Environmental Controls on Photosynthetic Microbial Mat Distribution and Morphogenesis on a 3.42 Ga Clastic-Starved Platform

Abstract

Three morphotypes of microbial mats are preserved in rocks deposited in shallow-water facies of the 3.42 Ga Buck Reef chert (BRC). Morphotype alpha consists of fine anastomosing and bifurcating carbonaceous laminations, which loosely drape underlying detrital grains or form silica-filled lenses. Morphotype beta consists of meshes of fine carbonaceous strands intergrown with detrital grains and dark laminations, which loosely drape coarse detrital grains. Morphotype gamma consists of fine, even carbonaceous laminations that tightly drape underlying detrital grains. Preservation of nearly uncompacted mat morphologies and detrital grains deposited during mat growth within a well-characterized sedimentary unit makes quantitative correlation between morphology and paleoenvironment possible. All mats are preserved in the shallowest-water interval of those rocks deposited below normal wave base and above storm wave base. This interval is bounded below by a transgressive lag formed during regional flooding and above by a small condensed section that marks a local relative sea-level maximum. Restriction of all mat morphotypes to the shallowest interval of the storm-active layer in the BRC ocean reinforces previous interpretations that these mats were constructed primarily by photosynthetic organisms. Morphotypes alpha and beta dominate the lower half of this interval and grew during deposition of relatively coarse detrital carbonaceous grains, while morphotype gamma dominates the upper half and grew during deposition of fine detrital carbonaceous grains. The observed mat distribution suggests that either light intensity or, more likely, small variations in ambient current energy acted as a first-order control on mat morphotype distribution. These results demonstrate significant environmental control on biological morphogenetic processes independent of influences from siliciclastic sedimentation.

Archean (3.33 Ga) microbe-sediment systems were diverse and flourished in a hydrothermal context

“…The occurrence of carbonaceous films in the photic zone, their morphological characteristics, and in-situ-measured stable carbon isotope ratios of -22‰ to -13‰ d 13 C VPDB (Vienna Peedee belemnite) are consistent with the type 1 films representing microbial mats utilizing a phototrophic metabolism (cf. Tice, 2009). The previously reported association of one of these biofilms with chemoorganotrophic sulfate-reducing bacteria (SRB) (Westall et al, 2011a), the activities of which induced still-preserved in-situ calcification, is supported in this study by in-situ measurements on the same fossil biofilm yielding d 13 C VPDB signatures as light as -45‰ and depleted d 34 S CDT (Canyon Diablo troilite) values of -24‰ (cf.…”

“…Nevertheless, phototrophic microbial communities in early to mid-Archean (ca. 3.5-3.2 Ga) strata of South Africa and Australia have been well documented (Tice, 2009;Allwood et al, 2006;Heubeck, 2009;Westall et al, 2006Westall et al, , 2011aWestall et al, , 2011b. On the other hand, no previous investigations have recognized and addressed the syngenetic diversity of Archean microbial paleocommunities-both phototrophs and chemotrophs -within their sedimentary habitats at the microbial scale and using in-situ methods.…”

Morphological adaptations of 3.22 Ga-old tufted microbial mats to Archean coastal habitats (Moodies Group, Barberton Greenstone Belt, South Africa)

“…In the strata of the BGB, a variety of microbial traces are preserved, e.g., carbonaceous chert containing filamentous, coccoidal and spindle-shaped microfossils (Walsh andLowe, 1985, 1999;Walsh, 1992;Westall et al, 2006), planar microbial mats (Tice et al, 2004;Tice and Lowe, 2006;Tice, 2009), pseudocolumnar, stromatolite-like structures (Byerly et al, 1986;Walsh and Westall, 2003 and references therein), and organic-walled, spheroidal microfossils (Javaux et al, 2010). Moreover, the BGB includes the world's oldest known regionally mappable record of microbial mats in a siliciclastic tidal setting, exposed in the 3.22 Ga, and remarkably well-preserved Moodies Group (Noffke et al, 2006;Heubeck, 2009;Gamper et al, 2012).…”

Life: the first two billion years

“…Continuing research has tilted interpretation back toward biogenesis (e.g. [46]), especially in consideration of microscopic textures indicative of continuous organic mats and millimetre-scale tufts [47][48][49][50].…”