The role of microbes in accretion, lamination and early lithification of modern marine stromatolites

Abstract

For three billion years, before the Cambrian diversification of life, laminated carbonate build-ups called stromatolites were widespread in shallow marine seas. These ancient structures are generally thought to be microbial in origin and potentially preserve evidence of the Earth's earliest biosphere. Despite their evolutionary significance, little is known about stromatolite formation, especially the relative roles of microbial and environmental factors in stromatolite accretion. Here we show that growth of modern marine stromatolites represents a dynamic balance between sedimentation and intermittent lithification of cyanobacterial mats. Periods of rapid sediment accretion, during which stromatolite surfaces are dominated by pioneer communities of gliding filamentous cyanobacteria, alternate with hiatal intervals. These discontinuities in sedimentation are characterized by development of surface films of exopolymer and subsequent heterotrophic bacterial decomposition, forming thin crusts of microcrystalline carbonate. During prolonged hiatal periods, climax communities develop, which include endolithic coccoid cyanobacteria. These coccoids modify the sediment, forming thicker lithified laminae. Preservation of lithified layers at depth creates millimetre-scale lamination. This simple model of modern marine stromatolite growth may be applicable to ancient stromatolites.

Origin and significance of lamination in Lower Cretaceous stromatolites and proposal for a quantitative approach

“…This difference in grain-size has been noted in present-day marine coarse-grained carbonate stromato lites formed by stabilisation (trapping and binding) of previously mo bile grains by microbial mats (Logan, 1961;Monty, 1976;Dravis, 1983;Reid and Browne, 1991;Riding et al, 1991a;Reid et al, 1999). In these examples, grain-trapping is produced by erect filaments, mat ir regularities, and extracellular polymeric substances (EPS) secreted by cyanobacteria and other mat microbes (Logan, 1961;Playford and Cockbain, 1976;Dravis, 1983;Dill et al, 1986;Awramik and Riding, 1988;Riding et al, 1991a;Visscher et al, 1998;Reid et al, 2000;Decho et al, 2005;Dupraz et al, 2009;Browne, 2011;Bowlin et al, 2012;jahnert and Collins, 2012). Rare filaments in some mixed laminae of the Leza coarse-grained stromatolites (Fig.…”

“…Predominance of alternating lamination distinguishes Leza coarse grained stromatolites from most recent examples of coarse-grained car bonate stromatolites, which generally lack thick micrite-rich laminae (Logan, 1961;Dill et al, 1986;Reid et al, 1995Reid et al, , 2000Feldmann and McKenzie, 1998). Planavsky and Ginsburg (2009) describe four succes sive processes responsible for the development of Bahamian microbialites: "1) sediment trapping, binding and initial lithification; 2) disruption and truncation of the initial fabric; 3) pervasive cementa tion and clot formation; and 4) late-stage boring".…”

“…in Bahamian coarse-grained stromatolites (Reid and Browne, 1991;Reid et al, 1995Reid et al, , 2000Macintyre et al, 1996Macintyre et al, , 2000Feldmann, 1997;Feldmann and McKenzie, 1998;Visscher et al, 1998Visscher et al, , 2000.…”

“…Visscher et al (1998Visscher et al ( , 2000 related formation of these micritic crusts to carbonate precipitation dissolution processes induced by sulphate-reducing bacteria, a few mm below the surface of mats. In a detailed study, Reid et al (2000) re lated thin micritic crusts in Bahamian stromatolites to successive mat processes in which Schizothrix mats promoted grain-trapping, hetero trophic bacteria induced precipitation of thin micritic horizons when accretion ceased, and endolithic Solentia cyanobacteria bored and micritised grains during longer hiatal periods (see also Macintyre et aL, 2000).…”

Determining the specific microbial populations and their spatial distribution within the stromatolite ecosystem of Shark Bay

“…However, the limited information preserved in fossil stromatolites has prompted studies on extant systems as crucial to understanding the process of their formation. Prominent examples of modern stromatolites can be found in the hypersaline region of Hamelin Pool, Western Australia (Hoffman, 1976;Playford and Cockbain, 1976), and in the open marine waters of Exuma Sound, Bahamas (Dravis, 1983;Reid et al, 2000). These modern analogues provide us with a template of how Earth and its early biosphere may have co-evolved.…”