Greatest mass extinction driven by changes to oceans

252 million years ago there was the greatest mass extinction in Earth history, which wiped out more than 90 per cent of marine species and more than two-thirds of the animals living on land. At that time – also known as the boundary between the ages of Permian and Trias – huge amounts of carbon dioxide from volcanic eruptions were emitted into the atmosphere and also absorbed by the oceans. This changed the chemical composition of the oceans – making them more acidic – with catastrophic consequences for life on Earth, researchers say.

A new study, now published in the journal Science, is the first to show that highly acidic oceans were to blame for the marine mass extinction. “Scientists have long suspected that an ocean acidification event occurred during the greatest mass extinction of all time, but direct evidence has been lacking until now. This is a worrying finding, considering that we can already see an increase in ocean acidity today that is the result of human carbon emissions.” said Dr Matthew Clarkson of the University of Edinburgh’s School of GeoSciences, who co-ordinated the study.

To test the ocean acidification theory, the researchers reconstructed the acidity level of the seawater at that time. “It was quite challenging to reconstruct ocean pH values and to reveal an ocean acidification record out of rocks, that were deposited in an ocean 250 million years ago”, said professor Simone Kasemann from the Center for Marine Environmental Sciences (MARUM) at the University of Bremen. Therefore the scientists measured the isotopes of the trace element boron. From the ratio of the boron isotopes they could draw conclusions about the ocean pH value. The data obtained in the Bremen labs were combined at the University of Exeter with computer models of the Earth System.

The Permian-Triassic Boundary extinction took place over a 60,000 year period, researchers say. Acidification of the oceans lasted for around 10,000 years. Ocean acidification was the driving force behind the deadliest phase of the extinction, which dealt a final blow to an already unstable ecosystem, researchers say. Increased temperatures and widespread loss of oxygen in the oceans had already put the environment under pressure. The mass extinction of both marine and land-based animals demonstrates that extreme change took place in all of Earth’s ecosystems.

The findings are helping scientists understand the threat posed to marine life by modern-day ocean acidification. The level of carbon in the atmosphere that triggered the mass extinction was approximately five times higher than emission levels today, the team says. However, the carbon was released at a rate similar to modern emissions. This fast rate of release was a critical factor driving ocean acidification. Oceans can absorb some carbon dioxide but the large volume released – at such a fast rate – changed the chemistry of the oceans, the team says.

The team analysed rocks unearthed in the United Arab Emirates – which were on the ocean floor at the time – to develop a climate model to work out what drove the extinction. The rocks preserve a detailed record of changing oceanic conditions at the time.

The study, published in the journal Science, was carried out in collaboration with the University of Bremen, Germany, and the University of Exeter, together with the Universities of Graz, Leeds, and Cambridge. “This work was highly collaborative and the results were only possible because we assembled a unique team of geochemists, geologists and modellers to tackle an important and long-standing problem.” said Professor Rachel Wood of the University of Edinburgh’s School of GeoSciences.

Funding was provided by the the International Centre for Carbonate Reservoirs, Natural Environmental Research Council, Leverhulme Trust, German Research Foundation and the Marsden Fund.

Publication:
Ocean acidification and the Permo-Triassic mass extinction
M. O. Clarkson, S. A. Kasemann, R. Wood, T. M. Lenton, S. J. Daines, S. Richoz, F. Ohnemueller, A. Meixner, S. W. Poulton, E. T. Tipper
Published April 10, 2015 in Science, DOI: 10.1126/science.aaa0193

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