Résumé
During Integrated Ocean Drilling Program Expedition 339, five sites were drilled in the Gulf of Cadiz and two sites were drilled off the West Iberian margin from November 2011 to January 2012. Total length of recovered core is 5447 m, with an average recovery of 86.4%. The Gulf of Cadiz was targeted for drilling as a key location for the investigation of Mediterranean Outflow Water (MOW) through the Strait of Gibraltar gateway and its influence on global circulation and climate. The gulf is also a prime area for understanding the effects of tectonic activity on evolution of the Strait of Gibraltar gateway and margin sedimentation. Drilling penetrated into the Miocene at two sites in the Gulf of Cadiz, where sedimentary record showed a strong MOW signal following the opening of the Strait of Gibraltar gateway. Preliminary results indicate contourite deposition from 4.2 to 4.5 Ma, although subsequent research will establish whether this deposition dates from the first onset of MOW. The Pliocene succession, penetrated at four sites, displays characteristics consistent with low bottom-current activity linked with weak MOW. Significant widespread unconformities at 3.0-3.2 and 2.2-2.4 Ma are interpreted as a signal of intensified MOW, especially from approximately 2.4 Ma. The Quaternary succession displays characteristics consistent with a much more pronounced phase of contourite drift development, with two distinct periods of increased MOW activity separated by a widespread unconformity at approximately 0.9 Ma related to even higher MOW. Following this unconformity, the final phase of drift evolution established the contourite depositional system architecture we see today. There is significant climate control on this evolution of MOW and bottom-current activity. However, from the closure of the Atlantic-Mediterranean gateways in Spain and Morocco around 6 Ma to the opening of the Strait of Gibraltar gateway at 5.3 Ma, even stronger tectonic control affected margin development, downslope sediment transport, and contourite drift evolution. Based on the timing of events recorded in the sedimentary record, we propose tectonic pulsing in the region linked with small movements of the African and Iberian plates. The Gulf of Cadiz is the world's premier contourite laboratory and thus presents an ideal testing ground for the contourite paradigm. Following recovery of >4.5 km of contourite cores, existing models for contourite deposition are found to be sound. Further study of these models will undoubtedly allow us to resolve outstanding issues of depositional processes, drift budgets, and recognition of fossil contourites in the ancient record onshore. The expedition also verified the presence of a more than expected quantity and extensive distribution of contourite sands that are clean and well sorted. These sands represent a completely new and important exploration target for potential oil and gas reservoirs. Preliminary work has shown a remarkable record of orbital-scale variation in bulk sediment properties of contourites at several of the drift sites and good correlation between all sites. Climate control on contourite sedimentation is clearly significant at this scale; further work will determine the nature of controls at the millennial scale.