Benthic foraminiferal assemblage changes across the Eocene–Oligocene transition in an astronomically calibrated and isotopically characterized Paratethyan section

The Eocene–Oligocene transition (EOT) marks a critical period of global palaeoclimatic and palaeoenvironmental change. Although global trends of the EOT are well established, regional basins, such as the Paratethys, remain underexplored. However, multiproxy datasets from such regions are crucial to evaluate the regional expression of global events. Here, we reconstruct the palaeoenvironmental, palaeoecological, and palaeoclimatic evolution of the EOT in the Brebi section of the Central Paratethyan Transylvanian Basin using benthic foraminiferal assemblages, stable isotope geochemistry, and cyclostratigraphy. We conducted high-resolution quantitative analyses of foraminiferal assemblages, stable isotopes, and cyclicity, integrated with revised calcareous nannoplankton and planktonic foraminifera biostratigraphy to place the Eocene–Oligocene boundary (EOB). We distinguish three successive phases within the EOT, including a distinct perturbation interval (Phase 2) around the EOB. Phase 2 is characterized by reduced sedimentation rate, weakened expression of cyclicity, increased faunal volatility, and a negative bulk carbonate δ 18 O anomaly that likely reflects regional freshwater influx, while δ 13 C evolution broadly parallels the global signal. The persistent dominance of epifaunal Heterolepa eocaena , high foraminiferal diversity near the EOB, and increased ratio of infaunal forms during Phase 2 indicate fluctuations in nutrient supply, salinity, and oxygenation, despite overall oxic conditions. Multivariate analyses distinguish Eocene and Oligocene assemblages and highlight a shift towards Cycloforina–Quinqueloculina assemblage and miliolinid-dominated morphogroups in the earliest Oligocene. These findings demonstrate that the Brebi section captures the global signatures with unique regional overprint across the EOT, emphasizing the importance of semi-restricted marine basins for understanding the complexity of past climate transitions.