Organically enriched sediments and foraminiferal species from the Açu Reef, indicators of upwelling in NE Brazil?

Abstract

Shallow and deep-water oceanographic confluence over shelf-edge environments affects the development of benthic habitats. We investigated these oceanographic influences on a narrow (6 km wide) and shallow (25–80 m water depth) outer shelf with warm waters (27–30 °C) through 84 sediment samples, CTD profiles, and underwater photographs. We analysed benthic foraminiferal content, sedimentary facies, carbonates, and organic matter in the inter-reef sediments of the newly discovered Açu Reef in north-eastern Brazil. Univariate and multivariate statistical analyses on biotic (foraminiferal assemblages) and abiotic data (temperature, salinity, sediments, depth) were carried out. The results highlight similarity with the Caribbean-type reef community. Among 45 species, the spatial distribution of living Buccella peruviana is directly associated with organically enriched sediments underlying areas of canyon heads and the terminal portion of shelf valleys, where temperature and salinity vertical gradients depict deflection patterns on the near-bottom seafloor, evidencing an upwelling of colder waters and nutrients between 25 m and the shelf edge. Furthermore, Quinqueloculina patagonica, which predominates inside the shelf valley, and Quinqueloculina lamarckiana, surrounding the reef field, reveal that variations in currents energy in the Açu Reef and the valley morphology constrain the transport and deposition of organic carbonate-enriched sediments. This is consistent with the high abundance and diversity of living foraminifera in the inner reef environment, including both large-sized foraminiferal species and colder water-related foraminiferal species that are autochthonous, supporting “in situ” carbonate production and deposition. Therefore, local upwelling associated with canyon heads on the Brazilian Equatorial shelf might be responsible for maintaining remains of living coral-algal systems on mesophotic outer shelves such as the Açu Reef. Our study also shows that foraminifer-derived upwelling signatures at the sediment-water interface could serve as a potential tool to reconstruct paleoenvironmental and climate changes.