Experimental evidence supports early silica cementation of the Ediacara Biota

Abstract

Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

Pyrite-lined shells as indicators of inefficient bioirrigation in the Holocene–Anthropocene stratigraphic record

“…1d) and the 16-20 cm thick mixed layer at Po sites observed at the time of sampling in 2013 and 2014 indicate that local-scale biomixing remains active in the uppermost sediment zones. The thickness of the mixed layer documented here and by other studies (Frignani and Langone, 1991;Frignani et al, 2005;Alvisi et al, 2006, Alvisi, 2009 is typical of the mixed-layer thickness observed in other marine shelf environments (Moodley et al, 1998;Teal et al, 2010), and the depth of mixing is also not smaller than the mixed-layer thickness (< 5 cm) typical of persistently hypoxic environments on continental slopes (Meadows et al 2000;Smith et al, 2000;Levin et al, 2003). Second, our inference about the role of inefficient bioirrigation in sequestration of pyrite linings in the subsurface stratigraphic record, as envisioned in the scenario in Fig.…”

“…The lack of bioturbation that limits O 2 exposure and allows preservation of intact, articulated, multi-element skeletal remains under anaerobic degradation of organic matter can induce early pyritization, phosphatization, or silicification of organic tissues (Gabbott et al, 2004;Zhu et al, 2005;Cai et al, 2012;Saleh et al, 2019Saleh et al, , 2020Schiffbauer et al, 2014;Novek et al, 2016). Early pyritization or silicification induced by reactive organics may also generate death masks that stabilize and cement sediment and blanket benthic organisms (Gehling, 1999;Strang et al, 2016;Tarhan et al, 2016;Gibson et al, 2018;Liu et al, 2019;Slagter et al, 2021). Similar pathways stimulated by the decay of labile organics or further sustained by microbial and mucus-forming processes in anaerobic microenvironments (microniches or pockets; Emery and Rittenberg, 1952;Jørgensen, 1977;Borkow and Babcock, 2003;Stockdale et al, 2010;Anderson et al, 2011;Virtasalo et al, 2010Virtasalo et al, , 2013Lehto et al, 2014) can lead to distinctive clusters of authigenic minerals concentrated in intra-skeletal voids initially filled with organic tissues in otherwise well-preserved skeletal remains (Brett et al, 1991(Brett et al, , 2012a.…”

Biofilms as agents of Ediacara-style fossilization

“…In order to investigate what factors, on a molecular level, were responsible for the greater silicification and higher-fidelity (more anatomically detailed) moldic preservation we observed under silica-enriched solutions and in the presence of biofilms, we also analysed experimental samples using potentiometric titrations and Fourier transform infrared spectroscopy (FTIR). Potentiometric titration data indicate that the algal and cyanobacterial taxa populating these mats are characterized by higher surface reactivities than the marine invertebrate animals investigated in this and previous studies 16 , including scyphozoan medusae, anemones, and sponges (Fig. 3 ).…”

“…An important consideration stemming from this experimental work is that the spatial extent and ecological maturity of matground- and biofilm-associated communities likely influenced the fossilization potential of Ediacara macroorganisms. Under higher-DSi conditions, but in the absence or relative paucity of microbial mats (e.g., as in certain lower Palaeozoic instances of Ediacara-style fossilization) 6 , moldic preservation of macroorganisms via silicification may still have occurred 16 , but potentially with lower fidelity than seafloor ecosystems associated with well-developed matgrounds. Consequently, although high-DSi ocean waters would have provided a global-scale potential for Ediacara-style fossilization linked to early silica cementation, local taphonomic factors—such as the presence or absence of well-developed microbial matgrounds—likely played a secondary but critical role in shaping fossilization potential and fidelity.…”

“…Similarly, preservation of Ediacara Biota taxa in other Ediacaran deposits characterized by other taphonomic modes 20 (including modes extensively documented in Phanerozoic deposits), as well as the occurrence of biostratinomically deformed Ediacara soft tissues 21 , suggest that Ediacara Biota organisms did not possess unusually strong integuments nor were these essential for their fossilization. Ediacara fossilization by means of authigenic mineralization—specifically, the potential importance of rapid precipitation of iron minerals 10 – 12 , clays 13 – 15 , or silica 6 , 16 in facilitating capture of soft-tissue features prior to carcass collapse and degradation—has of recent years been a topic of particular interest and contention. Although debate has continued regarding the syngeneity and relative importance of specific minerals 6 , 22 – 24 , both geological 6 and experimental 16 recent work has indicated that high dissolved silica (DSi) concentrations in the Ediacaran oceans 25 , 26 —potentially an order of magnitude higher than present-day marine dissolved silica levels—may have created a taphonomic window that favoured the moldic preservation of soft tissues via early diagenetic precipitation of silica cements, even from undersaturated solutions (Fig.…”