The Multiproxy Records of Braarudosphaera Acmes in the Campos Basin: Implications for Mid-Oligocene Paleoceanography of the South Atlantic

The Oligocene Epoch (33.9–23.04 Ma) was marked by significant cooling events associated with the opening of the Tasmanian and Drake gateways and declining atmospheric CO₂ concentrations. These climatic changes drove complex oceanographic responses that may have contributed to the formation of enigmatic calcareous nannofossil-rich deposits known as Braarudosphaera chalk. These deposits represent an important regional stratigraphic level in the southeastern Atlantic Ocean and are recognized as a seismic marker in Brazilian sedimentary basins. Similarly, seven Braarudosphaera acmes were previously identified at Walvis Ridge, along the African margin. However, the mechanisms responsible for these acmes remain debated and may involve orbital forcing, water-column stratification, and enhanced monsoonal rainfall. Here, we present a multiproxy study integrating calcareous nannofossils, cyclostratigraphy, bulk carbon and oxygen isotopes (δ¹³C_bulk and δ¹⁸O_bulk), inorganic geochemistry, and Total Organic Carbon (TOC) to investigate the relationship between mid-Oligocene paleoceanographic changes and marine biotic responses. The study is based on 91 samples from a 30-m-thick drill-core section of the Campos Basin, complemented by preliminary TOC data from 25 samples. The studied succession consists of marine mudstones and shales, with TOC contents ranging from 0.07 to 0.17%. The three samples with the highest TOC (2850.10, 2847.43, and 2827.04 m) show low Braarudosphaera recovery; however, seven Braarudosphaera acmes were identified and are associated with increased CaO/Fe₂O₃ ratios and TOC values ​​ranging from 0.07 to 0.11. Three acmes occur within Zone CP18 and four within CP19a. Cyclostratigraphic integration constrains deposition of the studied interval to approximately 30–27 Ma. Bulk isotope data reveal positive excursions between ~30 and ~29.15 Ma, followed by negative excursions extending to ~28.1 Ma. δ¹³C_bulk values range from −0.07 to 1.72‰, whereas δ¹⁸O_bulk values range from −3.99 to −0.24‰. Notably, the δ¹³C_bulk excursions observed in the Campos Basin closely resemble those previously documented at Walvis Ridge, suggesting a significant regional oceanographic connection between the Brazilian and African margins during the mid-Oligocene. [Financial Support Petrobras/ANP Project: Cicloestratigrafia Multiproxy do Eoceno–Mioceno do Atlântico Sul].