EXPERIMENTAL SILICA PRECIPITATION ONTO ORGANIC TISSUES: IMPLICATIONS FOR THE PRESERVATION OF THE EDIACARA BIOTA

Taphonomic experiments provide fundamental insights into the nature of the fossil record. The Ediacara Biota provides a critical window into the emergence of early complex, multicellular life. However, the affinities of these soft-bodied organisms have remained debated—due in part to poor understanding of the mechanisms responsible for their exceptional, “Ediacara style” fossilization (moldic preservation of soft tissues in sandstones). Various minerals have been proposed to have facilitated Ediacara-style preservation, particularly iron sulfides (the “death mask” model; Gehling, 1999) or clay minerals (e.g., Wade, 1968), and previous taphonomic experiments have largely focused on those phases. It was recently proposed that high dissolved silica (DSi) concentrations in the Ediacaran oceans, prior to the early Paleozoic radiation of silica biomineralizers, promoted the diffusion of silica-rich fluids from the water column to the sandy seafloor, preferentially nucle­ating silica onto organic tissues and surrounding pore spaces, welding the sand grains into coherent molds (Tarhan et al., 2016). We performed taphonomic experiments to further test whether high DSi concentrations could facilitate early silicification of a wide range of organisms, including sponges, anemones, medusae, plants and cyanobacteria, and to assess the role of organic substrates in silica precipitation. We provide the first empirical demonstration that organism soft tissues can rapidly silicify under biogeochemical conditions characteristic of Ediacara seawater. DSi concentration was observed to decrease with time, coincident with the precipitation of silica nano- and microspheres onto carcass surfaces and almost total loss of organic carbon. Moreover, our results indicate that the organic tissues themselves facilitated silica sorption, even in solutions undersaturated with respect to amorphous silica. Surface reactivity models based on acid-base potentiometric titration data provide evidence that silica kinetics in the presence of organic ligands exert a primary control on “Ediacara-style” preservation. This observation bolsters the case that seawater chemistry created unique conditions conducive to early silica cementation, resulting in the exceptional fossilization of the first complex animals on Earth.