In locations around the world, the geologic record exhibits large gaps in the sediment record that remain unexplained. One of the largest of these unconformities—the Great Unconformity—can be seen in the Grand Canyon, where age differences between neighboring rock layers can span more than a billion years.
In a new study in Proceedings of the National Academy of Sciences of the United States of America, researchers make the case that large-scale glaciation during parts of the Neoproterozoic era, between 720 million and 635 million years ago, led to extensive erosion of Earth’s crust, causing the Great Unconformity.
During this period, glaciers may have reached equatorial regions, creating a snowball or slushball Earth.
“We think that the glaciers eroded at least 1.6 vertical kilometers of Earth’s crust, on average, across all the landmasses,” says Brenhin Keller, lead author of the study and a postdoctoral fellow at the University of California, Berkeley.
According to the researchers’ model, the eroded crust was deposited into the ocean and ultimately made its way into the mantle by subduction, disappearing forever from the stratigraphic record. But signatures of the eroded crust could be preserved in other places.
“Subducted sediment in the mantle…is often incorporated into new arc magmas,” Keller says.
Ancient Evidence of Erosion
So records of large amounts of subducted crustal material that subsequently became part of the mantle could be preserved in igneous minerals, such as zircon.
Zircon is a tough mineral, able to withstand the ravages of time. Grains of zircon found in western Australia have been dated using uranium-lead dating to 4.2–4.3 billion years old—almost as old as Earth itself.
To determine whether a zircon crystal originated in the crust or the mantle, scientists can compare two isotopes of hafnium. In the crust, the ratio of hafnium-176 to hafnium-177 is low, whereas material from the mantle has a relatively high hafnium-176/177 ratio. When Keller and colleagues looked at the hafnium ratios in 29,523 zircons spanning the past 4.35 billion years from a global data set, they noticed a large drop in the hafnium-176/177 ratios beginning in the Neoproterozoic era, right around the time of the extensive glaciation events, which persisted for hundreds of millions of years.
Levels of another isotope, oxygen-18, can also indicate whether crustal material originated at or near the surface, where oxygen-18 levels would be relatively high, or whether the material originated in the deep crust, where oxygen-18 levels are stably low. Analysis of zircon crystals showed a jump in oxygen-18 levels concomitant with the decrease in hafnium-176/177 ratios, indicating that the crustal material incorporated into the magma was from Earth’s surface.
Taken together, these lines of evidence mean that “during this time, old crustal material was entering new magma on a huge scale,” Keller says.
Another piece of evidence for large-scale global erosion during the Neoproterozoic is the lack of impact craters from before that time. According to the study authors, extensive glaciation during the Neoproterozoic could have wiped out evidence of ancient meteor impacts, although evidence of a couple of immense impact craters does still exist.
Could glaciers have caused this extensive level of global erosion, sweeping away hundreds of millions of cubic kilometers of rock and erasing evidence of impact craters? Graham Shields, a geologist at University College London, thinks so.
“The global nature of glacial evidence [during the Neoproterozoic] supports the authors’ conclusions that erosion rates overall would have been exceptionally high for long periods,” Shields says. “But this is a story that will inevitably evolve as current efforts move forward to merge databases and digitize global rock areas, volumes, and thicknesses.”
Other researchers aren’t so sure, however. Karl Karlstrom, a geologist at the University of New Mexico in Albuquerque, says that the study authors propose an unrealistically high global average of erosion of continents by glaciers. “Erosion is never uniform, and I am skeptical that continental ice sheets bite that deeply into the underlying continents.”
Karlstrom is also skeptical of the team’s use of the global database for hafnium isotopes. Hafnium isotope data from the Grand Canyon–Death Valley region don’t match the global data presented by Keller and colleagues, Karlstrom says. “It raises the question of whether global averages are more significant than local tectonic signals,” he says.
But Keller notes that the hafnium isotope patterns found in the study “are expected only in areas of active Neoproterozoic arc magmatism.” Because much of the U.S. Southwest “was likely closer to a rift environment at this time, we do not expect a sediment subduction signature there.”
Whatever the actual amount, the high rates of erosion during the Neoproterozoic could have had literally a life-changing impact on Earth’s biosphere.
“The pulverization of large amounts of crust would have released phosphorus, which may have been a limiting nutrient in earlier time spans,” Keller says. That could have led to the Cambrian explosion: a period about 540 million years ago marked by the appearance of a huge variety of life on Earth.
Shields agrees that the biogeochemical changes brought about by Neoproterozoic glaciation would have influenced life on Earth, but during an earlier period. “I think it’s more likely that the extensive erosion provided nutrients to nurture Ediacaran communities (about 635 million to 542 million years ago), which were beginning to contain the first animal-based ecosystems, rather than the Cambrian explosion itself, which came quite a long time afterward,” he says.
—Adityarup Chakravorty (email@example.com), Science Writer
This article is part of a series made possible through the generous collaboration of the writers and editors of Earth magazine, formerly published by the American Geosciences Institute.
Chakravorty, A. (2019), Did global glaciation cause the Great Unconformity?, Eos, 100, https://doi.org/10.1029/2019EO120289. Published on 09 April 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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