Fenestrate Microbialites

Archean* microbial communities created complex structures that reflect both the behavior of bacteria in the community as well as environmental conditions. Some of these are called "fenestrate microbialites", which are found in various 3 to 2.5 billion-year-old carbonates. We are studying ones from the 2.6-2.5 billion-year-old Carawine Formation, Australia, and the 2.6-2.5 billion-year-old Reivilo, Gamohaan and equivalent formations, South Africa (see Sumner, 2000 and 1997). The morphology of fenestrate microbialites suggests that two distinct microbial communities coexisted in close proximity and affected local calcite* precipitation differently. The polished slab on the left shows an example of a fenestrate microbialite. The complex geometry of the microbial communities (black) outlines void-filling calcite cements (grey and white). Some of the microbial communities grew up vertically whereas others formed more common, horizontal and draping mats. The grey calcite cements consist of herringbone calcite. The scale bar at the bottom right is 1 cm long.

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 (Figures 3-21 to 3-23). 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 or bacteria behaving differently.

We have recently produced structures in the lab that mimic some of the characteristics of specific fenestrate microbialites. It appears that at least cyanobacteria can migrate into appropriate patterns and grow upward for a few millimeters, possibly providing a model for growth of the supports. See the modern microbial morphology page.

Dawn's Research Page
Dawn's Home Page
Department of Geology Home Page

Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
dysumner@ucdavis.edu