Silicification of trilobites and biofilm from the Cambrian Weeks Formation, Utah: Evidence for microbial mediation of silicification

We report on silicified trilobite sclerites with associated silicified biofilm from the Cambrian Weeks Formation, Utah (USA), that support a role for microbial biofilms in silicification. Silicified sclerites are typically small (<3 mm) and incompletely preserved. All studied specimens are partly coated in 5–20 μm (rarely >500 μm) of silica-cemented matrix. High-resolution scanning electron microscope (SEM) study reveals the presence of two different forms of carbon-rich threads, ribbons and mats, coating both sclerites and silica-cemented matrix. Crystalline-looking biofilm threads and ribbons composed of Si, O, and C are interpreted as silicified biofilm associated with the trilobite silicification. Rippled to smooth biofilm mats composed of more C, less Si and O, and a trace of N are post-silicification. Embedded in the silica of the sclerites and matrix are molds of framboids that we interpret as originally framboidal pyrite that was engulfed by silica. These data indicate that silica precipitation continued into the surrounding matrix following the propagation of sulfate-reducing bacteria feeding on the organic matter present in the sclerite and the neighboring sediment. This strongly supports the model that bacterially mediated decay is key to the silicification of carbonate bioclasts and provides the first direct evidence of a microbial community (biofilm). A literature review reveals that silica extends past the fossil more frequently than is recognized, suggesting that silicified biofilm might be common but overlooked.