April 16, 2009

Sulfur in rocks tells early oxygen story

U.S. researchers say a study of sedimentary rocks created more than 2.4 billion years ago suggests the Earth had an early, oxygen-rich atmosphere.

Pennsylvania State University researcher Yumiko Watanabe said rocks sometimes have an unusual sulfur isotope composition thought to be caused by the action of ultra violet light on volcanically produced sulfur dioxide in an oxygen poor atmosphere.

But the team of geochemists said they can show an alternative origin for that isotopic composition that might point to an early, oxygen-rich atmosphere.

The researchers, who included University of Maryland Associate Professor James Farquhar and Penn State Professor Hiroshi Ohmoto, said there's a possibility the rocks with an anomalous sulfur isotope fractionation came from the ocean floor where hydrothermal fluids seeped up from submarine vents through organic carbon rich sediments and mixed with the ocean water.

People never thought that anomalous sulfur isotope fractionation could be caused by a process other than atmospheric reactions, said Ohmoto. Our study significantly shifts possibilities to something different, to a biological and thermal regime. There are now at least two ways that the anomalous sulfur isotope fractionation seen in some rocks could be achieved.

The study is reported in the journal Science.