A RECIPE FOR CYANOBACTERIAL FOSSILIZATION IN SILICICLASTIC ENVIRONMENTS

The exceptional preservation of microbes and microbial textures in coarse-grained sediments (e.g. sandstones, siltstones) is unique to the Ediacaran. Microbial fossils in Ediacaran siliciclastics have also been found near and/or in association with soft-bodied macrofossils, suggesting a role for microbes in the preservation of macroscopic organisms. The decline in this mode of preservation after the terminal Proterozoic is often attributed to the rise of grazing fauna and increase in bioturbation. However, the apparent rarity of similarly preserved microbial fossils and structures before the Ediacaran suggests that additional factors may play a role. Here, we aim to identify environments most conducive to the fossilization of filamentous cyanobacteria. We test the influence of: (1) silica concentrations in seawater (0-0.4 mM), (2) the abundance of clay particles (0-50 mg of clay per 1L of media), and (3) agitation (0-300 rpm) on the development of continuous mineral coating around cyanobacterial sheaths. The relative contributions of mineral precipitation and biological trapping of suspended mineral grains are determined by physically separating the cyanobacteria from the mineral substrate. We find that minerals, composed of Ca, Fe, K, Mg, and Na – alumino-silicates, coat cyanobacterial filaments within days. Cyanobacteria also trap suspended clay and silt grains in the presence of moderate to high sediment loads (5 to 50 mg per L of media) and moderate agitation (200 rpm). Precipitation of minerals around cyanobacterial filaments occurs over longer timescales (>1 month) and is facilitated by high silica concentrations (0.1-0.4 mM). Thus, the most continuous, smooth coating develops in the presence of elevated concentrations of dissolved silica and a steady delivery of clay-sized, suspended mineral grains, identifying these factors as the most conducive to microbial fossilization in sandstones, siltstones and shales.