The Genesis of Oxygen

Nancy Marie Brown
September 01, 1994

In the beginning, was there air?

The answer depends, of course, on how you define "beginning," and what you mean by "air." According to Hiroshi Ohmoto and Takeshi Kakegawa of Penn State's department of geosciences, however, "by about 3.4 billion years ago" (more than three billion years before the earliest dinosaurs) "the atmosphere contained appreciable amounts (more than approximately one percent of the present atmospheric level) of free oxygen."

Ohmoto and Kakegawa, working with Donald Lowe of Stanford University, reached this conclusion after analyzing the sulfur contained in nuggets of pyrite found in rocks known to be 3.4 billion years old.

rocks with pyrite

Pyrite and calcite from Penn State's Earth and Mineral Sciences Museum

Sulfur in the form of sulfate [SO4], as they explain in the 22 October 1993 Science, is abundant in present-day seawater. It is generated primarily on land, through the weathering of gypsum and pyrite in rocks, and is washed down rivers to the sea. There, it is converted again into gypsum (by evaporation), or into pyrite. Pyrite, a mineral made of iron and sulfur, forms in marine sediments with the help of certain sulfate-reducing bacteria.

The weathering process depends on free oxygen in the atmosphere, the pyrite-making process on the existence of these bacteria. Oxygen and sulfate-reducing bacteria, therefore, control the present-day cycling of sulfur on the Earth.

Did they always do so? Ohmoto and his colleagues note that in earlier investigations of pyrite's geochemistry, the ratio of sulfur isotopes (the different atomic forms of the element) in 3.4-billion-year-old samples more closely matched that of modern igneous rocks than that of bacteria-produced pyrite. Therefore, some scientists had argued, the 3.4-billion-year-old pyrite must have formed volcanically. This explanation implied, note Ohmoto, Kakegawa, and Lowe, that "sulfur-reducing bacteria had not evolved, the oceans were free of sulfate, and the atmosphere contained virtually no free oxygen."

"However, some investigators have interpreted the same isotope data differently." These scientists attributed the contradictory isotope signatures to "differences in environmental parameters for sulfate-reducing bacteria, such as the sulfate content and temperature of seawater."

Settling the dispute required more precise measurements. The earlier methods, Ohmoto and his colleagues note, called for a quantity of at least five milligrams of crystals, 100 to 1,000 grains of typical pyrite; the individual crystal-to-crystal variation in chemistry was therefore lost. Having developed new laser and mass-spectrometry techniques, however, Ohmoto and his colleagues were able to analyze single grains of pyrite, each 150 microns to one millimeter in size.

"We analyzed a total of 123 grains of pyrite from the four rock samples," they write. The amount of isotope variation they found far exceeded their expectations, and was much wider than the range possible for rocks formed volcanically. "The observed variations in the [isotope ratio] values among pyrite crystals occurring in a small volume of a sedimentary rock can be explained most easily by models involving bacterial sulfate reduction," they conclude.

"The results of this study imply that the geochemical cycle of sulfur in near-surface environments has been virtually unchanged since at least 3.4 billion years ago."

Hiroshi Ohmoto, Ph.D., is professor of geochemistry in the College of Earth and Mineral Sciences, 208 Deike Building, University Park, PA 16802; 814-865-4074. Takeshi Kakegawa is a Ph.D. candidate in geosciences at Penn State. This work was funded by the National Science Foundation, the Japanese Ministry of Science, Education, and Culture, and the NASA Exobiology Program.

Last Updated September 01, 1994