A shift in redox conditions near the Ediacaran/Cambrian transition and its possible influence on early animal evolution, Corumbá Group, Brazil

Abstract

The Ediacaran–Cambrian transition witnessed some of the most important biological, tectonic, climatic and geochemical changes in Earth’s history. Of utmost importance for early animal evolution is the likely shift in redox conditions of bottom waters, which might have taken place in distinct pulses during the late Ediacaran and early Paleozoic. To track redox changes during this transition, we present new trace element, total organic carbon and both inorganic and organic carbon isotopes, and the first iron speciation data on the Tamengo and Guaicurus formations of the Corumbá Group in western Brazil, which record important paleobiological changes between 555 Ma to < 541 Ma. The stratigraphically older Tamengo Formation is composed mainly of limestone with interbedded marls and mudrocks, and bears fragments of upper Ediacaran biomineralized fossils such as Cloudina lucianoi and Corumbella werneri. The younger Guaicurus Formation represents a regional transgression of the shallow carbonate platform and is composed of a homogeneous fine-grained siliciclastic succession, bearing meiofaunal bilateral burrows. The new iron speciation data reveal predominantly anoxic and ferruginous (non-sulfidic) bottom water conditions during deposition of the Tamengo Formation, with Fe_HR/Fe_T around 0.8 and Fe_Py/Fe_HR below 0.7. The transition from the Tamengo to the Guaicurus Formation is marked by a stratigraphically rapid drop in Fe_HR/Fe_T to below 0.2, recording a shift to likely oxic bottom waters, which persist upsection. Redox-sensitive element (RSE) concentrations are muted in both formations, but consistent with non-sulfidic bottom water conditions throughout. We interpret the collected data to reflect a transition between two distinct paleoenvironmental settings. The Tamengo Formation represents an environment with anoxic bottom waters, with fragments of biomineralized organisms that lived on shallower, probably mildly oxygenated surficial waters, and that were then transported down-slope. Similar to coeval successions (e.g., the Nama Group in Namibia), our data support the hypothesis that late Ediacaran biomineralized organisms lived in a thin oxygenated surface layer above a relatively shallow chemocline. The Guaicurus Formation, on the other hand, records the expansion of oxic conditions to deeper waters during a sea level rise. Although the relationship between global biogeochemical changes and the activities of early bioturbators remains complex, these results demonstrate an unequivocal synchronous relationship between oxygenation of the Corumbá basin and the local appearance of meiofaunal bioturbators.

Keywords

Atmospheric oxygenation / Gondwana / Redox proxies / Iron speciation / Ediacaran biota / Redox-sensitive elements

CRediT authorship contribution statement

Fabricio A. Caxito: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Visualization, Project administration, Funding acquisition. Erik Sperling: Conceptualization, Methodology, Validation, Writing – review & editing, Resources, Funding acquisition. Gabriella Fazio: Resources, Writing – review & editing, Visualization. Rodrigo Rodrigues Adorno: Resources, Writing – review & editing, Visualization. Matheus Denezine: Resources, Writing – review & editing, Visualization. Dermeval Aparecido Do Carmo: Resources, Writing – review & editing, Visualization. Martino Giorgioni: Resources, Writing – review & editing, Visualization. Gabriel J. Uhlein: Resources, Writing – review & editing, Validation, Data curation. Alcides N. Sial: Resources, Methodology, Writing – review & editing, Validation, Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is supported by CNPq-Brazil through grants nb. 408815/2021-3 and 304509/2021-3, by Instituto Serrapilheira through Project “MOBILE: Mountain Belts and the Inception of Complex Life on Earth (geolifemobile.com)”, grant no. Serra-1912-31510, and by the Worldwide Universities Network – WUN through their Research Development Fund (RDF 2022). The first author is part of Instituto GeoAtlântico, a National Institute of Science and Technology, CNPq-Brazil process nb. 405653/2022-0. Most of the data presented here was acquired during a seven-month visiting research appointment of FAC at the Stanford Doerr School of Sustainability supported by CAPES-Brazil through their PRINT program (88887.682318/2022-00). EAS and some geochemical analyses at Stanford were supported by U.S. National Science Foundation grant EAR-2143164. MG was supported by the Research Foundation of the Federal District (FAPDF) - process no. 0193.001609/2017. We thank Simon Poulton for the fruitful discussions about interpretation of the iron speciation data, Douglas Turner of the Environmental Measurements Facility in Earth Sciences, Stanford University for support in acquiring the TOC dataset, Peter Michael Blisniuk of the Stable Isotope Biogeochemistry Laboratory, Stanford University for support in acquiring the organic carbon isotope dataset, and Hunter Olson for lab assistance. A previous version was improved after comments and suggestions by three anonymous reviewers.

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