The cataclysmic extinctions that scoured Earth 200 million years ago might have been easier to trigger than expected, with potentially troubling contemporary implications.
Rather than 600,000 years of volcanic activity choking Earth's atmosphere with carbon dioxide, just a few thousand years apparently sufficed to raise ocean temperatures so potent greenhouse gases trapped in seafloor mud came bubbling up.
Much of everything alive on Earth was soon wiped out. Another half-million years of vulcanism were just icing on the cake. The immediate question: What lessons, if any, can be drawn?
"Scientists have been worried about the current release of methane from seafloors. What this study shows is that it already happened in the past," said paleoecologist Micha Ruhl of Utrecht University, whose findings are published July 21 in Science. "It could happen again. It's only the boundary conditions that we don't know."
In what scientists call the end-Triassic mass extinction, at least half of all living species simply disappear from the fossil record. The die-off didn't merely cause ecological disruption. It was so sudden and profound that planetary chemical cycles went haywire for the next several million years.
The leading explanation for the extinction invokes extended, climate-altering volcanic activity caused by splitting continental plates, but earlier research by Ruhl suggested a more nuanced and jarring narrative.
By calculating how changes in ancient sediment composition corresponded to natural cycles in Earth's distance to the Sun, he could study the end-Triassic's onset in fine-grained chronological detail.
Ruhl found that limestone -- the geological remains of corals and shellfish -- vanishes within the first 20,000 years. Similar terrestrial disruption can be inferred from changes in fossilized plant spores. The end-Triassic extinction must have occurred far more suddenly than believed.
>'It could happen again. It's only the boundary conditions that we don't know.'
In the latest study, Ruhl's team examined chemical traces left by dying plants on the shores of the Tethys Sea, a body of water that separated the ancient continents of Laurasia and Gondwana. Today those shores are sedimentary layers in the Austrian Alps.
The researchers concentrated on changes to carbon isotopes, or subtly different elemental formations that betray whether carbon in plants came from carbon dioxide or methane. At 201.4 million years ago, in that narrow 20,000-year window of their updated end-Triassic cataclysm, they found a rise in CO2 followed by a tremendous spike of methane.
"A small release of CO2 from volcanoes triggered a small change in the global climate, raising land and ocean temperatures. That led to the release of methane from the seafloor," said Ruhl.
Methane, familiar to most people as the main component of natural gas fuels, is a greenhouse gas less common than carbon dioxide in the atmosphere but far more potent. Most of Earth's supply is contained in soils and seabeds.
Scientists have raised the possibility that rising global temperatures could release trapped methane into the atmosphere, further raising temperatures and releasing more methane in a feedback loop of warming and planetary disruption. That's apparently what happened during the end-Triassic extinction.
According to paleobiologist Jessica Whiteside of Brown University, a leading researcher on end-Triassic vulcanism, more evidence is needed before Ruhl's interpretations are conclusively supported. It's possible that the new study reflects localized rather than planetary patterns.
But though Whiteside is uncertain about methane's end-Triassic role, she said the possibility of a CO2-methane warming loop is real and likely fueled climate change leading into the Eocene, an epoch lasting from 55 to 35 million years ago.
"It was a very hot time, so hot there were crocodiles in the Arctic Circle and palm trees extending to the North Pole," said Whiteside. "It's the most recent time in Earth's history that greenhouse gases were at the level where we're predicted to be in the next century."
Exactly how much warming would be needed to start the loop anew, and how much methane would flow forth, are open questions.
"We could potentially trigger a small increase in ocean temperatures, which triggers methane release," said Ruhl. "But it's difficult to quantify how much methane is in the ocean these days. Maybe we have less methane in seafloors now. Maybe we have more."
Image: Mastodonsaurus, a giant amphibian that was the largest terrestrial animal before the end-Triassic extinction, and Rhynchosaurus, a reptile that also went extinct. (Joseph Smit/Wikimedia Commons)
See Also:
- Dinosaurs Rode Volcanic Armageddon to Victory
- Ancient Mass Extinctions Hint at Possible Ocean Future
- Mass Extinctions Change the Rules of Evolution
- A New Explanation for Ancient Mass Extinction
- Death Star Off the Hook for Mass Extinctions
Citation: "Atmospheric Carbon Injection Linked to End-Triassic Mass Extinction." By Micha Ruhl, Nina R. Bonis, Gert-Jan Reichart, Jaap S. Sinninghe Damsté, Wolfram M. Kürschner. Science, Vol. 333 No. 6041, July 22, 2011.
