About the time, 201 million years ago, that the super-continent Pangaea – the single land mass made up of all the present continents – started to break up, life on Earth was hit by a severe crisis that killed off numerous animal species.
On land, the devastation was not so bad, although many plants did wither and die.
Until now, this catastrophe has often been explained by violent volcanic actions started by the break-up of Pangaea.
This volcanic activity released both carbon dioxide and methane to the Earth’ atmosphere – greenhouse gases that warmed the climate and possibly caused methane ice at the bottom of the oceans to melt.
Both processes might have released so much of the greenhouse gas methane that global warming accelerated.
Many researchers believe that animals and plants were not greatly affected by the volcanic eruptions and their release of carbon dioxide.
For a long time, the accepted theory was that life on Earth was strangled by the greenhouse effect of the release of methane from the melting methane ice on the ocean floors.
But that theory has itself been strangled after a Danish research group at GEUS – the Geological Survey of Denmark and Greenland – published the results of their study.
In an article in the scientific journal Geology, the GEUS researchers document that the Earth was hit by mass extinction some time before the assumed release of methane.
“According to our studies, the widespread mass extinction occurred between two peaks in the atmosphere’s content of methane and carbon dioxide and not concurrently with the second peak, as has been presumed until now,” says Sofie Lindström, a senior researcher at Stratigraphy Department at GEUS, who led the study.
Lindström and her colleagues reached their conclusion after they studied a core taken from drilling operations in the underground beneath the town of Stenlille, 30 km northwest of Copenhagen.
The subterranean layers were formed here over a period covering the transition from the Triassic to the Jurassic geological periods.
By analysing carbon isotopes in the core’s many layers, they could draw a detailed graph of how the carbon composition of the atmosphere changed over time.
The scientists compared this graph with numerical analyses from the remains of animals and plants in the various layers.
When combined, the information from these two sources reveals both how and to what extent climate changes affected animal and plant life on Earth.
As the carbon composition in the atmosphere greatly influences the Earth’s climate, it also determines the conditions for life on Earth.
The drill core from Stenlille was chosen because it is complete: none of the layers from the time of the transition from the Triassic geological period to the Jurassic period is missing.
The layers contain both fossils of animals living in the sea and material from land.
The atmosphere of that time is no longer found, but vital information about its composition is stored in the various subterranean layers – which means they are a gigantic climate archives (see Factbox).
Subterranean layers are made of organic material, and the carbon variations in this material give the researchers an indication of sudden changes in the atmosphere’s composition.
The researchers’ graph of the composition of carbon isotopes in the drilling core confirms earlier studies, which have shown that the transition from the Triassic period to the Jurassic period had two peaks in the atmosphere’s content of light carbon isotopes. These peaks have been interpreted as releases of large amounts of carbon dioxide and/or methane to the atmosphere.
The first increase in greenhouse gases in the atmosphere was perhaps a longer-lasting trend resulting from volcanic activity that did not in itself directly cause any reduction in the number of animal species, the researchers say.
The catastrophic situation for life on Earth did not arise until some time later, in the period between the two peaks, when plant life on Earth has badly hit.
To ensure that their measurements do not just reflect local conditions in the Stenlille area, the researchers compared the data from this drill core with data from a well-known series of layers in England, which were formed at the same time and under similar conditions.
No matter whether the researchers analysed data from England or Stenlille they found the same results.
Very little pollen from trees on land has been found – only ferns and other spore plants appear to have been unaffected by the extreme conditions that prevailed. The catastrophe spread to the oceans, where more or less all life was affected.
In this period, Lindström and her colleagues no longer see any traces of life in the drill core – even plankton have disappeared.
The scientists see the new results as a great surprise and they are still deciding how to interpret them.
“We still don’t know how to explain all the details in the chain of events, nor all the causes of the mass extinction,” says Lindström. “We believe the first increase in methane and carbon dioxide in the atmosphere was started by a chain reaction of processes, ‘feedback mechanisms’, which, following a delay, led to the catastrophe.”
One hypothesis is that volcanoes’ release of sulphur dioxide caused acid rain and acidification of lakes and wetlands. When plants and trees die, there is nothing to keep the layers of soil in place, so they are washed into the ocean. Sediments muddied the water, which made it difficult for aquatic animals to filter their food from it. This made their survival difficult.
Remains of dead plants and plankton, which had ingested carbon from the atmosphere throughout their lives, were found in the drill core’s various layers.
Plants always prefer the light carbon variant isotope 12, found in large amounts in methane and carbon dioxide, rather than the heavier isotope 13.
The carbon composition in the individual layers of the drill core tells us what the climatic conditions were like on Earth over the ages.
“With the data we have collected so far, it appears that all the trees on land disappeared at the same time as the organisms in the water, but the question is whether this is actually correct,” says the researcher. “Future studies will hopefully shed more light on this.”
The second increase in methane in the atmosphere came later and could therefore not have caused the catastrophe, according to the study. On the contrary, this seems to have given new life to the plants.
Measurements indicate that, following the second peak, the vegetation re-established itself, although several species that were common during the Triassic period did disappear completely.
“After the second peak the plants apparently started to grow again,” says Lindström. “This contradicts our expectations, which were based on the very great changes in the environment and climate that characterised this period. But plants and animals do not necessarily react in the same way to climate change.”
The researchers are still considering why climate change was so beneficial for plants.
Life on Earth has been hit by mass extinction five times in the course of the last 540 million years of Earth’s history:
At the transition from the Ordovician period to the Silurian period (443 million years ago), during the Late Devonian period (about 370 million years ago), at the end of the Permian period (252 million years ago), at the end of Triassic period (201 million years ago) and at the Cretaceous/Tertiary transition (66 million years ago).
The Triassic period was generally really warm, dry and desert-like, but that changed towards the end of the period, when the climate in northern Europe became wetter and forests and marshes dominated the landscapes.
“When the Jurassic period started, the climate became even warmer and perhaps wetter in some seasons,” says Lindström. “As many plants had already adapted to intense heat, changes in the climate could not kill them. On the contrary, the large amounts of carbon dioxide probably strengthened the growth of these plants.”
While plant life on land quickly re-established itself after the catastrophe, life in the oceans continued to suffer.
It took a long time for ocean life to recover. Researchers believe that this difference in how plants and animals recovered from the catastrophe can result from the high content of sulphur dioxide in the atmosphere that acidified the oceans and made life difficult for calcareous-shelled organisms, as is the case today with tropical coral reefs.
The transition from the Triassic to the Jurassic period was a dramatic time and had great consequences for life on Earth – and everything is due to comparatively small fluctuations in the carbon composition of the atmosphere. This is something that Lindström sees as thought provoking at a time when the Earth is also experiencing carbon dioxide emissions and global warming.
“We live in an age when mankind itself is causing a large number of environmental catastrophes,” she says.
“We are changing the Earth’s environment, and this is happening quickly compared with the natural changes in previous ages. Life-affecting catastrophes on Earth exemplify how very quickly this has occurred, seen in a geological perspective. This can teach us how our world will react over time when we release so much carbon dioxide and sulphur into the atmosphere.”