Throughout the history of the planet, there have been a number of mass extinction events. The largest was the "Great Dying," which occurred at the border of the Permian and Triassic periods—during this event, over half of the species on Earth at the time went extinct. Thanks to a recent find in southwestern China, researchers have been able to nearly pinpoint the events that appear to have precipitated the Great Dying.

Major volcanic activity is often a leading candidate for causing mass extinctions, but pinpointing a specific eruption or sequence of eruptions that caused a mass die-off of marine life is difficult. Typically, indirect geochronological methods must be employed, and they have known timing inaccuracies that complicate the analysis. In the case of the Great Dying, it appears that there may have been several pulses of extinctions.

The finds from China are described in a letter in this week's edition of Science, and they come very close to providing a direct cause-and-effect relationship between volcanic activity and the earliest of these mass extinction pulses, the End-Guadalupian, which occurred 8 million years before the primary event at the Permian-Triassic boundary.

The researchers were studying the Maokou Formation, a set of limestone rocks in the Emeishan flood basalt province in south China (flood basalts are formed by massive volcanic activity). They found that the rocks contained evidence of both a massive marine extinction and a major carbon isotope excursion occurring within the same geological temporal and spatial frames.

At the onset of the eruptions, the rocks reveal that the carbonate deposition, which is driven by oceanic life, collapsed. The fossil record shows the disappearance of shallow water creatures such as foraminifers and algae. Approximately 50 kilometers to the East of the Emeishan volcanic range, a large amount of fauna went extinct in a similar time frame.

A substantial and widespread shift in the carbon isotope ratios suggest that the eruptions here had a global impact. The large shift in the carbon isotope ratios are an indication that the Earth's carbon cycle was knocked out of whack for some time after the extinction, with a likely impact on climate. The immediate effects of the eruptions—acid rain, short-term cooling, and other environmental effects—would also have helped extend the extinction event that was in progress.

The authors do not go so far as to conclude that they have unearthed a definite cause-and-effect relationship between the Emeishan volcanic eruptions and the global marine mass extinction at the End-Guadalupian, but they feel that the evidence makes a compelling case. The findings presented in this letter clearly give a big boost to the proposal that volcanism was responsible for one of the extinctions near the Permian-Triassic boundary. Mass extinction may not come only from above, but also from within.

Science,

2009. DOI: 10.1126/science.1171956

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