EVIDENCE FOR MICROBE-ENVIRONMENT INTERACTIONS: A NEOARCHEAN CASE STUDY SPANNING SCALES OF KILOMETERS TO MICRONS

The search for life requires models of where life should be and how it should look, either morphologically or geochemically. Such models can be developed by characterizing demonstrably biological communities. 2.55-2.52 Ga fenestrate microbialites in the Campbellrand-Malmani carbonate platform, South Africa, provide such a system. They consist of complex arrangements of ~3 micron-thick laminae defined by organic inclusions draped over vertically oriented >200 micron-thick organic supports and encased in marine calcite cements that also fill primary voids. Variations in abundance and geometry of laminae, supports, and cements create various microbialite structures. The microbialites grew predominantly below wave base. In slope environments, large bioherms with exposed lengths of ~400 m and stratigraphic heights of ~30 m consist of columnar and cuspate fenestrate microbialites that lack bedding. Depositional relief was >1 m based on onlapping relationships. Similar fenestrate microbialites in well defined beds onlap bioherms. The main textural difference between bedded and biohermal microbialites is a greater abundance of horizontal laminae in bedded facies. In addition, bedded facies sometime transition between fenestrate and columnar stromatolitic forms with increased mat disruption. Variations in microbial texture correlate to topography, sediment influx, and current disruption. The biohermal facies arrangement is distinctly different from fenestrate microbial deposition during the later drowning of the platform where individual <10 cm-thick beds are recognizable for 140 km obliquely across strike with little variation in topography and an absence of sediment influx. These relationships suggest that microbial communities are responding to regional environmental conditions that may be independent of the specific community. Petrographically, variations in microbialite morphology correlate with variations in geochemistry. Some microbial supports are associated with earlier calcite nucleation, higher Mg concentrations, and isotopically heavier carbonate than thin laminae and void-filling calcite. Variations in chemistry on the 10 micron scale may correlate to microbial processes, diffusion variations, or differences in diagenetic chemistry due to organic inclusions.