Archean Stromatolites as Microbial Archives

Abstract

Stromatolites are morphologically circumscribed accre-tionary growth structures with a primary lamination that is, or may be, biologically influenced (biogenic). They are found in Ar-chean sedimentary carbonate rocks, almost always associated with extensive volcanic sequences. Thirty-two occurrences have been reported from n small regional clusters representing the world's principal preserved Archean cratons: North America 16, Africa 7, Australia 5, Asia 3, and Europe 1; none are presently known from Archean rocks of South America and Antarctica; less than two dozen of the occurrences are viewed as definitely Archean and stro-matolitic. The earliest stromatolite records date back to nearly 3.5 Ga, and their worldwide distribution and abundance increase as time progresses. Morphological types include structures with flat, convex-up, concave-up, and globoidal laminae; stacking patterns producing nodular, columnar (unbranched as well as branched), and oncoidal forms are represented. The observed diameters of the structures show a gradual increase in size as the stratigraphic column is as-cended, spread over two orders of magnitude in geon 34 (centime-tric to decimetric), but ranging over six orders of magnitude by geon 25 (sub-millimetric to dekametric). Unlike Proterozoic stro-matolites, most are developed in limestones rather than dolostones, with sideritic/ankeritic and cherty types also present. Microfossils are only very rarely preserved. Ministromatolites with radial-fi-brous microstructure, probably almost exclusively the result of chemical precipitation, developed after 3.0 Ga, as did mesoscopic aragonite/calcite crystal fans, indicating carbonate supersaturation of ambient Meso-and Neoarchean ocean waters.