LATE ARCHEAN CALCITE-MICROBE INTERACTIONS - 2 MORPHOLOGICALLY DISTINCT MICROBIAL COMMUNITIES THAT AFFECTED CALCITE NUCLEATION DIFFERENTLY

Microbialites in the 2521 +/- 3 Ma Gamohaan and Frisco formations, Sou th Africa, consist of three components: very thin, filmy laminae that are interpreted as remnants of microbial mats; irregular surfaces that acted as supports over which filmy laminae draped, which also are int erpreted as microbial in origin; and cement-filled voids that formed c ontemporaneously with microbialite growth. The structure of the microb ialites varies with the proportions of filmy laminae and supports, dis ruption of had filmy laminae, and abundance of voids. Seven end-member morphologies have been defined: planar laminae, contorted laminae, te nted microbialites, cuspate microbialites, irregular columnar microbia lites, plumose structures, and herringbone calcite beds. Similar struc tures have not been reported in younger rocks, but they may have parti al analogs in Yellowstone hot springs, ice covered lakes in Antarctica , and Proterozoic conoform stromatolites. Herringbone calcite, a fibro us marine cement, precipitated contemporaneously with microbial growth . It preferentially precipitated on the supports over the laminated ma t as demonstrated by: the concentration of herringbone calcite near su pports; growth banding in herringbone calcite, which indicates that ca lcite nucleated on and grew away from supports but not laminated mat; and the abutment of herringbone calcite coatings against laminated mat attached to supports. These observations suggest that the laminated m at inhibited nucleation. of calcite crystals. The differences in the m orphology and relationships to herringbone calcite in the supports and laminated mat imply that they were composed of distinct microbial com munities that interacted with their environment differently. Also, the complexity of the microbialites demonstrates that a remarkably divers e set of microbe-substrate interactions evolved by late Archean time.