Issaku Kohl

Because oxygen-bearing sulfate minerals trap and preserve signals from Earth’s atmosphere, scientists closely study how they form. Sulfates are stable over billions of years, so their oxygen isotopes are seen as a time capsule, reflecting atmospheric conditions while they were evolving on early Earth—and possibly on its planetary neighbor Mars.

A new NASA-funded study led by a University of Utah geochemist examines how sulfate forms when pyrite, commonly known as “fool’s gold,” is oxidized in environments teeming with microbes versus those without them. The researchers focused on Spain’s Rio Tinto, a contaminated river passing through a region where iron and copper were mined for thousands of years. What’s left in the hills of Andalusia may be an environmental calamity, but scientists now regard it as an analog for what the Martian surface may have once been like.

This acidic mine drainage is rich in sulfates and bacteria known to oxidize both sulfur and iron. The research team measured the “triple oxygen isotopes” (ratios of ¹⁷O/¹⁶O and ¹⁸O/¹⁶O) in sulfate to figure out how much of the oxygen comes directly from air compared to water.

“This is the first time where we’ve seen outdoors, not in the lab, that we can perpetuate this direct reaction between O₂ and pyrite sulfur if the environmental conditions are just right,” said lead author Issaku Kohl, associate research professor in the Department of Geology & Geophysics. “Because we’ve been able to identify that niche, we now have geochemical markers or criteria that would allow you to find a similar environment or remnants of a similar environment in the rock record, either on Earth or in an extraterrestrial setting.”

The study homed in on a bacterium called Acidithiobacillus ferrooxidans, believed to be among the earliest clades of microbes, potentially producing energy prior to the evolution of photosynthesis. The research team discovered that in microbe-rich, acidic environments, A. ferrooxidans drives pyrite oxidation in a way that preserves a remarkably high amount, exceeding 80% and up to 90%, of atmospheric oxygen (O₂) in sulfate.

Unlike lab experiments, where this signal fades quickly as sulfate incorporates O₂ from water, the Rio Tinto microbial-active ecosystem maintains this strong atmospheric imprint.

Researcher Issaku Kohl recorded the video below at a historic mining district in Spain, which scientists now an analog for the surface of Mars. It shows the mixing zone on the Rio Tinto, where green water, rich in the Fe2+ ion of iron, containing very high O₂ content sulfate, is discharging from a mine tailings pile. This water is mixing into the river’s main branch red waters, where most of the iron occurs as Fe3+ and sulfate oxygen is mostly sourced from water.

Accordingly, sulfate deposits don’t just preserve atmospheric and environmental conditions—they may also carry a microbial “biosignature.” Such signatures could help scientists interpret sulfate minerals on Mars or in ancient Earth rocks as a potential record of both atmospheric conditions and microbial activity.

Martian sediments hosted evaporites containing abundant sulfate minerals, but scientists don’t yet know how those sulfates formed.

“The current favored hypothesis is that it’s through atmospheric oxidation of volcanic sulfur dioxide (SO₂). But environments like that have telltale geochemical signatures that indicate whether this was likely aerosolized and oxidized in the atmosphere at relatively high temperature and therefore, unlikely to have had life involved,” Kohl said.

by Brian Maffly

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