The Eugen Seibold coral mounds offshore western Morocco: oceanographic and bathymetric boundary conditions of a newly discovered cold-water coral province

Abstract

This study reports a new cold-water coral (CWC) province covering ~410 km2 off western Morocco (ca. 31°N) ~40 nautical miles north of the Agadir Canyon system between 678 and 863 m water depth, here named the Eugen Seibold coral mounds. Individual mounds are up to 12 m high with slope angles varying between 3° and 12°. Hydroacoustic data revealed mound axes lengths of 80 to 240 m. Slope angle, mound height, and density of mounds decrease with increasing water depth. The deepest mounds are composed of dead and fragmented Lophelia pertusa branches. Living CWCs, mainly L. pertusa, were sampled with box cores between 678 and 719 m water depth. Conductivity-temperature-depth (CTD) measurements revealed living CWC colonies to occur within the deeper part of the North Atlantic Central Water (NACW; conservative temperature Θ of 9.78–9.94 °C, absolute salinity SA of ca. 35.632 g/kg, and seawater density σΘ of 27.31–27.33 kg/m3). Comparable CWC reefs off Mauritania (17°N–18°N) and on the Renard Ridge (35°N) in the Gulf of Cadiz, the latter consisting only of a dead CWC fabric, are also located in the deeper layer of the NACW slightly above the Mediterranean Outflow Water. The new CWC province, with its thin cover of living corals and much larger accumulations of dead thickets and fragmented coral rubble, was successfully discovered by CTD reconnaissance applying seawater density as a potential indicator of CWC occurrences, followed by hydroacoustic mapping. U-Th isotope systematics for macroscopically altered buried Lophelia material (25 cm sediment depth) yielded absolute ages dating back to the late Holocene at least.