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

The Romualdo Formation (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 paleoceanographic reconstruction of the region. Two sections were studied: Santo Antônio in the south-central portion of the basin and Serra do Inácio in the southwest. This study is distinguished by its quantitative character based on calcareous nannofossil specimens (N = 37,817), which enabled reliable paleoenvironmental and paleoecological interpretations regarding the development of an epicontinental sea during the Aptian-Albian transition. Analysis of the nannofossil assemblages revealed low diversity in the sections, with 13 species and 12 genera, and high dominance of opportunistic taxa (Thoracosphaera, Watznaueria, and Nannoconus). Biozone NC8b was identified through the marker species Hayesites albiensis, recognized in both sections. Four ecozones were identified in the analyzed sections; in both cases, the ecozones recorded periods of stability in nutrient availability and seawater temperature. The ecozones corresponding to biozone NC8b demonstrate characteristics of the Aptian-Albian boundary, with a shift from the warm and oligotrophic conditions of the Aptian to cooling and eutrophic conditions in the early Albian. Thoracosphaera blooms were identified and correlated. Consequently, distinct episodes of the Romualdo Formation Sea were identified: periods of instability correlated with the Aptian-Albian boundary, periods of stabilization of the Albian Sea and closure conditions of this epicontinental sea, in addition to recognizing taxa with Tethyan affinity (Hayesites, Nannoconus, and Rhagodiscus), demonstrating influence from both the Tethys and Central Atlantic.