IODP Expedition 339 in the Gulf of Cadiz and off West Iberia: decoding the environmental significance of the Mediterranean outflow water and its global influence

摘要

IODP Expedition 339 drilled five sites in the Gulf of Cadiz and two off the westIberian margin (November 2011 to January 2012), and recovered 5.5 km ofsediment cores with an average recovery of 86.4%. The Gulf of Cadiz wastargeted for drilling as a key location for the investigation ofMediterranean outflow water (MOW) through the Gibraltar Gateway and itsinfluence on global circulation and climate. It is also a prime area forunderstanding the effects of tectonic activity on evolution of the GibraltarGateway and on margin sedimentation. We penetrated into the Miocene at twodifferent sites and established a strong signal of MOW in the sedimentaryrecord of the Gulf of Cadiz, following the opening of the Gibraltar Gateway.Preliminary results show the initiation of contourite deposition at 4.2–4.5 Ma,although subsequent research will establish whether this dates the onsetof MOW. The Pliocene succession, penetrated at four sites, shows low bottomcurrent activity linked with a weak MOW. Significant widespreadunconformities, present in all sites but with hiatuses of variable duration,are interpreted as a signal of intensified MOW, coupled with flowconfinement. The Quaternary succession shows a much more pronounced phase ofcontourite drift development, with two periods of MOW intensificationseparated by a widespread unconformity. Following this, the final phase ofdrift evolution established the contourite depositional system (CDS) architecture we see today. There is asignificant climate control on this evolution of MOW and bottom-currentactivity. However, from the closure of the Atlantic–Mediterranean gatewaysin Spain and Morocco just over 6 Ma and the opening of the Gibraltar Gatewayat 5.3 Ma, there has been an even stronger tectonic control on margindevelopment, downslope sediment transport and contourite drift evolution.The Gulf of Cadiz is the world's premier contourite laboratory and thuspresents an ideal testing ground for the contourite paradigm. Further studyof these contourites will allow us to resolve outstanding issues related todepositional processes, drift budgets, and recognition of fossil contouritesin the ancient record on shore. The expedition also verified an enormousquantity and extensive distribution of contourite sands that are clean andwell sorted. These represent a relatively untapped and important explorationtarget for potential oil and gas reservoirs.