New Research Explores Cause of End-Ordovician Mass Extinction
The end-Ordovician mass extinction, the first of the ‘Big Five’ mass extinctions occurred 445 million years ago and was characterized by the disappearance of 85% of marine species. In new research, a team of scientists from the United States, Canada, China, Mexico and France has investigated the ocean environment before, during, and after the end-Ordovician extinction in order to determine how the event was brewed and triggered. Their results appear in the journal Nature Geoscience.
“Ordovician seas were full of biodiversity,” said Dr. Seth Finnegan, a researcher in the Department of Integrative Biology at the University of California, Berkeley.
“Oceans contained some of the first reefs made by animals, but lacked an abundance of vertebrates.”
“If you had gone snorkeling in an Ordovician sea you would have seen some familiar groups like clams and snails and sponges, but also many other groups that are now very reduced in diversity or entirely extinct like trilobites, brachiopods and crinoids.”
Unlike with rapid mass extinctions, like the end-Cretaceous extinction event where dinosaurs and other species died off suddenly some 65.5 million years ago, the end-Ordovician mass extinction played out over a substantial period of time, with estimates between less than 500,000 years to almost two million years.
One of the major debates surrounding the end-Ordovician mass extinction is whether lack of oxygen in seawater caused the event.
To investigate this question, Dr. Finnegan and colleagues integrated geochemical testing with numerical simulations and computer modeling.
The researchers measured iodine concentration in carbonate rocks from that period. The concentration of this element in carbonate rocks serves as an indicator for changes in oceanic oxygen level in Earth’s history.
The new data, combined with computer modeling simulations, suggested that there was no evidence of anoxia — or lack of oxygen — strengthening during the extinction event in the shallow ocean animal habitat where most organisms lived, meaning that climate cooling that occurred during the end-Ordovician period combined with additional factors likely was responsible for the extinction event.
“On the other hand, there is evidence that anoxia in deep oceans expanded during that same time, a mystery that cannot be explained by the classic model of ocean oxygen,” said Dr. Alexandre Pohl, a researcher in the Department of Earth and Planetary Sciences at the University of California, Riverside, and the Université Bourgogne Franche-Comté.
“Upper-ocean oxygenation in response to cooling was anticipated, because atmospheric oxygen preferentially dissolves in cold waters.”
“However, we were surprised to see expanded anoxia in the lower ocean since anoxia in Earth’s history is generally associated with volcanism-induced global warming.”
The authors attribute the deep-sea anoxia to the circulation of seawater through global oceans.
“A key point to keep in mind is that ocean circulation is a very important component of the climatic system,” Dr. Pohl said.
The team’s computer modeling results show that climate cooling likely altered ocean circulation pattern, halting the flow of oxygen-rich water in shallow seas to the deeper ocean.
“Recognizing that climate cooling can also lead to lower oxygen levels in some parts of the ocean is a key takeaway from our study,” said Professor Zunli Lu, a resaercher in the Department of Earth Sciences at Syracuse University.
“For decades, the prevailing school of thoughts in our field is that global warming causes the oceans to lose oxygen and thus impact marine habitability, potentially destabilizing the entire ecosystem.”
“In recent years, mounting evidence point to several episodes in Earth’s history when oxygen levels also dropped in cooling climates.”
While the causes of end-Ordovician extinction have not been fully agreed upon, nor will they for some time, the current study rules out changes in oxygenation as a single explanation for this extinction and adds new data favoring temperature change being the killing mechanism for the end-Ordovician mass extinction.
A. Pohl et al. Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nat. Geosci, published November 1, 2021; doi: 10.1038/s41561-021-00843-9