Huge volcanic eruptions 233 million years ago pumped carbon dioxide, methane and water vapor into the atmosphere. This series of violent explosions, on what we now know as Canada’s west coast, led to massive global warming. Our new research revealed that this was a planet-changing mass extinction event that killed many of the dominant tetrapods and heralded the dawn of the dinosaurs.
The best known mass extinction occurred at the end of the Cretaceous period, 66 million years ago. This is when the dinosaurs, pterosaurs, marine reptiles and ammonites went extinct. This event was mainly caused by the impact of a giant asteroid which obscured the sunlight and caused darkness and freezing, followed by other massive perturbations of the oceans and atmosphere.
Geologists and paleontologists agree on a list of five of these events, of which the last Cretaceous mass extinction was the last. So our new discovery of a previously unknown mass extinction may seem unexpected. Yet this event, called the Carnian Pluvial Episode (CPE), appears to have killed as many species as the giant asteroid did. Terrestrial and marine ecosystems have been profoundly changed as the planet has become hotter and drier.
On land, this has triggered profound changes in plants and herbivores. In turn, with the decline of dominant plant-eating tetrapods, such as rhinocosaurs and dicinodons, dinosaurs were given their chance.
Dinosaurs originated about 15 million years earlier, and our new study shows that, as a result of CPE, they expanded rapidly over the next 10 million to 15 million years and became the dominant species in terrestrial ecosystems. The CPE triggered the “age of the dinosaurs” which lasted for another 165 million years.
It wasn’t just the dinosaurs that had a foothold. Many modern tetrapod groups, such as turtles, lizards, crocodiles and mammals date back to this recently discovered period of revolution.
Following the clues
This event was first noticed independently in the 1980s. But it was thought to be limited to Europe. First, geologists in Germany, Switzerland and Italy recognized an important turnover in marine fauna around 232 million years ago, called the Rheingraben event.
Five mass extinctions and what we can learn from them on the planet today
Then, in 1986, I independently recognized this as a global turnover between tetrapods and ammonites. But at the time, age dating was much weaker than it is now and it was impossible to be sure it was the same event.
The pieces of the puzzle began to fall into place when a roughly 1 million-year episode of wet weather was recognized across the UK and parts of Europe by geologists Mike Simms and Alastair Ruffell. So the geologist Jacopo dal Corso has identified a coincidence in the timing of the CPE with the peak of the eruptions of the basalts of Wrangellia.
Wrangellia is a term geologists give to a narrow tectonic plate attached to the west coast of the North American continent, north of Vancouver and Seattle.
Finally, in a review of the evidence from the Triassic rocks, the signature of the CPE was found – not only in Europe, but also in South America, North America, Australia and Asia. This was far from being a Europe-only event. It was global.
Massive Wrangellia eruptions pumped carbon dioxide, methane and water vapor into the atmosphere, leading to global warming and increased rainfall around the world. There have been up to five eruption pulses associated with 233 million-year-old warming peaks. The eruptions led to acid rain as volcanic gases mixed with rainwater to flood the Earth in dilute acid. The shallow oceans have also become acidified.
Strong warming drove plants and animals away from the tropics and acid rain killed plants on land, while ocean acidification attacked all marine organisms with carbonate skeletons. This ripped off the surfaces of the oceans and the earth. Life may have started to recover, but when the eruptions ceased, temperatures remained high while the tropical rains ceased. This is what caused the subsequent drying out of the soil on which the dinosaurs thrived.
The most extraordinary thing was the recast of the marine carbonate factory. This is the global mechanism by which calcium carbonate forms large thicknesses of limestone and provides material for organisms such as corals and mollusks to build their shells. The CPE marked the beginning of modern coral reefs, as well as many of the modern plankton groups, suggesting profound changes in ocean chemistry.
Before the CPE, the main source of carbonate in the oceans came from microbial ecosystems, such as limestone-dominated mud mounds, on continental shelves. But after the CPE, it was led by coral reefs and plankton, where new groups of microorganisms, such as dinoflagellates, appeared and flourished. This profound shift in fundamental chemical cycles in the oceans marked the beginning of modern marine ecosystems.
And there will be important lessons on how we will help our planet recover from climate change. Geologists must investigate the details of the volcanic activity of the Wrangellia and understand how these repeated eruptions have driven the climate and changed the ecosystems of the Earth. There have been a number of volcano-induced mass extinctions throughout Earth’s history, and physical disturbances, such as global warming, acid rain and ocean acidification, are among the challenges we see today.
Paleontologists will need to work more closely on marine and continental fossil record data. This will help us understand how the crisis unfolded in terms of biodiversity loss, but also explore how the planet has recovered.