EXPERIMENTAL TAPHONOMIC INSIGHTS INTO EDIACARA-STYLE FOSSILIZATION

Earth’s earliest fossils of complex macroscopic life are recorded in Ediacaran-aged siliciclastic deposits as exceptionally well-preserved three-dimensional casts and molds, known as “Ediacara-style” preservation. Recent geological and experimental work has indicated that high dissolved silica (DSi) concentrations in the ocean may have been responsible for the fossilization of these soft-bodied organisms by means of early silica cementation, resulting in their extraordinary preservation. Experiments conducted under conditions replicating proposed Precambrian seawater conditions, characterized by substantially elevated DSi concentrations (2 mM; an order of magnitude higher than the modern ocean) indicate that silica readily precipitates onto the organic surfaces of a wide range of invertebrate animals and microbial organisms, forming silica cements that facilitate their moldic preservation in sandy sediments. However, recent geochemical modeling suggests that marine DSi might have decreased through the later portion of the Proterozoic, and that early Paleozoic silica levels may have been less than ~150 μM, significantly lower than previous estimates. Here, we build upon previous experimental work to explore various sedimentary substrates in which the Ediacara Biota fossil assemblages are preserved (quartzose sands and clays) and a wider range of initial DSi concentrations (0.5 to 2 mM). Across all experiments, we observed development of amorphous silica coatings along the surfaces of various experimental organisms and in intergranular pore spaces, accompanied by a progressive drawdown DSi concentrations of the experimental solutions to levels well below amorphous silica saturation. Our results provide evidence that soft tissues can be rapidly preserved by silicate minerals precipitated under a wide range of predicted scenarios for Ediacaran seawater DSi concentrations and variable substrate compositions. These observations suggest that interactions between organic substrates and seawater DSi likely played a significant role in the exceptional fossilization of the first complex ecosystems on Earth.