Isotopic evidence for microbial sulphate reduction in the early Archaean era

Sulphate-reducing microbes affect the modern sulphur cycle, and may be quite ancient1,2, though when they evolved is uncertain. These organisms produce sulphide while oxidizing organic matter or hydrogen with sulphate3. At sulphate concentrations greater than 1 mM, the sulphides are isotopically fractionated (depleted in 34S) by 10–40‰ compared to the sulphate, with fractionations decreasing to near 0‰ at lower concentrations2,4,5,6. The isotope record of sedimentary sulphides shows large fractionations relative to seawater sulphate by 2.7 Gyr ago, indicating microbial sulphate reduction7. In older rocks, however, much smaller fractionations are of equivocal origin, possibly biogenic but also possibly volcanogenic2,8,9,10. Here we report microscopic sulphides in ∼3.47-Gyr-old barites from North Pole, Australia, with maximum fractionations of 21.1‰, about a mean of 11.6‰, clearly indicating microbial sulphate reduction. Our results extend the geological record of microbial sulphate reduction back more than 750 million years, and represent direct evidence of an early specific metabolic pathway—allowing time calibration of a deep node on the tree of life.