Meteor Impact Site Holds 200-Million-Year-Old Atmospheric Snapshot – GWC Mag

About 200 million years ago, a meteorite smashed into what is now Rochechouart, France. The site of the impact has been so thoroughly eaten away by erosion that the crater is barely recognizable. But minerals left behind by the collision survived and hold a snapshot of Earth’s atmosphere at that time, according to research published in Earth and Planetary Science Letters.

“That’s a very clever suggestion.”

An impact event can set off short-lived circulation of hot water in the crust, known as a hydrothermal system, which deposits minerals such as quartz. In the new study, scientists showed that fluids trapped in these hydrothermal minerals lacing the Rochechouart crater contain inert gases incorporated from the ancient atmosphere. These dissolved gases can preserve atmospheric snapshots for hundreds of millions of years, making impact craters a rare record of physical processes that shaped our planet over time.

“That’s a very clever suggestion in terms of a potential new repository [of ancient atmospheric samples],” said geochemist Sujoy Mukhopadhyay of the University of California, Davis, who was not involved in the study. The authors “did their due diligence in showing that this is not modern air.”

Stubbornly Stable Gases

Noble gases such as xenon and argon are the snobs of the periodic table. Colorless, tasteless, and odorless, these gases are so stable on their own that they’re loathe to react. Chemically speaking, they rarely do much of anything.

“They are not sensitive to biological activities, to biogeochemical processes, to any chemical reactions,” said study author Guillaume Avice, a geochemist at the Institut de Physique du Globe de Paris. “They are totally transparent to that; they don’t care. It means that they track only physical processes.”

This characteristic is precisely why noble gases are so interesting to scientists who want to understand how Earth evolved over deep time. Physical processes such as the delivery of mantle material to the surface and atmosphere and the formation of the continents release different gases into the air, with varying isotope abundances. And because noble gases don’t react easily, their isotopic ratios have a long, reliable memory for these physical processes.

The problem, however, is that there aren’t many reliable archives of noble gases from the ancient atmosphere. Ice cores are great, Mukhopadhyay said, but they extend back only about 1 million years.

Avice and his colleagues are trying to change that. Years ago, as a doctoral student, Avice wondered whether hydrothermal minerals in impact craters might be the missing archive he’d been looking for. Fluid inclusions in hydrothermal minerals such as quartz are known to preserve ancient atmospheric gases.

“But it’s never a pure signal,” Avice said. “It’s fluid that’s been circulating in the Earth’s crust for ages—sometimes for millions of years. And of course, it got contaminated by all the surrounding rocks.”

Impact hydrothermal minerals are different—or, at least, Avice hoped they would be. Hydrothermal circulation set off by impact events is ephemeral, lasting less than a few million years, what Avice calls a mere “snapshot” in geologic time. Without a long time to circulate and pick up isotopes in the crust, the chemical makeup of the fluids should be close to that of the atmosphere at the time they were trapped in a hydrothermal mineral.

To look for the first impact snapshot, Avice and his colleagues sliced and visually inspected hydrothermal quartz veins from the Rochechouart crater. They crushed up samples under vacuum to extract noble gases and bombarded the material with neutrons to produce new noble gas isotopes that are helpful for dating the material.

Argon in the Rochechouart minerals contained isotope ratios that couldn’t have come from modern air, meaning the fluid inclusions really did preserve a 200-million-year-old atmospheric snapshot.

Specifically, the low ratio of heavier argon-40 to lighter argon-36 in the samples offered strong evidence that the air was ancient. Argon-40 is made when potassium-40, a radioactive element, decays over billions of years. Modern air has higher ratios of argon-40 to argon-36 than ancient air because argon-40 made in the solid Earth has slowly been released and built up in the atmosphere.

Measuring Ancient Atmospheres

The results are a successful proof of concept for an entirely new archive of atmospheric noble gases, Mukhopadhyay said.

“We should be looking at other impact craters.”

“The most significant aspect is the potential for using these kinds of materials as repositories for reconstructing past records of atmospheric noble gases,” Mukhopadhyay explained. “We should be looking at other impact craters.”

Testing the method on more samples from more craters should be a next step, Avice agreed. If the method proves reliable, it could also be useful for learning about the evolution of other planets. Mukhopadhyay pointed out that most of the Mars rovers are already sitting in impact craters, where samples could be collected.

Avice said he hopes that collecting atmospheric snapshots from more craters will help settle a mystery closer to home: the long-standing question of how Earth developed its unusual nitrogen-dominated atmosphere.

Nitrogen isn’t a noble gas, but it behaves a lot like one in that it’s very inert, Avice said. And if ancient atmospheric snapshots can reliably capture nitrogen, too, sampling more craters could fill in the missing history of this abundant and important gas.

Today, nitrogen accounts for nearly 80% of the atmosphere. That extra atmospheric pressure affects everything from climate to clouds. If it wasn’t like that in the past, Avice said, “everything would be different.”

—Elise Cutts (@elisecutts), Science Writer

Citation: Cutts, E. (2023), Meteor impact site holds 200-million-year-old atmospheric snapshot, Eos, 104, https://doi.org/10.1029/2023EO230399. Published on 19 October 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0
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