Earth has already undergone five mass extinctions in its history, which scientists call the "Big Five." Its first mass extinction is known as the end-Ordovician mass extinction that occurred 445 million years ago, which killed 65% of marine species.
An international team of researchers from the US, Canada, China, Mexico, and France investigated ocean environments before, during, and after the first mass extinction to study its cause. Their study, titled "Vertical Decoupling in Late Ordovician Anoxia Due to Reorganization of Ocean Circulation," was published in the journal Nature Geoscience.
Seen here is a model-based reconstruction of life on an ancient seafloor in the CIncinnati, Ohio area during the Late Ordovician.
Earth's First Mass Extinction
The end-Ordovician mass extinction is the oldest of the Big Five mass extinctions, according to Discover Magazine. About 445 million years ago, simple life forms on Earth first knelt to death. These creatures are mostly bacteria and archaea that flourished for 3 billion years before going extinct.
The Ordovician period follows the Cambrian Period, an era well-known for the evolutionary explosion of animals that populated the world whose descendants still live today.
Many vertebrates, mollusks, arthropods, and other taxonomic groups that lived in the Cambrian period were growing and diversifying at an extraordinary rate until their abrupt downfall because of the end-Ordovician mass extinction.
Paleobiologist Seth Finnegan from the University of California, Berkeley told the online magazine that the Ordovician period was a very interesting time because it had the largest and most rapid biodiversification events and one of the largest mass extinction events. He added that these two major events during this period are tied up in the physical changes in the environment.
What Caused the End-Ordovician Mass Extinction?
In Dr. Finnegan's new study, he and his team investigated the oceans to uncover what caused the end-Ordovician mass extinction. He said the oceans during this period contain some of the first reefs made by animals but lack the abundance of vertebrates.
Sci-News reported that one of the major debates surrounding the cause of the first mass extinction is that anoxia or lack of oxygen in seawater caused the event that killed 85% of marine species.
Dr. Finnegan and his colleagues integrated geochemical testing with numerical simulations and computer modeling to determine whether this theory is true. Then they measured iodine concentration in carbonate rocks from the Ordovician period that will serve as an indicator of changes in oceanic oxygen levels.
They found no evidence that supports the theory of anoxia during the extinction event in the shallow ocean animal habitat where most marine species lived. That means climate cooling that occurred during that time and other factors are likely responsible for the extinction event.
However, they found evidence of anoxia in the deeper parts of the ocean, which remains a mystery for the tea. They said that upper-ocean oxygenation in response to cooling is expected, but expanded anoxia in the lower ocean is generally associated with global warming caused by volcanism.
Deep Ocean Anoxia
According to Science Daily, the team attributed the deep-sea anoxia to the circulation of seawater throughout the oceans around the world, an important component of the climactic system. Researchers noted that their computer modeling results show that climate cooling likely altered ocean circulation patterns that halted oxygen-rich water from flowing to the deep ocean.
Although scientists may not fully agree upon the causes of end-Ordovician mass extinction, this study rules out the changes in oxygenation as the only reason for this event and adds new data favoring climate cooling as a killing mechanism during that period.
RELATED ARTICLE: The Planet's First Mass Extinction Was Caused by the Production of Oxygen
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