The breakup of the ancient supercontinent Nuna during Earth’s “boring billion years” may have shaken up the planet, and its reorganization may have created the conditions for complex life, a new study shows in unprecedented detail.
The “Bowling Billion” refers to the period from 1.8 billion to 800 million years ago. Although this period included the breakup and assembly of two ancient supercontinents, Nuna and Rodinia, scientists gave the period this name because of its perceived lack of upheaval.
“The term was coined to describe what appears to be a long period of geochemical, climatic and biological stability in Earth’s history,” University of Sydney geophysics professor Dietmar Müller, who led the new study, told Live Science via email. “However, we now know that from the perspective of plate tectonics and evolutionary changes, this period was not as boring as previously thought.”
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According to a study published in the journal Earth and Planetary Science Letters on October 27, Nuna’s departure triggered a series of events that made Earth more hospitable to life. As Nuna’s fragments drift away from the supercontinent’s center, a shallow ocean appears in the gap between them, warmer and richer in oxygen than the previous ocean, the first simulation of its kind shows.
Researchers used a recently published state-of-the-art model to reconstruct plate tectonic movements and associated changes in carbon storage and emissions over the past 1.8 billion years. The novelty of this method lies in its ability to reconstruct carbon fluxes in more detail than previously possible, the researchers say in their study.
Over the course of 350 million years during the Boring Billion era, the total length of the shallow ocean around the landmass doubled to about 81,000 miles (130,000 km), more than three times the circumference of the Earth at the equator, the researchers say. At the same time, the study shows that because of how the plates moved, subduction zones (where one tectonic plate slides beneath another) became shorter overall.
Subduction zones cause volcanic activity at the Earth’s surface because they inject seawater into the mantle, the layer beneath the Earth’s crust, which lowers the melting temperature of rocks. This promotes the formation of magma, which rises into the Earth’s crust and erupts from the volcano along with debris and gases such as carbon dioxide (CO2).
As subduction zones became shorter, less CO2 leaked from Earth’s interior into the atmosphere. The researchers suggested that this cooled the Earth and established oxygen-rich conditions in the newly formed shallow oceans, and that these relatively stable ecosystems gave rise to more complex life than had previously existed.
“We think these vast continental shelves and shallow oceans were important ecological incubators,” study co-author Juraj Farkas, associate professor in the School of Physics, Chemistry and Earth Sciences at the University of Adelaide in Australia, said in a statement. “They probably provided a tectonically and geochemically stable marine environment with high levels of nutrients and oxygen, which were essential for the evolution and diversification of more complex life forms on Earth.”
Specifically, shallow oceans may have accelerated the diversification of eukaryotes. Eukaryotes are organisms whose cells have specialized structures called organelles and membrane-bound nuclei that contain DNA. Because all animals, plants, and fungi are eukaryotes, the study authors proposed that the emergence of eukaryotic cells in the Boring Billion was an important step in the evolution of complex life.
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Researchers already knew that eukaryotes evolved during the Boring era thanks to fossil evidence from 1.05 billion years ago. However, the conditions under which these creatures appeared remained unknown.
“The collapse of Nuna created a large amount of new ocean floor in a young ocean basin that did not exist before,” Muller explained. And this ocean floor is contributing to the reduction in atmospheric CO2 already caused by the shortening of subduction zones, he said. That’s because when seawater seeps into cracks in the ocean floor, it strips away carbon and forms limestone.
“This ocean floor was modified by hydrothermal circulation, which accumulated carbon in the form of carbonate cement in the voids and cracks, removing CO2 from the atmosphere,” Muller said.
In short, the breakup of the ancient supercontinent Nuna triggered three major changes that benefited complex life. Shallow oceans formed, gas emissions from volcanoes decreased, and carbon was trapped in ocean sediments, leading to a more oxygen-rich atmosphere and warmer environment.
“The next step will be to discover better-preserved eukaryotic fossils and document their early evolution,” Müller concluded.
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