A type of sulphur found far underground could rewrite theories of early Earth.
A form of sulphur rarely seen on Earth's surface, similar to that which gives the mineral lapis lazuli its deep-blue colour, may be the predominant form of the element in Earth's lower crust and upper mantle.
Known as the trisulphur ion, or S3−, it is quite different from the better-known sulphate (SO42-) and sulphide, S2− sulphur ions. "It's an intermediate form," says Gleb Pokrovski, an experimental geochemist at Géosciences Environnement Toulouse in France, who is first author of a study on the ion, published today in Science1.
Pokrovski's team did not sample this deep sulphur directly. It is found too far down in Earth's crust to be reached by drilling — not to mention that it would change chemically when drawn to the surface.
Instead, the team used a device known as a diamond anvil cell to heat sulphur-rich fluids to temperatures and pressures comparable to those 10–100 kilometres below the surface, using infrared Raman spectroscopy to monitor the types of chemical produced. They found that the trisulphur ion is the most stable form of the element under such conditions. Diamond anvil cells are common in geochemical research, but using them to study fluids is tricky. "There are not many groups doing this," says Pokrovski.
Breath of fresh air
Although the finding sounds esoteric, it could be important in understanding the development of early Earth.
Conventional theory holds that Earth's atmosphere was extremely low in oxygen before photosynthesis got going, about 2 billion–2.4 billion years ago.
This theory is based in part on ratios of the four stable isotopes of sulphur — 32S, 33S, 34S and 36S — found in mineral deposits from that era. These ratios, geochemists have found, can be affected by the amount of oxygen in the atmosphere, so they are useful proxies for oxygen levels. But studies of the ratios did not take into account how they might have been affected by S3−, says Pokrovski.
"Sulphur isotope geochemistry has been based on a fundamental assumption that sulphate (or sulphur dioxide) and hydrogen sulphide are the two principal sulphur-bearing aqueous species at high temperatures," says Hiroshi Ohmoto, a geochemist at Pennsylvania State University, University Park, Pennsylvania. "This study showed that it was a too simplistic assumption, and suggests the importance of studying the kinetics of sulphur isotopic reactions among H2S, trisulphur ion, sulphate, and SO2 in order to correctly interpret sulphur isotope data on ore deposits and igneous rocks."
Ariel Anbar, a biogeochemist at Arizona State University in Tempe, agrees. "Sulphur-isotope variations have become a critical tool in assessing the evolution of oxygen in Earth's ancient atmosphere," he says. "The discovery that S3− can be an important form of sulphur in subduction-zone settings will probably lead to new ideas about the causes of some sulphur-isotope variations, particularly in sulphur derived from volcanic sources."
A big deal
Craig Manning, a geochemist at the University of California, Los Angeles, adds that the finding could also be useful to those studying the formation of precious ores.
Sulphur compounds are vital to the geological process that forms high-grade ores of gold, copper, platinum and other precious metals. Mining companies know where to find such metals, but geochemists don't fully understand why the ores form, says Manning; this finding could change that. "We always have a problem" with gold ores, says Manning, "which is that the amount of these metals that can dissolve in geologic fluids is low. Finding new ways to get gold into solution is a big deal."
What's more, geophysicists sometimes use sulphur-isotope ratios to trace the sources of magmas and ores. "It's complicated stuff," says Pokrovski, and it might be further complicated by the discovery of S3−.
The next step is to add metals into the solutions being tested by Pokrovski's team. "We have to demonstrate that this new sulphur form can bind to these metals and carry them," he says.
Meanwhile, Manning, writing in a commentary on Pokrovski's research in the same issue of Science2, notes that the find has one more intriguing implication. If we were to peer beneath Earth's surface, we might discover that the fluids there are a beautiful ultramarine blue.
Pokrovski, G. S. & Dubrovinsky, L. S. Science 331, 1052-1054 (2011).
Manning, C. E. Science 331, 1018-1019 (2011).
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Lovett, R. Rare sulphur dominates the deep. Nature (2011). https://doi.org/10.1038/news.2011.124