New study suggests ‘missing billions of years’ in ‘Snowball Earth’ glacier-engraved rock record
“The Great Discordance” has puzzled geologists since it was first described nearly 150 years ago. It is an erosional surface found worldwide, but best exposed at the bottom of the Grand Canyon, where the 500 million year old Paleozoic Tapeats Sandstone directly overlies the 1 year old Proterozoic Vishnu Shale. .4 to 1.8 billion years, which represents a difference. between 100 million and 1 billion years old in Earth’s rock records.
Research has shown the complex story behind this missing time gap, and theories range from the break-up of an ancient supercontinent, causing tectonic uplift and erosion, to a drastic change in Earth’s climate and climate cycle. ‘alteration.
According to a new study published in Proceedings of the National Academy of Sciencesabout five kilometers (3 miles) of rock strata were carved out by ancient glaciers during the planet’s cryogenic (“Snowball Earth”) period about 700 to 635 million years ago.
“The fact that sedimentary rocks from this period are missing so many places has been one of the most puzzling features of the rock record,” said C. Brenhin Keller, assistant professor of earth sciences at Dartmouth College and principal investigator on the study. “With these results, the model is starting to make a lot more sense.”
“It was a fascinating time in Earth’s history,” said Kalin McDannell, postdoctoral researcher and lead author of the paper. “The Great Unconformity sets the stage for the Cambrian Explosion of Life, which has always been puzzling because it is so abrupt in the fossil record – geological and evolutionary processes are generally gradual.”
Research by C. B. Keller in 2019 first proposed that widespread erosion by continental ice sheets during the Cryogenic Glacial Interval caused the rock loss. This was based on geochemical proxies that suggested large amounts of mass erosion corresponded to the Snowball Earth period.
“The new research verifies and advances the results of the previous study,” Keller said. “Here we provide independent evidence of rock cooling and miles of exhumation during the Cryogenic Period across much of North America.”
The study is based on a detailed interpretation of the thermochronology to take stock.
Thermochronology allows researchers to estimate the temperature that mineral crystals experience over time as well as their position in the continental crust based on a particular cooling path. These histories can provide evidence of when the missing rock was removed and when rocks currently exposed on the surface may have been excavated.
The researchers used several measurements from previously published thermochronometric data taken at four North American locations. The areas, called cratons, are parts of the continent that are chemically and physically stable and where plate tectonic activity would not have been common at that time.
Running simulations that looked for the time-temperature path experienced by rocks, the research recorded a widespread signal of rapid, large-scale cooling that corresponds to about 2-3 miles of erosion during the Snowball Earth glaciations at the interior of North America.
“While other studies have used thermochronology to question glacial origin, a global phenomenon like the Great Unconformity requires global assessment,” McDannell said. “Glaciation is the simplest explanation for erosion over a large area during the Snowball Earth period, as ice sheets were thought to cover most of North America at that time and could be effective excavators Rock.”
According to the research team, the competing theory that tectonic activity carved out the missing rock was advanced in 2021 when a separate research group questioned whether ancient glaciers were eroding enough to cause the mass loss of rock. Although this research also used thermochronology, it applied an alternative technique to a single tectonically active location and suggested that the erosion occurred before Snowball Earth.
“The underlying concept is quite simple: something removed a lot of rock, which resulted in a lot of missing time,” Keller said. “Our research demonstrates that only glacial erosion could be responsible at this scale.”
According to the researchers, the new findings also help explain the links between rock erosion and the emergence of complex organisms around 530 million years ago during the Cambrian explosion. Erosion during the Snowball Earth period is thought to have deposited nutrient-rich sediments into the ocean that may have provided a fertile environment for the building blocks of complex life.
The study notes that the two hypotheses for how the rock eroded are not mutually exclusive – it is possible that tectonics and glaciation contributed to the disruption of the global Earth system during the formation of the Great Discrepancy. However, it appears that only glaciation can explain the erosion in the center of the continent, far from the tectonic margins.
“The fact that there was tectonic erosion along the craton margins does not rule out glaciation,” McDannell said. “The unconformities are composite features, and our work suggests that cryogenic erosion was a key contributor, but it is possible that both earlier and later erosion were involved in the formation of the unconformity surface at different locations. overall review will tell us more.”
This is the first research that uses their thermochronological modeling approach to study a time period that extends well beyond a billion years. Going forward, the team will repeat their work on other continents, where they hope to further test these hypotheses about how the Great Unconformity was created and preserved.