MICROBIAL MANIFESTATIONS AND PRIMEVAL STROMATOLITES

Precambrian stromatolites are morphologically circumscribed accretionary structures with a primary lamination that is, or may be, biogenic. The structures have attributes and components at different scales (viz., biostrome, column, lamina, microstructure, texture, grain, fossil, molecular, and atomic levels), at each of which a biologic input may be manifested. The challenge is to conclusively establish the biogenic contribution to any part of this spectrum. Morphology, petrography, mineralogy, and organic, inorganic, and isotope chemistry, and micropaleontology play significant roles in assessing biogenicity. Computer simulations help in visualizing the geometric development of the edifice, whether biogenic or abiogenic.

The laminae, fundamental to stromatolite development, represent erstwhile surfaces of equilibrium between interacting physical, chemical, and biologic influences, while the successively stacked laminae in the aggregate log the growth history and conditions over a given time interval. The biologic contributions (cell growth, division, concentration of isotopically light organic matter, biofabric, biomarker compounds) are likely to be more evident at the lamina and sub-millimetric levels, given the limit of microbial size.

The two prevalent lamina configurations are convex-up, and inflexed (concave-up), both of which are found as far back as the oldest stromatolites in Geon 34. However, the absence of biofabrics and biomarkers in these oldest stromatolites leaves their biogenicity unproven. The presence of biofabrics in younger occurrences (e.g., Belcher Supergroup and Gunflint Formation of Geon 18), including preserved pustular and filamentous mats, not only substantiates their biologic heritage, but also allows the contained microfossils and community structure to be compared with modern counterparts, and biologic affinities to be determined. Some Neoarchean coniform stromatolites with roll-over laminae have fabrics resulting from deformation of thin flexible entities characteristic only of microbial mats. Certain >3 Ga stromatolites lacking microfossils and diagnostic fabrics are interpreted as biogenic based on the comparison of morphologic attributes developed in younger occurrences.