By Tim Radford / Climate News Network
German scientists are now sure they can explain the biggest extinction of life on this planet: a catastrophe at the end of the Permian Epoch 252 million years ago.
It happened because atmospheric carbon dioxide (CO2) levels rose dramatically over thousands of years, turning the seas ever more acidic, to precipitate dangerous changes in oceanic conditions. In this, so far the greatest of life’s periodic extinctions, 95% of all the sea creatures perished, along with three-quarters of all life on land.
The evidence of dramatic change at a particular level in the geological record − the Permian-Triassic boundary, now more precisely dated at 251.9 million years ago − has been teasing geologists for decades. And climate change through one agency or another has been a favoured candidate from the start.
That moment in vanished history was marked by a long, slow but world-changing series of volcanic eruptions − the evidence lies in great sheets of basalt known to geologists as the Siberian Traps − and since outgassing of carbon dioxide is associated with all volcanic eruption, a greenhouse effect driven by carbon dioxide has long been a favourite.
But circumstantial evidence was not the same thing as proof. Dramatic ozone loss − ozone forms a shield against potentially lethal ultra-violet radiation in the upper atmosphere − has also been named as a suspect. Until somebody could deliver clear evidence of the machinery that ensured the extinction, the verdict could only be “not proven.”
Right now, with human help, the planet Earth is undergoing what has been termed a sixth great mass extinction, and evidence from the Permian extinction provides a lesson for what could happen in a changing climate, but so far simply that: an awful warning, rather than a dreadful example.
Now European researchers report in Nature Geoscience that they think they have settled the matter. They used evidence from fossils exposed in the Italian Alps to recreate the conditions in the ancient ocean.
The story told by the changing isotopes of carbon, oxygen and most importantly boron in a series of brachiopod fossils was of “substantial” change in the chemistry of sea water 252 million years ago: a change that could be linked to the carbon dioxide from the Siberian Traps eruptions.
The researchers took the reasoning a step further: they used computer models to simulate the conditions of the time. The world warmed, the oceans became increasingly acidic, and right at the outset those organisms in the sea that build shells from calcium were snuffed out of existence.
But the higher temperatures, greater evaporation and ever-higher rainfall that mark a greenhouse world meant ever-greater chemical weathering on land itself. So more nutrients flowed from land to ocean, to deliver too much fertiliser, to encourage too much algal growth.
What followed was a dramatic depletion of dissolved oxygen: marine life was increasingly stifled, or poisoned by sporadic sulphide concentration.
“We are dealing with a cascading catastrophe in which the rise of CO2 in the atmosphere set off a chain of events that successively extinguished almost all life in the seas,” said Hana Jurikova, then of the Helmholtz Centre for Ocean Research in Kiel, Germany, but now at St Andrew’s in Scotland, who led the study.
It is an axiom of geological research that the present is key to the past, but her forensic account of ancient mayhem may not be a precise lesson for today.
“Ancient volcanic eruptions of this kind are not directly comparable to anthropogenic carbon emissions, and in fact all modern fossil fuel reserves are far too insufficient to release as much CO2 over hundreds of years, let alone thousands of years as was released 252 million years ago,” said Dr Jurikova.
“But it is astonishing that humanity’s CO2 emission rate is currently fourteen times higher than the annual emission rate at the time that marked the greatest biological catastrophe in Earth’s history”.