Antiquity of the biological sulphur cycle: evidence from sulphur and carbon isotopes in 2700 million-year-old rocks of the Belingwe Belt, Zimbabwe

Sulphur and carbon isotopic analyses on small samples of kerogens and sulph ide minerals from biogenic and non-biogenic sediments of the 2.7x10(9) year s(Ga)-old Belingwe Greenstone Belt (Zimbabwe) imply that a complex biologic al sulphur cycle was in operation. Sulphur isotopic compositions display a wider range of biological fractionation than hitherto reported from the Arc haean. Carbon isotopic values in kerogen record fractionations characterist ic of rubisco activity, methanogenesis and methylotrophy, and possibly anox ygenic photosynthesis. Carbon and sulphur isotopic fractionations have been interpreted in terms of metabolic processes in 2.7 Ga prokaryote mat commu nities, and indicate the operation of a diverse array of metabolic processe s. The results are consistent with models of early molecular evolution deri ved from ribosomal RNA.