An actualistic perspective into Archean worlds – (cyano‐)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South Africa

Abstract

Extensive microbial mats colonize sandy tidal flats that form along the coasts of today's Earth. The microbenthos (mainly cyanobacteria) respond to the prevailing physical sediment dynamics by biostabilization, baffling and trapping, as well as binding. This biotic-physical interaction gives rise to characteristic microbially induced sedimentary structures (MISS) that differ greatly from both purely physical structures and from stromatolites. Actualistic studies of the MISS on modern tidal flats have been shown to be the key for understanding equivalent fossil structures that occur in tidal and shelf sandstones of all Earth ages. However, until now the fossil record of Archean MISS has been poor, and relatively few specimens have been found. This paper describes a study location that displays a unique assemblage with a multitude of exceptionally preserved MISS in the 2.9-Ga-old Pongola Supergroup, South Africa. The 'Nhlazatse Section' includes structures such as 'erosional remnants and pockets', 'multidirected ripple marks', 'polygonal oscillation cracks', and 'gas domes'. Optical and geochemical analyses support the biogenicity of microscopic textures such as filamentous laminae or 'orientated grains'. Textures resembling filaments are lined by iron oxide and hydroxides, as well as clay minerals. They contain organic matter, whose isotope composition is consistent with carbon of biological origin. The ancient tidal flats of the Nhlazatse Section record four microbial mat facies that occur in modern tidal settings as well. We distinguish endobenthic and epibenthic microbial mats, including planar, tufted, and spongy subtypes. Each microbial mat facies is characterized by a distinct set of MISS, and relates to a typical tidal zone. The microbial mat structures are preserved in situ, and are consistent with similar features constructed today by benthic cyanobacteria. However, other mat-constructing microorganisms also could have formed the structures in the Archean tidal flats.

Early life on land and the first terrestrial ecosystems

“…Some of the oldest examples of life activity, which come precisely from aquatic, shallow marine (Klein et al 1987;Schopf and Klein 1992;Van Kranendonk et al 2008;Westall 2010;Van Kranendonk 2011;Hickman and Van Kranendonk 2012), shallow lacustrine (Awramik and Buchheim 2009;Hickman and Van Kranendonk 2012), and intertidal environments (e.g., Noffke et al 2006;Noffke 2010;Noffke et al 2011;Westall et al 2011), show signs of evaporation (e.g., Noffke et al 2008;Westall et al 2011;Hickman and Van Kranendonk 2012), which suggests that early microbial communities in shallow waters had to deal with periodic desiccation and UV radiation >3,400 Ma ago. This further implies adaptations to resist desiccation, salinity fluctuations, and UV radiation that could be successfully used even after prolonged desiccation.…”

Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings: Implications for Ediacaran taphonomic models

“…Gehling, 1999), as well as other sedimentary structures of putative microbial origin, including pustules, sinuous cracks and sand chips (Fig. 2, Schieber, 1999;Gerdes et al, 2000;Noffke et al, 2001;, have been recognised from siliciclastic bedding planes ranging from the Archean (Noffke et al, 2003;2006, a, b;Noffke, 2007;Noffke et al, 2008;Noffke, 2009 this volume) to the present day (Gerdes et al, 2000;Noffke et al, 2001;Noffke, 2009 this volume). As applied here, the term 'microbial' refers to organisms ranging from bacteria and archaea, to protozoans, algae (protoctists) and fungi (e.g.…”

Life: the first two billion years

“…Documenting a biological nitrogen cycle may be most challenging of all, but Stü eken et al [42] interpret distinctive d Stromatolites occur in carbonate rocks deposited throughout the Archean Eon, and those younger than about approximately 3000 Ma have textural features reliably associated with microbial trapping and binding in younger rocks (figure 3d, [31,43]). Microbial textures also occur on sandstone bedding surfaces in approximately 2900 Ma successions [44]. The oldest known stromatolites (3400-3500 Ma), however, are more challenging to interpret because they consist almost entirely of precipitated laminae, with textures that can be difficult to distinguish from abiotic precipitates (figure 3a-c).…”