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‘A bad time to be alive’: Mass extinction 444 million years ago linked to loss of oxygen in Earth’s oceans

‘By expanding our thinking of how oceans behaved in the past, we could gain some insights into oceans today,’ says scientist at Stanford University

Peter Stubley
Tuesday 14 April 2020 15:56 BST
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World facing first mass extinction since the dinosaurs

The first major mass extinction in Earth’s history was linked to a severe and prolonged lack of oxygen in the oceans, according to a new study which could help scientists understand modern climate change.

Some 85 per cent of all species perished during the Late Ordovician die-off about 444 million years ago – a time when the vast majority of life was marine-based and most of our present day continents formed a single land mass, Pangaea.

A first wave of extinctions was caused by global cooling. When that ice age ended, sea levels rose and oxygen levels plummeted, resulting in a deficiency of oxygen, or anoxia.

In a new study, researchers at Stanford University found evidence that these anoxic conditions lasted for more than three million years – significantly longer than similar extinction events.

“For most ocean life, it was indeed a really bad time to be alive,” said co-author Erik Sperling, an assistant professor of geological sciences at Stanford University.

The study, published in Nature Communications, examined the geological record on the boundary between the Hirnantian and Rhuddanian ages in an attempt to bolster the theory.

A new model was created by Richard George Stockey, a graduate student at Stanford Earth, to incorporate previously published metal isotope data as well as new data from samples of black shale from the Murzuq Basin in Libya.

Taking into account 31 different variables, including the amounts of uranium and molybdenum that settle on the sea floor, it concluded that severe and prolonged ocean anoxia must have occurred across large volumes of Earth’s oceans.

“We can confidently say a long and profound global anoxic event is linked to the second pulse of mass extinction in the Late Ordovician,” said Mr Sperling.

The researchers said the findings have relevance for today given that global climate change is contributing to declining oxygen levels in the open ocean and coastal waters.

Last December, another study found that the overall level of oxygen in the oceans has dropped by roughly 2 per cent, while the number of known hypoxic “dead zones” has skyrocketed from 45 known sites in the 1960s to at least 700 areas today, some encompassing thousands of square miles.

“We actually have a big problem modelling oxygenation in the modern ocean,” Mr Sperling said. “And by expanding our thinking of how oceans have behaved in the past, we could gain some insights into the oceans today.”

Mr Stockey, whose research was supported by the Alfred P Sloan Foundation, National Science Foundation, Packard Foundation and Nasa, added: “There is no way that low oxygen conditions are not going to have a severe effect on diversity.”

Lack of oxygen in the oceans may also have played a part in the Devonian mass extinction 375 million years ago.

The most famous mass extinction is the Cretaceous-Paleogene event that wiped out all non-avian dinosaurs some 65 million years ago as a result of an asteroid strike that acidified the planet’s oceans.

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