TAPHONOMY AND PALEOENVIRONMENTAL SIGNIFICANCE OF SILICIFIED CYANOBACTERIAL FOSSILS IN THE EARLY CAMBRIAN HARKLESS FORMATION, NEVADA, USA

Abstract Primary producers play a critical role in marine ecosystems and global biogeochemical cycling, but the limited preservation of soft-bodied organisms in the fossil record makes it difficult to understand the identity and diversity of primary producers through geologic time. Here we present a new occurrence of the fossil cyanobacteria Obruchevella delicata and Girvanella wetheredii? from the outer shelf facies of the Cambrian Stage 4 Harkless Formation in Nevada, USA. Specimens were examined in thin section and after acid maceration using scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy. Elevated Si and O concentrations within fossil material, combined with the presence of D and G band peaks in fossil Raman spectra, suggest that Obruchevella and Girvanella filaments are silicified in a carbonate matrix and that the silica preserves remnant organic matter from the cyanobacterial sheaths. We suggest that silicification of original organic cyanobacterial tissues was facilitated by a combination of several mechanisms: storm sedimentation processes, organism-specific organo-silica interactions, and sedimentary redox conditions. Sedimentary processes, likely related to storm events, concentrated and transported cyanobacterial filaments to deeper waters, producing grainstone deposits with relatively higher porosity than the surrounding shale. Once transported, suboxic to anoxic bottom waters slowed decay of cyanobacterial filaments while Fe- and Si-rich fluids permeated into the sediment and facilitated early silicification of Obruchevella and Girvanella. Organo-silica interactions on the surface of decaying cyanobacterial filaments promoted Si nucleation on cyanobacteria specifically. The confluence of some or all of these processes resulted in a unique taphonomic window, allowing us to document cyanobacteria as one key primary producer in the Harkless paleoecosystem.