Early Archean fossil bacteria and biofilms in hydrothermally-influenced sediments from the Barberton greenstone belt, South Africa

SEM imaging of HF-etched, 3.3-3.5 Ga cherts from the Onverwacht Group, Sout h Africa reveals small spherical (1 mum diameter) and rod-shaped structures (2-3.8 mum in length) which are interpreted as probable fossil coccoid and bacillar bacteria (prokaryotes), respectively, preserved by mineral replac ement. Other, possibly biogenic structures include smaller rod-shaped bacte riomorphs ( < 2 <mu>m in length) and bacteriomorph moulds. The identificati on of these structures as fossil bacteria is based on size, shape, cell div ision, distribution in colonies and occurrence in biolaminated sediments. T he exceptionally fine conservation has preserved textures such as wrinkled outer walls on the coccoid fossils, while the bacillar fossils are turgid. Carbon isotope analyses support the presence of bacteria in these cherts wi th one delta C-13 Value around - 27 per mil. The cherts are characterised b y fine, wavy laminae created by granular to smooth or ropy-textured films c oating bedding planes, interpreted as probable bacterial biofilms, which ha ve also been pseudomorphed by minerals. Although most of the Onverwacht Gro up was deposited in relatively deep water (> 900 m), textures in the sedime nts in which these biogenic structures occur suggest that they were probabl y deposited in a shallow water environment which was subjected to intermitt ent subaerial exposure. Pervasive hydrothermal activity is evidenced by oxy gen isotope studies as well as the penecontemporaneous silicification of al l rock types by low temperature (less than or equal to 220 degreesC) hydrot hermal solutions. (C) 2001 Elsevier Science B.V. All rights reserved.