Washington, April 17: A team of geochemists has come across evidence that Earth''s atmosphere 2.4 billion years ago was oxygen-rich, as pointed out by the origin of sulfur in rocks.

Sedimentary rocks created more than 2.4 billion years ago 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.

Now, a team of geochemists can show an alternative origin for this isotopic composition that may point to an early, oxygen-rich atmosphere.

"The significance of this finding is that an abnormal isotope fractionation (of sulfur) may not be linked to the atmosphere at all," said Yumiko Watanabe, research associate, Penn State.

"The strongest evidence for an oxygen poor atmosphere 2.4 billion years ago is now brought into question," she added.

The researchers present the possibility that the rocks with an anomalous sulfur isotope fractionation came from locations on the ocean floor where hydrothermal fluids seeped up from submarine vents through organic carbon rich sediments and mixed with the ocean water.

Other scientists have previously determined that the sulfur dioxide, ultraviolet light reaction in the absence of oxygen can produce the anomalous isotope fractionation.

Watanabe looked at samples of amino acids and sodium sulfur compounds to try to recreate the anomalous sulfur isotope composition in another way.

She chose amino acids as a proxy for organic material because the anomalous sulfur isotopes often come from sedimentary rock, black shale, which also contains abundant mature kerogen - a mixture of organic compounds.

She chose sodium compounds because of the large amounts of sodium and sulfate in the ocean. Initial experiments used two amino acids - alanine and glycine - and sodium sulfite, which is less oxidized compared to sulfate.

When heated, these did not produce abnormal fractionation.

Watanabe then tested five amino acids, adding histidine, arginine and tryptophan, and mixed them with sodium sulfate.

In this case, alanine and glycine produced the anomalous isotope composition found in the rocks. In all, she ran 32 series of experiments with more than 100 individual samples.

"At high temperatures it sometimes took 24 hours for the sulfate to reduce to sulfide," said Watanabe. "At lower temperatures it took about two months, 1,000 hours," she added.

"People never thought that anomalous sulfur isotope fractionation could be caused by a process other than atmospheric reactions," said Hiroshi Ohmoto, professor of geoscience, Penn State.

"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," Ohmoto added. (ANI)