Archean Microbial Structures

Dawn Y. Sumner

The influence of microbial communities on the morphology and internal texture of stromatolites has been a hotly debated topic at the center of studies of Precambrian for decades (see discussions in Cloud and Semikhatov, 1969; Walter, 1976; Semikhatov, et al., 1979; Ginsburg, 1991). Early Soviet workers documented temporal variations in stromatolite morphology and internal textures which they generally interpreted as representing the evolution of microbial communities through time (summarized in Semikhatov, 1976). In contrast, many western workers considered stromatolite morphology a function of depositional environment rather than the composition of microbial communities (e.g. Cloud, 1942; Logan, et al., 1964). More recently, focus has shifted from stromatolite morphology, which is now generally considered a complex function of both biological and environmental processes (e.g. Ginsburg, 1991; Buick, et al., 1981; Krumbein, 1983), to stromatolite microtextures. Due to the complexity of both biotic and abiotic chemistry, the specific roles of microbial and environmental processes in shaping stromatolite microtextures also are difficult to distinguish, particularly for stromatolites consisting of carbonate that precipitated in place rather than trapped-and-bound detrital carbonate (Grotzinger and Rothman, 1996; Grotzinger and Knoll, 1999). Documenting the role of microbial communities in shaping stromatolite microtextures becomes increasingly difficult with increasing age due to the decreasing abundance of well preserved stromatolites. More care must also be taken with increasing age to carefully constrain biological influences since interpretations of the presence of specific biological processes (such as photosynthesis), have significant evolutionary implications if they can be identified in early microbial communities (e.g. Schopf, et al., 1971; Walter, 1983; Buick, 1992). Although Archean stromatolites have been used to support interpretations of the evolution of photosynthetic communities, the specific role of microbial communities, if any, in the formation of many of the oldest stromatolites is controversial (e.g. Buick, et al., 1981; Lowe, 1994; Lowe, 1995; Buick, et al., 1995). Some Archean stromatolites, however, contain compelling evidence for the presence of microbial communities.

The 2521+/- 3 Ma Gamohaan and Frisco formations contain a wide morphological range of microbialites (or stromatolites) and associated structures. All of the morphological variations in the microbialites can be described in terms of the proportion and geometrical arrangement of three components: fine filmy laminae, supports, and voids filled with carbonate cement. Filmy laminae consist of planar, draped, and recumbently folded layers that are interpreted as the remnants of microbial mats. The supports are somewhat irregular surfaces that are interconnected in three dimensions. In most microbialites, they are oriented vertically. Infrequently, they branch outward or even downward beneath overhangs. Supports are also interpreted as microbial in origin. A complex framework with abundant primary voids is created by the branching of supports, draping of filmy laminae over supports, and folding of filmy laminae. These voids are filled with herringbone, bladed, and blocky calcite. Herringbone calcite precipitated as the first generation of carbonate crystals within many voids, particularly those defined by supports draped with filmy laminae and by branching of supports. It is less common in voids created by folding of filmy laminae.

The differences in morphology of the supports and laminated mat suggest that they were composed of different microbial communities. Differences in the motility and metabolic needs of microbes may have led to their segregation into the two communities. For example, the supports may have been constructed of microbes with a strong upward phototactic or chemotactic response, whereas the laminated mats may have been composed of either less mobile microbes or microbes lacking the same taxis as those in the supports. The interpretation that different microbial communities composed the supports and the laminated mat is supported by differences in the precipitation of early marine calcite on the supports and laminated mat. Herringbone calcite preferentially precipitated on the supports over the laminated mats as demonstrated by the concentration of herringbone calcite near supports, growth banding in herringbone calcite which indicates that calcite nucleated on and grew away from supports but not the laminated mat, and the abutment of herringbone calcite coatings against laminated mat attached to supports. These observations suggest that herringbone calcite preferentially nucleated on the supports over the laminated mat. This difference in calcite nucleation could be due to differences in the composition or charge distribution in the extracelluar secretions produced by microbes in the supports versus those in the laminated mat, or it could be due to the release or uptake of different metabolic products by the two communities. Either cause suggests that the supports and laminated mat contained different microbial communities. Further work investigating these differences may lead to constraints on the evolution of metabolic or structural features of Archean microbes.

Publications on Archean Microbial Structures:

Schroeder, S., N. J. Beukes, and D. Y. Sumner
Microbialite-sediment interactions on the slope of the Campbellrand carbonate platform (Neoarchean, South Africa). Precambrian Research. (Accepted)

Waldbauer, Jacob R., Laura S. Sherman, Dawn Y. Sumner, and Roger E. Summons
Late Archean molecular fossils from the Transvaal Supergroup record the antiquity of microbial diversity and aerobiosis. Precambrian Research. (Accepted)

Murphy*, Megan, and Dawn Y. Sumner, 2008. Abstract
Variations in Neoarchean microbialite morphologies: Clues to controls on microbialite morphologies through time. Sedimentology, DOI: 10.1111/j.1365-3091.2007.00942.x

Murphy*, Megan, and Dawn Y. Sumner, 2007. Abstract
Tube structures of probable microbial origin in the Neoarchean Carawine Dolomite, Hamersley Basin, Western Australia. Geobiology, DOI: 10.1111/j.1472-4669.2007.00114.x

Sumner, Dawn Y., 2000. Abstract
Microbial versus environmental influences on the morphology of Late Archean fenestrate microbialites, in Microbial Sediments (R. Riding and S. Awramik, eds.), Springer, Berlin, p. 307-314.

Sumner, Dawn Y., 1997. Abstract (at the AJS site)
Carbonate precipitation and oxygen stratification in late Archean seawater as deduced from facies and stratigraphy of the Gamohaan and Frisco formations, Transvaal Supergroup, South Africa. American Journal of Science, v. 297, p. 455-487.

Sumner, Dawn Y., 1997. Abstract (at the Palaios site)
Late Archean calcite-microbe interactions: Two morphologically distinct microbial communities that affected calcite nucleation differently. Palaios, v. 12, p. 302-318.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616