At the end of the Permian 252 million years ago, the Earth was devastated by a mass extinction that wiped out more than 90% of the planet’s species. Unlike after other mass extinctions, the recovery from the “Great Dying” was slow: TK took millions of years to repopulate the planet and restore its diversity.
Now scientists may have figured out what delayed Earth’s recovery. A group of tiny marine organisms called radiolarians disappeared after becoming extinct. Their absence radically altered ocean geochemistry, allowing for a type of clay formation that released carbon dioxide. That release of carbon dioxide would have kept the atmosphere warm and the oceans acidic, thereby slowing life’s recovery, the scientists explained in an Oct. 3 article published in the journal nature geosciences (opens in new tab).
These are extreme conditions not seen on Earth hundreds of millions of years before the emergence of widespread life, said study co-author Clément Bataille, now a professor of earth and environmental sciences at the University of Ottawa in Canada live science.
“It just goes to show how much we don’t know about these biogeochemical cycles and how a small change can throw the system out of balance really, really quickly,” Bataille said.
An unfriendly earth
Bataille worked on the research as a postdoctoral researcher in the lab of Xiao-Ming Liu, a geochemist at the University of North Carolina at Chapel Hill. The researchers tried to understand the changes in the Earth’s climate at the end of the Permian (298.9 million to 251.9 million years ago) and at the beginning of the Triassic (251.9 million to 201.3 million years ago). At that time, all continents were called to one huge landmass Pangeaand a huge block of volcanoes known as the Siberian traps, planetary warming belched greenhouse gaseswhich likely contributed to the extinction event that led to the death of almost everything.
The team wanted to study a process called chemical weathering – when rocks on land break down, releasing calcium that is eroded into the oceans. There, the calcium combines with carbon dioxide (CO2) to form carbonate rock. The warmer the climate, the faster weathering occurs, since chemical reactions occur faster in warmer temperatures and more flowing water means more erosion. This creates a feedback loop that keeps global temperatures in check, Bataille said: When it’s warmer and weathering is faster, more CO2 flows into the sea and becomes trapped in ocean rocks, helping to cool the climate. As the climate cools, weathering slows and less CO2 is trapped in sea rocks, preventing it from getting too cold.
But there is another process that can occur in the ocean called reverse weathering. This happens when the mineral silica becomes plentiful and forms new clays on the sea floor. During reverse weathering, these clays release more CO2 than carbonate rocks can absorb.
Silica isn’t abundant in today’s oceans because tiny planktonic organisms snag it to build their shells, so reverse weathering doesn’t happen much. Similarly, in the Permian, tiny organisms called radiolarians consumed almost all of the silica, keeping reweathering to a minimum.
A sudden change
However, all that may have changed by the end of the Permian and the beginning of the Triassic. At this point, siliceous rocks disappeared from countless radiolarian shells, indicating that the radiolarian may have been wiped out. At the same time, Bataille, Liu and their colleagues found that the equilibrium of certain variants of molecules in ocean rocks was thrown off balance.
The researchers studied the ratios of lithium isotopes. Isotopes are versions of an element with slightly different atomic weights than the norm because they have a different number of neutrons in their nuclei. Because of their different weights, different lithium isotopes are incorporated in different ratios during the formation of new clays, which occurs during reweathering. The researchers found that some lithium isotopes virtually disappeared from the ocean just before the Great Dying, and did not recover for about 5 million years into the Triassic. This paints a picture of a world where the loss of radiolarian species resulted in an ocean filled with silica, allowing reverse weathering, Bataille said. The CO2 released by reverse weathering could have overwhelmed the CO2-trapping chemical weathering that was taking place at the time, and in turn kept the climate particularly hot. In such conditions, life would have struggled.
This is the first direct evidence that reverse weathering was occurring at that time, said Hana Jurikova, a marine biogeochemist at the University of St Andrews in Scotland. Yurikova did not take part in the research, but she wrote one Editorial to the newspaper (opens in new tab) in the journal Nature Geoscience.
“There’s obviously still work to be done,” Jurikova told Live Science, “but it’s kind of an elegant theory.”
Questions that remain to be answered include what killed the Radiolarians? Evidence shows that reverse weathering began a few million years before the mass extinction, Jurikova said, suggesting these microorganisms might have been struggling before the Siberian traps did their worst. Perhaps even before the deadly volcanic eruptions, the conditions became challenging for life.
“We have traditionally been very excited about the mass extinction and have tried to zoom in as much as possible,” Jurikova said, “but we may find that we need to zoom out.”
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