Methane: the Great Dying?
What caused the worst mass extinction in Earth's history 251 million years ago? This event is one of the most catastrophic in life's history: the P/T extinction.
An asteroid or comet colliding with Earth? A greenhouse effect? Volcanic eruptions in Siberia? Or an entirely different culprit? Scientists have suggested many possible causes for this "Great Dying": severe volcanism, a nearby supernova, environmental changes wrought by the formation of a super-continent, the devastating impact of a large asteroid -- or some combination of these. Whatever happened during this period left no form of life undisturbed: No class or species was spared from devastation. Trees, plants, lizards, proto-mammals, insects, fish, mollusks, and microbes -- all were nearly wiped out. More than 9 in 10 marine species and 7 in 10 land species vanished. Life on our planet almost came to an end.
|The painting titled "K/T Hit" by artist Donald E. Davis. This impact occured 65 million years ago, ending the reign of the dinosaurs, and is not to be confused with the P/T event 250 million years ago.
Image Credit: Don Davis
This catastrophe - marked in the geologic record as the Permian/Triassic boundary - occurred about 250 million years ago--and is not to be confused with the better-known Cretaceous-Tertiary (K/T) extinction that signaled the end of at least fifty percent of all species, including the dinosaurs, 65 million years ago. During this earlier cataclysmic period in Earth's history, known as "The Great Dying," up to 96 percent of marine species and about 70 percent of land species were wiped out. Scientists have not been able to determine what caused this cataclysm to life, although theories of asteroid impacts, climatic changes, and the greenhouse effect have all been suggested. Many paleontologists have been skeptical of the theory that an asteroid caused the extinction, since early studies of the fossil record suggested that the die-out happened gradually over millions of years -- not suddenly like a single, catastrophic event. But as their methods for dating the disappearance of species has improved, estimates of its duration have shrunk from millions of years to between 8,000 and 100,000 years--a very quick event in geological terms.
A Northwestern University chemical engineer believes the culprit may be an enormous explosion of methane (natural gas) erupting from the ocean depths. This explanation is closer to the inverse of an external impact, like an asteroid, and more like a disgorging of trapped energy that erupts from deep below the oceans. Such a global catastrophe has a more local precedent, as a similar eruption happened in Africa at Lake Nyos in 1986, killing 1700 people and rippling as far away as 25 kilometers.
|Self-sustained soda fountain (21 m height), Lake Nyos, Cameroon, Africa, is part of a project to degas gradually soda lakes.
Credit: Bernard Canet
In an article published in the September issue of Geology, Gregory Ryskin, associate professor of chemical engineering at Northwestern, suggests that huge combustible clouds produced by methane gas trapped in stagnant bodies of water and suddenly released could have killed off the majority of marine life and land animals and plants at the end of the Permian era -- long before dinosaurs lived and died.
The mechanism also might explain other extinctions and climate perturbations (ice ages) and even the Biblical flood, as well as be the cause of future catastrophes.
Ryskin calculated that some 10,000 gigatons of dissolved methane could have accumulated in water near the ocean floor under high pressure. If released quickly, perhaps triggered by an earthquake, the resulting cloud of methane would have an explosive force about 10,000 times greater than the world's entire stockpile of nuclear weapons. The huge conflagrations plus flooding and overturned oceans would cause the extinctions. (Approximately 95 percent of marine species and 70 percent of land species were lost.)
"That amount of energy is absolutely staggering," said Ryskin. "As soon as one accepts this mechanism, it becomes clear that if it happened once it could happen again. I have little doubt there will be another methane-driven eruption -- though not on the same scale as 251 million years ago -- unless humans intervene."
|What the world looked like 250 million years ago. Plate tectonics pushed the continents together to form the super-continent Pangea and the super-ocean Panthalassa. Weather patterns and ocean currents shifted, many coastlines and their shallow marine ecosystems vanished, sea levels dropped. Credit: Chris Scotese. [more]
Ryskin writes in Geology: "Focusing on the Permian-Triassic (P/T) boundary, ca. 251 Ma, I explore the possibility that mass extinction can be caused by an extremely fast, explosive release of dissolved methane (and other dissolved gases such as carbon dioxide and hydrogen sulfide) that accumulated in the oceanic water masses prone to stagnation and anoxia (e.g., in silled basins)." Such events have climatic effects. After the explosion, both dust and the vaporized sulfur smog would have darkened the atmosphere and blocked sunlight. Acid rains - created by the mixing of vaporized sulfur and water - acidified lakes and streams. With such a long-lasting sulfur smog, temperatures would have remained cool worldwide and photosynthesis would have been suspended for several lifetimes. Both methane (CH4) and carbon dioxide (CO2) are greenhouse gases, so temperatures - which had experienced a period of cooling because of the sulfur - would have immediately started to rise.
There is a recent precedent for the kind of explosive eruptions described by Ryskin: a lake in Cameroon, Africa--Lake Nyos--went through just such a rapid change in 1986. Lake Nyos is a water-filled throat of an old volcano and it is deep and funnel-shaped. Although no longer erupting, there is still gas being released by the old plumbing system under the lake. "The mechanism of the explosive release," continues Ryskin, "is the same as in the Lake Nyos disaster of 1986, i.e., a water-column eruption caused by the interplay of buoyancy forces and exsolution of dissolved gas. The eruption brings to the surface deep anoxic waters that cause extinctions in the marine realm."
In 1986, the Lake Nyos explosion killed more than 1700 people and livestock up to 25 km away.
Carbon dioxide gas was released directly into the deepest waters of the lake, where it could remain in solution (the way that carbon dioxide stays in solution in an unopened carbonated soda). In this situation the lake could build up a large amount of carbon dioxide dissolved in the deeper water. This was a stable situation. The carbon-dioxide charged water was slightly denser than the normal water in the upper levels of the lake, and the weight of the overlying water kept the carbon dioxide in solution in the deeper parts of the lake.
But that stable situation changed rapidly: somehow some of that carbon dioxide-rich water was displaced upward into shallower depths to the point where the overlying water pressure was lower and carbon dioxide bubbles could start to form (like when you lower the pressure on a soda by opening the bottle and suddenly bubbles start to form). At Lake Nyos, once these bubbles started to form they wanted to rise to the top, this brought up more carbon dioxide-rich water which then also started to develop bubbles, and pretty soon there was a big rush of carbon dioxide bubbles to the surface. Once all this carbon dioxide reached the surface, it splashed some lake water out of the lake, like a big bubble bursting.
As deep water becomes gas-charged, a sudden release is possible: the resulting dense bubble then hugs the ground and drifts until in the case of methane, a spark triggers the kind of huge explosions that might have global and not just local effects--all from the depths of a soda lake like Lake Nyos.
Ryskin speculates on what happened next, when "terrestrial extinctions are caused by explosions and conflagrations that follow the massive release of methane (the air-methane mixture is explosive at methane concentrations between 5% and 15%) and by the eruption-triggered floods. This scenario accounts well for the available data, and may be relevant to other phenomena."
Studying biological catastrophes like the P/T extinction can help astrobiologists understand the close connection between life, geology, chemistry - and how such events may disrupt this sometimes delicate relationship.
Related Web Pages
Great Impact: Part I
Impact Hazards Website
NASA/JPL Near Earth Object Program
Do We Know What Killed the Dinosaurs?
Massive Volcanic Eruptions in Siberia Linked with Mass Extinction
Theories of Causes of the PT extinction