Your ecosystem engineer was a dinosaur

(Press-News.org) ANN ARBOR—Dinosaurs had such an immense impact on Earth that their sudden extinction led to wide scale changes in landscapes—including the shape of rivers—and these changes are reflected in the geologic record, according to a University of Michigan study.

Scientists have long recognized the stark difference in rock formations from just before dinosaurs went extinct to just after, but chalked it up to sea level rise, coincidence, or other abiotic reasons. But U-M paleontologist Luke Weaver shows that once dinosaurs were extinguished, forests were allowed to flourish. This had a strong impact on rivers: The newly dense forests stabilized sediments and corralled water into rivers with broad meanders.

Weaver and colleagues examined locations throughout the western United States that depicted sudden geologic changes that occurred at the boundary between the age of dinosaurs and the age of mammals. 

Studying these rock layers, Weaver and colleagues suggest that dinosaurs were likely enormous "ecosystem engineers," knocking down much of the available vegetation and keeping land between trees open and weedy. The result was rivers spilled openly, without wide meanders, across landscapes. Once the dinosaurs perished, forests were allowed to flourish, helping stabilize sediment and corralling water into rivers with broad meanders. 

Their results, published in the journal Communications Earth & Environment and supported by the National Science Foundation, demonstrate how rapidly the Earth can change in response to catastrophic events.

"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," said Weaver, assistant professor in the U-M Department of Earth and Environmental Sciences. "It's rarely thought that life itself could actually alter the climate and the landscape. The arrow doesn't just go in one direction."

The Chicxulub asteroid impact

Dinosaurs became extinct after a large asteroid slammed into the Yucatan Peninsula. Scientists looking for evidence of the asteroid saw that the rocks overlying the fallout debris were starkly different from the rocks below. 

Weaver and his co-authors Tom Tobin of the University of Alabama and Courtney Sprain of the University of Florida began investigating this sudden geologic change in the Williston Basin, an area that spans eastern Montana and western North and South Dakota, as well as north-central Wyoming's Bighorn Basin. 

The early career scientists' interest in the geologic mystery was piqued during fieldwork they conducted together as graduate students. While investigating a previous paper, the research team examined a rock layer called the Fort Union Formation. 

The Fort Union Formation was deposited after the extinction of dinosaurs, and looks like it's composed of stacks of different colored rocks—"pajama-striped looking beds," Weaver said. The brightly colored rock layers were thought to be pond deposits caused, some researchers thought, by a time of rising sea levels.

The rock formation was a stark contrast to the formations lying beneath it, which had waterlogged, poorly developed soils reminiscent of what you might see in the outer edges of a floodplain. The researchers began to suspect the change in geology was somehow related to the mass extinction of dinosaurs, called the Cretaceous-Paleogene, or K-Pg, mass extinction. Moreover, they began to examine what types of environments were represented by these different rock formations.

"What we realized was 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," said Weaver, also assistant curator of fossil mammals at the U-M Museum of Paleontology. "So instead of looking at a stillwater, quiet setting, what we're actually looking at is a very active inside of a meander."

The large river deposits were bracketed by layers largely composed of lignite, a low-grade form of coal formed by carbonized plant matter. Weaver and colleagues believed these formed because with the stabilizing effect of dense forests, rivers flooded less frequently.

"By stabilizing rivers, you cut off the supply of clay, silt and sand to the far reaches of the floodplain, so you're mostly accumulating organic debris," Weaver said. 

The evidence that would clinch whether the change occurred right after the K-Pg mass extinction? A fine layer of sediment loaded with iridium, an element typically only delivered to Earth by cosmic rays. However, when the asteroid slammed into Earth, it carried with it a payload of the element, which settled over much of the planet in a fine layer. This iridium-rich layer of sediment, which defines the K-Pg boundary, carries about three orders of magnitude more iridium than typical sediments, and is called the iridium anomaly.

The research team then focused on an area the Bighorn Basin where the K-Pg boundary hadn't been located. Looking at places of geologic change between the dinosaur-bearing formation and Paleocene-mammal-bearing formations, Weaver took samples of a fine line of red clay about a centimeter in width.

"Lo and behold, the iridium anomaly was right at the contact between those two formations, right where the geology changes," he said. "That discovery convinced us that this isn't just a phenomenon in the Williston Basin. It's probably true everywhere throughout the Western Interior of North America."

The land before time

Still, the mystery of why the geology of landscapes should have changed so much before and after dinosaurs' extinction remained. But then Weaver encountered a series of talks about how present-day animals such as elephants influence the ecosystem in which they live.

"That was the light bulb moment when all of this came together," Weaver said. "Dinosaurs are huge. They must have had some sort of impact on this vegetation."

Weaving together prior literature and the work of co-author Mónica Carvalho, assistant curator at the U-M Museum of Paleontology and assistant professor of earth and environmental science, who studies how vegetation changed across the K-Pg boundary, Weaver and the research team suggested the sudden disappearance of dinosaurs allowed forests to flourish, helping to trap sediment, build point bars and structure rivers.

"To me, the most exciting part of our work is evidence that dinosaurs may have had a direct impact on their ecosystems," said U-F's Courtney Sprain. "Specifically, the impact of their extinction may not just be observable by the disappearance of their fossils in the rock record, but also by changes in the sediments themselves."

Weaver says the K-Pg extinction event is also a lesson in how the record of Earth might change in light of human-caused climate change and loss of biodiversity.

"The K-Pg boundary was essentially a geologically instantaneous change to life on Earth, and the changes we're making to our biota and to our environments more broadly are going to appear just as geologically instantaneous," Weaver said. "What's happening in our lifetimes is the blink of an eye in geologic terms, and so the K-Pg boundary is our best analog to our very abrupt restructuring of biodiversity, landscapes and climate."

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