March 23, 2010 | 5
When Pangaea finally broke up, some 200 million years ago, the result was a lot of heat. Specifically, volcanism, as enormous flows of basalt burst to the surface, ultimately covering more than nine million square kilometers. It wasn’t just the death of a supercontinent; it was also one of Earth’s five major extinction events—and the one that paved the way for the dinosaurs.
Now scientists have linked this great volcanism to catastrophic climate change via an analysis of carbon isotopes in wood and soil preserved in rocks. In short, geologist Jessica Whiteside of Brown University and her colleagues show in a paper published March 22 in Proceedings of the National Academy of Sciences that the extinction event at the end of the Triassic occurred at the same time as carbon dioxide levels jumped and shell-forming animals in the ocean suddenly had a much harder time forming their homes thanks to an eruption that lasted for more than 500,000 years.
Looking at the geologic record, Whiteside and her colleagues record a drop in carbon 13 roughly 200 million years ago. That suggests more of the lighter isotope of carbon (C-12) had suddenly become available, since plants prefer to use it, which in turn suggests soaring levels of CO2 in the atmosphere. Those levels persisted for at least 20,000 years, followed by another at least 20,000-year period when plants were forced to cope with higher C-13 levels. Records found in rocks that were underwater all those years ago show a similar pattern. All in all, it adds up to "strong direct evidence by direct superposition that the eruption of a giant flood basalt province could cause a climatic catastrophe resulting in a major mass extinction."
But mysteries remain about what specifically caused the jump in CO2 levels. As the scientists write, "it is unlikely that [the changes in carbon isotope levels] was generated by mantle CO2 alone." Other variables could include a massive burp of methane from the sea bottom that had been trapped in ice (clathrates) or the increasing levels of sulfuric acid as a result of all the volcanism. Nor can the scientists "exclude a role for a bolide impact as a killing mechanism" though no asteroid crater has been found to date.
It does not paint a pretty picture of what happens when CO2 levels rise. Terrestrial and marine life, such as tetrapods and pollinating plants, dwindled as a result of the sudden rise in CO2 levels in the atmosphere, with attendant ocean acidification. It remains to be seen who might benefit from today’s ongoing sixth extinction and its related climate change, as theropod dinosaurs benefited from the end-Triassic extinction. But it usually isn’t the dominant life forms on the planet at the time.
Image: North Mountain basalt of the Central Atlantic Magmatic Province (brown) resting above the level of the end-Triassic extinction event (in white layer), in turn, resting on red shallow lake deposits at Red Head, Five Island Provincial Park, Nova Scotia, Canada. Photo is by Jessica H. Whiteside.