Unveiling early to Middle devonian biotic crises in Eastern Gondwana's Paraná Basin, Brazil: Inorganic geochemistry and clay mineral insights

1. ABSTRACTThe Devonian period was marked by significant biological and climatic shifts, including widespread biotic crises that affected marine ecosystems. The Ponta Grossa Formation (Lochkovian to Frasnian) of the Paraná Basin in Eastern Gondwana records critical paleoenvironmental transitions that impacted the Malvinokaffric fauna, a low-diversity marine assemblage adapted to cold-water conditions. This study integrates mineralogical and geochemical analyses, employing techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), to reconstruct the depositional environment during the Early to Middle Devonian, emphasizing the Daleje and Kačák biotic events. The Daleje event, characterized by a sea-level drop and freshwater influx, resulted in water column stratification and increased input of terrigenous material of organic and inorganic origin, creating chemically unstable conditions. The Kačák event, marked by sea-level rise and rare earth element (REE) enrichment, enhanced connectivity between the Paraná and Parnaíba Basins, altering salinity and fostering anoxic conditions characterized by the results of the V/Ni and Pr/Phy diagnostic ratios, and illite concentration. The correlation between the biomarker gammacerane and makatite, alongside sulfur and the biomarker C30 αβHopane, supports evidence of microbial activity influencing paleoenvironmental shifts. The geochemical data also reveal the presence of toxic compounds like perylene, a polycyclic aromatic hydrocarbon, and gadolinium, potentially contributing to the Lilliput Effect observed in the Malvinokaffric fauna. These findings highlight the role of inorganic and organic geochemical proxies in understanding ancient marine crises and their long-term ecological impacts.