An investigation of seamount phosphorites in the Eastern Equatorial Atlantic

Abstract

Phosphorites with P 2O 5 contents up to 33% have been recovered from the eastern Atlantic between 6°N and 9°N. Formed from the replacement of shallow-water reef limestones of Middle Eocene age, the deposits are exposed on the crests of three subsided seamounts, Lighthill, Hirondelle and Carter, at depths of 794–1246, 1161–1176 and 552–1158 m, respectively. Phosphates exposed on the sea floor are associated with high levels of acoustic backscatter recorded using a ship’s echo-sounder and a 10-kHz pinger towed ∼30 m above the bottom. Seabed scintillometer measurements on Carter Seamount near 9°N, 21°W revealed high γ-ray emission where phosphorites are known to crop out from photography and sampling. Uranium contents of the deposits lie in the range 9–46 ppm, values which are lower than those found in continental margin phosphates, but similar to concentrations in Pacific seamount phosphorites. 87Sr/ 86Sr ratios indicate that phosphatisation of the Middle Eocene reef limestones occurred in the Early–Middle Miocene (∼16–23 Ma) on Carter and Hirondelle Seamounts and in the Late Miocene (∼6 Ma) on Lighthill Seamount. Upwelling near the palaeo-equator probably played a vital role in diagenesis by promoting high biological productivity and a large flux of phosphorus to pore waters in the seamount limestones. Phosphorites are probably widespread on other sediment-capped seamounts and fracture-zone ridges in the Equatorial Atlantic.