(ORDO NEWS) — A new study provides additional evidence that rocks representing a billion years of geologic time were carved by ancient glaciers during the Snow Earth period, according to a study published in the Proceedings of the National Academy of Sciences.
The study represents the latest in a debate over the causes of Earth’s “Great Discontinuity” – a time gap in the geological record associated with the erosion of rocks up to 3 miles thick in areas around the globe.
“The fact that sedimentary rocks from this period are missing in so many places was one of the most puzzling features of the geologic record,” said C. Brenhin Keller, assistant professor of geosciences and senior research fellow for the study. “With these results, the picture starts to make a lot more sense.”
The massive volume of missing rock that became known as the Great Disparity was first named in the Grand Canyon in the late 1800s. This conspicuous geological feature is seen where layers of rocks dating back to distant time periods intersect with each other, and it is often identified where rocks with fossils are directly above those that do not contain fossils.
“It was an exciting time in Earth’s history,” says Calin McDannell, a Dartmouth postdoctoral fellow and lead author on the paper. “The great discrepancy sets the stage for the Cambrian explosion of life, which has always been puzzling because it was so pronounced in fossils – geological and evolutionary processes usually occur gradually.”
For more than a century, researchers have tried to explain the reason for the absence of geological time.
Over the past five years, two opposing theories have emerged: One explains that the rock was carved by ancient glaciers during the “snowy earth” period approximately 700-635 million years ago. Another theory considers a series of plate tectonic events over a much longer period during the formation and breakup of the supercontinent Rodinia from about 1 billion to 550 million years ago.
Research led by Keller in 2019 suggested for the first time that widespread erosion of continental ice sheets during the Cryogenic Ice Age resulted in rock loss. This was based on geochemical proxies that suggested that large amounts of mass erosion corresponded to the “Snowball Earth” period.
“The new study confirms and builds on the findings of the previous study,” Keller said. “Here we provide independent evidence of rock cooling and many kilometers of cryogenic exhumation over a large area of North America.”
The study relies on a detailed interpretation of thermochronology to make an assessment.
Thermochronology allows researchers to estimate the temperature that mineral crystals experience over time, as well as their position in the continental crust at a particular thermal structure. This story may provide evidence of when the extinct rock was removed and when the rocks currently exposed on the surface could have been exhumed.
The researchers used numerous measurements from previously published thermochronometry data from four locations in North America. These areas, known as cratons, are chemically and physically stable parts of the continent where plate tectonic activity was not prevalent at the time.
By running simulations that looked for a time-temperature path traveled by rocks, the researchers captured a widespread signal of rapid and severe cooling that corresponds to roughly 2-3 miles of erosion during Snowball Earth’s glaciation throughout the interior of North America.
“While other studies have used thermochronology to cast doubt on a glacial origin, a global phenomenon such as the Great Discrepancy requires a global assessment,” McDannell said. “Glaciation is the simplest explanation for erosion over a vast area during the Snowball Earth period, as it is believed that ice sheets covered most of North America at the time and may be effective excavators of rock.”
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