THE BIAS OF SILICIFICATION IN THE PRESERVATION OF PROTEROZOIC MICROBIAL MATS

Modern hypersaline cyanobacterial mats are commonly composed of a base community of Phormidium sp. with a secondary, lower-light community composed of Microcoleus chthonoplastes, and a variety of unicellular coccoids. These cyanobacterial mats can vary in structure from smoothly laminated mats to small tufts, to a blistery appearance that results from the presence of gas bubbles trapped within the mat and its associated EPS. Within mats, strong vertical zonation is reflected in the chemistry, with strong gradients in both oxygen and hydrogen sulfide concentration, and in the physiological structure of cyanobacterial constituents, where active photosynthesis occurs dominantly in the upper 1-2 mm of the mat and deeper cyanobacterial communities experience a rapid migration of trichomes from their host sheaths. Once sheaths are no longer maintained by active metabolism of the cyanobacteria, they become susceptible to degradation. As a result, the physical structure of the mat becomes increasingly incoherent at depths of greater than approximately 5 millimeters.

Here we use petrographic observations of hypersaline microbial mat communities from the 1.1 Ga Angmaat Formation to explore the preservation of cyanobacterial mats in early diagenetic chert. Filamentous mats from the Angmaat Formation bear striking resemblance to modern hypersaline microbial mats — including a base community of Siphonophycus sp. with a secondary community composed of Eomicrocoleus sp. Despite the scarcity of preserved trichomes, cyanobacterial sheathes can remain exquisitely well-preserved through more than 20 mm of vertical mat. Here we propose that the silicification process itself—particularly the formation of a silica gel—may have played a critical role in migration of cyanobacterial trichomes, in protection of mat elements from heterotrophic degradation, and in the ultimate arrest of sheath degradation. In this way, silicification may fundamentally bias the types of information able to be preserved in microbial mats.