Newly discovered evidence of microbial activity in tiny grains of the iron sulfide mineral pyrite indicate that life took hold on land as early as 3.2 billion years ago, a team of German researchers reported this week in Geology, the official journal of the Geological Society of America.
Sami Nabhan of the Free University of Berlin and his colleagues were studying rock formations, some up to 3.5 billion years old, at South Africa’s Barberton greenstone belt when they detected trace element distributions and different sulfur isotope ratios in different parts of the pyrite.
In addition to Sulfur-32, the stable isotope of the element that comprises approximately 90% of it in nature, they found Sulfur-34, an isotope with two additional neutrons that make up only 4% of the element in nature, according to the study. Their analysis showed that the fraction of the sulfur in the core of some crystals had different characteristics from those found in their rims.
This discovery, the researchers explained in a statement, indicates that the exterior of the grains experienced a process called biogenic fractionation, meaning that they were affected by microbes some 300 million years earlier than the oldest previously known evidence had indicated.
Soil-based microbes responsible for altering pyrite’s structure
The sulfur isotope ratios were determined through an analysis of sample masses of less than one billionth of a gram at the GFZ German Research Centre for Geosciences. Their analysis showed that, based on the rock’s composition the shape of the crystals, the sequence was derived from an ancient soil profile that developed on a flood plain some 3.22 billion years ago.
Furthermore, field data collected as part of the research implied that sediment containing the iron sulfide crystals was transported by a braided river system. Based on their observations, Nabhan’s team concluded that microbes living in soil that was constantly changing between wet conditions and dry conditions were responsible for producing the pyrite crystals’ unusual outer edges.
“The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and [Sulfur-32 depleted] sulfur for the growth of authigenic pyrite,” the authors wrote, adding that this indicates that soil-forming processes of the era involved “microbiological modification” as well as “physical and chemical modification.”
Based on these findings, the German researchers concluded that they had discovered evidence of biological activity in this 3.22 billion year old geological formation, which indicates that life had gained a foothold on land approximately 300 years earlier than scientists previously believed.
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Image credit: Sami Nabhan/FSU Jena
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