A rapid decline in oxygen levels in the Earth's oceans around 251 million years ago could be the primary cause of the planet's largest extinction event - a crisis that occurred at the Permian-Triassic boundary and resulted in the loss of upwards of 90% of marine species.
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DUBLIN: A sudden and dramatic drop in oxygen levels in Earth's oceans caused the largest mass extinction ever, some 251 million years ago, a new study suggests.
The findings, published today in Proceedings of the National Academy of Sciences, show that the extent of oxygen-free water (anoxia) in the ancient ocean expanded rapidly at this time, coinciding with an extinction event that wiped out nearly 90% of marine species.
"Anoxia has been thought to be one of the possible causes of the world's largest mass extinction, but until now, the timing and severity of the anoxia was not precisely known," said lead author and geochemist Greg Brennecka from Arizona State University.
Earth's biggest extinction
Earlier studies of the Permian-Triassic boundary (251 million years ago) identified a big shift toward oxygen-deficient conditions at the time of the end-Permian crisis, which devastated marine life in Earth's oceans.
These studies, however, relied on local measures and some indicated changes happened either before or after the extinctions. The new study is the first to employ a measure that relates to global conditions, rather than conditions at a specific location.
It shows conclusively that the major changes in the global ocean occurred at the time of the crisis, not before or after it, said geologist Thomas Algeo at the University of Cincinnati, who was involved in the study.
"It shows that, given the correspondence in timing, these redox changes may have been a major cause of the extinction itself," said Algeo.
Global changes more reliable indicators
Changes in the ratio of two uranium isotopes (Uranium 235 and 238) can tell scientists about changes in oxygen levels throughout entire oceans. Uranium has a very long residence time and so the same ratio occurs everywhere in the global ocean at a given time.
Ocean oxygen levels are important because life in the ocean depends on oxygen. The study looked at sediments in China and identified a 6-fold expansion of seafloor experiencing complete anoxia (lacking oxygen). This happened in a short space of time - geologically speaking.
"We show this drastic change in ocean redox chemistry happened in the tens of thousands of years or even less," said Brennecka.
One favoured theory of end-Permian extinctions is that it was caused by the eruption of the Siberian Traps, a massive eruption of flood basalt lava which can coat massive stretches of land, or in this case, the ocean floor. Some argue that this triggered a deadly volcanic winter, with warming and anoxia following after the extinctions.
Deoxygenation occurred rapdily
This new study offers compelling evidence that ocean deoxygenation developed quickly at the time of the end-Permian extinction, commented Lee Kump, a geoolgist at Pennsylvania State University.
"Previous evidence suggested a more protracted period of anoxia preceded and followed the extinction," he explained, adding that it's possible that anoxia could have been the kill mechanism because its onset coincides with the extinction.
