Revealing the surprising secrets behind Earth’s first major mass extinction

Hirnantian Branchiopoder

Detail photos of fossils from the Ordovician period on Anticosti Island, Quebec, Canada. Credit: André Desrochers, University of Ottawa

A team of researchers publishes a new study exploring the cause of the late Ordovician mass extinction.

We all know that the dinosaurs died in a mass extinction. But did you know that there were other mass exterminations? There are five most significant mass extinctions, known as the “Big Five,” in which at least three-quarters of all species that exist across the Earth faced extinction during a given geological period. With current trends in global warming and climate change, many scientists now believe we can be in sixth place.

Discovering the root cause of the Earth’s mass extinction has long been a hot topic for scientists, as understanding the environmental conditions that led to the elimination of most species in the past could potentially help prevent a similar event from occurring in the future.

A team of researchers from Syracuse University’s Department of Earth and Environmental Sciences University of California, Berkeley and the University of California, Riverside, the Université Bourgogne Franche-Comté, the University of New Mexico, the University of Ottawa, the University of Science and Technology of China, and Stanford University were recently co-authors of a paper exploring the late Ordovician mass extermination (LOME), which is the first or oldest of the “Big Five (~ 445 million years ago).” About 85% of marine species, most of which lived in shallow oceans near continents, disappeared during that time.

Hirnantic fossils

Detail photos of fossils from the Ordovician period on Anticosti Island, Quebec, Canada. Credit: André Desrochers, University of Ottawa

Lead author Alexandre Pohl, from UC Riverside (now postdoctoral fellow at the Université Bourgogne Franche-Comté in Dijon, France) and his co-authors studied the marine environment before, during and after the extinction to determine how the event was brewed and triggered. The results of their study were published in the journal Nature Geoscience today (November 1, 2021).

To paint a picture of the oceanic ecosystem during the Ordovician period, mass extinction expert Seth Finnegan, an associate professor at UC Berkeley, says the oceans were full of biodiversity. The oceans contained some of the first reefs made of animals, but lacked an abundance of vertebrates.

“If you had been snorkeling in an Ordovician sea, you would have seen some well-known groups like mussels and snails and fungi, but also many other groups that are now greatly reduced in diversity or completely extinct like trilobites, brachiopods and crinoids,” says Finnegan.

In contrast to rapid mass extinctions, such as Chalk– Tertiary extinction event where dinosaurs and other species died suddenly about 65.5 million years ago, Finnegan says that LOME unfolded over a significant period, with estimates ranging from less than half a million to almost two million years.

Fossils from the Ordovician period

Detail photos of fossils from the Ordovician period on Anticosti Island, Quebec, Canada. Credit: André Desrochers, University of Ottawa

One of the major debates surrounding LOME is whether the lack of oxygen in seawater caused the period’s mass extinction. To investigate this issue, the team integrated geochemical tests with numerical simulations and computer modeling.

Zunli Lu, professor of soil and environmental sciences at Syracuse University, and his students took measurements of the iodine concentration in the carbonate rocks from that period, which contributed important findings about oxygen levels at different sea depths. The concentration of the element iodine in carbonate rocks serves as an indicator of changes in oceanic oxygen levels in Earth’s history.

Their data, combined with computer modeling simulations, suggested that there was no evidence that anoxia – or lack of oxygen – was enhanced during the extinction event in the shallow sea creature’s habitat, where most organisms lived, meaning that climate cooling that took place in the late Ordovician period combined with additional factors likely to be responsible for LOME.

On the other hand, there is evidence that anoxia in deep oceans expanded over the same period, a mystery that cannot be explained by the classic model of havilt, says climate modeling expert Alexandre Pohl.

“Overseas oxygenation in response to cooling was expected because atmospheric oxygen is primarily dissolved in cold water,” says Pohl. “But we were surprised to see expanded anoxia in the lower oceans, as anoxia in Earth’s history is generally associated with volcanism-induced global warming.”

They attribute deep-sea anoxia to the circulation of seawater through global oceans. Pohl says that an important point to keep in mind is that ocean circulation is a very important component of the climatic system.

He was part of a team led by senior model Andy Ridgwell, a professor at UC Riverside, whose computer modeling results show that climate cooling is likely to change ocean circulation patterns and halt the flow of oxygen-rich water in shallow seas to the deeper oceans.

According to Lu, the recognition that climate cooling can also lead to lower oxygen levels in some parts of the ocean is an important part of their study.

“For decades, the prevailing thinking within our field has been that global warming causes the oceans to lose oxygen and thus affect the habitability of the ocean, potentially destabilizing the entire ecosystem,” says Lu. “In recent years, rising evidence has pointed to several episodes in Earth’s history where oxygen levels also dropped in cooling climates.”

While the causes of late Ordovician eradication have not been fully agreed, nor will they for some time, the team’s study excludes changes in oxygenation as a single explanation for this eradication and adds new data favoring temperature changes, which is the killing mechanism for LOME.

Pohl hopes that as better climate data and more sophisticated numerical models become available, they will be able to offer a more robust representation of the factors that may have led to the late Ordovician mass extinction.

Reference: “Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation” by Alexandre Pohl, Zunli Lu, Wanyi Lu, Richard G. Stockey, Maya Elrick, Menghan Li, André Desrochers, Yanan Shen, Ruliang He, Seth Finnegan and Andy Ridgwell, November 1, 2021, Nature Geoscience.
DOI: 10.1038 / s41561-021-00843-9

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