Roughly 200 million years ago, at least half the species on Earth died off over the course of about 100,000 years, both on land and in sea. This mass extinction, at the boundary between the Triassic and Jurassic periods and one of five known such events in Earth's geologic history, set the stage for dinosaurs to rise to prominence and dominate the planet's terrestrial life for the next 135 million years.
The quest for answers regarding dinosaurs and the end-Triassic mass extinction had previously led paleontologist Paul Olsen and his colleagues at Columbia University's Lamont –Doherty Earth Observatory (LDEO) to globe-trot to around cities in Morocco and along sea cliffs in the U.K . digging up clues from that past era. This summer, it brought them to the Kell family's yard in western New Jersey—the area holds one of the most visible examples of the Triassic– Jurassic boundary.
The sub surface dig for revealing sediment cores required a lightweight, portable drill called a Winkie. D iamond-tipped and gas-powered , it can be carried and operated by only two people—often paleomagnetist Dennis Kent and geochemist Morgan Schaller, both at Rutgers University. The goal was to unearth evidence of what might have triggered the mass extinction.
"The dinosaurs had actually first evolved about 25 million years before the mass extinction, but after their competition got wiped out, it looks like they came in like gangbusters," Olsen says. Such competition included extinct relatives of modern crocodilians, such as the large and carnivorous land-based rauisuchians and semiaquatic phytosaurs as well as plant-eating aetosaurs and revueltosaurs.
Many scientists blame the end-Triassic mass extinction on exceptionally massive volcanic eruptions over the course of less than 20,000 years that occurred about when the one-time supercontinent of Pangea began rifting apart. These eruptions coated what was to become Africa and the Americas with a million cubic kilometers of lava and doubled the level of carbon dioxide in the air causing massive global warming, "about a 3- degree Celsius increase on average in temperature, if the climate system was as sensitive as models suggest," Olsen says, citing research published by Schaller, Kent and their colleague James Wright in 2011.
Those 3 degrees "could have translated to lethally high summer temperatures, especially for some kinds of broad-leafed vegetation, which in turn could have led to extinctions of animals dependent on the plants," Olsen explains. "In the oceans, a rapid rise in carbon dioxide would have resulted in acidifying the oceans and badly impacting animals that make calcium carbonate skeletons, such as corals, bivalves and ammonites, all of which suffered massive drops in diversity."
Volcanic eruptions also release large amounts of sulfur-laden compounds that reflect sunlight, however, causing cooling. "The cooling only lasts a short time because the sulfur is removed quickly from the atmosphere, but the effect can be very intense, and for a world in which there were no ice caps and there were forests at the poles, such dramatic drops in temperature cold have been devastating on land," Olsen says.
"One can envision the carbon dioxide raising temperatures multiple times for tens of thousands of years, fading away over 100,000 years or so, but these periods of great warmth would be punctuated by many intervals of abrupt and intense cooling caused by the sulfur," Olsen continues. "This one-two punch may have been too much for terrestrial ecosystems and the warmth and acidity in the oceans too much for marine life. Result: mass extinction."
Still, puzzling details abound regarding the end-Triassic mass extinction, such as when fern spores appeared. Ferns are often the first plants to appear after a natural disaster. But in this case, although massive quantities of fern spores coincide with lava flows in some areas, the spike occurs before the extinctions in others.
Other evidence suggested that a huge meteorite strike may have also played a role in the end-Triassic extinction event— akin to the impact that created Chicxulub crater in what is now Mexico's Yucatan Peninsula and is thought to have wiped out the dinosaurs during the Cretaceous–Tertiary (K –T) mass extinction about 65 million years ago. The giant hit that left behind the Manicouagan Crater in what is now Quebec apparently occurred about the time of the end-Triassic extinction. Research eventually showed, however, that collision was about a million years too early to have caused a die-off. But Olsen and his colleagues have investigated a crater of the right age to be the culprit—a hole about 40 kilometers wide discovered in France at Rochechouart.
Yet, a number of questions remain about the impact idea, overall, especially with regard to iridium, the relatively rare metallic element that typically accumulates as a signature layer in the geologic strata and is exposed at such sites . Scientists had apparently found multiple iridium layers, and "since multiple impacts spread over tens of thousands of years seems very unlikely, we became much less enamored of the impact hypothesis, especially because volcanic ashes can produce the same kind on anomaly," Olsen says. Over the past couple of years, though, research has also revealed that one of these iridium layers possessed ratios of other platinum group elements that were much more similar to those associated with meteorites than with most volcanic processes. The results have put the impact hypothesis back in the ring.
The researchers went digging in New Jersey looking for more fern spores, iridium traces and other potential clues that might help resolve all these questions regarding the end-Triassic mass extinction.
The region back then may have been very hot by modern standards, with climate fluctuating between humid and arid. It was covered by small-leaved conifers and was home to herbivorous and carnivorous relatives of crocodilians called the crurotars ans, in addition to small dinosaurs, lizards and the like, Olsen says. Nowadays the area is mostly rolling farmland. The researchers managed to convince the Kells to let the team drill beside the family garage, right next to a cornfield.
'The entire layer cake'
Using a Winkie is relatively inexpensive compared with the average geologic drilling project, and it can drill to a depth of about 150 meters, Olsen says.
The drill uses water for lubrication, leading to muddy jeans and sneakers all around. The engine often proved fussy, stalling when the oil– gas mix wasn't quite right. Still, over the course of a week in August, as cicadas hummed over the chainsaw buzz of the rig's motor, the researchers managed to drill down 40 meters into red mudstone. "We may have lucked out and recovered the entire layer cake—sediment layers deposited before and after the big extinction event as well as those recording the event itself," Kent says.
To see whether the suspected geologic and extraterrestrial events happened before, during and after the mass extinction, Kent will look in the core samples for signs of reversals in Earth's magnetic field, which happen periodically over hundreds of thousands of years. These reversals are captured by magnetically sensitive minerals, which preserve the way Earth's magnetic poles once pointed much like compasses. The regular nature of these reversals make them useful markers of time, and therefore help shed light on when other details found in the same rocks might have happened. By getting a better idea of when any eruptions and impacts might have occurred, the researchers hope to see what events might have directly preceded the end-Triassic mass extinction.
Also, the scientists will look for carbonate nodules and the density of tiny openings known as stomata on fossil plant leaves. Carbon dioxide captured in the carbonate nodules could give an idea of what the atmospheric gas levels were like back then—higher levels would also leave behind more carbonate nodules and more stomata to absorb the gas.
Even if the researchers discover that the iridium layer in western New Jersey did result from an impact and occurred right before climate disruptions and the mass extinction, it may not have caused them. Still, meteorite hit may have stressed ecosystems, Olsen says, such that they were more sensitive to the warming and cooling events from later volcanic eruptions
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