WELLINGTON, July 20 (Xinhua) -- Ancient rocks unearthed in the Middle East have shed light on why life on Earth took millions of years to recover from the greatest mass extinction ever, the New Zealand scientist who led the international study said Wednesday.
The study provided fresh insight into how Earth's oceans became starved of oxygen in the wake of the "Great Dying" event 252 million years ago, delaying the recovery of life by 5 million years.
It was also helping scientists to better understand how environmental change could have disastrous consequences for life on Earth, said Otago University chemistry researcher Dr Matthew Clarkson.
The Permian-Triassic Boundary extinction wiped out more than 90 percent of marine life and around two thirds of animals living on land, and the Earth's oceans became starved of oxygen conditions known as anoxia during the recovery period.
Previous research suggested the mass extinction and delayed recovery were linked to the presence of anoxic waters that also contained high levels of harmful sulfide compounds.
But the study found anoxic conditions at the time were more complex, and the toxic, sulfide-rich state was not present throughout all the world's oceans, Clarkson said in a statement.
The team analyzed rocks unearthed in Oman that were formed in an ancient ocean around the time of the extinction, and found that while the water was lacking in oxygen, toxic sulfide was not present.
Instead, the waters were rich in iron, indicating that iron-rich, low-oxygen waters were a major cause of the delayed recovery of marine life following the mass extinction.
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.
The precise cause of the long recovery period remained unclear, but increased run-off from erosion of rocks on land caused by high global temperatures likely triggered anoxic conditions in the oceans.
"We knew that the lack of oxygen in the oceans played a key role in the extinction and recovery processes, but we are still discovering how exactly it was involved. 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," said Clarkson.
