Spatial heterogeneity of organic matter during OAE1b in the Araripe Basin: controls on source and preservation

Oceanic Anoxic Events (OAEs) represent major perturbations of the global carbon cycle, yet their expression in epicontinental basins can be strongly modulated by local depositional conditions. This study investigates how contrasting marine connectivity and continental input controlled organic matter (OM) composition, accumulation, and preservation during OAE1b in the Romualdo Formation, Araripe Basin, Brazil. We focus exclusively on concretion-bearing black-shale intervals from the Estiva (6–17 m) and Sobradinho (37–44 m) sections, integrating Rock-Eval pyrolysis, total organic carbon (TOC), δ¹³Corg, δ¹⁵N, C/N, Hg, elemental geochemistry, and palynofacies. Both successions record OM enrichment associated with the OAE1b carbon-cycle perturbation but display contrasting responses. Sobradinho shows higher mean amorphous organic matter (AOM; 49.3 vs. 29.2%), median TOC (7.03 vs. 3.95%), and hydrogen index (HI; ~540 vs. ~426 mg HC/g TOC) than Estiva. The strong TOC–HI correlation at Sobradinho (ρ = 0.87; p < 0.001) indicates accumulation of hydrogen-rich OM associated with favorable preservation under more distal anoxic conditions. In contrast, Estiva displays greater palynofacies heterogeneity, with mean phytoclast dominance (38.3%), abundant palynomorphs (32.5%), and pronounced dinocyst pulses reaching 45%, indicating marked variations in terrestrial and aquatic/marine OM input, suggesting greater proximity to the terrestrial source area. Despite stronger paleontological evidence for marine connectivity at Sobradinho, its preserved OM is not characterized by greater dinocyst abundance. These results reveal a partial decoupling between environmental marine influence, OM source, and preservation efficiency. We propose that regional OAE1b promoted organic enrichment at both sites, whereas local differences in marine connectivity, continental input, and preservation conditions controlled contrasting expressions of OM within the same basin.