Water toxicity after extinction event clue to impact of climate change on life forms.

It was the greatest extinction event the world had ever known, wiping out more than 90 per cent of marine life and around two thirds of animals living on land.

In a major study out today, led by a Kiwi researcher, scientists suggest it was iron-rich, low oxygen waters that delayed the planet's recovery from the so-called "Great Dying" by around five million years.

The apocalyptic event, which took place around 252 million years ago, was understood to have been triggered by volcanic eruptions loading the Earth's oceans with massive amounts of carbon dioxide, increasing the acidity of the great water masses with catastrophic results.

That highly acidic oceans were to blame for what is formally known as the Permian-Triassic Boundary extinction was first demonstrated in a study last year co-authored by University of Otago researcher Dr Matthew Clarkson.

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In a new investigation, published in the journal Nature Communications, Clarkson has unravelled the factors behind the planet's slow comeback - something that could help us better understand how environmental change can have disastrous consequences for life.

While previous research linked the event and recovery to the presence of anoxic waters that also contained high levels of harmful sulphide compounds, the study authors say anoxic conditions at the time were more complex, and that this toxic, sulphide-rich state was not present throughout all the world's oceans.

The team used chemical techniques to analyse rocks unearthed in Oman that were formed in an ancient ocean around the time of the extinction. Data from six sampling sites, spanning shallow regions to the deeper ocean, revealed that while the water was lacking in oxygen, toxic sulphide was not present. Instead, the waters were rich in iron, suggesting it was this and low oxygen in the waters behind the delay.

The study further showed how oxygen levels varied at different depths in the ocean. While low oxygen levels were present at some depths and restricted the recovery of marine life, shallower waters contained oxygen for short periods, briefly supporting diverse forms of life.

While the precise cause of the long recovery period remains unclear, increased run-off from erosion of rocks on land, caused by high global temperatures, likely triggered anoxic conditions in the oceans.

Said Clarkson: "Our findings about the chemistry of the ocean at the time provide us with a clearer picture of how this complex process delayed the recovery of life for so long."

Apocalypse then

• A mass extinction event around 252 million years ago.

• Followed by a recovery period that lasted five million years.

• Wiped out 96 per cent of all marine species and 70 per cent of terrestrial vertebrate species.

• Some 57 per cent of all families and 83 per cent of all genera became extinct.