An experimental look at the taphonomy of cyanobacterial mats in siliciclastic sediments

Cyanobacteria are ubiquitous in a variety of modern habitats, and siliciclastic sediments in particular are home to a wide diversity of microbial communities. Benthic microbial mats, typically established by cyanobacteria on modern Earth, were likely prevalent on Archean Earth, yet explicit traces of their ancestors in Archean siliciclastic rocks are difficult to detect. To understand the taphonomy of benthic microbial mats in sandy, subaquatic environments, cyanobacterial mats were incubated for five months under a range of temperatures representative of ambient (25 degrees C) and eogenetic conditions (37 degrees C, 70 degrees C, and 100 degrees C). Cyanobacterial materials including trichomes, sheaths, and extracellular polymeric substances (EPS) were analyzed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and micro Raman spectroscopy. Textures were permineralized in all temperature regimes with phases that included mixed silicates, Na-carbonate, clays, gypsum-anhydrite, pyrrhotite, anatase, akaganeite, magnetite, natrojarosite, and ankerite. Pigments including chlorophyll, beta -carotene, and scytonemin were identified in the lower temperature regimes, but were not easily detected in the samples incubated at 100 degrees C. The morphological characteristics of trichomes and sheaths were maintained to some degree in all temperature regimes, but there was a higher relative abundance of EPS as temperatures increased. The profusion of EPS obscured the absolute differentiation between individual trichomes and sheaths at higher temperatures. The results indicate that over time, morphological, mineralogical, and carbonaceous features that formed at the end of these incubation experiments could collectively create the laminations characteristic of fossilized microbial mats found in sandstones throughout the geologic record. In Archean sandstones, where very little is preserved, these collective features may prove to be especially important in the detection of ancient life.