F.J. Hernández-Molina

Contourite processes associated with the Mediterranean Outflow Water after its exit from the Strait of Gibraltar: Global and conceptual implications

We characterize the eastern Gulf of Cadiz, proximal to the Strait of Gibraltar, using a multidisciplinary approach that combines oceanographic, morphosedimentary, and stratigraphic studies. Two terraces (upper and lower) were identified along the middle slope. They are composed of several associated morphologic elements, including two large erosive channels, which allow us to determine a new and more detailed understanding of the Mediterranean Outflow Water (MOW) pathway and its deceleration upon exiting the Strait of Gibraltar. There is evidence for along-slope circulation and additional secondary circulation oblique to the main flow. The present upper core of the MOW flows along the upper terrace and the lower core flows along the lower terrace. However, the lower terrace shows larger and better defined erosive features on the seafloor than does the upper terrace; we attribute this to a denser, deeper, and faster MOW circulation that prevailed during past cold climates. Development of the present features started ca. 3.8–3.9 Ma, but the present morphology was not established until the late Pliocene–early Quaternary (3.2 to older than 2.0 Ma), when the MOW was enhanced, coeval with global cooling, a sea-level fall, and an increase in thermohaline circulation. We propose a direct link between the MOW and the Atlantic Meridional Overturning Circulation and therefore between the MOW and both the Northern Hemisphere and global climate. Our results have enabled a better understanding of a major overflow related to an oceanic gateway, and are of broad interest to geologists, climatologists, oceanographers, and petroleum geologists.

Faro-Albufeira drift complex, northern Gulf of Cadiz

Abstract The northern margin of the Gulf of Cadiz is swept by Mediterranean Outflow Water between about 500 and 1000 m water depth. This warm, saline, thermohaline, bottom current attains velocities in excess of 1 m s-1 through the narrow and relatively shallow Gibraltar gateway, and then descends and slows as it moves towards the north and west around the Iberian margin. It was established in its present form in the latest Miocene, following tectonic re-opening of the Gibraltar gateway, and has since helped to sculpt the slope region in conjunction with downslope processes and diapiric intrusion. The principal area of contourite deposition, up to 600 m in thickness, is the Faro-Albufeira drift complex in a mid-slope setting some 30 km south of Faro. This comprises an elongate low-mounded drift (Faro-Albufeira) and adjacent broad sheeted drifts (Faro and Bartolomeu Dias Planaltos), flanked and partly dissected by deep, erosional, bottom-current channels and buried channels. The seismic character is one of progradational-aggradational depositional units with laterally extensive sub-parallel reflectors, widespread discontinuities and a large-scale cyclicity in seismic facies. The upper 10 m of cored section comprises muddy, silty and sandy contourites of mixed terrigenous and biogenic composition, that show small-scale cyclicity in grain size and associated sedimentary features. Rates of accumulation varied from < 1 to 14.5 cm ka-1 (cores), and 3.5 to 29.5 cm ka-1 (seismics). The large and small-scale cyclicity noted can be related to fluctuation in bottom current velocity related to climate and sea-level changes, although the precise correlation between these events remains uncertain.