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

Abstract

SEM imaging of HF-etched, 3.3–3.5 Ga cherts from the Onverwacht Group, South Africa reveals small spherical (1 μm diameter) and rod-shaped structures (2–3.8 μm in length) which are interpreted as probable fossil coccoid and bacillar bacteria (prokaryotes), respectively, preserved by mineral replacement. Other, possibly biogenic structures include smaller rod-shaped bacteriomorphs (<2 μm in length) and bacteriomorph moulds. The identification of these structures as fossil bacteria is based on size, shape, cell division, distribution in colonies and occurrence in biolaminated sediments. The 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 with one δ 13C value around −27 per mil. The cherts are characterised by fine, wavy laminae created by granular to smooth or ropy-textured films coating bedding planes, interpreted as probable bacterial biofilms, which have also been pseudomorphed by minerals. Although most of the Onverwacht Group was deposited in relatively deep water (>900 m), textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure. Pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature (⩽220°C) hydrothermal solutions.