MICROBIAL FACIES IN THE NEOPROTEROZOIC LOWER BECK SPRING FORMATION

The Neoproterozoic Beck Spring Formation (~787–750 Ma) is a unit that preserves diverse clotted and laminated carbonate microbialites. Beck microbialites have experienced both dolomitization and silicification offering unique insight into different taphonomic windows in Neoproterozoic marine environments. Samples from the lower Beck Spring Formation reveal abundant putative ~100 µm coccoidal fossils that record similar preservation styles in both chert and dolomite: the two mineral types preserve equally distinct structures of comparable scale and morphology. In thin section, coccoidal structures contain sharply defined walls and central spheres, and Raman spectroscopy confirms the presence of organic carbon in these structures. EDS point analysis and mapping similarly indicate that lower Beck Spring Formation samples contain carbonaceous material embedded in both minerals. EDS also reveals that carbonate and chert are associated in these samples, and boundaries between mineral types are often nebulous. To further examine conditions that foster co-precipitation of different minerals, we conducted a taphonomy experiment that approximately replicated Neoproterozoic marine chemistry, with elevated silica compared to modern oceans. Coccoidal and filamentous cyanobacteria were inoculated in seawater media of varying Mg:Ca ratios. We used EDS analyses of dehydrated biomass to evaluate the roles of microbial morphology and water chemistry in mineral nucleation. Under all conditions, nanoscopic colloidal silica phases precipitated within the exopolymeric substances coating the cells. However, higher seawater [Mg] resulted in a greater abundance of biofilm-coating precipitates and a higher overall Mg:Si ratio in the precipitates, suggesting that elevated concentrations of cations may promote preservation by silica. These combined investigations help to better constrain the taphonomic processes that fostered organic preservation in the Beck Spring Formation.