Post-Depositional Fluids as a Primary Control on Carbonate Isotopic Variability in the Tamengo Formation (Ediacaran, Brazil)

The emergence of complex life in the Ediacaran underscores the need to constrain paleoceanographic conditions, which have traditionally been inferred from apparently consistent global δ¹³C and ⁸⁷Sr/⁸⁶Sr curves. However, in carbonate successions with complex diagenetic histories, separating primary marine signatures from regional and post-depositional overprints remains problematic. Exposed in west-central Brazil, the Tamengo Formation records the Ediacaran–Cambrian transition. It provides an opportunity to evaluate how diagenetic processes influence carbonate records associated with early skeletal metazoans, including Cloudina. This study is based on the analysis of 330 thin sections from seven key outcrops and two drill cores. The main petrographic constituents: a predominantly calcitic matrix composed of well-preserved and neomorphosed micrite, bioclasts, and ooids. Diagenetic phases include calcite, dolomite, and silica cement, as well as pyrite, iron and manganese oxides, and gypsum pseudomorphs. Additional features comprise primary and secondary porosity and pressure-solution structures such as stylolites and dissolution seams. Petrographic, cathodoluminescence, and SEM–EDS analyses indicate that carbonate factories were modified through multiple diagenetic stages, beginning during sedimentation and intensifying during burial. Syndepositional micrite precipitation dominated low-energy settings, whereas ooid formation reflects more agitated sectors of the carbonate ramp. Local gypsum precipitation indicates episodic evaporitic conditions and transient hydrodynamic constraints. Early diagenesis involved shallow-burial calcite cementation, locally accompanied by brecciation and calcitization of evaporitic phases, preserving gypsum pseudomorphs, and indicating fluid circulation still closely linked to seawater chemistry. With progressive burial, mesodiagenesis became the dominant driver of mineralogical and textural reorganization. This stage is marked by neomorphism, micrite-to-microspar recrystallization, modification of ooids and bioclasts, and the development of vuggy porosity associated with selective dissolution and pressure-solution features. Cathodoluminescence reveals heterogeneous cement generations, while SEM–EDS maps document partial dolomitization and localized Fe- and S-enrichment along seams and fractures. Bulk carbonate δ¹³C values (40 samples) show a wide positive range ( +2.8 to +5.7‰ ), whereas δ¹⁸O values are consistently depleted (- 4.2 to -11.1‰), resulting in weak C–O covariance. The coexistence of strongly negative δ¹⁸O values (< - 9‰) with still positive δ¹³C indicates decoupled isotopic behavior, consistent with burial-related fluid–rock interaction and mesodiagenetic recrystallization rather. The isotopic variability reflects polyphase diagenetic overprinting controlled by depositional setting and burial-related fluid flow, emphasizing the limitations of bulk geochemical proxies and the need for petrographically constrained approaches.