Sedimentary rock layers at the bottom of Grand Canyon have long entombed a geologic record of a period of evolutionary frenzy some 540 million years ago known as the “Cambrian Explosion.” During this biologic shift, which marks the beginning of the Cambrian Period, a burst of new animal life emerged, or at least ‘visible life’ emerged, including trilobites and other animals that made a hard skeleton.
The Tonto Group, a package of sedimentary strata within the canyon, is among the best preserved sections of this history in the world, yet it hasn’t been studied in detail for nearly 75 years – until now.
Utah State University geologist Carol Dehler is among a team of scientists from the University of New Mexico, Boise State University, Germany’s Julius Maximilian University of Würzburg, the Museum of Northern Arizona, the Utah Field House of Natural History State Park Museum and the Denver Museum of Nature and Science, who have undertaken a detailed study of the Tonto Group, including high-precision dating of rock layers and meter-by-meter study of layers, which not only contain information about life, but also about ecology and changes in the carbon cycle.
The team published findings in two papers in the May 2020 issue of Geology, journal of the Geological Society of America: “Asynchronous trilobite extinctions at the early to middle Cambrian transition,” and the cover feature, “Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale.”
Further, the team was awarded an $815,000 grant from the Sedimentary Geology and Paleobiology program of the National Science Foundation to fund a three-year study to characterize and date layers above and below key fossil layers in Grand Canyon and across the western United States.
“Our findings and our award are a welcome breakthrough,” says Dehler, professor in USU’s Department of Geosciences and an GSA Fellow. “Our paper redefines the Tonto Group and reveals it is much younger than previously thought. People come to the Grand Canyon and assume all of the geology has been figured out. We, as Grand Canyon geologists, have learned these rocks are the gift that keeps on giving.”
Over the course of the continuing study, Dehler will coordinate geochemical and environmental analyses of the rock strata.
“The carbon encased in these amazingly well-exposed strata will provide us with a ‘barcode’ of changes in the carbon cycle alongside the changes in biota,” she says. “These records, when calibrated by cutting-edge, high-precision U-Pb dating, will provide a window into the causes, styles and rates of evolutionary change in the Grand Canyon area during Cambrian time and will also be compared to other records around the world.”
Dehler notes one of the best parts of the study, though, is the field work.
“We’re mentoring students ranging from undergraduates just getting started in research to graduate and post-doctoral scholars,” she says. “Many of these students are first-generation college students, from underrepresented groups, including Hispanics and Native Americans.”
Her students’ efforts will have even greater outreach impacts, as they develop tools and resources to provide science outreach to elementary, middle school and high school students, as well as many of the annual six million visitors to Grand Canyon National Park.
“We’ll get as much mileage as we can out of our efforts,” Dehler says. “We hope our work will inspire new generations of scientists. It’s too fascinating not to share.”
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