The rocks of the Barberton Greenstone Belt are among the oldest known rocks on the planet. They are up to 3.5 billion years old. In a layer that has been dated at 3.22 billion years old, the scientists found tiny grains of the mineral pyrite, an iron sulfide. The granules show clear signs of influence by microorganisms, including the distribution of trace elements as well as the ratio of the sulfur isotopes 34S and 32S in the pyrite.
The specific isotope ratios were determined through an analysis in a laboratory of the GFZ. The necessary prerequisites for the analysis were provided by scientists from the Institute of Geological Sciences at Freie Universität Berlin: They mapped the various elements of the pyrite grains to characterize the chemical composition of the mineral. This work was done by Sami Nabhan and Ralf Milke of Freie Universität using an electron probe micro-analyzer. The selected samples were then examined in the GFZ laboratory in Potsdam.
The composition of the rock, the shape of the crystals, and the layering visible in the field all indicate that the studied rock sequence was derived from an ancient soil profile. This so-called paleosol developed on a river flood plain more than 3 billion years ago.
The river transported sediments containing iron sulfides and deposited them in the plane. The data collected imply that microorganisms lived in a soil zone that was alternately moist and dry, causing distinctive overgrowths on the pyrite crystals. "This indicates that there was terrestrial life outside of the oceans at least 3.2 billion years ago – about 300 million years earlier than previously believed," says Sami Nabhan, who currently does research and teaches at Friedrich-Schiller-Universität Jena, a position he has held since May 2016.
Original Publication: Sami Nabhan, Michael Wiedenbeck, Ralf Milke, and Christoph Heubeck: Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols. In: Geology, vol. 44, No. 9 (DOI: 10.1130/G38090.1)
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Caption for both photos:
The hills of the Barberton Greenstone Belt are composed of rocks that are up to 3.5 billion years old. Where today there is grassland, about 3.2 billion years ago there was a braided river system. Microorganisms lived in the sediments of the river beds.