Experiment considers chemical composition of prehistoric ocean

The research may force scientists to reconsider their interpretations of ancient sedimentary rocks.
By Brooks Hays  |  March 9, 2018 at 3:47 PM
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March 9 (UPI) -- How can you study conditions inside Earth's ancient ocean? You recreate it, but on a much smaller scale.

Scientists at Iowa State University did just that, mixing up a biochemical concoction designed to replicate the ocean waters of the Archean Era, 2.5 billion years ago, inside a graduated cylinder.

The experiment is helping scientists better understand the conditions under which iron-rich sedimentary rocks form.

Until now, scientists believed iron-rich sedimentary rocks form under anoxic conditions, in ocean waters with limited amounts of oxygen. Chemical signatures inside rocks can help researchers better understand the changing chemical conditions in ancient oceans.

Iron-rich sedimentary rocks have traditionally served as a proxy for the ocean as it was before cyanobacteria began pumping large amounts of oxygen into the prehistoric seas.

The latest experiment -- detailed this week in the journal Science -- showed cyanobacteria-powered oxygenation doesn't trap as much iron as previously thought.

The graduated cylinder featured iron-rich sea water with no added oxygen. The only oxygen in the conception was the small amount produced by the added cynobacteria.

Though some of the iron in the iron-rich seawater was oxidized, it didn't remain, and was reduced back to its dissolvable form. Scientists hypothesized the iron reduction -- which cycles the iron back into the seawater -- is fueled by an enzyme produced by cyanobacteria cells.

"Such steady-state iron redox cycling may have maintained dissolved, and hence mobile [iron] in oxygenated seawater above ferruginous deep basins such as those inferred for many Precambrian [iron formations]," researchers wrote in their paper.

The findings may force researchers to reconsider their interpretations of ancient sedimentary rocks.

"Traditionally if you see iron enriched in sediments from the ocean you tend to think it was deposited under anoxic conditions," Iowa State University's Elizabeth Swanner said in a news release. "But potentially we have a way to still deposit a lot of iron but have it be deposited under fully oxygenated conditions."

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