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Suspect killer: volcanism

Approximately 250 million years ago, almost 90 percent of the planet’s species disappeared, marking the largest mass extinction in Earth’s history. For years scientists have debated what happened at the Permian-Triassic (P-Tr) boundary. New evidence from geochronological data on Siberian basalts published by Mark Reichow of the University of Leicester and colleagues in the June 7 Science points to volcanism.

This research is not the first to charge volcanism with mass destruction. However, what is new about Reichow and his team’s work is that they record the timing and volume of the largest known volcanic province, the Siberian Traps. Reichow’s team estimates the province is twice as large as previously thought giving it an area equal to half the size of Australia. They suggest that because the Traps are so large, they provide compelling evidence that volcanism caused climate perturbation and in turn the P-Tr extinctions.

“This is a type example of a trend within flood basalt research. Once geologists recognize coherent provinces in terms of geochemistry and ages, new discoveries keep showing them as bigger and bigger. However, this is by far the biggest and most significant,” says Paul Renne, a geologist at the Berkeley Geochronology Center.

Approximately a dozen large igneous provinces (LIPs), such as the Siberian Traps, have occurred in the last 250 million years. LIPs are suspects in the case of mass extinction because they tend to coincide with these events. Volcanologists believe that LIPs spewed out magma, particulates and volcanic gases, such as sulfur dioxide and carbon dioxide, for periods of 1 to 3 million years and at rates higher than seen anywhere on Earth today.

Scientists widely accept the view that present day volcanic activity, such as on Iceland, shows that volcanic gases modify the climate. How much gas erupts will regulate how much the climate and subsequently ecosystems are destabilized, Renne explains.

“We know that the killing mechanisms for the P-Tr extinction seem to be related to global warming — oceanic anoxia and changes in terrestrial ecosystems. Our discovery that the Siberian Traps are much larger than previously realized fits in with this scenario very well — double the size of the Traps and you double the volcanic carbon dioxide output,” says Andrew Saunders, co-author and also of the University of Leicester.

The basalt samples Reichow and his team used are from the Western Siberian Basin. Although this basin lies adjacent to and west of the previously known extent of the Siberian Traps, it lacks basalt exposures, which is why it was not considered part of the province. Drill cores reaching to depths of 2 to 3 kilometers revealed basalts that underlie the exposed sedimentary rocks. Some had speculations that these basalts were related to those of the Siberian Traps, but without accurate age dating, researchers could not confirm a connection. Reichow and his team’s isotopic dating revealed ages coincident with the Siberian flood basalts. Major and trace element geochemistry also indicated the basalts had a source similar to the Trap basalts.

The doubling of the Siberian Flood Basalt Province might explain why the P-Tr boundary extinctions were also the largest in Earth’s history. However some scientists believe that, while volcanism may play a part, it may not be the primary cause for these extinctions. Instead they favor a scenario of many catastrophes, including a meteorite impact. “For an extinction to occur, the evidence — specifically paleontological — suggests it must be more complex than from a single event,” says Luann Becker, a geologist from the University of California-Santa Barbara and leading proponent for meteorite impact as a cause of the P-Tr extinction (Geotimes, April 2001).

Few scientists dispute that an impact was associated with the Cretaceous-Tertiary (K-T) boundary extinctions and the demise of dinosaurs 65 million years ago. Also well recognized at the K-T is the presence of another LIP, the Deccan Traps in India. Scientists such as Becker hypothesize a link among all three — volcanoes, meteorite impacts and extinction events — with meteorite impacts being the primary cause. Becker suggests that when a meteorite crashes into Earth, it causes perturbations not only in the climate, but also in the solid earth, triggering volcanism that in turn compounds climate change.

Although, Becker adds, there is no smoking gun for an impact at the P-Tr boundary as at the K-T boundary, she believes that the presence of cosmic tracers points to a potential impact at the P-Tr boundary. “There is work on the Triassic-Jurassic boundary which also suggests there could be an extraterrestrial impact at this time. This makes three extinction events in the history of Earth’s life associated with volcanism and potentially with an impact. These are significant catastrophes and the question is whether there is a causal link between all these events.” Other scientists remain skeptical. “The causal links themselves are rather complex, poorly understood and very difficult to prove,” Saunders says. Renne expresses similar doubts in reference to the current debate on whether cosmic tracer signatures are conclusively observed in P-Tr boundary sediments.

One way to distinguish between a meteorite impact and volcanism as the primary cause for extinctions is to study extinction rates. Impacts cause rapid (tens of thousands of years) extinction rates. In contrast, extinctions caused by volcanoes alone take millions of years. As Saunders explains, estimates for the duration of the P-Tr extinctions vary widely.

Despite the debate, Renne says, Reichow and colleagues’ work strengthens the causal link between the Siberian Traps and the P-Tr extinctions. “The discussion of whether or not a meteorite impact occurred is completely separate from, and unaffected by, our research,” Saunders says. “The jury is still out on this one.”

Salma Monani


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