Peering into the Cradle of Life: multiple sulphur isotopes reveal insights into environmental conditions and early sulphur metabolism some 3.5 Ga ago

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One of the most intriguing questions in Earth and Life Sciences is where and when life emerged on our planet. Well preserved sediments with an age of 3500 Ma contain clear traces of life: microscopic structures composed of carbonaceous material and interpreted as microfossils, stromatolites, and geochemical and isotopic evidence that document the early presence of a rich microbial ecosystem on Earth. Any older record of life is unfortunately lost due to severe postdepositional alteration of an already sparse rock record. Among the isotopic evidence, multiple sulphur isotopes have become a prime biosignature, both in the modern world but more so in ancient rocks, including one of the oldest well preserved succession, i.e. sedimentary rocks from the Barberton Greenstone Belt, South Africa. In addition, mass-independent sulphur isotope fractionation appears to be a prime recorder of Earth’s atmospheric composition, most prominently in respect to the presence of free oxygen. Consequently, multiple sulphur isotopes and complementary geochemical analyses will be used in order to peer into the cradle of life.