Carbonate Mounds in the Gulf of Cadiz

Mound Clusters

An exploratory cruise of R/V Belgica in 2002 off Larache (Morocco) has led to the discovery of small mounds topping ridges and structural heights (respectively on Pen Duick Escarpment, Renard Ridge, Vernadsky Ridge and Al Idrisi Ridge). These mounds are found amidst 9 giant mud volcanoes: the El Arraiche mud volcano field (Van Rensbergen et al. 2005a). They occur in a setting where focused fluid seepage is observed (Baraza and Ercilla 1996; Pinheiro et al. 2003; Somoza et al. 2003; Van Rooij et al. 2005). Because of its unique setting, the carbonate mound site on the Moroccan margin became rapidly involved in a developing stage of focused multidisciplinary research. In May 2004, a 9m-long Kasten core was recovered with R/V Marion Dufresne on top of one of the largest build-ups – 60 m high. It contained cold-water corals over its whole length, a striking similarity with the Porcupine mound cores (Foubert et al. 2007) but a very strong smell of hydrogen sulphide was reported on deck. The core featured a well-delineated zone of heavily dissolved cold-water corals and a zone of diagenetic carbonate precipitation. Pore water analyses gave evidence of a sharp sulphate-methane transition (SMT) zone at 3.5 m below the mound top. Cruises in summer 2004 and 2005 have confirmed these observations, and have expanded the exploration of this cluster of mounds.