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

Abstract

The 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 is not recorded in the basin, due to sea-level fall, but associated levels (pre-Daleje) are characterized by water-column stratification and increased input of terrigenous material of organic and inorganic origin, creating chemically unstable conditions. The interval interpreted as related to Kačák-type conditions shows evidence of sea-level rise and REE enrichment, suggesting enhanced connectivity between the Paraná and Parnaíba Basins. These changes altered salinity and promoted anoxic conditions, as indicated by the V/Ni and Pristane/Phytane diagnostic ratios and by increased 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 potentially toxic compounds, such as perylene—a polycyclic aromatic hydrocarbon—and gadolinium, indicating environmental stress within the depositional setting. These signals may reflect early stressors analogous to those that later contributed to the development of patterns such as the Lilliput Effect in younger intervals. These findings highlight the role of inorganic and organic geochemical proxies in understanding ancient marine crises and their long-term ecological impacts.