Marine sediments from mesophotic reefs as indicators of offshore vortex in the Açu reef (Northeast, Brazil)

Abstract

hallow and deep-water oceanographic influences over shelf-edge environments affect the development of benthic habitats. We investigated the influence of an offshore vortex 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, organic matter, and carbonates in the inter-reef sediments of the newly discovered Açu reef in north-eastern Brazil between 25 m and the shelf edge closer to a recently described vortex. Benthic living Buccella peruviana, Peneroplis carinatus, P. pertussis, and the planktonic Globigerina rubra is directly associated with organically enriched sediments, and carbonate production where offshore vortices are likely to occur. The sedimentation pattern is evidenced by the deposition in one side and erosion in the other side on its forecasted pathway. Therefore, local vortices in thermocline associated with upwelling of cold waters in 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. Here we show that foraminifer-derived signatures at the sediment-water interface could serve as a potential tool to reconstruct paleo environmental and climate changes of habitats close to very dynamic water masses pathways.