Collapse and recovery of the foraminifera during Oceanic Anoxic Event 1b of the Araripe Basin, in Central Gondwana

The Aptian–Albian transition (113 Ma), marked by the Kilian sub-event of Oceanic Anoxic Event 1b (OAE-1b), is investigated here through the foraminiferal assemblages of the Romualdo Formation (Araripe Basin, Brazil), an epicontinental system of Central Gondwana. At Santo Antônio, a pronounced negative carbon-isotope excursion (Δδ 13 C ∼ − 7‰) coincides with hypersalinity and anoxia (Sr/Ba >100; V/Cr > 15) and persistently low total organic carbon (TOC: 0.11–0.70%), demonstrating that anoxia does not necessarily lead to organic carbon enrichment in restricted settings. This interval coincides with a collapse of the foraminiferal assemblage, marked by extremely low diversity and near-monospecific dominance of porcelaneous miliolids ( Quinqueloculina d'Orbigny, 1826). Recovery is recorded upward by the replacement of miliolids by Conorboides minutissima and Patellina subcretacea , accompanied by increased diversity. The Serra do Inácio section records an anoxic interval (V/Cr > 13; 2 Sr/Ba >10) associated with the two highest diversity levels of the profile, together with increased marine influence, including planktonic taxa such as Microhedbergella miniglobularis and M. renilaevis , as well as marine macrofauna, indicating episodic marine incursions. The presence of these taxa constrains the Aptian–Albian boundary. Correlation between both sections supports the placement of the Aptian–Albian boundary and the identification of the Kilian sub-event. The results demonstrate that the expression of OAE-1b in the Romualdo Formation is highly heterogeneous, reflecting the influence of paleogeographic and hydrological factors in epicontinental systems.