Rock samples from Quebec's Anticosti Island are offering new clues about Earth's first major mass extinction event, suggesting that it may have been caused by a cooling climate.
A team of scientists from the U.S., China, France and the University of Ottawa published a paper in the exploring the Late Ordovician mass extinction event, which took place around 445 million years ago. It is the oldest among the "big five" mass extinction events and saw around 85 per cent of marine species disappear during that time.
鈥淚f you had gone snorkeling in an Ordovician sea you would have seen some familiar groups like clams and snails and sponges, but also many other groups that are now very reduced in diversity or entirely extinct like trilobites, brachiopods and crinoids,鈥 said study co-author Seth Finnegan in a news release.
But when these species disappeared, they didn't die off suddenly, like how the dinosaurs did during the Cretaceous extinction 65 million years ago. Instead, the Late Ordovician mass extinction event played over a period of 500,000 to two million years.
The researchers sought to investigate whether a lack of oxygen in the seawater, also known as anoxia, played a part in the mass extinction. Anoxia and global cooling are two of the prevailing hypotheses on the cause of the extinction event.
Measurements of iodine concentration in carbonate rocks from that period were taken from Anticosti Island as well as the Copenhagen Canyon in Nevada. These samples, combined with computer modelling simulations, would offer clues regarding the oxygen levels at various oceanic depths.
The data showed no evidence that oxygen levels decreased in the shallow ocean, where most organisms lived.
"Upper-ocean oxygenation in response to cooling was anticipated, because atmospheric oxygen preferentially dissolves in cold waters,鈥 said lead author Alexandre Pohl in a news release.
These findings show that a cooling climate, rather than a lack of oxygen in the ocean, was the likely cause of the mass extinction event, the researchers say.
However, researchers were also surprised to find a lack of oxygen in the lower ocean, an outcome that would normally have been associated with volcano-induced global warming.
The researchers say their findings show that no simple relationship exists between temperatures and oxygen levels in the water.
鈥淔or decades, the prevailing school of thoughts in our field is that global warming causes the oceans to lose oxygen and thus impact marine habitability, potentially destabilizing the entire ecosystem,鈥 co-author Zunli Lu said in a news release. 鈥淚n recent years, mounting evidence points to several episodes in Earth鈥檚 history when oxygen levels also dropped in cooling climates.鈥
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