Triassic End Times
The End Triassic extinction event was an extinction event that occurred over 200 million years ago. At least half of the species known to have been living on Earth at that time went extinct. This event vacated terrestrial ecological niches, allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Panagea started to break apart. Scientists examining evidence across the world from New Jersey to North Africa say they have linked the abrupt disappearance of half of earth’s species 200 million years ago to a precisely dated set of gigantic volcanic eruptions. The eruptions may have caused climate changes so sudden that many creatures were unable to adapt—possibly on a pace similar to that of human-influenced climate warming today. The extinction opened the way for dinosaurs to evolve and dominate the planet for the next 135 million years, before they, too, were wiped out in a later planetary cataclysm. This study provides the tightest link yet, with a newly precise date - 201,564,000 years ago, exactly the same time as a massive outpouring of lava.
Several explanations for this event have been suggested, but all have unanswered challenges:
Gradual climate change, sea-level fluctuations or a pulse of oceanic acidification during the late Triassic reached a tipping point. However, this does not explain the suddenness of the extinctions in the marine realm.
Asteroid impact, but so far no impact crater of sufficient size has been dated to coincide with the Triassic—Jurassic boundary.
Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province (CAMP), would release carbon dioxide or sulfur dioxide and aerosols, which would cause either intense global warming (from the former) or cooling (from the latter).
The new study, appearing today in an early online version of the leading journal Science, unites several pre-existing lines of evidence by aligning them with new techniques for dating rocks. Lead author Terrence Blackburn used the decay of uranium isotopes to pull exact dates from basalt, a rock left by eruptions. The basalts analyzed in the study all came from the Central Atlantic Magmatic Province (CAMP), a series of huge eruptions known to have started around 200 million years ago, when nearly all land was massed into one huge continent.
The Central Atlantic magmatic province (CAMP) is a large connected magma flow formed during the breakup of Pangaea during the Mesozoic Era. The initial breakup of Pangaea in early Jurassic time provided a legacy of basaltic dikes, sills, and lavas over a vast area around the present central North Atlantic Ocean.
The eruptions spewed some 2.5 million cubic miles of lava in four sudden spurts over a 600,000-year span, and initiated a rift that evolved into the Atlantic Ocean; remnants of CAMP lavas are found now in North and South America, and North Africa. The scientists analyzed samples from what are now Nova Scotia, Morocco and the New York City suburbs.
Previous studies have suggested a link between the CAMP eruptions and the extinction, but other researchers’ dating of the basalts had a margin of error of 1 to 3 million years. The new margin of error is only a few thousand years—in geology, an eye blink. Blackburn and his colleagues showed that the eruption in Morocco was the earliest, with ones in Nova Scotia and New Jersey coming about 3,000 and 13,000 years later, respectively. Sediments below that time contain pollen, spores and other fossils characteristic of the Triassic era; in those above, the fossils disappear. Among the creatures that vanished were eel-like fish called conodonts, early crocodilians, tree lizards and many broad-leaved plants. The dating is further strengthened by a layer of sediment just preceding the extinction containing mineral grains providing evidence of one of earth’s many periodic reversals of magnetic polarity. This particular reversal, labeled E23r, is consistently located just below the boundary, making it a convenient marker, said coauthor Dennis Kent. With the same layers found everywhere the researchers have looked so far, the eruptions "had to be a hell of an event," said Kent.
The third piece of chronological evidence is the sedimentary layers themselves. Sedimentary rocks cannot be dated directly—one reason why the timing of the extinction has been hard to nail. Olsen and some others have long contended that the earth’s precession—a cyclic change in the orientation of the axis toward the sun and resulting temperature changes—consistently created layers reflecting the alternate filling and drying of large lake basins on a fairly steady 20,000-year schedule. This idea is well accepted for more recent time, but many scientists have had doubts about whether it could be applied much farther back. By correlating the precisely dated basalts with surrounding sedimentary layers, the new study shows that precession operated pretty much the same way then, allowing dates with a give or take of 20,000 years to be assigned to most sediments holding fossils, said Olsen.
Olsen has painstakingly cataloged the layers around the time of the End Triassic, and the initial phase of the extinction occurs in just one layer—meaning the event took 20,000 years at most. But, he said, "it could have taken much less. This is the level of resolution we have now, but it’s the less part that is the more important, and that’s what we are working on now."
Many scientists assume that giant eruptions would have sent sulfurous particles into the air that darkened the skies, creating a multi-year winter that would have frozen out many creatures. A previous study by Kent and Rutgers University geochemist Morgan Schaller has also shown that each pulse of volcanism doubled the air’s concentration of carbon dioxide—a major component of volcanic gases. Following the cold pulses, the warming effects of this greenhouse gas would have lasted for millennia, wiping out creatures that could not take too much heat. (It was already quite hot to begin with at that time; even pre-eruption CO2 levels were higher than those of today.) Fossils show that heat-sensitive plants especially suffered; there is also evidence that the increased CO2 caused chemical reactions that made the oceans more acidic, causing populations of shell-building creatures to collapse. As if this were not enough, there is also some evidence that a large meteorite hit the earth at the time of the extinction--but that factor seems far less certain. A much stronger case has been made for the extinction of the dinosaurs by a meteorite some 65 million years ago—an event that opened the way for the evolution and dominance of mammals, including human beings. Volcanism may have been involved in that extinction as well, with the meteorite delivering the final blow.)
Paul Renne, a researcher at the Berkeley Geochronology Center in California, who studies the End Triassic but was not involved in the Science paper, said the study was “part of a growing pattern in which we see that the major ecosystem crises were triggered by volcanism. Of the actual mechanism that killed creatures, he said climate change was the most popular suspect. But, he added, "We still don’t have any way yet of knowing exactly how much CO2 was put into the atmosphere at that time, and what it did. If we did, we would then be able to say to people, Look folks, this is what we’re facing now, and here’s what we have to do about it. But we don’t know that yet."
For further information see End Time.
Map image via University of Bristol.