Palaeozoic World

In a boost to the CO2 mass extinction idea, a computer simulation of the Earth's climate 250 million years ago suggests that global warming triggered the so-called "great dying" in the Permian-Triassic boundary extinction.

A dramatic rise in carbon dioxide caused temperatures to soar to between 10 to 30 degrees Celsius higher than today, US researchers found. The warming had a profound impact on the oceans, cutting off oxygen to the lower depths and extinguishing most lifeforms.

The research added to the growing body of evidence that higher temperatures, rather than a giant space rock hitting the planet, led to the greatest mass extinction in history.

The extinction, at the end of the Permian Period and the beginning of the Triassic, had puzzled scientists for many years. Some 95% of lifeforms in the oceans became extinct, along with about three-quarters of land species.

Many possible reasons for this catastrophic event have been proposed - including impacts, volcanism, climate change and glaciation. Hard evidence, however, has been difficult to find.

This latest data from scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, supported the view that extensive volcanic activity over the course of hundreds of thousands of years released large amounts of carbon dioxide and sulphur dioxide into the air, gradually warming up the planet.

Deep impact

The NCAR team used a research tool known as the Community Climate System Model (CSSM) which looks at the combined effects of atmospheric temperatures, ocean temperatures and currents.

Their work indicated that temperatures in higher latitudes rose so much that the oceans warmed to a depth of about 3,000m (10,000ft).

This interfered with the circulation process that takes colder water, carrying oxygen and nutrients, into lower levels. The water became depleted of oxygen and was unable to support marine life.

"The implication of our study is that elevated CO2 is sufficient to lead to inhospitable conditions for marine life and excessively high temperatures over land would contribute to the demise of terrestrial life," Jeffrey Kiehl and colleagues wrote in Geology.

Until shortly before this study, computer models of past climate had been hampered by the difficulty of accounting for complex interactions between the various components of the Earth's climate system.

Professor Paul Wignall, of the University of Leeds, UK, who studies the Permian-Triassic boundary, said the models had not been sophisticated enough to recreate such "lethal super-greenhouse climates".

"I suspect many in the modelling community have been sceptical about just how bad conditions were 250 million years ago, even though the evidence is in the rocks; but now the latest climate system modelling is able to replicate climatic conditions that came close to destroying life on Earth," he said.

Other lines of research

At about the same time as the climate models were being finalised, a joint UK-Chinese team reported that the disaster that befell the planet 250 million years ago must have happened in phases.

Their conclusion was based on the abundance of "organic fossils" found in rocks at Meishan in southern China. These suggested that there were at least two episodes to the mass die-off that saw up to 95% of lifeforms disappear.

"And this fits with a growing body of literature that now points to a complex sequence of changes on Earth," Richard Pancost, from the University of Bristol, said.

Complex route to death

Some scientists have proposed the idea that the "great dying" at the boundary of the Permian and Triassic Periods could have occurred quite abruptly - the result of environmental changes brought on by the impact of a giant space rock. It is a similar argument to the one put forward to explain the demise of the dinosaurs at the much later date of 65 million years ago.

A geological structure, known as the Bedout High, in the seabed off what is now Australia, has even been suggested as the possible crater remains from the impactor. But it is an argument that has struggled to find favour.

The prevailing theory is that several factors - including supervolcanism and extensive climate warming - combined over thousands of years to strangle the planet's biodiversity. Earth may well have been hit by extraterrestrial objects, but it is unlikely there was some killer punch from space.

Ring of existence

The new data from China supports this view. It is based on the traces left in rocks by cyanobacteria. These photosynthetic, mostly single-celled organisms existed in vast blooms in the Permian oceans. They are one of the major groups of phytoplankton, which form the basis of the marine food chain.

However, the phytoplankton not eaten by higher organisms would have fallen to the seafloor over time to be incorporated into the sedimentary rocks we see today. And chemical components in their cell membranes have left telltale signs of their past existence.

Specifically, a lipid molecule, known as 2-methylhopane, has left ring structures in the Meishan rock. "These ring structures are the 'hydrocarbon skeleton' - that is how we would refer to them - and they can be preserved for a very long time," explained Dr Pancost.

The research team found two peaks of abundance in the Chinese rocks which are believed to indicate periods immediately following biotic crises in the oceans - times when the collapse of higher marine lifeforms allowed the cyanobacteria populations to boom.

"What we think happened was that the grazing pressure changed," explained Dr Pancost. "A lot of the fauna that went extinct went through larval stages that would have fed on the phytoplankton.

"Changes in the faunal assemblages would have changed predation patterns, and this led to the phytoplankton prospering."

Land of turmoil

The Permian-Triassic mass extinction killed off about 95% of all marine species and about three-quarters of all land families. It is the boundary at which the famous water-dwelling arthropods known as the trilobites were wiped out.

The Permian saw the creation of the supercontinent known as Pangea, and the geological evidence suggests this landmass experienced huge volcanic turmoil.

The Siberian Traps were built during the period - millions of cubic kilometres of basalt lavas were spilled on to the Earth's surface.

"This is the first time that we know what's happening to the very base of the food chain at this time. We had no handle on plankton populations before this because they don't fossilise," commented Dr Paul Wignall of the University of Leeds, who studies the Permian-Triassic extinction.

"It's very interesting, because to affect the base of the food chain takes a lot of doing. It shows a world in crisis."

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