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Constraints on the Archaean Environment |
1 Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, UK e.nisbet{at}gl.rhul.ac.uk
2 Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK
3 Department of Chemistry, University of Rhode Island, Kingston, RI 02881, USA
4 Department of Chemistry, University of Buea, Buea, SW Cameroon, Cameroon
5 , 6 Autumn Close, Greendale, Harare, Zimbabwe
Sulphide-rich sediments, stromatolitic limestones and tidal-flat deposits in the late Archaean (2.7 Ga) Manjeri and Cheshire Formations, Belingwe greenstone belt, Zimbabwe show evidence for complex and extensive prokaryotic mat communities, including (1) shallow-water coastal sulphur mats; (2) mats, probably in somewhat deeper water; (3) nearby stromatolites that lived by oxygenic photosynthesis in shallow coastal settings. Petrological and geochemical (rare earth element; REE) evidence, coupled with high-resolution stable isotope results, identifies several complex interdependent metabolic consortia of bacteria and archaea. These microbial consortia would have exchanged nutrients and products both locally within prokaryotic mats and more widely via the waters of the Belingwe basin. This isotopic, sedimentological and REE evidence for a complex ecology of bacteria and archaea is consistent with metabolic inferences from rRNA phylogeny and is direct evidence that a diverse prokaryotic community, managing carbon on a global scale, had evolved by the late Archaean.