This review of the current knowledge of impact spherule layers in the Barberton greenstone belt, together with new petrographic, geochemical, and field data on a number of impact and volcanic spherical particle horizons, highlight a number of problems with the proposition of frequent and large meteorite impacts during mid-Archean times. Field data indicate that some of the four previously proposed impact spherule layers may be laterally correlative units that may have formed from the same impact event. Petrographic work reveals the presence of volcaniclastic particles associated with some spherule layers, while other layers not regarded as impact deposits contain clasts commonly observed in the spherule beds. Major and trace element compositions of spherule layers reflect the composition of the immediate host rocks. The existing platinum group element and chromium isotope data are difficult to reconcile with the current knowledge of the composition of meteoritic debris. A thorough discussion of these problems is necessary before meaningful estimates of Archean impact flux and bolide diameters should be attempted.

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, δ 34 S in sulfide minerals from sedimentary sequences range from −3.8‰ to +3.4‰. δ 13 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 34 S values from +1.5‰ to +5.6‰. Black shales have δ 13 C red values from −32.4‰ to −5.7‰, suggesting that oxygenic photosynthetic and sulfate-reducing bacteria were present by ca. 3.24 Ga. The δ 13 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 δ 34 S in sulfides from −21.1‰ to +16.7‰ and for δ 13 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 34 S and δ 13 C red ranges, respectively 37‰ and 36‰, indicate a great variety of biological processes interacting with each other.