Early ocean anoxia may have led to first mass extinction event

Sunday, July 1, 2018

Dengying Formation (South China)A new study by an international collaboration involving Earth and Environmental Sciences Professor Brian Kendall suggests the world’s first mass extinction event 540 million years ago was caused by a widespread loss of dissolved oxygen in the Earth’s oceans.

“This was essentially the first known major mass extinction of macroscopic animals,” says Kendall. 

How and why animal diversity took this sharp dip at the Ediacaran-Cambrian transition only a few tens of millions of years after macroscopic animals first appeared has been a debate between Earth scientists for decades. Part of the mystery lies in what ocean chemistry looked like that long ago. 

Which is why the team, led by scientists at Arizona State University, took a new approach to linking the fossil record with geochemical data. They measured uranium isotope variations from the fossil-bearing carbonates of the Dengying Formation (South China) to determine if an expansion of ocean anoxia contributed to the decline in early animal diversity.

Their study was published this month in Science Advances.

“This work builds on a recent previous study with several members of the same research team," says co-author Kendall, who helped with the uranium isotope modelling and interpretation on this project. "I used uranium and molybdenum isotope data from black shales of the Doushantuo Formation - which underlies the rocks of the Dengying Formation - to argue for an episode of widespread ocean oxygenation around 560-550 million years ago, just 10-20 million years prior to the extinction.”

Their results suggest not only that changes in ocean chemistry were responsible for the event, but that the mass extinction may have led to another major step in evolution: the emergence of mobile animals that could better seek O2-rich environments.

“It is possible that early mobile bilaterian animals may have evolved in response to the expanded ocean anoxia and these animals outcompeted the largely immobile Ediacaran metazoans,” says Kendall.

Mass extinction events have occurred with frightening regularity over the course of Earth’s history. After the initial mass extinction event at the Ediacaran-Cambrian transition, five additional mass extinctions have happened so far as indicated by the fossil record for the Phanerozoic Eon, including the infamous Cretaceous-Tertiary event 65 million years ago, which killed off a majority of the dinosaurs. Biologists have suggested that we could be entering another mass extinction now.

The study was funded by NASA and the U.S. National Science Foundation. Co-authors include Feifei Zhang, Stephen Romaniello, Geoffrey Gilleaudeau, and Ariel Anbar of Arizona State University, and researchers Shuhai Xiao, Virginia Tech; Huan Cui, University of Wisconsin-Madison; Mike Meyer, Carnegie Institution of Science; and Alan Kauffman, University of Maryland.

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