Scientists say this phenomenon likely led to first mass extinction on Earth
- A team of scientists studied the Late Ordovician mass extinction
- The extinction, likely caused by climate change, wiped out about 85% of all species around 445 million years ago
- Rock samples from the period helped scientists understand changes in oceanic oxygen levels
Researchers believe climate change may have caused the first mass extinction on Earth about 445 million years ago. The Ordovician mass extinction wiped out about 85% of all species. In comparison, the Devonian mass extinction that occurred about 375 million years ago killed 75% of the world’s species. The Permian mass extinction about 250 million years ago and the Triassic mass extinction 50 million years later accounted for the extinction of over 95% and about 80% of all species, respectively.
What caused the Ordovician mass extinction?
A paper published last week in the Nature Geoscience journal says that a mass extinction of marine creatures was caused by the cooling climate that likely changed the ocean circulation pattern, which in turn disrupted the flow of oxygen-rich water from the shallow seas to deeper oceans.
“If you had gone snorkelling 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,” according to one of the study's authors, Seth Finnegan, from the Department of Integrative Biology, University of California, Berkeley.
The clues offered by iodine concentration in rocks from that period helped scientists understand the changes in oceanic oxygen levels. While upper-ocean oxygenation in response to cooling was anticipated because atmospheric oxygen preferentially dissolves in cold waters, the researchers were surprised to see expanded anoxia in the lower ocean.
This is because "anoxia in Earth’s history is generally associated with volcanism-induced global warming,” said the first author, Alexandre Pohl, from the Department of Earth and Planetary Sciences, the University of California, in a release.
Their models that used data on the Ordovician climate and marine biogeochemical cycles during that period showed “seafloor and upper-ocean oxygenation in response to ongoing global cooling.”
This deep-sea anoxia affected ocean circulation. Pohl says that a key point to keep in mind is that ocean circulation is a very important component of the climatic system.