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Experimental constraints on “Ediacara-style” preservation
Silvina Slagter
Yale University

Earth’s earliest ecosystems –the Ediacara Biota– are extraordinarily well-preserved as casts and molds in Ediacaran-aged sandstones, also known as “Ediacara-style” preservation. The mode and mechanisms of preservation of these soft-bodied organisms in sandstones has been extensively debated. Recent studies have proposed that Ediacara-style fossilization was controlled by the high dissolved silica levels (DSi) prevalent in the ocean during the Precambrian—potentially as high as 2 mM, an order of magnitude higher than in today’s oceans—before the radiation of silica-biomineralizers resulted in substantially lower seawater DSi concentrations. Instances of Ediacara-style fossilization are represented in a variety of sedimentary facies characterized by clean quartzose sandstones (as in the Ediacara Member in South Australia) as well as less compositionally mature, sandstones. Taphonomic experiments were conducted to determine the conditions that may foster Ediacara-style preservation across a broader array of sedimentary facies, including a wide range of geochemical and substrate conditions representative of the Ediacaran seafloor. Our experiments were performed at multiple proposed DSi concentrations proposed to reflect Ediacaran seawater (ranging from 0.5-2 mM); a range of organisms, including microalgae, cyanobacteria, marine invertebrates; and a variety of different mineral substrates, with differing combinations of quartzose sand, kaolinite, and iron oxides. We observed the development of amorphous silica and authigenic clay coatings along the surfaces of various experimental organisms and in intergranular pore spaces accompanied by a progressive drawdown in the DSi concentration of the experimental solution. Furthermore, the reactivity of organic matter seems to have an important effect on triggering silica precipitation, as indicated by potentiometric acid-base titration and FTIR measurements. These experimental studies suggest that interactions between organic matter and DSi in seawater may have facilitated the rapid mineralization of organic tissues and promoted the preservation of the Ediacara Biota.