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

Abstract

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 ␦ 13 C 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.

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…The filamentous bacteria, typically cyanobacteria or chemotrophic sulfide-oxidising bacteria, such as Beggiatoa, that comprise the mats, trap and bind sediments with their exopolysaccharide (EPS) secretions as they grow upward; this biostabilisation method allows them to withstand erosion by waves and currents. MRS are one of the oldest forms of fossil evidence, having been found in rocks as old as the 3.2 Ga Moodies Group, South Africa (Noffke et al, 2006;Heubeck, 2009) and the 2.9 Ga Pongola Supergroup, South Africa (Noffke et al, 2003). In the case of the Moodies Group, recent stratigraphic and depositional facies analysis have documented the association of three principal mat morphotypes representing different environmental settings: (1) planar-type in coastal floodplains, (2) wavy-type in intertidal zones, and (3) tufted-type in upper inter-to supratidal zones.…”

Viruses as new agents of organomineralization in the geological record

“…M icrobial mats represent one of Earth's earliest ecosystems [1][2] . However, due to diagenetic effects during rock lithification and the possible abiotic pathways for carbonate formation, the biogenic origin of ancient putative microbial mats is often controversial 3 .…”

A case study of microbial mat-related features in coastal epeiric sandstones from the Paleoproterozoic Pretoria Group (Transvaal Supergroup, Kaapvaal craton, South Africa); The effect of preservation (reflecting sequence stratigraphic models) on the relationship between mat features and inferred paleoenvironment

“…The inference that MRS might also have sequence stratigraphic connotations, notably denoting transgression-regression cycles (e.g., Noffke et al, 2006;Noffke, 2007Noffke, , 2010 seems to be supported by the combination of the Daspoort-Silverton-Magaliesberg succession, which together mark such a second-order sequence stratigraphic cycle. As a caveat however, it should be noted that Noffke (op.…”

“…As many studies of microbial mat features have been carried out in modern and ancient presumed analogues of shallow marine tidal-supratidal as well as hypersaline lagoon environments (e.g., Gerdes et al, 1985a, b and c), there is a body of opinion that suggests they might be partially diagnostic of such settings, and that they might also even have sequence stratigraphic connotations, specifically as marking transgression-regression cycles (e.g., Noffke et al, 2006;Noffke, 2007Noffke, , 2010. Mats have been seen as contributing at a range of scales, to clastic sediment stacking patterns and concomitant architecture within the sequence stratigraphic context Catuneanu, 2007).…”