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A new window into Early Archean life: Microbial mats in Earth's oldest siliciclastic tidal deposits (3.2 Ga Moodies Group, South Africa)

¹ Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, Virginia 23529, USA
² Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
³ Carnegie Institution, Geophysical Laboratory, 5251 Broad Branch Road NW, Washington, DC 20015, USA
⁴ Department of Physical Sciences, University of Pennsylvania, Kutztown, Pennsylvania 19530, USA

Newly discovered sedimentary structures produced by ancient microbial mats in Early Archean sandstones of the 3.2 Ga Moodies Group, South Africa, differ fundamentally in appearance and genesis from Early Archean stromatolites and bacterial cell fossils preserved in chert. Wrinkle structures, desiccation cracks, and roll-up structures record the previous existence of microbial mats that effectively stabilized sediment on the earliest known siliciclastic tidal flats. In thin-section, the sedimentary structures reveal carpet-like, laminated fabrics characteristic of microbial mats. Negative ¹³C isotope ratios of –20.1 to –21.5 ± 0.2 are consistent with a biological origin for the carbon preserved in laminae. The biogenicity of the sedimentary structures in the Moodies Group is substantiated by comparative studies on identical mat-related features from similar tidal habitats throughout Earth history, including the present day. This study suggests that siliciclastic tidal-flat settings have been the habitat of thriving microbial ecosystems for at least 3.2 billion years. Independently of controversial silicified microfossils and stromatolites, the newly detected microbially induced sedimentary structures in sandstone support the presence of bacterial life in the Early Archean.

Key Words: microbially induced sedimentary structures • microbial mats • Moodies Group • Early Archean • siliciclastic tidal flats

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