Congo River signature and deep circulation in the eastern Guinea Basin

Cruises since 1976, have allowed identification of oxygen and silicate anomalies within the Guinea Basin at around 4000 m depth, particularly above the Congo River fan. The EQUALANT cruise in 2000 was an opportunity to revisit this area and to study such anomalies through hydrological and chemical analyses as well as current and turbidity measurements. The main goal was to determine the possible origins, the distribution and the fate of these signals and their spreading along the African continent. The shape of the oxygen, nutrient and turbidity profiles was the first evidence of this phenomenon and an observation of a silicate–oxygen anti-correlation indicates regeneration processes associated with recently deposited particulate material, carried to depth along the Congo channel. This is a special aspect of the morphology of this river. Turbidity events are observed to be dispersed offshore within the ocean and along the slope at around 4000 m depth. While previous studies have shown southward propagation of the Congo signature, this study shows it to occur as far as 7°W–5°N, off Ivory Coast, near the 4000 m isobath, therefore demonstrating a northward propagation in this depth range.