New Study Suggests ‘Missing Billion Years’ In Rock Record Carved Away By ‘Snowball Earth’ Glaciers

“The Great Unconformity” has perplexed geologists since it was first described nearly 150 years ago. It is an erosion surface found worldwide, but best exposed at the bottom of the Grand Canyon, where the 500-million-year-old Paleozoic Tapeats Sandstone is directly overlying the 1.4 to 1.8 billion-year-old Proterozoic Vishnu Schist, representing a gap between 100 million and 1 billion years long in Earth’s rock record.

Research has shown the complex history behind this gap of missing time, and theories range from the break up of an ancient supercontinent, causing tectonic uplift and erosion, to a dramatic shift in Earth’s climate and weathering cycle.

According to a new study published in Proceedings of the National Academy of Sciences, an estimated five kilometers (3 miles) of rock strata were carved away by ancient glaciers during the planet’s Cryogenian (“Snowball Earth“) period about 700 to 635 million years ago.

“The fact that so many places are missing the sedimentary rocks from this time period has been one of the most puzzling features of the rock record,” said C. Brenhin Keller, an assistant professor of Earth sciences at Dartmouth College and senior researcher on the study. “With these results, the pattern is starting to make a lot more sense.”

“This was a fascinating time in Earth’s history,” said Kalin McDannell, a postdoctoral researcher and the lead author of the paper. “The Great Unconformity sets the stage for the Cambrian explosion of life, which has always been puzzling since it is so abrupt in the fossil record—geological and evolutionary processes are usually gradual.”

Research led by C.B. Keller in 2019 first proposed that widespread erosion by continental ice sheets during the Cryogenian glacial interval caused the loss of rock. This was based on geochemical proxies that suggested that large amounts of mass erosion matched with the Snowball Earth period.

“The new research verifies and advances the findings in the earlier study,” said Keller. “Here we are providing independent evidence of rock cooling and miles of exhumation in the Cryogenian period across a large area of North America.”

The study relies on a detailed interpretation of thermochronology to make the assessment.

Thermochronology allows researchers to estimate the temperature that mineral crystals experience over time as well as their position in the continental crust given a particular cooling path. Those histories can provide evidence of when missing rock was removed and when rocks currently exposed at the surface may have been exhumed.

The researchers used multiple measurements from previously published thermochronometric data taken across four North American locations. The areas, known as cratons, are parts of the continent that are chemically and physically stable, and where plate tectonic activity would not have been common during that time.

By running simulations that searched for the time-temperature path the rocks experienced, the research recorded a widespread signal of rapid, high magnitude cooling that is consistent with about 2-3 miles of erosion during Snowball Earth glaciations across the interior of North America.

“While other studies have used thermochronology to question the glacial origin, a global phenomenon like the Great Unconformity requires a global assessment,” said McDannell. “Glaciation is the simplest explanation for erosion across a vast area during the Snowball Earth period since ice sheets were believed to cover most of North America at that time and can be efficient excavators of rock.”

According to the research team, the competing theory that tectonic activity carved out the missing rock was put forth in 2021 when a separate research group questioned whether ancient glaciers were erosive enough to cause the massive loss of rock. While that research also used thermochronology, it applied an alternate technique at only a single tectonically active location and suggested that the erosion occurred prior to Snowball Earth.

“The underlying concept is pretty simple: Something removed a whole lot of rock, resulting in a whole lot of missing time,” said Keller. “Our research demonstrates that only glacial erosion could be responsible at this scale.”

According to the researchers, the new findings also help explain links between the erosion of rock and the emergence of complex organisms about 530 million years ago during the Cambrian explosion. It is believed that erosion during the Snowball Earth period deposited nutrient-rich sediment in the ocean that could have provided a fertile environment for the building blocks of complex life.

The study notes that the two hypotheses of how the rock eroded are not mutually exclusive—it is possible that both tectonics and glaciation contributed to global Earth system disruption during the formation of the Great Unconformity. It appears, however, that only glaciation can explain erosion in the center of the continent, far from the tectonic margins.

“The fact that there may have been tectonic erosion along the craton margins does not rule out glaciation,” said McDannell. “Unconformities are composite features, and our work suggests Cryogenian erosion was a key contributor, but it is possible that both earlier and later erosion were involved in forming the unconformity surface in different places. A global examination will tell us more.”

This is the first research that uses their thermochronology modeling approach to study a period that extends well beyond a billion years. In the future, 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.