Greatest mass extinction event driven by ocean acidification

An increase in ocean acidity was responsible for Earth’s most severe mass extinction event, a recent study has confirmed.

The Permian-Triassic Boundary (P- Tr) extinction event – also known as The Great Dying, took place 252 million years ago and occurred as a result of Earth’s oceans absorbing vast amounts of carbon dioxide from powerful volcanic eruptions, altering the chemical composition of the oceans – making them more acidic. It is believed that extreme volcanic activity in the Siberian Traps released huge amounts of carbon dioxide into the atmosphere, triggering the start of the extinction event.

The event took place over a 60,000 year period, with acidification of the oceans lasting for around 10,000 years.This particular extinction event wiped out up to 96% of all marine species as well as 70% of terrestrial vertebrate species. Some 57% of all families and 83% of all genera became extinct.

The study co-ordinated by the University of Edinburgh, carried out in collaboration with the University of Bremen and the University of Exeter, together with the Universities of Graz, Leeds, and Cambridge is the first to show that increased ocean acidity was responsible for the event.

The team analysed rocks unearthed in the United Arab Emirates, which would have been 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. An abrupt shift in ocean pH levels, lasting for about 10,000 years, was discovered by comparing the ratios of boron isotopes in these Permian-age rocks. This data indicates that ocean acidification was the driving force behind the deadliest phase of the extinction, which dealt a final blow to an already unstable ecosystem, when increased temperatures and widespread loss of oxygen in the oceans had already put the environment under pressure.

Field work in the United Arab Emirates. Credit: D. Astratti

Field work in the United Arab Emirates.
Credit: D. Astratti

It is known that Earth’s oceans can and do absorb carbon dioxide (about a quarter of the CO2 we release into the atmosphere every year), however the extremely large volume released from massive volcanism 250 millions years ago, combined with the fast rate of release, drastically altered the oceans chemistry – with catastrophic results for marine and terrestrial organisms.

The extinction primarily affected organisms that possessed calcium carbonate skeletons, especially those reliant on stable CO2 levels to produce these skeletons.

These calcifying organisms span the food chain and include coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs. Under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ion is at supersaturating concentrations. However, with a massive decline in pH as exhibited during the P-Tr, the concentration of carbonate ions required for saturation to occur increases, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. The marine groups with the highest survival rates during the event generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; with more heavily calcified organisms possessing simpler breathing apparatus suffering the highest mortality.

How increased acidity impacts calcifying organisms - NOAA

How increased acidity impacts calcifying organisms – NOAA

The researchers findings have revealed a disturbing parallel with what is occurring to our oceans in present times, with ongoing modern-day ocean acidification. The amount of carbon in the atmosphere that triggered the mass extinction was probably greater than today’s fossil fuel reserves, the team says, however the carbon was released at a rate similar to modern emissions, which is the concerning factor.

Dr Matthew Clarkson, of the University of Edinburgh’s School of GeoSciences, who co-ordinated the study, said: “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.

Professor Rachel Wood, of the University of Edinburgh’s School of GeoSciences, said: “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.”

Life on earth: Mass extinctions

Life on Earth began about 3.8 billion years ago in the form of microbes that left few traces. As animals and plants became bigger, they left fossils behind – and the evidence of five previous mass extinctions:

Ordovician-Silurian (443m years ago): most of life lived in the seas. About 85 per cent of marine species were killed.

Late Devonian (359m years ago): about three-quarters of species became extinct over several million years. Much of the sea became starved of oxygen, possibly due to asteroid impacts.

Permian (252m years ago): nicknamed the Great Dying because it was the biggest mass extinction in history, killing off about 96 per cent of marine species. Volcanoes were thought to be involved.

Triassic-Jurassic (200m years ago): over several million years about half of all species at the time disappeared. But strangely plants were not badly affected.

Cretaceous-Tertiary (65m years ago): famous for being the end of the dinosaurs. An asteroid impact was almost certainly involved, preceded by volcanic eruptions.

Journal Reference

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

Photo Header Credit

Prehistoric Oceans – National Geographic

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