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The Role of Volcanic-Derived Clays in the Preservation Of www.nature.com/scientificreports OPEN The role of volcanic‑derived clays in the preservation of Ediacaran biota from the Itajaí Basin (ca. 563 Ma, Brazil) Bruno Becker‑Kerber1,2, Abderrazak El Albani2*, Kurt Konhauser3, Ahmed Abd Elmola2, Claude Fontaine2, Paulo S. G. Paim4, Arnaud Mazurier2, Gustavo M. E. M. Prado5, Douglas Galante6, Pedro B. Kerber7, Ana L. Z. da Rosa8, Thomas R. Fairchild5, Alain Meunier2 & Mírian L. A. F. Pacheco9,10 The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual ‘Ediacaran‑style’ preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifcally, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high‑fdelity preservation was made possible by rapid burial during volcanic events and the in‑situ templating of tissue by clays via microbially‑mediated mineralization. This newly described Lagerstätte demonstrates that a number of minerals can facilitate preservation, and that perhaps ‘Ediacaran‑style’ preservation result from diferent processes leading to the same broad style of preservation. Te typical moldic preservation of sof-bodied organisms in the Ediacaran Period (635–539 Ma)1–4, in particular the positive epirelief type where the fossils protrude up from bedding surfaces4, has few parallels in other periods of geologic time, hence the term “Ediacaran-style preservation.” One explanation points to rheological difer- ences amongst the sediments5, a hypothesis that has received recent support4. Gehling1 also proposed the so- called ‘death-mask’ model for this preservation, in which microbial mats stabilized the sediments and produced a veneer of authigenic pyrite (FeS 2) that replicated the morphology of the organisms. Evidence in favor of this hypothesis comes from the presence of iron oxide and pyrite concentrations along fossiliferous bedding planes in certain localities3,6. Other factors may include episodic event deposition 7, early cementation by amorphous 2,8 9 silica (SiO2) , and burial by volcanic ash—the so-called “Conception-style” preservation . It is similarly possible that multiple mechanisms could produce this type of moldic fossilization and/or that taphonomic factors varied according to the geologic setting. In this regard, the Itajaí Basin10–12 (ca. 563 Ma, Brazil) appears to be a promising locality in which to study the possible mechanisms of preservation operating during the Ediacaran Period. Tis geological unit is relatively unmetamorphosed, has similar ages to the Avalon 1Programa de Pós-Graduação em Ecologia e Recursos Naturais, Universidade Federal de São Carlos, Washington Luiz, 325km, São Carlos, SP 13565-905, Brazil. 2Unité Mixte de Recherche (UMR), Centre National de la Recherche Scientifque (CNRS), IC2MP 7285, University of Poitiers, 86073 Poitiers, France. 3University of Alberta, Edmonton, Canada. 4Programa de Pós-Graduação em Geologia, Universidade do Vale do Rio dos Sinos, São Leopoldo, RS 93022-750, Brazil. 5Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, Cidade Universitária, São Paulo, SP 05508-080, Brazil. 6Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for Research in Energy and Materials, Av. Giuseppe Maximo Scolfaro, 10000, Campinas CEP 13083-100, Brazil. 7Instituto de Química, Universidade Federal de Mato Grosso do Sul, Cidade Universitária, Av. Costa e Silva-Pioneiros, Campo Grande CEP 79070-900, Brazil. 8Petrobras, Santos, SP, Brazil. 9Departamento de Biologia, Universidade Federal de São Carlos-Campus Sorocaba, Rod. João Leme dos Santos km 110, Sorocaba CEP 18052-780, Brazil. 10Instituto de Física, Universidade de São Paulo, Rua do Matão, Travessa R 187, São Paulo CEP 05508-090, Brazil. *email: [email protected] Scientifc Reports | (2021) 11:5013 | https://doi.org/10.1038/s41598-021-84433-0 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. Ediacaran fossils and preservation styles in the Itajaí Basin. (a) Tree-dimensionally preserved tufed microbial mats (CAP-879). (b) 3D Arumberia-type microbial mats (CAP-880). (c) Aspidella in positive hyporelief (CAP-549). (d) Sub-millimetric to micrometric flaments in positive epirelief (CAP-881). (e) Exquisite preservation of flaments bearing putative cellular structures (arrows) (CAP-866). (f) Palaeopascichnus in positive epirelief. Image created using Inkscape 1.0 (https ://inksc ape.org/). Assemblage, and also possesses high-fdelity preservation of the fossils. Recently, Becker-Kerber et al.13 described the Itajaí biota, reporting the presence of Palaeopascichnus, discoidal forms, micrometric flaments, Arumberia and fossilized microbial tufs. In this work, we apply a diverse array of techniques to address the fossilization processes of this new Ediacaran locality. More specifcally, we demonstrate the close association of body fossils and microbial mats with volcanic sediments. We then provide a mechanism for the morphological preservation of tissue that includes the rapid formation of authigenic clays during early diagenesis as a result of the alteration of volcanic materials and microbial activity. Results Te Itajaí biota is composed of the well-known chambered taxon Palaeopascichnus, the discoidal forms Aspi- della and Nimbia, and micron-sized algal or bacterial flaments 13 (Fig. 1c–f). Based on the mineralized width (33–193 µm) and bedding plane disposition of the flaments it is suggested that these could be the fossilized remnants of giant flamentous sulphur oxidizing bacteria or eukaryotic algae. Well-preserved reticulated and Arumberia-type microbial mats have also been observed (Fig. 1a,b). We detected three modes of preservation: (1) primarily impressions in positive epirelief (Palaeopascichnus, Aspidella, and micrometric flaments) (Fig. 1d–f); (2) rare impressions as negative epirelief (Aspidella) (Fig. 1c); and (3) full-relief, three-dimensional microbial mats replicated by clays (Fig. 1a,b). To our knowledge, this is the frst case in the geological record of distinctive morphologies of microbial mats (e.g., reticulated tufs and Arumberia) being three-dimensionally replicated by clay minerals. Investigations by microtomography (µ-CT) corroborated the moldic preservation of the flaments (Supplementary Fig. S1a–c) as well as the unusual 3D nature of the microbial mats (Supplementary Fig. S1d–f, and Movie 1). Remarkably, very small flaments (c. 30 µm in width) are preserved as impressions, and some individuals seem to bear putative cellular structures (Fig. 1e). Tis minute detail indicates a level of morphologi- cal retention in molds that is unprecedented in the fossil record because flamentous microfossils are usually permineralized or occur as palynomorphs 14. All the preservation types mentioned above occur in the same lithology: pale-gray millimeter-scale rhyth- mites of mud and silt that were deposited in an upper slope setting subjected to rhyolitic volcaniclastic input (Supplementary Text 1, Supplementary Figs. S2, and S3). Tese layers additionally display abundant evidence of microbial mat development (e.g., reticulated mats, crinkly laminations; Fig. 1a, Supplementary Fig. S1d–f). Te fossil impressions are located in authigenic (i.e., formed in situ) clay-rich laminations characterized by opaque and microcrystalline clay minerals (Fig. 2a; Supplementary Figs. S4 and S5). Tese surfaces can either occur between detrital clay (i.e., inherited) layers or at the bottom or top of siltstone laminae (Fig. 2a, Supplementary Fig. S5). Te authigenic clays also compose the unique three-dimensional microbial mats at the same beds (Figs. 1a,b, 2b,c, Supplementary Figs. S4 and S6a). Identical textures are again observed in 3D clay-replicated microbial mats from distal delta-front millimetric-scale rhythmites (e.g., Arumberia; Supplementary Figs. S2 and S6b) associated with reworked volcaniclasts and positive epirelief impressions of Nimbia. By contrast, when Scientifc Reports | (2021) 11:5013 | https://doi.org/10.1038/s41598-021-84433-0 2 Vol:.(1234567890) www.nature.com/scientificreports/ Figure 2. Pyroclasts from the fossiliferous horizons of the Itajaí Basin. (a) Vertical thin section through micron-sized flament. Note the concentration of authigenic clays on the fossiliferous surface (FS). Vertical thin section under transmitted (b) and refected light (c) showing fossiliferous laminae (white arrows) and tuf (orange arrow) composed of authigenic clay minerals. (d, e) Micron-sized flaments covered by a layer of altered ash. (f) Accretionary pellet (white arrow) and scattered phenocrysts (black arrows). (g) Pyroclasts devitrifed to microcrystalline quartz. (h) Angular clast devitrifed to sericite. (i) Fibrous radial sericite in early-formed fractures. Note the molding relationship (arrow) with the host rock. Image created using Inkscape 1.0 (https :// inksc ape.org/). microbially induced sedimentary structures (MISS) are found in non-volcaniclastic sediments, they are preserved only as impressions in semi-relief without associated authigenic minerals (see Ref.13).
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