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. Expedition 339 recovered 5447 m of core, 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 (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 Gibraltar Gateway and margin sedimentation. Drilling penetrated into the Miocene at two sites in the Gulf of Cadiz and showed a MOW signal in the sedimentary record following the opening of the Gibraltar Gateway. The Pliocene succession, penetrated at four sites, displays characteristics consistent with low bottom-current activity and weak MOW since near the beginning of the Pliocene at approximately 4.5-4.2 Ma and a somewhat more pronounced contourite signal upward through the Pliocene. Significant widespread unconformities found at 3.0-3.2 Ma are interpreted as a signal of intensified MOW. The Quaternary succession was sampled at six sites and displays characteristics consistent with a much more pronounced phase of contourite drift development. There are three distinct periods of high MOW activity separated by two widespread unconformities at 2.2-2.4 Ma and approximately 0.9 Ma related to even higher MOW. Following the approximately 0.9 Ma unconformity, the final phase of drift evolution established the contourite depositional system architecture we see today. There is significant climatic and tectonic control on the evolution of MOW and bottom-current activity. 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. However, from the closure of the Atlantic-Mediterranean gateways in Spain and Morocco around 6 Ma and the opening of the Gibraltar Gateway at 5.3 Ma to the present day, even stronger tectonic control has affected margin development, downslope sediment transport, and contourite drift evolution. Based on the timing of these events recorded in the sedimentary record, we propose that tectonic pulsing in the region, resulting from small movements of the Africa and Eurasia plates and associated deep-seated asthenosphere activity, has played an important role in controlling sedimentation on this margin. 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 more than 4.5 km of contourite cores from six of the sites under direct MOW influence, 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 contourite sands that are clean and well sorted in a number of settings, including those created by local diapiric activity. These sands represent a completely new and important deepwater sand type that, where found deeper in the subsurface, could provide a significant exploration target for potential oil and gas reservoirs. At the deeper water site off southwest Portugal (the Shackleton site), we drilled five separate holes and recovered two complete splice sets of Quaternary hemipelagic sedimentation over the past 1.4 m.y. These will form the basis of detailed shore-based collaboration to establish a high-precision marine reference section of Quaternary climate change for comparison with other marine sections and with established ice-core and terrestrial records.