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Physical Chemistry

Casting Doubt On Old Rocks' Tales

Sediment's unusual sulfur signature may not have required oxygen-poor atmosphere

by Elizabeth K. Wilson
April 20, 2009 | A version of this story appeared in Volume 87, Issue 16

Credit: Hiroshi Ohmoto
This 2.46 billion-year-old formation, part of a mine in Western Australia, covers a large area of anomalously fractionated sulfur isotopes.
Credit: Hiroshi Ohmoto
This 2.46 billion-year-old formation, part of a mine in Western Australia, covers a large area of anomalously fractionated sulfur isotopes.

A SULFUR ISOTOPE SIGNATURE in rocks billions of years old might not have been produced in an oxygen-poor atmosphere, a new study shows. The finding could call into question sulfur isotopes' longtime use as a marker for Earth's shift to an oxygen-rich atmosphere.

Sedimentary rocks that are more than 2.4 billion years old often contain anomalous ratios of two stable sulfur isotopes, 33S and 36S, a profile that's wildly different from that of younger rocks. Until now, the only mechanism known to produce this particular isotope fractionation was the ultraviolet photolysis of volcanic sulfur dioxide in the absence of both ozone and O2.

The 2.4 billion-year-old geological boundary at which the isotope fractionation signature suddenly disappeared has always been thought to pinpoint the time when Earth shifted to an oxygenated atmosphere.

But now, Hiroshi Ohmoto, geochemistry professor and director of the NASA Astrobiology Research Center at Pennsylvania State University; center astrobiologist Yumiko Watanabe; and University of Maryland geology professor James Farquhar have created the same anomalous isotope signature through reactions of powdered amino acids and sulfate (Science 2009, 324, 370). Their study suggests that on ancient Earth, reactions between organic matter in sediments and sulfate in hydrothermal solutions could have produced the signature even in the presence of an oxygenated environment.

Watanabe says she and Ohmoto "think there is a strong possibility that most, if not all, signatures of anomalously fractionated sulfur isotopes in sedimentary rocks were created by chemisorption processes rather than by atmospheric reactions."

This view is by no means a consensus. Coauthor Farquhar says he doesn't believe the new mechanism necessarily indicates that the isotopic signatures in all such rocks were produced by this alternative mechanism. "My opinion may change, but I think that more work will be needed to understand how this mechanism works and that this will be required to make the necessary connections to the isotope record," he says.

Others are not convinced at all. James F. Kasting, another geosciences professor at Penn State, is concerned that the study does not provide a strong explanation for why the isotopic signal would have suddenly disappeared 2.4 billion years ago. "The conventional explanation of sulfur dioxide photolysis in a low-oxygen atmosphere explains the disappearance nicely and is in accord with other geologic O2 indicators," Kasting contends.

Watanabe says she and Ohmoto are considering the possibility that the disappearance of the isotopic signature in the sedimentary record may have been related to biological evolution as well as the thermal history of Earth.



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