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How to tell geologic time in the Triassic Period

Reporting from Argentina expedition, NHMU paleontologist Randy Irmis describes using zircon crystals to pinpoint age of sediments crucial for understanding Triassic fossils in the Southern Hemisphere.

This piece is adapted from Science Stories, a blog hosted by the Natural History Museum of Utah highlighting research associated with the museum’s scientists.

The basic question all paleontologists must address in the field is how old are the fossils they collect and the rocks that preserve them.

Randy Irmis

This posed a quandary for Randy Irmis of the Natural History Museum of Utah and his research team while studying sedimentary rocks and Triassic fossils recently in Argentina’s Santa Clara region in the northern Mendoza Province.

“This might seem like a basic question we should have already answered years ago for most fossil-bearing areas, but the answer is often not so straightforward,” Irmis wrote in the museum’s Science Stories blog with colleague Cecilia Benavente. Their expedition to Santa Clara is a case in point where the team used various dating techniques to pinpoint the age of Triassic Period sediments for a study to be published this year and came up with surprising results that could improve the dating of fossils in the Southern Hemisphere.

Benavente, the study’s lead author, is a researcher with Argentina’s Universidad Nacional de Cuyo, and Irmis is an associate professor of geology and geophysics at the University of Utah and the museum’s curator of paleontology.

First off, how do geoscientists fix dates to the rocks they study?

“One option, which has been in use for over 200 years, is called ‘relative dating,’ and uses the fossils themselves,” Irmis and Benavente wrote. “If we know what species of fossils are found in different rock layers in one area, then we can compare the types of fossils we find in a rock layer elsewhere to this first sequence to determine if the rocks in the second area are older, younger or similar in age.”

But this technique doesn’t provide the actual age of the fossils, but rather how old they are relative to fossils found in rock layers elsewhere. This is where radioisotopic dating comes in. A common example is radiocarbon dating, but it works only for organic samples less than 50,000 years old, and Irmis’s work looks back hundreds of millions of years.

A team of researchers inspects an outcrop the Santa Clara in northern Mendoza Province, Argentina. Photo courtesy of Randall Irmis.

For rocks going back to the time of dinosaurs, paleontologists use uranium-lead (U-Pb) dating and other forms of “absolute dating.”

“This method takes advantage of the fact that when the mineral zircon forms during a volcanic eruption, the crystal structure traps the element uranium, but not lead,” the pair wrote. “However, the radioactive decay of uranium produces lead, and therefore we know that any lead in the zircon crystal is from this radioactive decay.”

The rate of this decay is precisely known, so scientists can calculate how many millions of years ago it crystallized by measuring the ratio of uranium to lead in the crystal. And finally, volcanic ash layers containing zircon crystals are often found between fossil-bearing rock layers, so dating zircons from these ashes can provide accurate age estimates for the adjacent sediments and the fossils they contain.

The Santa Clara expedition studied rocks formed by sediments deposited during the Triassic Period (252 to 201 million ago) in lakes, rivers and streams, which contained a rich fossil record of plants, arthropods, fish, reptiles and mammal ancestors. The team needed to determine exactly when these sediments were deposited during the Triassic, a 51-million window marking the rise of dinosaurs. This period frames the end-Permian extinction and the Jurassic Period.

The first dating technique the team used relied on the microscopic fossil pollen and spores in the sediments, a relative dating method that suggested the rocks were from the Late Triassic Period, sometime between 237 and 209 million years ago. The team wanted greater precision so they applied U-Pb zircon dating, which yielded more satisfying and surprising results.

Fossil of a Triassic ray-finned fish from the Santa Clara area. Specimen IANIGLA PV-1242.

The fossils proved to be older, coming from the Middle Triassic, or 244 to 242 million years old.

“These results were exciting because there are very few fossil-bearing Middle Triassic rocks from the Southern Hemisphere that are well-dated, and this time interval is critical for understanding how ecosystems finally recovered from the end-Permian mass extinction at the start of the Triassic Period,” Irmis and Benavente wrote.

So why did relative dating using pollen and spores give such a different age than the absolute dates recorded in the zircon?

“We suspected it might be that the similarities and differences in species during the Triassic have less to do with geologic time, and more to do with the climate the plants are living in,” they wrote.

Because of continental drift, a region’s latitude as seen today may be very different than it was during the Triassic. Those differences could have a major influence on the climate at the time fossils were formed.

So the research team statistically compared Triassic pollen and spore data from many different areas across the Southern Hemisphere to the paleolatitudes of these places during the Triassic Period, as well as their inferred geologic age.

The results demonstrated that when two different areas had similar pollen and spores, it was more likely due to ancient latitude than geologic time. This confirmed Irmis’s suspicions that climate was playing a role since climate varies greatly across latitudes.

Accordingly, a long-term project for Irmis’ team is to conduct more absolute dating to improve the accuracy of using Triassic pollen and spores for relative dating.

This research was published in the journal Gondwana Research. The authors of the study are Benavente, Irmis, Tomas Pedernera and Adriana Mancuso of IANIGLA, CCT-CONICET in Mendoza, and Roland Mundil of the Berkeley Geochronology Center. Funding was provided by the Ann and Gordon Getty Foundation and the University of Utah. Fieldwork was conducted under permits issued by the Dirección de Patrimonio Cultural y Museos, Provincia de Mendoza.