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Compounds in ancient plant leaves tell the story of how an extinction event shaped our planet’s ecosystems.

Frenchman Valley in Chambery Coulee, Saskatchewan, above, was one of the sites at which researchers traced changes in plant ecology following the Chicxulub impact event. Credit: R. D. Bourque

By Hannah Thomasy 12 hours ago

Chambery Coulee is a window into the deep past, a place that holds the secrets of the mass extinction that ended the age of the dinosaurs 66 million years ago.

At first glance, Chambery Coulee might look like any other valley in the vast prairies of southern Saskatchewan. But for Robert Bourque and his colleagues at McGill University in Quebec, Chambery Coulee is a window into the deep past, a place that holds the secrets of the mass extinction that ended the age of the dinosaurs 66 million years ago.

Their recent study uses compounds in plant waxes to shed new light on how plant communities, the water cycle, and the carbon cycle changed after the dinosaur-killing Chicxulub asteroid impact, which marked the end of the Cretaceous period and the beginning of the Paleogene. Bourque, who received his master of science degree from McGill and is now a Ph.D. student at Rensselaer Polytechnic Institute in New York, said, “Plant waxes provide a unique opportunity in looking through certain lenses at the global ecosystem at the time, since plant waxes record atmospheric [carbon dioxide] as well as the hydrogen derived from rainfall. So we’re able to use them as proxies for looking at these environmental signatures.”

Even though it is now widely accepted that the asteroid impact was the root cause of the extinction event, the specific mechanism (or mechanisms) by which the impact wiped out the majority of all plant and animal species on Earth is still not fully understood. Studying environmental changes that took place around the time of the impact may help scientists figure out the answer.

Changing Ecosystems

Bourque and his colleagues analyzed leaf wax n-alkanes (a type of hydrocarbon) that had been preserved in ancient fluvial sediments in Chambery Coulee and another site near Saskatchewan’s Highway 37 to learn more about how the world changed. These leaf wax compounds, Bourque said, “come in a variety of…chain lengths, and different types of plants produce different ratios of these chain lengths. Typically, aquatic plants will produce shorter chain lengths, whereas terrestrial plants produce longer ones.… So we noticed that there’s a shift from more shorter-length plant waxes to longer ones going across the boundary.” This change implies that terrestrial plants were becoming more abundant compared with aquatic plants after the extinction event.

This finding aligns with previous research on prehistoric pollen samples, which showed an increase in angiosperm (flowering plant) pollen, especially species similar to today’s birch and elm trees, in the time following the extinction event.

Researchers also analyzed the carbon and hydrogen isotopes in the n-alkanes, which allowed them to reconstruct changes in the carbon cycle and the water cycle. In contrast to the long-term changes in plant communities they observed after the extinction event, the team did not find obvious long-term changes in precipitation or the carbon cycle subsequent to the event.

So what was responsible for the long-lasting changes in plant ecology? Researchers hypothesized that this burst of terrestrial plant abundance was due to another factor: the sudden disappearance of the dinosaurs. With just about every large terrestrial herbivore wiped from the face of Earth, terrestrial plants flourished.

Next Steps

Ken MacLeod, a geology professor at the University of Missouri, said he would like to see the new paper’s line of inquiry expanded to include more samples, across both time and geographic space. More samples from the time period would help provide greater resolution of exactly when these environmental changes occurred, whereas more samples from around the world would help determine how widespread this phenomenon was—did it occur only in southern Saskatchewan, across all of North America, or even perhaps around the entire planet?

Bourque had similar thoughts. “Looking at a finer scale across the boundary would be extremely interesting, to try and better estimate what happened over what time period,” he said. “And broadening the scope to other sites from the same time period would be incredibly interesting.”

MacLeod said there’s a lot of interest in improving our understanding of this time period, especially because “the Cretaceous-Paleogene impact is, I think, literally the only event in the Phanerozoic [the past 541 million years] with global-scale implications that manifest on timescales shorter than anthropogenic changes.” Studying how Earth systems responded to perturbations in the past, MacLeod said, might help us understand the response of these systems in the present, which is also a period of rapid change.

—Hannah Thomasy (@HannahThomasy), Science Writer