Impact of post-depositional fluids on paleoceanographic signatures of Ediacaran deposits: Implications for paleoceanographic constraints of the Cryogenian-Ediacaran-Cambrian transition

Abstract

The Ediacaran period (~ 635 - 538 Ma) marked the onset of animal diversification with complex ecological behaviors, the paleooceanographic and atmospheric conditions played a pivotal role in shaping these life forms. Paleoceanographic reconstructions primarily rely on isotopic records, with the resemblance of signals prompting the formulation of global curves. The use of 87Sr/86Sr ratios in seawater is a proxy for continental weathering and atmospheric CO2 partial pressure (pCO2), complemented by ´13C to discern changes in the carbon cycle. However, decoding the significance of ´13C anomalies and 87Sr/86Sr isotope ratios, distinguishing their global, regional, or diagenetic origin, poses a challenge in Neoproterozoic paleoenvironmental studies. The individual diagenetic history of these deposits can obscure primary isotopic signatures, casting doubts on global correlations. Diagenetic events, which may have been regional and intricately tied to the tectonic and thermal history of the basins where they were deposited, must be considered. A practical approach to evaluating and differentiating primary signals from post-depositional alterations involves multiproxies, focusing on carbonate textural, mineralogical, and geochemical aspects. This study encompasses high-resolution mapping of diagenetic effects and the superimposition of events in isotopic signals, employing whole-rock techniques such as X-ray diffraction and geochemical analysis of major and minor elements by ICPMS. Additional in situ studies include microprobe, Raman microscopy, cathodoluminescence, and high-resolution compositional mapping techniques such as nano-XRF using synchrotron light. Analyses will be carried out in drillcore samples related to the Bambuí Group, Minas Gerais, Cryogenian-Ediacaran transition, and drillcores in the western region of Mato Grosso do Sul (MS) and southern Namibia, related to the Corumbá and Nama Groups, respectively. These activities will provide unprecedented subsurface records of the Ediacaran-Cambrian transition. These occurrences are deemed crucial due to their exceptional preservation, well-understood geological framework, and outstanding geochronological, radiometric, and biostratigraphic calibrations, making them ideal candidates for constructing a reference curve for the ´13C and 87Sr/86Sr transition from the Criogeniano-Ediacarano-Cambrian. The multiproxy approach reveals the intricate influence of diagenesis on the molding of the isotopic signatures of these ancient deposits. When preserved, major and minor elements, along with ´13C and 87Sr/86Sr ratios and ongoing paleontological and sedimentological studies, offer insights for reconstructing paleoredox and paleoceanographic conditions. This constraint is essential for understanding the feedback mechanisms between planet Earth's chemical and biological evolution during the Ediacaran, a period distinguished by significant biological innovation.