years, and the annual rate was at least “14 times less than” current human emissions.