Archaean microbial consortia of the 2.7-2.6 Ga Ngesi Gp. (Belingwe) sediments, Zimbabwe

Isotopic evidence from 2.7-2.65 Ga old sediments in the Belingwe belt, Zimbabwe, shows that a complex microbial community existed. Two major sequences have been studied, the 2.7 Ga Manjeri Fm. and, separated from it by thick komatiitic and basaltic lavas, the overlying 2.65 Ga Cheshire Fm. Both sedimentary formations contain a wide range of facies including stromatolites, silts and shales. In 2.7 Ga Manjeri stromatolites, δ13C(reduced) is around -23% in material with 1-2wt% reduced carbon content, ranging to -35%. δ13C in stromatolitic carbonate is around 0% and in carbonates from black shales δ13C(carbonate) is around -8%, indicating crystallisation in sediment after methanotrophy. In shallow-water shales δ13C(reduced) varies from -30 to -23%,, suggesting carbon capture by aerobic bacteria. A large sulphide population has δ34S from -20 to -12%, suggesting available sulphate. δ34S in sulphides show a 40% range in total , from -23.7% to +16.7%. Manjeri black shales show small and mostly negative MIF-S, with Δ33S varying from -0.6 to 0.9% (average around 0.0%). In deeper water samples, with proximal volcanism, carbon-sulphide sapropel-like deposits have δ13C(reduced) around -34%, and accompanying carbonate blebs have δ13C around -12 to -10%. In these samples, δ34S is markedly heavy, ranging from + 7 up to +14%. In the overlying ~2.65 Ga Cheshire Fm., carbonate in stromatolitic rocks has a narrow range in δ13C around +0.2 ±0.3%, while δ13C(reduced) within these samples is typically -28.8% ± 2.6%, consistent with capture of carbon by oxygenic photosynthesis. In associated Cheshire shales, δ13C(reduced) values range as light as -43.8% with the average at -40.0±3.0%, and some carbonate has δ13C -9 to -8%, implying methanogenesis and methanotrophy in the microbial consortia of the sedimentary column. δ34S values range from -2.1 to +2.4%. Δ33S anomalies are near zero but consistently positive between 0.1 and 1.2 % (average 0.7 ± 0.3%). A diverse microbial ecology can be deduced. S isotopes imply the presence of sulphate reducers, and most probably sulphide oxidisers in sulphureta. C isotopes in stromatolites imply the presence of cyanobacteria carrying out oxygenic photosynthesis. C isotopes also record more extreme fractionation involving methanogenesis. The presence of methanotrophs to reconvert the methane to carbon dioxide and organic matter is also a likely inference, supported by the 'light' carbonate in the Cheshire shales. Both the Archaean carbonate reefs and the muddy bottoms probably hosted patchworks of microbial consortia, dependent on small-scale physical (and chemical) variation in the sediment reflecting local depositional facies and diagenetic processes, as well as external factors such as tides, currents and seasonality.