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This study reports a new cold-water coral (CWC) province covering ~410 km 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 S of ca. 35.632 g/kg, and seawater density σΘ of 27.31-27.33 kg/m). 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. [ABSTRACT FROM AUTHOR]

The hydrographic properties of bottom waters influencing living cold-water coral (CWC) reefs in the North Atlantic are discussed. Cold-water corals (CWCs) are sessile filter feeders and depend on hydrographic properties providing suspended, and ideally enriching nutrients as well as on the removal of resuspended sediments. Furthermore, living CWC reefs occur in a distinct density (σΘ) window as a result of ambient temperature and salinity properties. The development of a steep density gradient above living CWC reefs seems to be necessary to advect and accumulate nutrients. The density gradient is derived by calculating the difference in water mass density between 10 m intervals resulting in the term of ∆σΘ10m. This density gradient seems to be indicative for living CWC reefs as soon as its value is >0.02 kg/m3. This parameter does not depend on the health status or size of the CWC structure and is valid for large CWC reef structures or just thin living CWC covers.The investigated CWC structures on the southern North Atlantic margin such as Morocco and Mauretania, as well as those in the semi-closed basin of the Gulf of Mexico are composed of living CWC veneers or isolated smaller patches. There, the gradients are mainly driven by changes in temperature. In contrast, the living CWC reefs on the northern North Atlantic margin off Norway which form reef buildups, as well as in the Porcupine Seabight which form large scale biogenic frameworks, while those in the Bay of Biscay (St. Nazaire canyon) just exhibit patchy growth forms. At these sites, the density gradient is mainly driven by changes in salinity. The calculated maximum density gradients of ∆σΘ10m differ in their bathymetrical thickness right above the living CWC reefs. Healthy and pristine reef systems on the Norwegian margin exhibit a range of ~15 m maximum ∆σΘ10m thickness, whereas, the poorly developed structures of Mauretania, Morocco and in the Gulf of Mexico show a thickness of ~50 m.The development of density gradients favour nutrient accumulations. Thick, but less pronounced layers of maximum of ∆σΘ10m release nutrients obviously more rapidly, while in thin, but pronounced layers of maximum of ∆σΘ10m nutrients are more retained. [ABSTRACT FROM AUTHOR]