Global warming and mass extinction at the end of the Triassic
The research team behind the new study comprises geologists from the Netherlands, the UK, Denmark, the US, Belgium, Luxembourg and Sweden. Led by researchers Bas van de Schootbrugge and Teuntje Hollaar at Utrecht University, they have jointly investigated layers of sedimentary rock from the transition between the Triassic and Jurassic periods in four drill cores from the UK, Germany, Denmark and Luxembourg.
At the time these sedimentary layers were formed, large parts of north-western Europe were covered by an epicontinental sea – that is, a shallow inland sea that stretched across the landmasses. The drill cores document how life and the environment at each site changed when life on Earth was struck by one of the five greatest mass extinctions in the planet’s history – the mass extinction at the end of the Triassic period 201 million years ago.
This major crisis in the history of life was caused by extensive and prolonged volcanism that arose when the supercontinent Pangaea began to break apart. Large quantities of volcanic rocks – lava and basalts, remnants of this extensive volcanism – are preserved today on four continents around the central Atlantic: north-western Africa, Europe, eastern North America and northern South America. The volcanic activity occurred in bursts over hundreds of thousands of years and released enormous quantities of greenhouse gases (carbon dioxide and methane), sulphur dioxide and heavy metals. This led to global warming of at least three to four degrees Celsius and extensive climate change in many regions.
Three out of four animal species were lost
Both marine and terrestrial life were affected, and it is estimated that around 75 per cent of all species on Earth became extinct during or in the aftermath of the crisis. During the period representing the mass extinction itself, researchers have mapped changes in vegetation in north-western Europe.
Sofie Lindström, a state geologist at the Geological Survey of Sweden, an adjunct professor at the Department of Geosciences and Natural Resource Management at the University of Copenhagen, and a senior lecturer at the Department of Earth and Environmental Sciences at Lund University, is a co-author of the article and explains that north-western Europe was covered by forest and forested mires in the period prior to the mass extinction.
"During the mass extinction, widespread forest die-off then occurred. The vegetation came to be dominated instead by ferns, which formed open savannahs during the tens of thousands to hundreds of thousands of years that the crisis lasted. In the early Jurassic period, following the mass extinction, ferns remained a dominant feature of the vegetation for a long time, even as trees began to return. We wanted to gain a better understanding of why ferns remained dominant for such a long time," says Sofie Lindström.
Tracing forest fires in the deep past
The researchers used several different methods to analyse and compare data from the four drill cores.
“By studying the presence of fossilised charcoal and organic molecules formed in smoke from forest fires – known as polycyclic aromatic hydrocarbons (PAHs) – we were able to map the occurrence of fires in the vegetation across the Triassic–Jurassic boundary,” explains co-author Sylvain Richoz, senior lecturer at the Department of Earth and Environmental Sciences at Lund University.
Combined with pollen and spore data, these forest fire records show that forest fire activity was intense during the period of mass extinction and that this coincides with the widespread distribution of ferns.
“However, charcoal data can be misinterpreted if larger pieces of charcoal break down into smaller fragments within the sediments. Similarly, PAHs emitted during forest fires do not necessarily originate from nearby forest fires, or the molecules may not be preserved at all,” explains Sylvain Richoz.
The research team therefore developed a new method for tracing forest fire activity in the deep past. By analysing colour changes in the fossilised spores and pollen preserved in the sedimentary layers of the drill cores across the Triassic–Jurassic boundary, the researchers were able to determine whether the microscopic plant remains had been exposed to high temperatures.
“It is a simple and very inexpensive technique that complements the charcoal and PAH data well,” says Sylvain Richoz.
Colour patterns that deviated from expectations
Fossil spores and pollen consist of a substance called sporopollenin, which is made up of carbon, oxygen and hydrogen. When spores and pollen are buried in sediment, they are subjected to changes in pressure and temperature. As the sediments are buried deeper, the temperature rises. Over a long period, the fossils are affected by the heat, much as if they were slowly ‘baked’ into the bedrock. The organic material then becomes richer in carbon and darkens; in other words, the deepest layers should contain the darkest fossils.
“But here we found a completely different pattern. In the oldest, deepest samples in the cores, the pollen and spores are light yellow to yellow-brown in colour, but in the interval corresponding to the mass extinction itself, they are brown to extremely dark brown. After the extinction, the colours return to a light yellow again,“ says Sofie Lindström.
This phenomenon puzzled the researchers. As it occurs in all four drill cores, during exactly the same time interval, it could not be related to the burial depth of the sediments, since the four areas from which the drill cores originate have undergone very different geological histories.
The ‘dark zone’
Using a camera attached to a light microscope, the researchers measured the RGB colour spectrum exhibited by spores and pollen in each sample. The colour values were then converted into an average greyscale value that could be compared between samples within the same core and between cores from different sites. This enabled the team to map the colour changes.
The team generated 15,000 measurements from both pollen and spores from plants that lived before, during and after the mass extinction at the end of the Triassic period. By comparing the colour changes in both tree pollen and fern spores, it was possible to rule out any biological effects specific to different plant groups. Sofie Lindström explains:
“Since all plant groups exhibit the same colour changes, these must be the result of external factors. When we compared the colour changes with the other indications of forest fires, we realised that the dark zone represents prolonged and intense forest fires during the period of the mass extinction.” ‘The dark zone actually coincides exactly in time with the spread of fern savannahs, the mass extinction and the increased occurrence of charcoal and PAHs,’ she says.
A Triassic inferno
Ferns have survived many crises in the Earth’s history. During the Triassic and Jurassic periods, there were no flowering plants and the land was instead covered by ferns.
“They are truly remarkable. Some species can adapt to some of the most extreme environments today and can rapidly colonise disturbed habitats. They can be regarded as true pioneer species,” says Sofie Lindström.
The explosive spread of ferns at the time of the mass extinction at the end of the Triassic was the result of a combination of factors. These were caused by extensive volcanism and included global warming, forest die-off, soil erosion and forest fires. Sofie Lindström explains that certain ferns can rapidly take over disturbed landscapes and prevent other plants from establishing themselves; furthermore, forest fires can stimulate ferns to spread even faster. Although the ferns’ fronds are destroyed in the fires, they can quickly regrow via their underground root systems and outcompete other plants in the process.
This ability likely contributed to the dominance of ferns during the mass extinction event, which is estimated to have lasted between 40,000 and perhaps as long as 300,000 years. On the widespread fern savannahs, certain species acted as fire ladders whilst simultaneously smothering other vegetation.
“When thick mats of ferns dry out, the dry leaves become the perfect fuel for igniting and spreading intense forest fires,” says Sofie Lindström.
Awareness of the environments we humans affect
The ferns both benefited from and provided fuel for the repeated massive forest fires, and this helped to prevent the recovery of other vegetation during the mass extinction interval.
“It must have been a harsh world to live in for both plants and animals – at times a veritable inferno,” agree Sofie Lindström and Sylvain Richoz.
Although this happened more than 200 million years ago, it still feels relevant to today’s society.
“Like studies of modern wildfires, our research shows that rising temperatures, drought and changes in vegetation types increase both the risk of forest fires and their severity. The lesson we can draw from this is that we need to be much more aware of the plant habitats that we humans influence or create,” says Sofie Lindström.
Publication
Link to the scientific article in Nature Geoscience:
Continental-scale fern savannah wildfires during end-Triassic greenhouse warming
The article above is an adapted version of a press release from Utrecht University; see the link below.
Triassic Inferno: Fires raged across Europe for thousands of years during past global warming - News - Utrecht University