Press Release 11-002 
Widespread Ancient Ocean "Dead Zones" Challenged Early Life

Persistent lack of oxygen in Earth's oceans affected animal evolution

The oceans became oxygen-rich as they are today about 600 million years ago, during Earth's Late Ediacaran Period. Before that, most scientists believed until recently, the ancient oceans were relatively oxygen-poor for the preceding four billion years.

Now biogeochemists at the University of California-Riverside (UCR) have found evidence that the oceans went back to being "anoxic," or oxygen-poor, around 499 million years ago, soon after the first appearance of animals on the planet.  

They remained anoxic for two to four million years.   

The researchers suggest that such anoxic conditions may have been commonplace over a much broader interval of time

"This work is important at many levels, from the steady growth of atmospheric oxygen in the last 600 million years, to the potential impact of oxygen level fluctuations on early evolution and diversification of life," said Enriqueta Barrera, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.  The researchers argue that such fluctuations in the oceans' oxygen levels are the most likely explanation for what drove the explosive diversification of life forms and rapid evolutionary turnover that marked the Cambrian Period some 540 to 488 million years ago.

They report in this week's issue of the journal Nature that the transition from a generally oxygen-rich ocean during the Cambrian to the fully oxygenated ocean we have today was not a simple turn of the switch, as has been widely accepted until now.

"Our research shows that the ocean fluctuated between oxygenation states 499 million years ago," said paper co-author Timothy Lyons, a UCR biogeochemist and co-author of the paper.

Credit National Science Foundation

January 5, 2011