What were the differences and similarities between the Triassic extinction world and today’s Earth?
But it is difficult to arrange the end-Triassic rocks of different parts of the world in terms of time. The types of rocks vary from place to place, and the ammonites that paleontologists use to mark the end of the Triassic didn’t live in places like England’s much-studied Bristol Channel. Professor Paul Wignall of the University of Leeds said: “It’s easy to say what happened over the last half million or 100,000 years. But it is very difficult to say what happened in 10 or 100 years.”
The only recent radiometric date in marine rocks is when life began to recover after the extinction. At 201.36 million years ago, there is a very large uncertainty of plus or minus 170,000 years. On land, the radiometric extinction date is more precisely dated to 201,564 million years ago (plus or minus 15,000 years), but the change in pollen used to estimate this date elsewhere appears much later, the only thing What we can say is that the extinction occurred about 200,000 years between the first and second radiometric dates.
To complicate matters, consider that extinctions appear to have occurred in multiple events, and scientists disagree on the number of events. Wignall sees a similar pattern to other mass extinctions, and says there are two extinction crises on the evidence. But Dr Callum Fox of Khalifa University in Abu Dhabi disagrees and says there was an extinction event.
Intense volcanic activity
In contrast to species extinction, there is greater clarity with volcanic eruptions.
The sudden emergence of what is now called the Central Atlantic Magmatic Province, or CAMP, began 201.635 million years ago when the heart of Pangea split open. With the occurrence of this event, lava was thrown from France to Bolivia, a strong and vast flow of underground magma started and caused a flood of lava from basalt and the release of large amounts of carbon dioxide and pollutants. Sheets of molten rock made their way and flowed into the sediments, producing carbon dioxide, methane, and more halocarbons. The history of these volcanic rocks shows three distinct periods of volcanic activity. It would be nice if volcanic activity coincided with extinction events, but it doesn’t and nature isn’t cooperating.
Apparently, only the first period of volcanic activity coincided with an extinction event, while the second and third periods of volcanic activity are dated after the extinction event. However, Wignall thinks that the second period of volcanic activity may have overlapped with the second extinction event.
Capriolo and his colleagues wanted to see if the initial volcanic event could have produced enough carbon dioxide to change the Triassic climate. They found minerals in rocks that had trapped tiny bubbles of volcanic gases as they crystallized deep underground. These bubbles indicate that the CAMP magma was rich in carbon dioxide and entered the atmosphere through eruptions at the end of the Triassic. Capriolo’s team calculated that the first episode of eruptions consisted of a small number (perhaps 10) of intense eruptive activity, each equivalent to a century of human emissions on average. Capriolo says the amount is similar and therefore alarming.
Such a widespread release of carbon should change the balance of carbon isotopes in the atmosphere. Three distinct fluctuations of carbon isotopes are seen in a number of Late Triassic sediments, which are usually attributed to three times of carbon release from CAMP. Scientists have tried to match these global signals to end-Triassic rocks, but nature has obscured some events. This has caused ambiguity and different interpretations by different teams.
According to Fox and colleagues, the first fluctuation in carbon isotopes could have occurred more than a million years before the first CAMP eruptions. But Professor Sophie Lindström of the University of Copenhagen and others say the fluctuation could have occurred tens of thousands of years after the eruptions began.
The intermediate carbon isotope swing occurred either during the mass extinction (according to Lindström, Van Deschutbruge, Wignall et al.) or before it (according to Fox et al.), while the third isotope swing occurred after the extinction. This ambiguity is partly due to the fact that the carbon dioxide signal in carbon isotopes is obscured by local changes in organic matter washed into the oceans, especially in the shallow seas of the end of the Triassic, and this makes it difficult to use carbon isotope fluctuations to determine the sequence of global events.
Location, location, location
One of the big problems is that the available information is limited to English rocks that were once formed in a large shallow sea. As a result of the earthquake, the sediments on the bottom of this shallow sea moved along a long line and changed shape. At that time, the sea in that area became shallow and eventually dried up, leaving deep cracks. Shells and other marine life disappeared and microbial mats took over. Many scientists consider this microbial world to be the result of the first extinction event, but Fox and his colleagues believe it was local and not global. “Disappearances of marine organisms are the result of local environmental changes and do not indicate a global extinction event,” they report in the journal PNAS. In fact, when seawater returned to the sea before what might have been a second extinction event, some Triassic species also reappeared, only to become extinct by the end of the next event.
But other scientists argue that returning species were survivors of the first extinction event. According to Wignall, in many mass extinctions, many species manage to survive.
If Fox is right that the first extinction event was local, the apparently mild climate at the time seems reasonable. Dr Victoria Petryshyn of the University of California and colleagues found evidence of a milder climate when they estimated past water temperatures in sediments left behind by microbial mats. The researchers were able to detect seasonal temperature changes for the 201-million-year-old limestone masses, but they found no sign of extreme cooling or warming over several thousand years. We don’t know if the mentioned study, which was done in only one place, really shows the climate of the end of the Triassic.
But if the first extinction event was a regional phenomenon, what on Earth could cause such a drastic change that was limited to a local sea and the species that lived in it? Van Deschutbruge thinks the earthquakes and shallowing of the sea could be caused by crustal uplift above the mantle hotspot, which ultimately triggered the eruptions of the Central Atlas Magma Province.
Stinking acid ocean
Fortunately, there is more agreement on how marine animals perished in the second extinction event. Molly Trudgill of the University of St Andrews measured boron isotopes in oysters that lived after the second extinction event in England to estimate the pH of the ocean at the time. He noticed an apparent acidification, indicating that during that time, atmospheric carbon dioxide suddenly increased, causing ocean acidification. This explains why the fossilized shells that belong to the post-extinction period are weak because it was difficult for organisms to form shells during that period.
But there was another killer at work.
Fox and others found the remains of specific species of sulfur bacteria in shale (a type of sedimentary rock) created during the second extinction event. “We see an abundance of purple sulfur bacteria that require much higher light intensity than brown-green sulfur bacteria,” Fox said.
The presence of light-requiring sulfur bacteria suggests that the oceans were devoid of oxygen, and that the sun-exposed area where the greatest marine diversity would normally grow contained toxic hydrogen sulfide. In other words, the oceans were deadly and smelled like rotten eggs.