Evidence for the Great Oxidation Event Challenged

April 17, 2009 / Posted by: Daniella Scalice

The timing of the rise of oxygen in Earth’s atmosphere is a key question in astrobiology. It is coupled not only to the question of when organisms capable of oxygenic photosynthesis first evolved on Earth, but also what signs of life might be found on young Earth-like planets around other stars.

Members of NAI’s Penn State and Carnegie Institution of Washington Teams report in the current issue of Science that certain sulfur isotopes found in many sedimentary rocks older than 2.4 billion years may not be the result of photochemical reactions in an oxygen-free atmosphere as previously thought. Their research shows that the isotopic signature could instead be due to reactions between organic carbon-rich sediments and sulfate-rich seawater in ancient hydrothermal systems. If so, then the disappearance of the signature in sediments younger than 2.4 billion years may indicate changes in Earth’s hydrothermal system, rather than signaling the rise of oxygen in Earth’s atmosphere. This new paradigm of Earth’s early atmosphere accommodates the theoretical presence of oxygen prior to 2.4 billion years ago.

“The significance of this finding is that an abnormal isotope fractionation (of sulfur) may not be linked to the atmosphere at all,” says Penn State’s Yumiko Watanabe, the study’s lead author. “The strongest evidence for an oxygen poor atmosphere 2.4 billion years ago is now brought into question.” For more information, see Science’s News of the Week GEOCHEMISTRY.