Rocks formed immediately before and after non-avian dinosaurs went extinct are strikingly different, and now, tens of millions of years later, scientists think they’ve identified the culprit—and it wasn’t the Chicxulub asteroid impact.
In a study published Monday in the journal Communications Earth & Environment, researchers argue that dinosaurs physically influenced their surroundings so dramatically that their disappearance led to stark changes to the Earth’s landscape, and, in turn, the geologic record.
Specifically, their mass extinction—an event known as the Cretaceous-Paleogene (or K-Pg) mass extinction—enabled dense forests to grow, stabilizing sediments, and shaping rivers with broad meanders, or curves.
“Very often when we’re thinking about how life has changed through time and how environments change through time, it’s usually that the climate changes and, therefore, it has a specific effect on life, or this mountain has grown and, therefore, it has a specific effect on life,” Luke Weaver, a paleontologist at the University of Michigan, said in a statement.
“It’s rarely thought that life itself could actually alter the climate and the landscape. The arrow doesn’t just go in one direction.”
River deposits, not pond deposits
Weaver and his colleagues concentrated their studies on the Williston Basin, which spreads throughout parts of Montana, North Dakota, and South Dakota; and the Bighorn Basin, in north-central Wyoming. Williston Basin’s Fort Union Formation dates to after non-avian dinosaurs went extinct, and features colorful rock layers that Weaver described as resembling pajama stripes. Beneath the Fort Union Formation are water-rich soils similar to a floodplain’s outer edges.
Past research has posited that the colorful layers are evidence of pond deposits from rising sea levels. But the team’s new investigation, however, revealed that “the pajama stripes actually weren’t pond deposits at all. They’re point bar deposits, or deposits that form the inside of a big meander in a river,” Weaver said.
“So instead of looking at a stillwater, quiet setting, what we’re actually looking at is a very active inside of a meander,” he explained.
Above and below these river deposits were layers of a kind of coal created by plant matter, which the team thinks formed thanks to the stabilizing effect of thick forests, which can prevent rivers from frequent flooding. Stable rivers don’t distribute clay, silt, and sand across a floodplain, so the organic remains mostly pile up instead.
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