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UT News

Arizona Rock Offers Clues to the Chaotic Earth of 200 Million Years Ago

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AUSTIN, Texas — A rock core from Petrified Forest National Park in Arizona has given scientists a powerful new tool to understand how catastrophic events shaped Earth’s ecosystems before the rise of the dinosaurs.

The quarter-mile-long core is from an important part of the Triassic period when life on our planet endured a series of cataclysmic events: mountain-sized asteroids struck Earth at least three times, chains of volcanoes erupted to choke the sky with greenhouse gases and tectonic movement tore apart Earth’s single supercontinent, Pangea.

Among the chaos, many plants and animals vanished in a shake-up of life on Earth that scientists have yet to explain, including some of the long-snouted and armored reptiles that ruled Pangea in that time.

The study, published today in GSA Bulletin, offers scientists a foundation to explain the changes in the fossil record and determine how these events may have shaped life on Earth.

By determining the age of the rock core, researchers were able to piece together a continuous, unbroken stretch of Earth’s history from 225 million to 209 million years ago. The timeline offers insight into what has been a geologic dark age and will help scientists investigate abrupt environmental changes from the peak of the late Triassic and how they affected the plants and animals of the time.

Before the rock core was recovered, most of what was known about the Late Triassic came from searching for signs of the Chinle Formation on rocky outcrops such as this one, found throughout Petrified Forest National Park. The rock layers that make up the formation were deposited during the Late Triassic Period. Credit: Randall Irmis/NHMU

“The core lets us wind the clock back when the Petrified Forest National Park was a tropical hothouse populated by crocodilelike reptiles and turkey-size early dinosaurs,” said Cornelia Rasmussen, a postdoctoral researcher at the University of Texas Institute for Geophysics, who led the analysis that determined the age of the core. “We can now begin to interpret changes in the fossil record, such as weather changes at the time were caused by an asteroid impact or slow geographic changes of the supercontinent drifting apart.”

The research overcomes the broken puzzle problem by recovering every layer in the order it was deposited. Scientists can then match those layers, like tree rings, with the fossil and climate record.

To find the age of each layer, the researchers searched the rock core for tiny crystals of the mineral zircon, which are spewed into the sky during volcanic eruptions. Zircons are a date stamp for the sediments with which they are buried. Researchers then compared the age of the crystals with traces of ancient magnetism stored in the rocks to help develop a precise geologic timeline.

A close up of the quarter-mile-long rock core that represents a continuous geologic record from 225 million to 209 million years ago. Cornelia Rasmussen, a postdoctoral researcher at the University of Texas Institute for Geophysics, has worked on the core since it was drilled from Petrified Forest National Park in 2013. Credit: Cornelia Rasmussen

Petrified Forest National Park’s paleontologist Adam Marsh said that despite a rich collection of fossils from the period in North America, until now there was little information on the late Triassic’s timeline because most of what scientists knew came from studying outcrops of exposed rock pushed to the surface by tectonic movements.

“Outcrops are like broken pieces of a puzzle,” said Marsh, who earned his Ph.D. from UT Austin but was not an author of the study. “It is incredibly difficult to piece together a continuous timeline from their exposed and weathered faces.”

The research is the latest outcome of the Colorado Plateau Coring Project. The research and the coring project were funded by the National Science Foundation and the International Continental Scientific Drilling Program.