Recently, we discussed evidence that the Earth's biggest mass extinction, the End Permian (often called the Great Dying), was triggered by a physiological crisis: high levels of atmospheric carbon dioxide lowered the ocean's pH and created problems with anoxia. That doesn't, however, tell us much about what happened on land or about how quickly events transpired. Now, researchers have provided the most precise dates yet on the Great Dying and found that it took place over less than 200,000 years and was accompanied by very rapid changes in the atmospheric carbon dioxide.

The published dates for the End Permian roughly agree—it happened about 250 million years ago—but they use different methods and sites to arrive at those dates. The different sites shouldn't affect the dating, but they could include different ecosystems. If the timing of the mass extinction isn't uniform across these sites, it's easy to see how they could produce different dates.

The new study attempts to cut down on the uncertainty through volume: 300 samples were dated from 29 different volcanic ash beds in South China. Combined, these sites cover ocean, transitional, and terrestrial environments.

The uranium-lead dating used in these studies has a standard error of between 60-120 Kyrs (Kilo-years). However, with so many different samples, the authors were able to narrow the range of the extinction down to about 200,000 years, not much more than the error in the technique. In the end, they place the extinction at just over 252 million years ago.

To track what was going on in the biosphere, the authors turned to carbon isotope ratios. Carbon has two stable isotopes, 12C and 13C. Since the latter is marginally heavier, the carbon used by living entities tends to be biased in favor of the lighter isotope. By tracing the ratio of these two isotopes, it's possible to get a sense of where any changes in the carbon cycle are taking place.

The authors further subdivided the carbon cycle into two fractions. Carbonate deposits are derived from organisms that incorporate it into shells. It's not part of the metabolic cycle, so it tends to directly reflect the levels of CO 2 in the atmosphere (which in turn drives the amounts dissolved in the oceans). The other source is organic carbon, which does reflect what was happening to organisms at the time.

What they found was that, for a period of about 90,000 years before the extinction started, the atmospheric carbon was gradually shifting away from 13C. Then, immediately prior to the mass extinction, there's a sudden plunge that took place over the span of less than 20,000 years. As the extinction kicked into gear, organic carbon followed suit, indicating a massive die-off of plant matter.

As the authors note, "The synchroneity of [carbonate] isotope excursion and extinction suggest a causal link between the introduction of the light carbon and the extinction." What can suddenly pump this sort of light carbon into the atmosphere? The authors suggest that the gradual decline indicated large amounts of carbon ending up in the air through some combination of the loss of a biological system to remove it; through increased volcanic activity; and/or a change in the rates of weathering.

This eventually pushed the ecosystem to a tipping point, during which there was a sudden, large release of carbon with a biological origin. The authors suggest that the gradual warming driven by the earlier carbon release dried out tropical forests, which then burned. Additionally, the heat may have destabilized methane stored as a solid in the ocean. Whatever the cause, it pushed the ocean into anoxia, helping to trigger the mass extinction.

There is clearly evidence for widespread volcanic activity in advance of the Great Dying. In many cases, the carbon released by that activity has been given a starring role as the killer. Here, it seems the authors have given it the role of supporting actor, one that set up the conditions in which carbon from a biological source delivered the final, fatal blow.

Science, 2011. DOI: 10.1126/science.1213454 (About DOIs).