Calcareous nannofossils and paleoceanographic, paleoecological, and biostratigraphic insights into the Aptian/Albian of the Araripe Basin, central Gondwana

The Romualdo Formation, in the Araripe Basin, northeastern Brazil, is associated with a restricted marine environment related to the opening of the South Atlantic Ocean. Here, calcareous nannofossils were used as proxies for a paleoceanographic reconstruction. Two sections were studied: Santo Antônio, in the south-central portion of the basin, and Serra do Inácio, in the southwest. This study is based on a large sample of calcareous nannofossil specimens (N = 37,817), which supported reliable paleoenvironmental and paleoecological interpretations of the establishment of an epicontinental sea during the Aptian/Albian transition. The analysis of the nannofossil assemblages revealed low diversity (25 taxa) and a predominance of opportunistic taxa ( Thoracosphaera , Watznaueria , Nannoconus ). Sub-biozone NC8b was the only zone identified unequivocally, based on the Hayesites albiensis marker. Four ecozones were identified in both study sections, recording periods of stability in nutrient availability and water temperatures. The ecozones that correspond to sub-biozone NC8b are typical of the Aptian/Albian boundary, shifting from warm and oligotrophic conditions in the Aptian to cooler and more eutrophic conditions in the Early Albian. Thoracosphaera blooms were identified by correlating the ecozones at the top of the sections. Three main episodes in the evolution of the Romualdo Formation were identified: periods of instability correlated with the Aptian/Albian boundary, the stabilization of the Albian Sea, and the closure of this epicontinental sea. A number of taxa with Tethyan affinities were recognized ( Hayesites , Nannoconus , Rhagodiscus ), reflecting the influence of both the Tethys Sea and the Central Atlantic Ocean. • First quantitative approach to calcareous nannofossils in the Araripe Basin. • Paleoecological analysis of the Romualdo Formation based on nannoplankton. • Comparative approach between two stratigraphic sections of the Romualdo Formation. • First ecozonal study using calcareous nannofossils for the Albian. • Aptian-Albian boundary recorded at the base of the Romualdo Formation.