Carbonate mounds in a mud volcano province off north-west Morocco: Key to processes and controls

This paper presents a new cluster of carbonate mounds discovered in 2002 in the Gulf of Cadiz off Morocco (R/V Belgica 2002) in water depths of 500 to 600 m amidst a field of giant mud volcanoes. Multibeam bathymetry, side scan sonar imagery and 2D seismics are analyzed to present four mound provinces: (1) the Pen Duick Mound Province on the Pen Duick Escarpment, (2) the Renard Mound Province on the Renard Ridge, (3) the Vernadsky Mound Province on the Vernadsky Ridge and the Al Idrisi Mound Province on the gas-blanked sediments above the buried Al Idrisi Ridge. Video imagery and surface samples are described to ground-truth the different mound areas. The paradox is that nearly no live corals are presently being observed at the surface of the mounds, while the mound cores display throughout a high number of reef-forming cold-water coral fragments (scleractinians) in association with numerous associated fauna formerly inhabiting the econiches provided by the coral framework. Environmental and oceanographic conditions during the recent past (glacials/stadials) were probably more favourable for cold-water coral growth. Pore water analyses in on-mound cores at the south-eastern edge of Pen Duick Escarpment give evidence of focused, higher methane fluxes and sulphate reduction rates on mounds than in the surrounding sediments. Cores from several mounds display horizons of strong corrosion and dissolution of the coral fragments. A three-phase model for carbonate mound evolution in these settings is proposed. (1) In a first stage external controls (positive oceanographic and environmental conditions, the presence of an active planktonic food chain, based on a high primary production, and a suitable substrate) are responsible for the initiation of cold-water coral growth. (2) Once the cold-water corals established an initial framework, sedimentation becomes an important factor controlling mound growth: the cold-water corals baffle the sediments. (3) Throughout mound growth, the mound may episodically be affected by diagenetic processes responsible for aragonite dissolution and probably carbonate precipitation.