Early life signatures in sulfur and carbon isotopes from Isua, Barberton, Wabigoon (Steep Rock), and Belingwe Greenstone Belts (3.8 to 2.7 Ga)

Carbon and sulfur isotopes have been measured on samples from four Archean greenstone belts dating from 3.8 Ga to 2.7 Ga, in order to trace metabolic changes as life evolved over this one-billion-year period. In the Isua Greenstone Belt (3.8 Ga), Greenland, δ³⁴ S in sulfide minerals from sedimentary sequences range from -3.8‰ to +3.4‰. δ¹³ C _red measured in BIFs, turbidites and conglomerates vary from -29.6‰ to -14.7‰; this range permits us to hypothesize the presence of hyperthermophilic and chemotrophic species in transient settings, or possibly pelagic photoautotrophic microbes, or both. In the Barberton Greenstone Belt, South Africa, sulfide minerals show δ ³⁴S values from +1.5‰ to +5.6‰. Black shales have δ ¹³C _red values from -32.4‰ to -5.7‰, suggesting that oxygenic photosynthetic and sulfate-reducing bacteria were present by ca. 3.24 Ga. The δ¹³ C _red measured in the stromatolites of Steep Rock Lake (3.0 Ga), Ontario, Canada, are from -30.6‰ to -21.6‰, giving clear evidence for occupation of a shallow water environment by cyanobacteria. The wide isotopic ranges for δ³⁴ S in sulfides from -21.1‰ to +16.7‰ and for δ¹³ C _red in carbon-rich cherts and black shales from -43.4‰ to -7.2‰ in the Belingwe Greenstone Belt, Zimbabwe, indicate that photosynthetic microbial mat communities were well established at 2.7 Ga. In these well-preserved Late Archean formations, modern-style biological sulfur and carbon cycles may have been in operation. The δ ³⁴S and δ¹³ C _red ranges, respectively 37‰ and 36‰, indicate a great variety of biological processes interacting with each other.