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Observations of the Mediterranean outflow—I mixing in the Mediterranean outflow
Abstract
An identification of the two scores in the Mediterranean water outflow was made near the Straits of Gibraltar at about 7°W. It was estimated that the upper and lower cores constituted 95 and 72%, respectively, of the admixture with North Atlantic Central Water, whereas in proceeding around the Gulf of Cadiz towards Cape St Vincent these fractions were reduced to 50 and 38%. The most likely flow paths were determined by establishing the high coherence in the thermohaline properties associated with these cores. An interpretation of a short set of time series data, to account for the vertical periodicity that was observed in the outflow, was regarded as equally relevant to a possible explanation of a process by which the upper core might be formed within the straits.
... At deeper depth, between 500 and 1500 m below sea level, hydrographic features of the Gulf of Cadiz are dominated by the MOW (Fig. 1), a salty, high-density, warm water current composed of the Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW), and exiting the Mediterranean Sea through the Strait of Gibraltar (Ambar and Howe, 1979;Bryden and Stommel, 1984;Bryden et al., 1994). The dense MOW accelerates through the narrow gateway of the Strait of Gibraltar, locally reaching velocities of up to 300 cm/s and gradually decreasing northwestward, near Cape Saint Vincent, to around 15-20 cm s -1 (Ambar and Howe, 1979;Gasser et al., 2017;Sanchez-Leal et al., 2017). ...
... At deeper depth, between 500 and 1500 m below sea level, hydrographic features of the Gulf of Cadiz are dominated by the MOW (Fig. 1), a salty, high-density, warm water current composed of the Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW), and exiting the Mediterranean Sea through the Strait of Gibraltar (Ambar and Howe, 1979;Bryden and Stommel, 1984;Bryden et al., 1994). The dense MOW accelerates through the narrow gateway of the Strait of Gibraltar, locally reaching velocities of up to 300 cm/s and gradually decreasing northwestward, near Cape Saint Vincent, to around 15-20 cm s -1 (Ambar and Howe, 1979;Gasser et al., 2017;Sanchez-Leal et al., 2017). During its north-westward progression, the MOW flows along the mid-continental slope of the Gulf of Cadiz, entrains the overlying fresher ENACW, reducing its salinity and temperature, and flowing within the Atlantic at intermediate depth (Ambar and Howe, 1979;Baringer and Price, 1997; van Aken, 2001;Bashmachnikov et al., 2015;Sanchez-Leal et al., 2017), above the cold and less saline North Atlantic Deep Water (NADW). ...
... The dense MOW accelerates through the narrow gateway of the Strait of Gibraltar, locally reaching velocities of up to 300 cm/s and gradually decreasing northwestward, near Cape Saint Vincent, to around 15-20 cm s -1 (Ambar and Howe, 1979;Gasser et al., 2017;Sanchez-Leal et al., 2017). During its north-westward progression, the MOW flows along the mid-continental slope of the Gulf of Cadiz, entrains the overlying fresher ENACW, reducing its salinity and temperature, and flowing within the Atlantic at intermediate depth (Ambar and Howe, 1979;Baringer and Price, 1997; van Aken, 2001;Bashmachnikov et al., 2015;Sanchez-Leal et al., 2017), above the cold and less saline North Atlantic Deep Water (NADW). The sinking of the MOW in the Gulf of Cadiz seems to affect the surface circulation in the subtropical Atlantic Ocean, inducing an eastward zonal current, thus becoming a driving mechanism in the formation of the AzC (Jia, 2000;Ö zgökmen et al., 2001;Kida et al., 2008;Volkov and Fu, 2010;Comas-Rodríguez et al., 2011;Afanasyev et al., 2012;Carracedo et al., 2015). ...
... La MOWU qui constitue la composante géostrophique de la MOW longe la pente continentale moyenne du golfe de Cadix entre 300 et 800 m en se maintenant en contact avec le fond ( Fig. 1-9 et Fig. 1 -11 ;Madelain, 1970;Zenk, 1975;Ambar andHowe, 1979a, 1979b;Sánchez-Leal et al., 2017 (Ambar and Howe, 1979b;Zenk and Armi, 1990;Johnson et al., 2002;Sánchez-Leal et al., 2017 ...
... Elle est caractérisée par des Madelain, 1970;Ambar and Howe, 1979a;Zenk and Armi, 1990;Bower et al., 1997;Ambar et al., 1999Ambar et al., , 2002Sánchez-Leal et al., 2017). Au niveau du méridien 7°W, les vitesses moyennes de la MOWL sont relativement importantes (46-60 cm.s -1 ) puis elle diminuent graduellement le long de son écoulement dans le Golfe de Cadix jusqu'à atteindre des vitesses inférieures à 36 cm.s -1 au niveau du Cap St Vincent (Ambar and Howe, 1979b;Zenk and Armi, 1990;Bower et al., 1997;Ambar et al., 1999;Johnson et al., 2002;Sánchez-Leal et al., 2017). ...
... The present-day circulation in the Gulf of Cadíz is driven by exchanges through the Strait of Gibraltar where relatively low density (less saline) North Atlantic Water penetrates into the Mediterranean Sea at surface, while denser Mediterranean Water flows into the Atlantic Ocean at depth (e.g., Bryden and Stommel, 1984;Bryden et al., 1994;Baringer and Price, 1999;Naranjo et al., 2015). The MOW which circulates northward following the continental slope (Ambar and Howe, 1979b;Hernández-Molina et al., 2006, 2014a is the principal water mass at intermediate depths in the Gulf of Cadíz. This saline Mediterranean water is located above the North Atlantic Deep Water ( (Baringer and Price, 1999; and is overlaid by the sub-polar variety of North Atlantic Central Water (Voelker et al., 2015a) (Fig. 4-1C). ...
La sédimentation de la pente moyenne du Golfe de Cadix est fortement influencée par l’écoulement d’un courant de fond d’origine méditerranéenne : la Mediterranean Outflow Water (MOW). La compréhension de la relation entre l’écoulement de la MOW, l’Atlantic Meridional Overturning Circulation (AMOC) et les variations climatiques en Atlantique Nord représente un enjeu majeur et complexe. Ces travaux de thèse, en étudiant les caractéristiques sédimentaires des sédiments des drifts contouritiques du Golfe de Cadix, corps sédimentaires propices à l’étude de la variabilité des courants de fond, visent à comprendre la dynamique de la MOW depuis le Pléistocène inférieur. Cette étude s’appuie essentiellement sur des archives sédimentaires prélevées lors de l’Expédition Integrated Ocean Drilling Program 339 « Mediterranean Outflow » (2011-2012) et valorise également des carottes sédimentaires collectées durant la mission océanographique CADISAR (2001). Ce travail a permis d’identifier les sources des minéraux argileux composant les dépôts du système contouritique de Cadix (apports fluviatiles du Guadalquivir et poussières éoliennes nord-africaines) et mis en évidence les rôles de la circulation océanique de surface et l’écoulement de la MOW, dans la distribution des minéraux argileux dans le Golfe de Cadix. Les assemblages ostracodiques, la minéralogie des argiles ainsi que les mesures granulométriques, effectués à haute résolution sur des archives sédimentaires prélevées à différentes bathymétries, ont permis d’affiner le schéma de circulation de la MOW dans le système contouritique de Cadix pour le dernier cycle climatique. Des séries sédimentaires plus anciennes couvrant l’intervalle des Stades Isotopiques Marins (MIS) 12-11 ont montré que le modèle de fonctionnement général établi demeurait valable mais d’intensité moindre, pour l’analogue climatique le plus récent. Enfin les mesures granulométriques et des cortèges argileux réalisées à haute résolution sur une longue série sédimentaire du drift de Faro, mises en âges par une stratigraphie isotopique inédite, ont permis de déterminer trois phases de construction du drift de Faro ainsi que de reconstruire la dynamique de la MOW depuis le Pléistocène inférieur (1,2 Ma).
... Evaporation losses over the Mediterranean Sea generate a horizontal density gradient with respect to the North Atlantic Ocean that is regulated by a two-layer exchange flow along the Strait of Gibraltar (SoG; Tsimplis et al. 2006;Soto-Navarro et al. 2015). Specifically, the mean eastward, warmer, and fresher Atlantic inflow in the surface layer overlies the westward, cooler, and saltier Mediterranean outflow in the intermediate layer (Ambar and Howe 1979;Bryden et al. 1994; Baringer and Price 1997;Hopkins 1999;Tsimplis et al. 2006;Soto-Navarro et al. 2010;García-Lafuente et al. 2011;Soto-Navarro et al. 2015). The resulting Mediterranean Overflow Water (MOW), and especially its salinity, play a key role in the stability of the convective cells in the eastern North Atlantic and hence the meridional overturning circulation (Artale et al. 2006;Koltermann et al. 2011;Rogerson et al. 2012). ...
... MOW is the saltiest and warmest water mass in the intermediate layer of the eastern North Atlantic (36.3-36.5 and 11.78-12.98C, respectively) (Zenk 1970;Ambar and Howe 1979;Baringer 1993;Price et al. 1993;Price and O'Neil Baringer 1994;Baringer and Price 1997;Álvarez et al. 2004;Wu et al. 2007;Carracedo et al. 2016). It reaches neutral buoyancy and geostrophic balance at 78W and 800-1200-m depth in the Gulf of Cádiz (GoC; Fig. 1, Ochoa and Bray 1991;Price et al. 1993;Sánchez-Leal et al. 2017;de Pascual-Collar et al. 2019). ...
... The general circulation and variability of MOW have been explored using both observations and climate models (Ambar and Howe 1979;Bryden et al. 1994; Baringer and Price 1997;Hopkins 1999;Soto-Navarro et al. 2010;García-Lafuente et al. 2011;Soto-Navarro et al. 2015). Furthermore, thermohaline properties and volume transports at the SoG have been FIG 1. Twenty-year mean (1986-2005 salinity maxima (contours) and their corresponding depth levels (shading) between 500 (light blue) and 1500 (purple) m. ...
Accurate representation of the Atlantic-Mediterranean exchange in climate models is important for a reliable simulation of the North Atlantic ocean circulation. We evaluate the performance of ten global climate models in representing Mediterranean Overflow Water (MOW) over the recent period 1986 to 2005 by using various performance metrics. The metrics are based on the representation of the climatological mean state and the spatiotemporal variability of temperature, salinity and volume transports. Based on analyses and observations, we perform a model ranking by calculating absolute, relative and total relative errors ( E j ) over each performance metric and model. The majority of models simulate at least six metrics well. The equilibrium depth of the MOW, the mean Atlantic-Mediterranean exchange flow and the dominant pattern of the MOW are represented reasonably well by most of the models. Of those models considered, MPI-ESM-MR, MPI-ESM-LR, CSIRO-Mk3-6-0 and MRI-CGCM3 provide the best MOW representation ( E j = 0.14, 0.19, 0.19, 0.25, respectively). They are thus likely to be the most suitable choices for studies of MOW dependent processes. However, the models experience salinity, temperature and transport biases and do not represent temporal variability accurately. The implications of our results for future model analysis of the Mediterranean overflow are discussed.
... The present-day circulation in the Gulf of Cadíz is driven by exchanges through the Strait of Gibraltar where relatively low density (less saline) North Atlantic Water penetrates into the Mediterranean Sea at surface, while denser Mediterranean Water flows into the Atlantic Ocean at depth (e.g., Bryden and Stommel, 1984;Bryden et al., 1994;Baringer and Price, 1999;Naranjo et al., 2015). The MOW which circulates northward following the continental slope (Ambar and Howe, 1979;Hernández-Molina et al., 2006, 2014Mulder et al., 2006) is the principal water mass at intermediate depths in the Gulf of Cadíz. This saline Mediterranean water is located above the North Atlantic Deep Water (Baringer and Price, 1999;Hernández-Molina et al., 2014) and is overlaid by the sub-polar variety of North Atlantic Central Water (Voelker et al., 2015b). ...
... MOW L flows between 800 and 1400 m depth alongside the middle slope, and is subdivided into three minor branches due to slope morphology (e.g. Madelain, 1970;Ambar and Howe, 1979;Borenäs et al., 2002). Following the base of the upper slope at depths of 400-700 m (Ambar, 1983;Ambar et al., 2002;Hernández-Molina et al., 2011, 2014, the MOW U flows northward into the Gulf of Cadíz. ...
... Following the base of the upper slope at depths of 400-700 m (Ambar, 1983;Ambar et al., 2002;Hernández-Molina et al., 2011, 2014, the MOW U flows northward into the Gulf of Cadíz. MOW velocity gradually decreases northward from 300 cm.s − 1 in the Strait of Gibraltar to 10-30 cm.s − 1 at the latitude of Cape Saint Vincent (Ambar and Howe, 1979). Bottom current velocity is however highly variable locally due to seafloor irregularities García et al., 2009;Stow et al., 2009;Serra et al., 2010) The Contourite Depositional System of the Gulf of Cadíz results from the complex interaction between bottom water currents of the MOW and the middle slope of the southwest Iberian margin (Madelain, 1970;Faugères et al., 1984;Nelson and Maldonado, 1999;Stow et al., 2002;Mulder et al., 2006;Hernández-Molina et al., 2006;Hanquiez et al., 2007;Marchès et al., 2007). ...
High-resolution records from International Ocean Discovery Program (IODP) sites U1386 and U1387 drilled during IODP Expedition 339 into the Faro Drift, made it possible to assess the impact of intensifications of the upper core (MOWU) of the Mediterranean Outflow Water (MOW) and of changes in sediment supply on the sedimentation in the northern Gulf of Cadíz since the Middle Pleistocene. This work focuses on the comparison of records covering Marine Isotope Stage (MIS) 2–1 and MIS 12–11, in order to investigate the behaviour and circulation regime of the MOWU over two climatic cycles of similar astronomical configurations and their associated deglaciation. The analysis of facies established on the basis of grain-size, XRF core-scanning, and carbonate content revealed contourite beds formed by the MOWU during MIS 11 and MIS 1 and deglaciations (deglaciation V and I). Contourite sequences show that MOWU velocity at the seabed was higher during MIS 2–1 than during MIS 12–11, and that sediment supply was different between these two climatic cycles. While overall low during MIS 12–11, MOWU intensity increased during deglaciationV and MIS 11 and preceded large ice rafted events and cooling in the North Atlantic Ocean. As a major element of the MOW, MOWU strengthening during deglaciation V likely contributed to higher heat and moisture transport towards the high latitudes inducing a slight increase of calving and size of boreal ice sheets. The MOW-derived injection of heat and salt in the North Atlantic Ocean during deglaciation V might have contributed, through reactivation of the upper AMOC, to the switch of the Atlantic thermohaline circulation from a glacial to an interglacial mode.
... The onset of the MOW began with the opening of the Gibraltar gateway, 5.3 Ma, and gradually increased to become a major contributor to the North Atlantic oceanic circulation in the late Pliocene (Hernández-Molina et al., 2014b). Since then, it has had a major influence on the sedimentation of the entire SWIM, generating one of the most extensive contourite depositional systems in the world (Ambar et al., 1976;Ambar & Howe, 1979;Hernández-Molina et al., 2002;Mulder et al., 2003;Hanquiez et al., 2007;Llave et al., 2007a). The velocity of the MOW varies from 2.5 ms −1 at the Strait of Gibraltar, where the gateway narrows the current to a width of approximately 10 km (Baringer & Price, 2002;Mulder et al., 2003;Hernández-Molina et al., 2016a), to <0.2 ms −1 off Cape São Vicente (Johnson et al., 2002;Mulder et al., 2003) (Figs 3 and 4). ...
... While moving northward, the MOW flows above the colder (3 to 8°C) and less saline (34.95 to 35.2‰) NADW (Llave et al., 2006;Hernández-Molina et al., 2014a), a current that flows from the Greenland-Norwegian Sea region southward. The MOW begins to raft above the NADW in correspondence with Cape São Vicente, where it reaches a neutral buoyancy and detaches from the seabed (Ambar & Howe, 1979;Hernández-Molina et al., 2016a). ...
... Distinct channels located in the upper and mid-slope constrain spreading of the MOW and promote the formation of several MOW branches, that travel with velocity, salinity and density values that increase with depth (Sánchez-Leal et al., 2017). The two major MOW pathways that are generally distinguished in the literature are the MOW upper core (MU), warmer and less saline, that flows between 500 m and 800 m, and the deeper MOW lower core (ML), flowing between 800 m and 1500 m ( Fig. 1) (Ambar & Howe, 1979;Cacho et al., 2000;Mulder et al., 2003;Serra et al., 2005). The differentiation between the upper and lower core is most likely directly inherited from the two water masses exiting the Mediterranean Sea: the Western Mediterranean ...
Synchronous interaction between bottom currents and turbidity currents has been reported often in channel-levée systems where the thickness of the turbidity currents exceeds that of the levées. Such interplay between along-slope and down-slope sedimentary processes is one of the mechanisms by which 'mixed turbidite-contourite systems' can originate. However, bottom currents flow over large areas of the seafloor, including continental slopes characterized by deeply incised submarine canyons rather than channel levées. In these cases, a direct interaction between along-slope and down-slope currents is, theoretically, unlikely to take place. In this study, oceanographic, swath bathymetry, multichannel seismic data and sediment cores are used to investigate a 25 km long, 10 km wide and up to 0.5 km thick deep-sea late Quaternary deposit that seats adjacent to the northwest flank of one of the major canyons in the North Atlantic, the São Vicente Canyon, in the Alentejo Basin (south-west Iberian margin). The area receives the influence of a strong bottom current, the Mediterranean Outflow Water (MOW), which has swept the continental slope at different water depth ranges during glacial and interglacial periods. Architectural patterns and sediment characteristics suggest that this sedimentary body, named Marquês de Pombal Drift, is the result of the interaction between the Mediterranean Outflow Water (particularly during cold periods) and turbidity currents flowing along the São Vicente Canyon. Because the canyon is incised significantly deeper (ca 1.5 km) than the thickness of turbidity currents, an additional process, in comparison to earlier models, is needed to allow the interaction with the Mediterranean Outflow Water and transport sediment out of the canyon. In the São Vicente Canyon, and likely in other canyons worldwide, interaction of turbidity currents with contour currents requires intermediate nepheloid layers that export the finer-grained fraction of turbidity currents out of the canyon at the boundary between major water masses.
... Beyond Camarinal Sill, the principal pathway for the MOW is towards the W-SW, along a valley that arises as an extension of the Strait of Gibraltar. In this valley, the MOW becomes a salty wedge about 5 km wide and 150 m thick with time-averaged speeds near 1.5 m s À1 (Ambar and Howe, 1979) and instantaneous values as large as 3 m s À1 (Mulder et al., 2003). During its initial descent, the westward MOW is driven by pressure gradients arising from its excess density relative to the ambient NACW, and its path is constrained by the bottom topography. ...
... During its initial descent, the westward MOW is driven by pressure gradients arising from its excess density relative to the ambient NACW, and its path is constrained by the bottom topography. Eventually the MOW reaches the continental slope and propagates into the Gulf of Cadiz in approximate geostrophic equilibrium, splitting into at least two main branches or cores centered at depths of about 600 m and 1000 m, respectively (Madelain, 1970;Zenk, 1975;Ambar and Howe, 1979;Ochoa and Bray, 1991;Price et al., 1993;Price, 1997a, 1999;Ambar et al., 2002;Borenäs et al., 2002). The upper branch flows along the base of the upper slope (depths between 500 and 800 m) until Cape São Vicente, with average speed about 0.5 m s À1 , temperature 13.7°C and salinity 37.1 g kg À1 (Madelain, 1970;Zenk, 1975;Ambar and Howe, 1979;Ambar et al., 1999;. ...
... Eventually the MOW reaches the continental slope and propagates into the Gulf of Cadiz in approximate geostrophic equilibrium, splitting into at least two main branches or cores centered at depths of about 600 m and 1000 m, respectively (Madelain, 1970;Zenk, 1975;Ambar and Howe, 1979;Ochoa and Bray, 1991;Price et al., 1993;Price, 1997a, 1999;Ambar et al., 2002;Borenäs et al., 2002). The upper branch flows along the base of the upper slope (depths between 500 and 800 m) until Cape São Vicente, with average speed about 0.5 m s À1 , temperature 13.7°C and salinity 37.1 g kg À1 (Madelain, 1970;Zenk, 1975;Ambar and Howe, 1979;Ambar et al., 1999;. The lower branches are located further south -at depths 800-1200 m, with average speed about 0.25 m s À1 , temperature 13.6°C and salinity 37.4 g kg À1 (Madelain, 1970;Zenk, 1975;Zenk and Armi, 1990;Bower et al., 2002). ...
The Mediterranean Water leaves the western end of the Strait of Gibraltar as a bottom wedge of salty and warm waters flowing down the continental slope. The salinity of the onset Mediterranean Outflow Water (MOW) is so high that leads to water much denser (initially in excess of 1.5 kg m⁻³) than the overlying central waters. During much of its initial descent, the MOW retains large salinity anomalies – causing density anomalies that induce its gravity current character – and relatively high westward speeds – causing a substantial Coriolis force over long portions of its course. We use hydrographic data from six cruises (a total of 1176 stations) plus velocity data from two cruises, together with high-resolution bathymetric data, to track the preferential MOW pathways from the Strait of Gibraltar into the western Gulf of Cadiz and to examine the relation of these pathways to the bottom topography. A methodology for tributary systems in drainage basins, modified to account for the Coriolis force, emphasizes the good agreement between the observed trajectories and those expected from a topographically-constrained flow. Both contour avenues and cross-slope channels are important and have complementary roles steering the MOW along the upper and middle continental slope before discharging as a neutrally buoyant flow into the western Gulf of Cadiz. Our results show that the interaction between bottom flow and topography sets the path and final equilibrium depths of the modern MOW. Furthermore, they support the hypothesis that, as a result of the high erosive power of the bottom flow and changes in bottom-water speed, the MOW pathways and mixing rates have changed in the geological past.
... At present, the circulation in the Gulf of Cadiz is mainly driven by the exchange of the warm, high saline, near-bottom Mediterranean Outflow Water (MOW) and less saline, colder north Atlantic water masses (Fig. 1). The north Atlantic water mass comprises the North Atlantic Surface Water (NASW; 0–100 m) and the North Atlantic Central Water (NACW; 100–700 m) (Ambar and Howe, 1979a,b; Baringer and Price, 1999). The MOW water mass represents a mixture of water masses formed in the Mediterranean Sea including LIW and WMDW (Millot et al., 2006; Rogerson et al., 2012 ). ...
... Iberian margin, as a strong bottom current of ~ 1.78 Sv (Ambar, 1983; Ambar and Howe, 1979a,b). On entering the Gulf of Cadiz, MOW mixes rapidly with ambient NACW (Ambar and Howe, 1979a,b). ...
... Iberian margin, as a strong bottom current of ~ 1.78 Sv (Ambar, 1983; Ambar and Howe, 1979a,b). On entering the Gulf of Cadiz, MOW mixes rapidly with ambient NACW (Ambar and Howe, 1979a,b). The buoyancy of MOW being warm and high saline is not in equilibrium with the ambient NACW and it is held upwards by Coriolis force (Baringer and Price, 1997; Schönfeld, 2002a). ...
We studied variations in benthic foraminiferal assemblages and δ 13 C for the last 225 kyr at IODP site U1387 which is currently bathed by upper core of the Mediterranean Outflow Water (MOW). The MOW paleocirculation and sea-floor environment (oxygen level, trophic condition, bottom current strength) have been inferred from faunal composition; species diversity, abundances of selected index species/groups, microhabitat preferences combined with δ 13 C record of the epifaunal Cibicidoides pachyderma. The faunal and isotope records indicate relatively better ventilation at sea-floor and low trophic condition during MIS 1, 5 and 7 possibly due to increased influence of upper MOW in the Gulf of Cadiz. Our multi-proxy record reflects significant and rapid changes during cold (stadial) and warm (interstadial) phases within the interglacials MIS 5 and 7 and at Termination II. The faunal and isotope records reveal strong MOW flow and better ventilated, oligotrophic bottom-water conditions during stadials MIS 5b, 5d, 7b and 7d. The study further demonstrates weakened MOW intensity associated with poor ventilation and increased trophic level at sea-floor during interstadials MIS 5a, 5e, 7a and 7c. MOW flow was relatively sluggish at Termination II, followed by its strengthening at the end of MIS 5e. The chronology of these events suggests that periods of weakened MOW correlate with sapropel layers of the Mediterranean Sea, implying strong coupling between glacial–interglacial climate and MOW circulation in the Gulf of Cadiz.
... The MOW flows to the NW, underneath the ENACW, along the Iberian upper and middle continental slope of the GoC (Lacombe and Lizeray, 1959;Madelain, 1970;Mélières, 1974). Influenced by the rough bathymetry (Sánchez-Leal et al., 2017), 200 km west of the Strait of Gibraltar the MOW is divided into two main branches: the shallowest Mediterranean Upper Core (UC) and the deepest Mediterranean Lower Core (LC) (Madelain, 1970;Ambar and Howe, 1979;Serra et al., 2005Serra et al., , 2010Sánchez-Leal et al., 2017). The UC is divided into plunging branches that deepen to the NW (Madelain, 1970;Mélières, 1974;Ambar and Howe, 1979;Sánchez-Leal et al., 2017). ...
... Influenced by the rough bathymetry (Sánchez-Leal et al., 2017), 200 km west of the Strait of Gibraltar the MOW is divided into two main branches: the shallowest Mediterranean Upper Core (UC) and the deepest Mediterranean Lower Core (LC) (Madelain, 1970;Ambar and Howe, 1979;Serra et al., 2005Serra et al., , 2010Sánchez-Leal et al., 2017). The UC is divided into plunging branches that deepen to the NW (Madelain, 1970;Mélières, 1974;Ambar and Howe, 1979;Sánchez-Leal et al., 2017). The LC is influenced by contourite channels and furrows and this branch experiences small scale overflows which flow downslope (Hernández-Molina et al., 2006;Sánchez-Leal et al., 2017). ...
... This bottom-current activity is the result of water exchange between the Atlantic Ocean and Mediterranean Sea through the Strait of Gibraltar (or Gibraltar Gateway), which involves the overflow of warm saline Mediterranean Outflow Water (MOW) at depth and the surface influx of Atlantic Inflow Water (AIW) (Howe, 1982;Baringer and Price, 1999;Lobo et al., 2000;Ambar et al., 2002;Cabecadas et al., 2002;Garcia-Lafuente et al., 2006;Llave et al., 2007). The Mediterranean Outflow is intensified through the relatively narrow Gibraltar Gateway, reaching velocities up to 300 cm s − 1 (Ambar and Howe, 1979;Mulder et al., 2003) as it cascades over the shallow sill, causing turbulence, meso-scale eddies and seafloor erosion. It then veers towards the northwest as a result of the Coriolis Effect and separates into several distinct strands as it interacts with diapiric-induced seafloor topography (Fig. 2). ...
... It is well known that flow speed, direction and energy of the bottom currents associated with the MOW are affected by variable overflow through the Gibraltar Gateway as a result of climate, sea-level and tectonics, as well as by meso-scale eddies and vertical turbulence in part due to interaction with seafloor topography and the Coriolis effect. On a shorter timescale, velocity fluctuation is caused by interaction with internal tides and waves, and the periodic occurrence of benthic storms (Ambar and Howe, 1979;Farmer and Armi, 1988;Ambar et al., 1999Ambar et al., , 2002Baringer and Price, 1999;Cabecadas et al., 2002;Hernández-Molina et al., 2016). ...
The bi-gradational sequence (C1-C5) is the standard facies model for fine-grained, mixed mud-sand contourites. Drilling in the Gulf of Cadiz during IODP 339 recovered over 4.5 km of contourites with over 1600 distinct contourite sequences, having an average thickness of 3 m (range 0.5–7.5 m). This study documents the past 1.1 My of contourite succession at IODP Site U1389, in which there are a total of 299 full and partial sequences, with a variable thickness of 0.13–10.6 m (mean 2.65 m) and estimated duration of 0.4–32 ky (mean 8 ky).
Two complete bi-gradational sequences have been analysed in detail. Primary sedimentary structures are absent, apart from some bedding-parallel sharp contacts and abrupt omission surfaces. Bioturbation is pervasive throughout, and a distinctive pattern of ichnofacies change is observed through each sequence. Textural trends show reverse to normal bi-gradation through the sequence: mean size ranges from 7 μm to 55 μm, sorting from 1.8 to 2.9 phi, skewness from −0.3 to +0.6, and kurtosis from mesokurtic to very platykurtic. Compositional trends based on mineralogy, inorganic and organic geochemistry vary systematically with mean size. Of the biogenic fraction, the proportion of foraminifera increases with mean size and dominates the coarsest fraction (>150 μm). There is no discernable trend in planktonic/benthic ratio, and the benthic foraminifera are all characteristic of the upper bathyal zone. Between 30 and 60% of the tests are broken. Both terrestrial and marine sources of organic matter are present, with the former more abundant after 1 Ma and the latter dominant from 2 to 1 Ma.
Collectively, these facets of the contourite sequence validate the dual controls exerted by long-term variation in bottom-current velocity and episodic changes in sediment supply (both clastic and biogenic). Of these, bottom current velocity is the more important. Estimates of bottom current speeds, based on the standard sortable silt (SS) proxy, for the two sequences range from approximately 12–24 cm s−1 (with fluctuations of 1–5 cm s−1). However, we would support the case for a refined method that takes into account the very fine sand fraction moved by bottom currents. Where current speed is relatively high then non-deposition/erosion occurred and an omission surface results. Such omission surfaces probably account for between 20% and 35% of missing section in the two studied sequences. Where current speed was relatively low, thick featureless mud is deposited with a dominant hemipelagic sediment supply and slow alongslope drift. Such hybrid contourite-hemipelagite muds are believed to be widespread in the ocean.
... The lowest velocity (about 10 cm s − 1 ) is reached along the South Portuguese continental slope although locally it can reach values up to 50 cm s − 1 as in the Diogo Cão Deep . As the MOW spreads westwards its density progressively decreases by turbulent entrainment of overlying less-salty NACW at depths between 500 and 1500 m, until it approaches neutral buoyancy near 8°W at about 1000 m depth (Ambar and Howe, 1979a;Bower et al., 2002;Serra et al., 2005;Ambar et al., 2008). ...
... Westwards of 6°20′-7°W, the main core of the MOW separates into two major cores and two minor ones with distinct densities and flowing at different depths ( Fig. 1a) (Zenk, 1970;Howe, 1979a, 1979b;Ambar et al., , 2008. The two major cores are the Mediterranean Upper Water (MU) and the Mediterranean Lower Water (ML) and the minor cores are the Shallow Core (SC) and Deep Core (DC) (Madelain, 1970;Zenk, 1970;Ambar and Howe, 1979a;. The MU is warmer and less saline (13-14°C and 35.7-37‰) and flows at depths between 600 and 1000 m depth, centered at 700-800 m depth close to the continental slope of southwest Iberia. ...
The detailed analysis of the new bathymetric data (180.000 km2) collected mainly in the last decade, in the Gulf of Cadiz and SW Iberia Margin allowed the identification and mapping of a wide range of morphologic seafloor features. Based on the joint inspection of the swath bathymetry, seafloor reflectivity and multichannel seismic profiles (adding up to 20.700 km), a detailed characterization of the seafloor shaping processes was carried out. This allowed a classification of the seafloor morphology into twenty two seafloor morphotectonic domains.
A detailed characterization of the processes responsible for the shaping of the seafloor is proposed based on the joint interpretation of the bathymetry, backscatter and multichannel seismic reflection profiles.
The area considered in the Gulf of Cadiz and the SW Iberia comprises part of three Abyssal Plains (Seine, Horseshoe and Tagus abyssal plains) separated by two linear submarine mountains of tectonic compressive origin, the Gorringe Bank and the Coral Patch Seamount. The Seine Abyssal Plain displays several reliefs that correspond to hangingwall thrust folds that resulted from the collision of Africa with Eurasia, whilst the Horseshoe Abyssal Plain displays isolated folds associated with the development of a possibly transpressive plate boundary in Quaternary times. The submarine mountains rise various kilometers from the seafloor and together with the i) accretionary wedge of the Gulf of Cadiz, ii) the Gibraltar Straits, iii) the Horseshoe scarp, and iv) Príncipe de Avis Spur constitute the main submarine boundaries of the Pliocene-Quaternary sedimentary systems of SW Iberia.
The sediment transport encompasses, from source to sink, an interconnected scheme of three major systems made up by submarine canyons, submarine valleys and the Mediterranean Outflow Water system (MOW). These sedimentary systems initiated in the Late Miocene-Early Pliocene associated with the major paleogeographic modifications of SW Iberia, such as, the closure and opening of the Straits of Gibraltar, uplift of part of the south Portuguese Margin, westwards roll-back of the Gulf of Cadiz oceanic slab, and subsidence of the southeast Portuguese continental slope.
On the northeastern part of the Gulf of Cadiz, the MOW system drapes the shallower parts of the accretionary wedge and the recent tectonic features of the still deforming south Portuguese Margin. However, inside the Accretionary wedge domain, several features are present like mud volcanoes, salt domes, crests and troughs, extensional fault collapses and strike-slip faults attesting for important internal tectonic and fluid escape processes activity.
This work revealed the interaction involving several processes of different origins that are responsible for the seafloor shaping, at different time and spatial scales. Particularly obvious is the diachronic interaction of tectonics, fluid injection and halokinesis in the Seine Abyssal Plain and in the accretionary wedge and also mass transport processes, uplift and deep water contourite currents.
The combined inspection of several multichannel seismic profiles and the multibeam bathymetry of the SW Iberia allowed the mapping the major tectonic structures of the area. These are grouped in three classes; the NW-SE to N-S thrusts, the long WNW-ESE strike slip faults, and the accretionary wedge.
The S. Vicente Canyon lies in the transition between the South Portuguese and the Western Portuguese margins. Its head sector starts at the edge of the continental shelf and extends for 120 km (with a mean slopes of 2.3º, locally up to 25º) until it reaches the Horseshoe Abyssal Plain at depths of 5000 meters below sea level. The geomorphology study of the submarine canyon revealed four distinct sectors. The shallower one displays a wide and poorly incised bottom (walls with less than 800m command) with low gradient flanks (<6º). The following sector displays a narrower bottom with steeper (~8º) and higher flanks (>900m), except for the areas affected by mass movement processes that show lower gradient values for the walls. The third sector displays an increase in the thalweg pitch (~14º) and the walls are also steep and highly incised by gullies and display evidences of mass wasting processes. In this sector a major kink of about 60º is present in the trend of the canyon that results from a tectonic control. The deepest sector displays a wide bottom, low gradients for both the flanks (<4º) and the thalweg, and low flanks (~550m). The canyon initiated as a subsided area at the base of Late Miocene in its present day middle sector associated to the activity of the Marquês de Pombal Thrust. However, only in Late Miocene-Pliocene times the clear morphologic and sedimentary features of the canyon are well marked as it propagated upslope along the S. Vicente Canyon Fault. The upslope retreat of the canyon’s head is still an ongoing process at present as evidenced by the pattern defined by tributary channels and gullies around its head.
To the east, the Portimão Canyon displays an overall N-S trend. Its head starts incising the continental shelf at about 100 of water depth and extends for 50 km with a mean slope of 2.7º; it is sub-divided in three sectors. The first sector displays flanks with little command over the thalweg (less than 200m high) with moderate gradients. In the middle sector, the flanks are steeper and the walls are higher (about 300m high), the canyon axis displays sinuosities where the canyon captures the MOW and intersects the Alvares Cabral Moat and erodes the contourite drifts. The deeper sector displays some irregularities on the bottom, very high and steep walls that decrease their size and gradient as the canyon merges with the Portimão Valley at 2500 meters water depth, part of the orthogonal drainage system that originates in the Gibraltar approaches. The Portimão Canyon started to form in its present day middle sector by the convergence of bottom currents caused by localized salt ascent in post base of Miocene times linked with the Portimão Fault. A more accurate dating for these events is difficult to assign, although it was probably during the Lower Pliocene when an older drainage system (now buried under the Faro contourite drift) ceased its activity and the bottom currents started to erode the seafloor generating the present day Portimão Canyon. Afterwards, the canyon migrated upslope and downslope from this area along the Portimão Fault. The Quaternary development of the canyon is not only associated with movement in the Portimão Fault but rather with the overall deformation of the continental margin (vertical movements) and source to sink drainage of sediments. These processes account for the canyon upslope retreat and development of a localized erosion at the meeting point of the canyon mouth and the Portimão Valley (part of the east to west drainage system that extends from the Gibraltar approaches to the Horseshoe Abyssal Plain).
... The onset of the MOW began with the opening of the Gibraltar gateway, 5.3 Ma ago, and gradually increased to become a major contributor to the North Atlantic oceanic circulation in the Late Pliocene (Hernández-Molina et al., 2014b). Since then, it has had a major influence on the sedimentation of the entire SWIM, generating one of the most extensive contourite depositional systems in the world (Ambar et al., 1976;Ambar and Howe, 1979;Hanquiez et al., 2007;Hernández-Molina et al., 2002;Llave et al., 2007a;Mulder et al., 2003). The velocity of the MOW varies from 2.5 ms -1 at the Strait of Gibraltar, where the gateway narrows the current to a width of approximately 10 km (Baringer and Price, 2002;Hernández-Molina et al., 2016a;Mulder et al., 2003) to less than 0.2 ms -1 off Cape São Vicente Mulder et al., 2003). ...
... The velocity of the MOW varies from 2.5 ms -1 at the Strait of Gibraltar, where the gateway narrows the current to a width of approximately 10 km (Baringer and Price, 2002;Hernández-Molina et al., 2016a;Mulder et al., 2003) to less than 0.2 ms -1 off Cape São Vicente Mulder et al., 2003). While moving northward, the MOW warmer and less saline, that flows between 500 m and 800 m, and the deeper MOW lower core (ML), flowing between 800 m and 1500 m (Ambar and Howe, 1979;Cacho et al., 2000;Mulder et al., 2003;Serra et al., 2005). The differentiation between upper and lower core is most likely directly inherited from the two water masses exiting the Mediterranean ...
... Maintaining the sustainable functioning of the global biosphere requires protection of deep-sea ecosystems, particularly because they face major changes related to human and climate-induced impacts [1]. For an effective protection, an improvement in knowledge [8][9][10][11][12] are labelled as: North Atlantic Curren (NAC), Azores Current (AC). Regional bathymetric map extracted from the EMODnet Project (http://www.emodnet.eu/bathymetry, ...
... The second case study refers to chemosynthesis-based habitats and other vulnerab deep-sea habitats (aggregations of sponges, gorgonians, antipatharians, among others) i mud volcanoes (MVs) located in the Moroccan Atlantic margin of the Gulf of Cádiz (GoC [8][9][10][11][12] are labelled as: North Atlantic Current (NAC), Azores Current (AC). Regional bathymetric map extracted from the EMODnet Project (http://www.emodnet.eu/bathymetry, ...
In this work, we integrate five case studies harboring vulnerable deep-sea benthic habitats in different geological settings from mid latitude NE Atlantic Ocean (24–42° N). Data and images of specific deep-sea habitats were acquired with Remoted Operated Vehicle (ROV) sensors (temperature, salinity, potential density, O2, CO2, and CH4). Besides documenting some key vulnerable deep-sea habitats, this study shows that the distribution of some deep-sea coral aggregations (including scleractinians, gorgonians, and antipatharians), deep-sea sponge aggregations and other deep-sea habitats are influenced by water masses’ properties. Our data support that the distribution of scleractinian reefs and aggregations of other deep-sea corals, from subtropical to north Atlantic could be dependent of the latitudinal extents of the Antarctic Intermediate Waters (AAIW) and the Mediterranean Outflow Waters (MOW). Otherwise, the distribution of some vulnerable deep-sea habitats is influenced, at the local scale, by active hydrocarbon seeps (Gulf of Cádiz) and hydrothermal vents (El Hierro, Canary Island). The co-occurrence of deep-sea corals and chemosynthesis-based communities has been identified in methane seeps of the Gulf of Cádiz. Extensive beds of living deep-sea mussels (Bathymodiolus mauritanicus) and other chemosymbiotic bivalves occur closely to deep-sea coral aggregations (e.g., gorgonians, black corals) that colonize methane-derived authigenic carbonates.
... After crossing the Camarinal Sill in the Strait of Gibraltar, at 290 m water depth, this dense MOW cascades down into the Gulf of Cadiz, as an overflow (Legg et al., 2009), accelerating because of its density anomaly and mixes and entrains the overlying fresher eastern North Atlantic central water (ENACW). Because of this mixing, the MOW reduces its salinity and temperature to transform into the MOW that settled within the Atlantic between 500 and 1,500 m ( Figure 1) (Ambar & Howe, 1979;Baringer & Price, 1997;Bashmachnikov et al., 2015;Sanchez-Leal et al., 2017;van Aken, 2001). Most of this mixing and entrainment occurs within the first 100 km along the MOW path after the Strait of Gibraltar and as a result the MOW volume transport increases from 0.8 Sv at the strait to more than 2.7 Sv when the MOW reaches its equilibrium depth in the Atlantic (Baringer & Price, 1997;Legg et al., 2009;Sanchez-Leal et al., 2017). ...
... This lower core has a slightly different salinity than the upper core but a much colder temperature. The speed of the MOW gradually decreases from up to 300 cm s −1 at the Strait of Gibraltar to around 15-20 cm s −1 in Cape Saint Vincent (Ambar & Howe, 1979;Gasser et al., 2017;Price et al., 1993;Sanchez-Leal et al., 2017). As a result, sediments at the seafloor were profoundly affected by these bottom currents to form a huge contourite depositional system along the southern and west Iberian Margin (Faugeres et al., 1984;Hernández-Molina et al., 2003, 2014Llave et al., 2007;Nelson et al., 1993;Stow et al., 1986). ...
To investigate past changes in the Mediterranean Overflow Water (MOW) to the Atlantic, we analyzed the strength of the MOW and benthic δ¹³C along the last 250 kyr at Integrated Ocean Drilling Program (IODP) Site U1389 in the Gulf of Cadiz, near the Strait of Gibraltar. Both the strength of the MOW and the benthic δ¹³C were mainly driven by precession‐controlled fluctuations in the Mediterranean hydrologic budget. Reduced/enhanced Nile discharge and lower/higher Mediterranean annual rainfall at precession maxima/minima resulted in higher/lower MOW strengths at Gibraltar and stronger/weaker Mediterranean overturning circulation. At millennial scale, the higher heat and freshwater loss to the atmosphere during Greenland stadials increased buoyancy loss in the eastern Mediterranean. This enhanced the density gradient with Atlantic water, resulting in a higher MOW velocity in the Gulf of Cadiz. Unlike non‐Heinrich stadials, a lower‐amplitude increase in velocity was seen during Heinrich stadials (HSs), and a significant drop in velocity was recorded in the middle phase. This weak MOW was especially recognized in Termination I and II during HS1 and HS11. These lower velocities at the depth of Site U1389 were triggered by MOW deepening due to the lower densities of Atlantic intermediate water caused by freshwater released from the Laurentide and Eurasian ice sheets. The intrusion of salt and heat at deeper depths in the Atlantic during HSs and its shoaling at the end could have contributed to drive the changes in the Atlantic Meridional Overturning Circulation during Terminations.
... The waters of the GEOTRACES-GA01 section are also influenced by the saline Mediterranean Water (MW), and the relatively fresh Subarctic Intermediate Water (SAIW). MW enters the North Atlantic from the Mediterranean Sea after overflowing the Strait of Gibraltar (Ambar and Howe, 1979;Baringer and Price, 1997). SAIW originates in the western boundary of the subpolar gyre, i.e. the Labrador Current ( Fig. 1) (Arhan, 1990), by mixing between the warm saline waters of the NAC with the cold and fresher LSW (Iselin, 1936;Arhan, 1990;Read, 2000). ...
... The thermohaline properties of LSW were chosen from the thermohaline properties of LSW formed in 2008 (LSW 2008 ; Kieke and Yashayaev, 2015;Loder, 2009, 2017), which, according to the transit times proposed by Yashayaev et al. (2007), would have reached the Irminger and Iceland basins by 2014. The properties of MW were taken from Wüst and Defant (1936) near Cape St. Vicente, where MW has its θ -S characteristics established after overflowing the Strait of Gibraltar (Ambar and Howe, 1979;Baringer and Price, 1997). The thermohaline range of SAIW (4-7 • C and S < 34.9) was represented by two SWTs: SAIW of 6 • C (SAIW 6 ) and SAIW of 4 • C (SAIW 4 ), following the descriptions of Bubnov (1968) and Harvey and Arhan (1988). ...
We present the distribution of water masses along the GEOTRACES-GA01 section
during the GEOVIDE cruise, which crossed the subpolar North Atlantic Ocean
and the Labrador Sea in the summer of 2014. The water mass structure
resulting from an extended optimum multiparameter (eOMP) analysis provides
the framework for interpreting the observed distributions of trace elements
and their isotopes. Central Waters and Subpolar Mode Waters (SPMW) dominated
the upper part of the GEOTRACES-GA01 section. At intermediate depths, the
dominant water mass was Labrador Sea Water, while the deep parts of the
section were filled by Iceland–Scotland Overflow Water (ISOW) and North-East
Atlantic Deep Water. We also evaluate the water mass volume transports across
the 2014 OVIDE line (Portugal to Greenland section) by combining the water
mass fractions resulting from the eOMP analysis with the absolute geostrophic
velocity field estimated through a box inverse model. This allowed us to
assess the relative contribution of each water mass to the transport across
the section. Finally, we discuss the changes in the distribution and
transport of water masses between the 2014 OVIDE line and the 2002–2010 mean
state. At the upper and intermediate water levels, colder end-members of the
water masses replaced the warmer ones in 2014 with respect to 2002–2010, in
agreement with the long-term cooling of the North Atlantic Subpolar Gyre that
started in the mid-2000s. Below 2000 dbar, ISOW increased its contribution in
2014 with respect to 2002–2010, with the increase being consistent with
other estimates of ISOW transports along 58–59° N. We also
observed an increase in SPMW in the East Greenland Irminger Current in 2014
with respect to 2002–2010, which supports the recent deep convection events
in the Irminger Sea. From the assessment of the relative water mass
contribution to the Atlantic Meridional Overturning Circulation (AMOC) across
the OVIDE line, we conclude that the larger AMOC intensity in 2014 compared
to the 2002–2010 mean was related to both the increase in the northward
transport of Central Waters in the AMOC upper limb and to the increase in the
southward flow of Irminger Basin SPMW and ISOW in the AMOC lower limb.
... A l'est, en bordure de la marge européenne, un courant circule en direction du Nord (Shelf Edge Current, SEC ; Swallow et al., 1977;Ambar and Howe, 1979;Reid, 1994;Paillet & Mercier, 1997;Mazé et al., 1997). Il remonte le long du Portugal puis contourne le nord de 31 l'Espagne pour atteindre le Golfe de Gascogne. ...
... Ceci conduisit Pollard & Pu (1985) (Zenk, 1975a;Ambar and Howe, 1979;Howe, 1982;Coste et al., 1988). La MSW remonte préférentiellement le long de la marge européenne à des profondeurs comprises entre 700 et 1500m (27,3 < σ < 27,9) avec un noyau centré sur 1200m (σ ≈ 27,7) . ...
L’objectif de cette thèse est d’apporter de nouvelles contraintes sur l’hydrologie de l’Atlantique Nord-Est et de la Méditerranée occidentale depuis la dernière période glaciaire à partir de l’analyse de la composition isotopique du Nd (εNd) dans des échantillons d’eau de mer ainsi que des coraux profonds et des foraminifères, prélevés dans des carottes sédimentaires marines. Les changements de l’hydrologie des masses d’eau intermédiaire (LIW, MSW, AAIW et masses d’eau intermédiaire des gyres subtropical et subpolaire) ont été plus particulièrement étudiés car leur rôle sur les transferts de sels en Atlantique Nord et in fine sur l’AMOC est actuellement mal contraint. Ce travail a été mené au cours des périodes de changements hydrologiques majeurs et abrupts de l’océan qui se sont produits lors des variations climatiques rapides de la dernière période glaciaire (événements d’Heinrich et cycles de Dansgaard-Oeschger) et lors du dépôt du Sapropel S1 en Méditerranée orientale. Dans un premier volet, nous avons amélioré la couverture spatiale des valeurs d’εNd des masses d’eau de l’Atlantique Nord-Est et de la mer d’Alboran, préalable indispensable pour restituer l’hydrologie passée de ces régions avec le traceur εNd. Nous avons ensuite mis en évidence un changement majeur du schéma de circulation de la Méditerranée occidentale durant la période de dépôt du sapropel S1, marquée par une forte réduction des masses d’eau de la Méditerranée orientale (LIW) au sud de la Sardaigne au profit de celles provenant du Golfe du Lion (WIW). Ce changement hydrologique ainsi que ceux qui s’opèrent en Méditerranée depuis la dernière période glaciaire ne sont pas associés à de fortes modifications des valeurs d’εNd de la LIW de la mer d’Alboran et de la mer des Baléares, suggérant une stabilité de la signature isotopique en Nd de la MOW au cours du temps. Ceci a permis, à partir d’un enregistrement d’εNd obtenus sur des coraux profonds du Golfe de Cadix, de mettre en évidence une contribution plus importante de l’AAIW plus radiogénique et donc une pénétration plus marquée en Atlantique Nord de cette masse d’eau lors des périodes de fortes réductions de l’AMOC, liées à la déstabilisation des calottes de glace de l’Hémisphère Nord.
... The lowest velocity (about 10 cm s − 1 ) is reached along the South Portuguese continental slope although locally it can reach values up to 50 cm s − 1 as in the Diogo Cão Deep (Ambar and Howe, 1979b). As the MOW spreads westwards its density progressively decreases by turbulent entrainment of overlying less-salty NACW at depths between 500 and 1500 m, until it approaches neutral buoyancy near 8°W at about 1000 m depth (Ambar and Howe, 1979a;Ambar et al., 2002;Bower et al., 2002;Serra et al., 2005;Ambar et al., 2008). ...
... Westwards of 6°20′-7°W, the main core of the MOW separates into two major cores and two minor ones with distinct densities and flowing at different depths ( Fig. 1a) (Zenk, 1970;Howe, 1979a, 1979b;Ambar et al., 2002Ambar et al., , 2008. The two major cores are the Mediterranean Upper Water (MU) and the Mediterranean Lower Water (ML) and the minor cores are the Shallow Core (SC) and Deep Core (DC) (Madelain, 1970;Zenk, 1970;Ambar and Howe, 1979a;Ambar et al., 2002;2008). The MU is warmer and less saline (13-14°C and 35.7-37‰) and flows at depths between 600 and 1000 m depth, centered at 700-800 m depth close to the continental slope of southwest Iberia. ...
... Intermediate depths (500-1400 m) along the southern and western Iberian margin and within the NE Atlantic are characterized by MOW [41,48], which is sometimes also called (Atlantic) Mediterranean Water. The MOW is formed east of the Strait of Gibraltar (main sill depth of 280 m) by mixing the warm and saline subsurface outflow from the Mediterranean Sea with the surrounding North Atlantic water masses, whereby ENCAW has the highest contribution, but Antarctic Intermediate Water and upper Northeast Atlantic Deep Water also contribute [49,50]. The varying mixing of the water masses results in the formation of two MOW cores, an upper core between 500 and 800 m and a more saline, denser lower core between 1000 and 1400 m that spread along pathways in the Gulf of Cadiz ( Figure 1b) and into the NE Atlantic [48,51,52]. ...
In order to better understand interglacial climate variability within the 41 kyr world, we produced high-resolution climate records for interglacial Marine Isotope Stage (MIS) 47 (1424–1452 ka) at IODP Site U1387 (36°48′ N, 7°43′ W) on the southern Portuguese margin. Using benthic and planktonic foraminifera stable isotope records, Uk’37 sea-surface temperature (SST), and plankton assemblage data we investigated Mediterranean Outflow Water (MOW) and surface water conditions. The MOW-level records indicate a poorly ventilated and sluggish bottom current during the MIS 48/ MIS 47 transition in association with the insolation maximum, whereas a well-ventilated MOW formed a contourite layer during the second insolation maximum. The benthic δ18O record shows a fairly abrupt change during the deglaciation of MIS 48, while the surface waters experienced a terminal stadial event that was associated with initial cooling and freshening followed by stepwise warming until interglacial SST was reached at 1450 ka. Interglacial conditions with SST of 24 °C or higher persisted until 1427 ka, although warm SST prevailed into MIS 46. The persistent and prolonged warmth is attributed to a northward expansion of the subtropical gyre during MIS 47 as reflected by the dominance of subtropical-tropical planktonic foraminifera species and the presence of warm water coccolithophores taxa.
... It is located along the main path of the MOW, which is the result of mixing of the Mediterranean outflow with East North Atlantic Central Water (ENACW) in the Gibraltar Strait. The higher density of the Mediterranean outflow today compared with the ENACW forced the MOW to rapidly descends into the Atlantic to reach its equilibrium depth between 500 and 1500 m, above the North Atlantic Deep Water (NADW) (Ambar and Howe, 1979;Sanchez-Leal et al., 2017) (Fig. 1). The fast flow of the MOW along the southern margin of Iberia interacts with bottom sediments to form the Gulf of Cadiz contourite system (Hernandez-Molina et al., 2016;Llave et al., 2001;Stow et al., 2013b). ...
The study of planktic and benthic δ¹⁸O at site U1389 in the Gulf of Cadiz allowed us to reconstruct climate variability during the penultimate glacial period at an unprecedented millennial scale resolution. Hereby, a sequence of interstadial-stadial episodes similar to the Dansgaard-Oeschger events recorded during Marine Isotope Stage (MIS)3, were recognized in MIS6. After a detailed correlation with millennial-scale variability recorded along the Iberian margin and the North Atlantic we were able to link the millennial changes next to the Iberian Peninsula with Antarctic climate variability due to the interhemispheric seesaw response of the Atlantic circulation. The straight coupling at site U1389 of a) the planktic δ¹⁸O that reflects climate change in the Atlantic with b) the benthic δ¹⁸O, which records the temperature and oxygen isotope composition of the mixture of Mediterranean Outflow and Atlantic intermediate water, indicates that millennial climate variability also had a strong impact on the eastern Mediterranean. The detailed analysis of the benthic δ¹³C and the fine sand content in the sediments from site U1389 led us to recognize the response of the Mediterranean overflow water (MOW) to millennial scale changes in Mediterranean overturning circulation during the penultimate glaciation. Both Mediterranean Overturning Circulation and MOW strength increased at times of cool and arid climates and weakened during warm and more humid episodes, similar to what has been described for MIS3. The tuning of MOW weakening events occurring at times of precession minima with eastern Mediterranean sapropels and enhanced Asian monsoon allowed the elaboration of a new chronology for MIS6. It was compared with the Epica Dome C time scale, which was also based on the tuning of atmospheric δ¹⁸O with Asian Monsoons speleothem records.
Mediterranean overturning during MIS6 was also strongly affected by freshwater perturbations occurring at times of insolation maxima, leading to the formation of the “glacial sapropel” S6. Two major drops in seawater δ¹⁸O centered at 175 and 150 ky were observed near the entrance of the Mediterranean that were related to meltwater released from the ice sheets, coinciding with periods of insolation maxima. The entry of this meltwater anomaly through the Strait of Gibraltar is clearly registered in the western and eastern Mediterranean, especially at the onset of sapropel S6, suggesting that it was a major component of the freshwater that initiated the buoyancy gain preceding deep water stagnation.
... On both the Atlantic and Western Mediterranean sides, the water column is composed of low-salinity Atlantic surface waters and dense (highly saline) Mediterranean deep waters. These masses interact together, giving rise to a specific hydrodynamic regime in the area (Ambar & Howe, 1979). While the AS is characterised by large anticyclonic gyres, some important rivers, including the Guadalquivir, discharge into the coastal areas of the GoC. ...
The Gulf of Cadiz (GoC) in the Atlantic Ocean and the Alboran Sea (AS) in the Mediterranean Sea are part of an oceanographic complex system of extreme importance in the exchange of waters between the Atlantic and the Mediterranean through the Strait of Gibraltar. Besides its significant role as ecological transition system, it is unknown whether regional and local environmental drivers affect similarly species inhabiting these two adjacent and connected ecosystems. This study analyses the spatial shifts of representative demersal species and their response to the environmental and demographic drivers in these two regions using information from two trawl surveys carried out in the GoC and in the AS from 1994 to 2015. Species distribution trends were observed in both basins. However, they were more evident for GoC, where six out of eleven species presented temporal trends, than in AS, where only two species displayed significant distribution changes. These species showed geographic displacements in both basins towards Strait of Gibraltar. Also, a high percentage of species showed significant differences in mean depth distribution on both sides of Strait of Gibraltar. Our study shows a general heterogeneity and independence in the drivers influencing species distributions at both areas. GoC showed a high influence of both large and regional scale climate influence affecting several species, while in AS the diversity of drivers affecting was higher with species density being the most common driver. This study provides scientific basis to improve ecosystem-based management measures in these important transitional ecosystems.
... This is valid for the Portimão canyon (e.g. Ambar and Howe, 1979), where the present oceanic model simulated slight cross-shore deflections of the bottom currents, with water flowing onshore under upwelling conditions. Previous studies have proposed that the canyon alters the westward bottom currents (Ambar et al., 2002;Marchès et al., 2007) by inducing an upstream (east) detachment of the flow from the bottom and the decrease of its velocity (Peliz et al., 2009). ...
Deep-water shrimps remain some of the less understood decapod crustacean taxa, despite the high commercial and ecological importance. The lack of knowledge on adults and larval stages of many species results in an incomplete view of the biology, distribution, dispersal and connectivity, essential knowledge for conservation and stock management. In zooplankton samples, larvae of deep-water shrimps are usually found in low abundances. Here, we address the insufficiently studied topics of distribution and transport of deep-water shrimp larvae, providing information on potential pathways and mechanisms of dispersal. The work focuses on southern Portuguese waters, giving important indications for deep-water shrimp species worldwide. Physical-ecological modelling was used to examine the dispersal of larvae of the deep-water rose shrimp, Parapenaeus longirostris, that were found distributed over shelf waters in the study region. In situ spatial abundances were examined, and ontogenetic vertical distributions and sites of larval release were included in the models. The circulation in the vicinity of submarine canyons and deep moats promoted relatively shallower positions of simulated particles. Passive drifts were insufficient for larvae to reach the uppermost water layers, while the vertical distribution adopted, in the upper or lower water layers, was highly important, dictating the direction of the transport and the retention over shelf waters. Slope or shelf larval releases influenced the dispersal patterns and the concentration of particles over productive waters, suggesting that the onshore movement of females during spawning may benefit the larval retention over shelf waters.
... The pattern of oceanographic circulation in the GoC is controlled by the exchange of water masses through the Strait of Gibraltar, with the surficial North Atlantic Central Water flowing into the Alboran Sea and the deeper Mediterranean Outflow Water (MOW hereinafter) flowing out to the Atlantic Ocean (Lacombe and Lizeray, 1959;Ochoa and Bray, 1991;Sánchez-Leal et al., 2017) (Fig. 1A). The MOW, which shows high salinity (38.45 psμ) and temperature (12.9 • C), sweeps along the southwestern Iberian margin as a seafloor bottom current between 300 m and 1200 m water depth, flowing to the north and west due to the Coriolis deflection, and splitting into two main branches, the Mediterranean Upper Core and the Mediterranean Lower Core (Madelain, 1970;Ambar and Howe, 1979;Sánchez-Leal et al., 2017). The present study area is under the influence of the Mediterranean Upper Core (Fig. 1A), with bottom currents reaching a maximum speed of 0.3-0.5 m s − 1 along the contourite channels, in contrast to the minimum speeds of between 0.01 and 0.1 m s − 1 observed between the diapiric ridges (Díaz-del-Río et al., 2014; Sánchez-Leal et al., 2017). ...
Highlights
Gazul MV presents a high geomorphological variability and diversity of bottom types.
Six main epibenthic assemblages have been identified conforming habitats.
Hard bottoms of the MV edifice contain the most biodiverse assemblages.
Cold-water coral communities are dominated by Madrepora oculata.
Gazul MV harbors different vulnerable marine ecosystem elements and indicators.
... The exchange of water masses through the Strait of Gibraltar, with a surficial Atlantic Inflow Water current towards the Alborán Sea and a deep Mediterranean Outflow Water current towards the Atlantic Ocean, represents the main oceanographic context of this area of the GoC (Lacombe and Lizeray, 1959;Ochoa and Bray, 1991;Nelson et al., 1999;Sánchez-Leal et al., 2017). The Mediterranean Outflow Water current flows in a north-west direction and divides into two main cores: a Mediterranean Upper Water and a Mediterranean Lower Water core (Madelain, 1970;Ambar and Howe, 1979). The shallowest part of the Shallow Field of Fluid Expulsion (less than 300 m depth) is mainly exposed to the North Atlantic Central Water, with a complex circulation and moderate salinity and temperature (Sánchez-Leal et al., 2017). ...
The Gulf of Cádiz represents an area of extensive seepage of the south-western Iberian Peninsula that is located between two continents and basins, where a high geomorphologic, sedimentary and environmental complexity occurs. In the present study, epibenthic communities were investigated in a mud volcano field containing four mud volcanoes (Gazul, Anastasya, Pipoca and Tarsis), one diapir/mud volcano complex (Chica) and several diapirs that are all located in the upper and middle slope of the Spanish margin of the Gulf of Cádiz (300–700 m depth). Faunistic samples were collected in 2010 and 2011 using a beam-trawl in different sectors of the mud volcano field, including fluid venting submarine structures and their adjacent seabeds. Environmental variables from the sediment and water column as well as the commercial bottom trawling activity were also measured in the same sampled areas for studying relationships with the fauna. A total of 325 taxa were found in the samples, including sponges, molluscs, cnidarians and decapods the most diverse groups. The epibenthic communities displayed differences between sectors that were related to depth, sediment features, characteristics of the water masses and the trawling activity. Six different assemblages were found including deep-sea sponge aggregations and cold-water coral banks in Gazul, echinoid and solitary coral aggregations in Gazul, sea-pen communities that were sometimes intermixed with sponge and gorgonian aggregations in Tarsis, Chica and Pipoca, and low density sea-pen communities with components of chemosynthesis-based communities in Anastasya. Species richness was higher in Gazul, Pipoca and Chica due to the presence of complex habitats (e.g. cold-water coral banks, sponge aggregations) and low in Anastasya due to the scarce epifauna, high trawling activity and low near-bottom current speed. Typical cold seep chemosymbiotic bivalves of the Gulf of Cádiz were not detected in beam-trawl samples due to their infaunal habit. The studied mud volcano field was declared a Site of Community Importance due to the presence of habitats and species with a conservation status, but several commercial species are exploited by bottom trawling. Therefore, a sustainable management of this mud volcano field is still needed according to current EU directives for combining fisheries and habitat conservation (Habitats Directive of the European Union, Council Directive - 1992/43/EEC; Marine Strategy Framework, Council Directive - 2008/56/EC).
... Traditionally, only two intermediate water masses are considered in the GoC (Ambar and Howe, 1979a;Baringer and Price, 1997;Van Aken, 2000): the Eastern North Atlantic Central Water (ENACW) and the Mediterranean Water (MW) (Fig. 2). The ENACW is formed by strong evaporation first and further winter cooling later along the Azores Front (Pollard and Pu, 1985;Rios et al., 1992). ...
Global circulation of intermediate water masses has been
extensively studied; however, its regional and local circulation along
continental margins and variability and implications on sea floor
morphologies are still not well known. In this study the intermediate water
mass variability in the Gulf of Cádiz (GoC) and adjacent areas has been
analysed and its implications discussed. Remarkable seasonal variations of
the Antarctic Intermediate Water (AAIW) and the Subarctic Intermediate Water
(SAIW) are determined. During autumn a greater presence of the AAIW seems to
be related to a reduction in the presence of SAIW and Eastern North Atlantic
Central Water (ENACW). This interaction also affects the Mediterranean Water
(MW), which is pushed by the AAIW toward the upper continental slope. In the
rest of the seasons, the SAIW is the predominant water mass reducing the
presence of the AAIW. This seasonal variability for the predominance of
these intermediate water masses is explained in terms of the concatenation
of several wind-driven processes acting during the different seasons. Our
finding is important for a better understanding of regional intermediate
water mass variability with implications in the Atlantic Meridional
Overturning Circulation (AMOC), but further research is needed in order to
decode their changes during the geological past and their role, especially
related to the AAIW, in controlling both the morphology and the
sedimentation along the continental slopes.
... On both the Atlantic and Western Mediterranean sides, the water column is composed of low-salinity Atlantic surface waters and dense (highly saline) Mediterranean deep waters. These masses interact together, giving rise to a specific hydrodynamic regime in the area (Ambar & Howe, 1979). While the AS is characterised by large anticyclonic gyres, some important rivers, including the Guadalquivir, discharge into the coastal areas of the GoC. ...
... Fluctuations in the physical characteristics and volume of the MOW result from the interannual to decadal variability in WMDW and LIW formation [17,18]. As the MOW enters the Atlantic through the Strait of Gibraltar (main sill depth of 280 m) the dense Mediterranean water entrains fresher North Atlantic Central Water (NACW) increasing the transport volume by a factor of 3, decreasing the density, and turning from a density driven plume into an intermediate depth jetstream, the Mediterranean undercurrent [19,20]. The downstream modification affects the MOW's hydrodynamic and energetic behavior and thereby the lateral spreading [20]. ...
The presence of contourite drifts in the southern Gulf of Cadiz (GoC) along the Moroccan margin raises questions about the (re)circulation of Mediterranean Outflow Water (MOW) in the GoC and the origin of the currents depositing them. Here, we compare two cores representative of Iberian and Moroccan contourite drifts, covering the last 22 kyr. Although the whole sequence is contouritic in character, it reflects the interaction of distinctive silty-contourite facies (high flow velocity periods) imbedded in muddy-contourite facies (low flow velocity periods). Evidence from benthic foraminifera δ¹³C, sortable silt grain-size, oceanographic CTD profiles and numerical simulations, indicate the Mediterranean water mass as the source of the southern contourite deposits. Our data, therefore, suggests an additional branch of upper-MOW veering southwards off the Straits of Gibraltar along the Moroccan margin. During MIS-(Marine Isotope Stage) 2, upper-MOW was a sluggish current while in the Holocene upper-MOW dominated as a fast, semi-steady flow. Throughout the deglaciation, silty contourites associated with higher flow speeds were deposited in the northern and southern GoC during cold events such as Heinrich Stadial 1 (HS1) and the Younger Dryas, forced by global millennial-scale climate variability. Millennial variability also appears to drive the deposition of silty-contourites in the Holocene. We estimated an average duration of 1 ka for the process of depositing a fast contourite unit. The case of silty-contourite I6 (within HS1) allows us to illustrate with extremely high resolution a “rapid” sequential change in circulation, with gradual slow-down of dense Mediterranean water while surface was freshening (HS1), provoking injection of high-salinity intermediate waters (via contour-currents) into the GoC, and hence the North Atlantic. The subsequent brief collapse of dense water formation in the Mediterranean Sea triggered a major increase in sea surface temperatures (10 °C/ka) in the GoC, developing into the next interstadial (Bølling/Allerød). The impact of Mediterranean intermediate waters is manifested here by triggering a substantial rearrangement of intermediate and deep circulation in the North Atlantic, which would have further impacted the Atlantic Meridional Overturning Circulation (AMOC).
... Mortality is also high from the southern release zones, consistent with the regions of dispersion found by Arístegui et al. (2009), and is due to the consistent mesoscale activity to the southwest of Lisbon in zones 19, 20, 21, and 24 (e.g., Peliz et al., 2004;Arístegui et al., 2009) that is observed in the ROMS model, which sweeps larvae offshore. The release zone offshelf of the Algarve (zone 24) consistently shows the highest mortality, which may also be due to the influence of the Atlantic Inflow that directs water in the top 300 m to the southeast towards the Strait of Gilbraltar (Stevenson, 1977;Ambar and Howe, 1979;Nelson et al., 1999) and offshore of the 1000 m isobath. Larvae transported to the open ocean are far less likely to encounter sufficient food resources to enable their survival untill metamorphosis (Parrish et al., 1981;Bakun and Parrish, 1982;Oliveira and Stratoudakis, 2008). ...
The European sardine (Sardina pilchardus) is the most important small pelagic fishery of the Western Iberia Upwelling Ecosystem (WIUE). Recently, recruitment of this species has declined due to changing environmental conditions. Furthermore, controversies exist regarding its population structure with barriers thought to exist between the Atlantic-Iberian Peninsula, Northern Africa, and the Mediterranean. Few studies have investigated the transport and dispersal of sardine eggs and larvae off Iberia and the subsequent impact on larval recruitment variability. Here, we examine these issues using a Regional Ocean Modeling System climatology (1989–2008) coupled to the Lagrangian transport model, Ichthyop. Using biological parameters from the literature, we conduct simulations that investigate the effects of spawning patchiness, diel vertical migration behaviors, and egg buoyancy on the transport and recruitment of virtual sardine ichthyoplankton on the continental shelf. We find that release area, release depth, and month of release all significantly affect recruitment. Patchiness has no effect and diel vertical migration causes slightly lower recruitment. Egg buoyancy effects are significant and act similarly to depth of release. As with other studies, we find that recruitment peaks vary by latitude, explained here by the seasonal variability of offshore transport. We find weak, continuous alongshore transport between release areas, though a large proportion of simulated ichthyoplankton transport north to the Cantabrian coast (up to 27%). We also show low level transport into Morocco (up to 1%) and the Mediterranean (up to 8%). The high proportion of local retention and low but consistent alongshore transport supports the idea of a series of metapopulations along this coast.
... The MOW is composed of a mixture of water masses sourced from the Mediterranean Basin (Millot, 2009). It accelerates through the narrow Strait of Gibraltarlocally reaching velocities up to 300 cm sec À1 (Ambar & Howe, 1979;Mulder et al., 2003) within the Strait. It forms a turbulent density-driven flow before veering towards the north-west along the mid-continental slope of the Gulf of Cadiz in multiple distinct cores of differing properties ( Fig. 1). ...
This work presents a detailed study of CONTOURIBER and Integrated Ocean Drilling Program 339 sediment data targeting sand-rich contourites in the Eastern Gulf of Cadiz. All of the collected sediments are interpreted as contourites (deposited or reworked by bottom currents) on the basis of oceanographic setting, seismic and morphometric features and facies characteristics. A variety of sandy and associated facies are found across the study area including: (i) bioturbated muddy contourites; (ii) mottled silty contourites; (iii) very fine mottled and fine-grained bioturbated sandy contourites; (iv) massive and laminated sandy contourites; and (v) coarse sandy/gravel contourites. The thickest sands occur within contourite channels and there is a marked reduction in sand content laterally away from channels. Complementary to the facies descriptions, grain-size analysis of 675 samples reveal distinctive trends in textural properties linked to depositional processes under the action of bottom currents. The finest muddy contourites (<20 μm) show normal grain-size distributions, poor to very poor sorting, and zero or low skewness. These are deposited by settling from weak bottom currents with a fine suspension load. Muddy to fine sandy contourites (20 to 200 μm) trend towards better sorting and initially finer and then coarser skew. These are typical depositional trends for contourites. As current velocity and carrying capacity increase, more of the finest fraction remains in suspension and bedload transport becomes more important. Clean sandy contourites (>200 μm) are better sorted. They result from the action of dominant bedload transport and winnowing at high current speeds. The results highlight the importance of bottom current velocity, sediment supply and bioturbational mixing in controlling contourite facies. Despite growing interest in their hydrocarbon exploration potential, contourite sands have remained poorly understood. This research therefore has important implications for developing current understanding of these deposits and aiding the correct interpretation of deep marine sands and depositional processes.
... Upon exiting the Strait of Gibraltar, MOW follows the shelf down slope through the Gulf of Cadiz and flows northwestward settling as an intermediate contour current in several layers within the North Atlantic. Its salinity from 36.5 to 37.5 and temperature from 10.5 to 14.0°C (Baringer & Price, 1997;Borenäs et al., 2002;Cabeçadas et al., 2002) are observed between 300 and 1,400 m depth (Ambar & Howe, 1979). Note that the temperature signature of MOW is less prominent than its salinity signature in the Gulf of Cadiz (Figure 1; Ambar et al., 2002;Borenäs et al., 2002;Cabeçadas et al., 2002). ...
The state of Atlantic Meridional Overturning Circulation (AMOC) is influenced by both the strength and the location of the Mediterranean Outflow (MOW) plume in the Gulf of Cadiz (Gulf of Cadiz). To evaluate the influence of MOW on AMOC over deglaciations, precise and accurate salinity and temperature reconstructions are needed. For this purpose, we measured Mg/Ca and clumped isotopes of several benthic foraminiferal species at IODP Site U1390 in the Gulf of Cadiz. The clumped isotope results of Cibicidoides pachyderma, Uvigerina mediterranea and Pyrgo spp. are consistent between species and record no significant difference in Glacial to Holocene DWT. Over the deglaciation, the Mg/Ca-based temperatures derived from U. mediterranea indicate three periods of MOW absence at site U1390. Mg/Ca-based temperatures of Hoeglundina elegans and C. pachyderma are on average 6 °C too cold when compared to the present core-top temperature, which we explain by a carbonate ion effect on these epibenthic species related to the high alkalinity of the MOW. Combining DWT estimates with the benthic oxygen isotope data and considering different relationships between seawater oxygen isotopes and salinity, we infer a salinity decrease of MOW by 3 to 8 units over the deglaciation, and 4 units during S1, accounting for the global δ18O depletion due to the decrease in ice volume. Our findings confirm that the Mediterranean Sea accumulates excess salt during a glacial low stand, and suggest that this salt surged into the Atlantic over the deglaciation, presumably during HS1.
... Once LIW and WMDW exit the Strait of Gibraltar and form MOW, this water mass splits into two plumes due to the complex morphology of the continental slope in the Gulf of Cadiz. The upper plume flows between 500 and 800 m while the lower plume flows between 800 and 1400 m (Ambar and Howe, 1979;Borenäs et al., 2002;García et al., 2009;Llave et al., 95 2007;Madelain, 1970;Marchès et al., 2007;Serra et al., 2005;Zenk, 1975). Surface circulation in the Gulf of Cadiz is governed by the Gulf of Cadiz slope current, flowing eastward along the western Iberian margin and the offshore inflow which meet at the Strait of Gibraltar and enter the Mediterranean basin (Peliz et al., 2009). ...
Mediterranean-Atlantic exchange through the Strait of Gibraltar plays a significant role in the global ocean-climate dynamics in two ways. On one side, the injection of the saline and warm Mediterranean Outflow Water (MOW) contributes to North Atlantic deep-water formation. In return, the Atlantic inflow is considered a sink of less saline water for the North Atlantic Ocean. However, while the history of MOW is the focus of numerous studies, the latter has received little attention so far. The present study provides an assessment of the Mediterranean–Atlantic exchange with focus on the Atlantic inflow strength and its response to regional and global climate from 3.33 to 2.60 Myrs. This time interval comprises the mid-Pliocene warm period (MPWP, 3.29–2.97 Myr) and the onset of the Northern Hemisphere Glaciation (NHG). For this purpose, gradients in surface δ¹⁸O records of the planktonic foraminifer Globigerinoides ruber between the Integrated Ocean Drilling Program (IODP) Hole U1389E (Gulf of Cadiz) and ODP Site 978 (Alboran Sea) have been evaluated. Interglacial stages and warm glacials of the MPWP revealed steep and reversed (relative to the present) W-E δ¹⁸O gradients suggesting a weakening of Mediterranean–Atlantic exchange likely caused by high levels of relative humidity in the Mediterranean region. In contrast, periods of stronger inflow are indicated by flat δ¹⁸O gradients due to more intense arid conditions during the severe glacial Marine Isotope Stage (MIS) M2 and the initiation of the NHG (MIS G22, G14, G6–104). Intensified Mediterranean–Atlantic exchange in cold periods is linked to the occurrence of ice-rafted debris (IRD) at low latitudes and weakening of the Atlantic Meridional Overturning Circulation (AMOC). Our results thus suggest the development of a negative feedback between AMOC and exchange rates at the Strait of Gibraltar in the latest Pliocene as it has been proposed for the late Quaternary.
... and 36.5-37.5), flowing deeper between 800 and 1400 m toward the NW Atlantic (Madelain, 1970;Zenk, 1975;Ambar and Howe, 1979;Borenäs et al., 2002;Serra et al., 2005;Llave et al., 2007;Marchès et al., 2007;. The influence of MOW in the Gulf of Cadiz has been documented since the mid-Pliocene, which together with tectonic activity results in a complex depositional environment characterized by moats, channels, contourite drifts, turbidites, and debrites (Hernández-Molina et al., 2006, 2014. ...
The presence and high abundances of the benthic foraminiferal group “elevated epifauna” has been proposed as indicator of the existence of Mediterranean Outflow Water (MOW) in the Gulf of Cadiz. Here we intend to use this potential proxy to reconstruct MOW in the early Pliocene at the Integrated Ocean Drilling Program (IODP) Hole U1387C. Cibicides lobatulus, Planulina ariminensis and Cibicides refulgens were found at this site, but high abundances of C. lobatulus and C. refulgens coincide with increases of shelf taxa and grain size, related to episodes of downslope transport. This issue calls into question the applicability of these species as MOW proxies. The present study therefore reassesses the role of these three elevated epifaunal species by means of stable isotope analyses (δ¹⁸O, δ¹³C) by contrasting their isotopic signature with that of shelf dwellers and deep-water taxa from Pliocene and Recent sediments from the Gulf of Cadiz and the western Iberian margin. Since foraminiferal calcite does not always precipitate in equilibrium with seawater, substantial efforts have been taken in order to determine species-specific offset values and to correct the isotopic signature for vital effects. Our corrected results demonstrate that C. lobatulus and C. refulgens are isotopically similar to the shelf dwellers in the Pliocene data-set, eliminating them as MOW indicators. Recent samples from the Gulf of Cadiz and the western Portugal shelf corroborate the wide bathymetric range of C. lobatulus from the shelf to the slope. Our results thus warrant for caution when considering C. lobatulus and C. refulgens as indicators of bottom current strength in unstable slope settings, particularly when co-occurring with allochthonous shelf dwellers. In contrast, P. ariminensis shows a signature close to deep-water taxa, corroborating its reliability as indicator of MOW.
... Surrounding GB, at depths sampled in this study (to 1800 m), we intercepted 3 water masses direct/indirectly related with the communities sampled: i) the Eastern North Atlantic Central Water (ENACW) occupies the level of hydrographic structures found over the GB summit; ii) Below ENACW and to ca. 1500 m, we found the Mediterranean Outflow Water (MOW) characterised by an increase of salinity. MOW had a maximum core, with higher salinity and lowoxygen concentration at depths between 800 and 1100-1200 m (Ambar and Howe, 1979;Iorga and Lozier, 1999;INDEMARES data). This water mass comes from the Strait of Gibraltar, and reaches velocities of 5-10 cm/s (Iorga and Lozier, 1999), and iii) below MOW, the Labrador Sea Water (LSW) that moves southwards from northern latitudes to GB and has a core at 1800 m (Iorga and Lozier, 1999). ...
This study presents the results of seafloor habitat identification and mapping of a NE Atlantic deep seamount. An “assemble first, predict later” approach has been followed to identify and map the benthic habitats of the Galicia Bank (NW Iberian). Biotic patterns inferred from the survey data have been used to drive the definition of benthic assemblages using multivariate tools. Eight assemblages, four hard substrates and four sedimentary ones, have been described from a matrix of structural species. Distribution of these assemblages was correlated with environmental factors (multibeam and backscatter data) using binomial GAMs. Finally, the distribution model of each assemblage was applied to produce continuous maps and pooled in a final map with the distribution of the main benthic habitats. Depth and substrate type are key factors when determining soft bottom communities, whereas rocky habitat distribution is mainly explained by rock slope and orientation. Enrichment by northern water masses (LSW) arriving to GB and possible zooplankton biomass increase at vertical-steep walls by “bottom trapping” can explain the higher diversity of habitat providing filter-feeders at slope rocky breaks. These results concerning vulnerable species and habitats, such as Lophelia and Madrepora communities and black and bamboo coral aggregations were the basis of the Spanish proposal of inclusion within the Natura 2000 network. The aim of the present study was to establish the scientific criteria needed for managing and protecting those environmental values.
... Surrounding GB, at depths sampled in this study (up to 1800 m), there are 3 water masses direct/indirectly related with the communities sampled: i) Eastern North Atlantic Central Water (ENACW) occupies the level of hydrographic structures found over the GB summit; ii) below ENACW and to ca. 1500 m, Mediterranean Outflow Water (MOW) characterized by an increase of salinity. MOW has a maximum core, with higher salinity and low-oxygen concentration at depths between 800 m and 1100-1200 m (Ambar and Howe, 1979;Iorga and Lozier, 1999;INDEMARES data). This water mass comes from the Strait of Gibraltar, and reaches velocities of 5-10 cm/s (Iorga and Lozier, 1999), and iii) below MOW, Labrador Sea Water (LSW) that moves southwards from northern latitudes to GB and has a core at 1800 m (Iorga and Lozier, 1999). ...
Galicia Bank is a deep seamount included as Site of Community Importance (SCI) in the Spanish Natura 2000 Network proposal. In the present study, epibenthic assemblages of sedimentary habitats have been described, together with the main environmental factor explaining species and communities distribution. Five epibenthic assemblages have been identified. Depth was the main factor explaining assemblage distribution, and the role of sediment type, water masses, and coral framework presence is also discussed. Three assemblages are located in the summit: the shallowest one (730–770 m), in the boundary between Eastern North Atlantic Central Water (ENACW) and Mediterranean Overflow Water (MOW) water masses is typified by ophiuroids and characterized by medium sands. The second assemblage (770–800 m) typified by the bivalve Limopsis minuta and the solitary coral Flabellum chunii correspond with medium sands and MOW core; and the third typified by the presence of cold-water coral communities dominated by Lophelia pertusa and Madrepora oculata, also on the MOW influence. In the border of the summit, in the bank break, an assemblage located in the range 1000–1200 m is dominated by the urchin Cidaris cidaris and the sponge Thenea muricata. In the flat flanks around the bank, the deepest assemblage (1400–1800 m) is dominated by the holothurian Benthogone rosea, in a depth range dominated by the Labrador water (LSW) and in fine sands with highest contents of organic matter. Most of species appeared in a depth range smaller than 25% of total depth range sampled and in < 10% of samples. Differential preference of species is evident in the different trophic guilds, with a higher dominance of filter-feeders in the summit and of deposit-feeders in the deepest assemblage, and have clear links with nutrient dynamics in the bank.
... • The Gulf Cadiz is the birthplace of the first contourite facies model Gonthier et al., 1984). • The MOW (Fig. 9.17) and related properties have been well studied (Zenk, 1975;Ambar and Howe, 1979;Zenk and Armi, 1990;Pinardi and Masetti, 2000; Criado-Aldeanueva , 2003, 2006García et al., 2009;Alves et al., 2011;Mulder et al., 2013;Stow et al., 2013), with salinity, temperature, and velocity measurements (Price et al., 1993;Baringer and Price, 1999). • Internal waves and internal tides have been documented in the Gulf of Cadiz (Cairns, 1980;Armi and Farmer, 1988;LaViolette and Lacombe, 1988;Apel, 2000;Bruno et al., 2006;Alvarado-Bustos, 2011;Sanchez-Garrido et al., 2011;Quaresma and Pichon, 2013). ...
Contourites are deposits of thermohaline-driven geostrophic contour currents in deep-water environments. However, the contourite domain is still in a state of flux after nearly 60 years of research because of 14 fundamental problems: (1) dual forcing mechanisms (i.e., atmospheric and thermohaline) of contour currents, (2) process continuum between turbidity currents and contour currents, (3) the broad application of the term “contourite” to represent deposits of all four types of bottom currents (i.e., thermohaline, wind, tide, and baroclinic), (4) regional hiatuses, (5) recognition of contourite channels, (6) Gulf of Cadiz as the type locality, (7) the contourite facies model, (8) the lack of measured grain-size data for the IODP Expedition 339 cores, (9) recognition of ancient contourites, (10) bedform-velocity matrix, (11) seismic wave geometries, (12) the lack of sedimentological knowledge on deposits of oceanic waves (i.e., internal waves/tides, cyclonic waves, and tsunami waves), (13) reservoir quality, and (14) sediment provenance.
... (b) A close-up of the 25-50 ka BP interval allows for better consideration of the pointers selected (dotted vertical lines) for this new age model. and Howe, 1979;Ambar et al., 2002). North Atlantic Deep Waters are found below 1500 m . ...
New dinoflagellate cyst (dinocyst) analyses were carried out at high resolution in core MD99-2339, retrieved from a contouritic field in the central part of the Gulf of Cadiz, for the Marine Isotope Stage (MIS) 3 interval, allowing for discussion of palaeohydrological changes over the last 50 ky in the subtropical NE Atlantic Ocean. Some index dinocyst taxa, according to their (palaeo)ecological significance, shed light on significant sea-surface changes. Superimposed on the general decreasing pattern of dinocyst export to the seafloor over the last 50 ky, paralleling the general context of decreasing aeolian dust fertilization, a complex variability in dinocyst assemblages was detected at the millennial timescale. Enhanced fluvial discharges occurred during Greenland Interstadials (GIs), especially GI 1, 8 and 12, while enhanced upwelling cell dynamics were suggested during the Last Glacial Maximum and Heinrich Stadials. Finally, during the early Holocene, and more specifically during the Sapropel 1 interval (around 7-9 ka BP), we evidenced a strong decrease in dinocyst fluxes, which occurred synchronously to a strong reduction in Mediterranean Outflow Water strength and which we attributed to an advection of warm and nutrient-poor subtropical North Atlantic Central Waters. Over the last 50 ky, our study thus allows for capturing and documenting the fine tuning existing between terrestrial and marine realms in North Atlantic subtropical latitudes, in response to not only the regional climate pattern but also monsoonal forcing interfering during precession-driven Northern Hemisphere insolation maxima. This mechanism, well expressed during the Holocene, is superimposed on the pervasive role of the obliquity as a first major trigger for explaining migration of dinocyst productive centres in the NE Atlantic margin to the subtropical (temperate) latitudes during glacial (interglacial) periods.
... Surrounding GB, at depths sampled in this study (to 1800 m), we intercepted 3 water masses direct/indirectly related with the communities sampled: i) the Eastern North Atlantic Central Water (ENACW) occupies the level of hydrographic structures found over the GB summit; ii) Below ENACW and to ca. 1500 m, we found the Mediterranean Outflow Water (MOW) characterised by an increase of salinity. MOW had a maximum core, with higher salinity and lowoxygen concentration at depths between 800 and 1100-1200 m (Ambar and Howe, 1979;Iorga and Lozier, 1999;INDEMARES data). This water mass comes from the Strait of Gibraltar, and reaches velocities of 5-10 cm/s (Iorga and Lozier, 1999), and iii) below MOW, the Labrador Sea Water (LSW) that moves southwards from northern latitudes to GB and has a core at 1800 m (Iorga and Lozier, 1999). ...
Habitat identification and habitat mapping are fundamental in determining locations of potential protected areas although both requisites are challenging, especially in the deep sea. This study presents the results on sea floor habitat identification and mapping of a NE Atlantic seamount (Galicia Bank, NW Iberian), the deepest SAC of the Spanish Natura 2000 proposal. An “‘assemble first, predict later” approach has been followed to identify and map the benthic habitats of the GB. Biotic patterns inferred from the survey data have been used to drive the definition of benthic assemblages using multivariate tools. 9 assemblages, 5 in hard substrates and 4 in sedimentary ones, have been described from a matrix of structural species. Rocky habitats clustering pattern is not determined by depth as first factor, but for a combination of slope and orientation. Cluster groups are typified by the combination of structural taxa as are cold-water corals, bamboo corals, black corals, gorgonians and large sponges. Five rocky assemblages are: i) Bank Summit Plain Rock (SPR); habitat is typified by antipatharians as Parantipathes sp, Trissopathes sp and Schizopathes sp, and Octocorallia as the gorgonian species Acanthogorgia sp, Swiftia rosea and Narella sp. ii) Summit and Bank Break Rock (SBBR): typified by Antipatharia, Octocorallia and large Porifera. The sponges Aphrocallistes beatrix, other Hexactinellida undet., and Geodidae undet., the black coral Bathypathes sp., the gorgonians Acanthogorgia sp. and Swiftia rosea, and the seastar Brissinga endecacnemos were the most discriminating species. iii) Bank Break Rock subgroup 1 (BBR1): located in the northern and eastern slopes of the bank. Typified by gorgonians (Swiftia rosea, others), bamboo corals (Acanella arbuscula, Isididae undet.) and black corals (Schizopathes sp, Trissopathes sp, Parantipathes sp). iv) Bank Break Rock subgroup 2 (BBR2): Deepest rock samples. Typified by gorgonians (Swiftia rosea, Anthothela grandiflora), bamboo corals (Acanella arbuscula), black corals (Parantipathesssp) and large sponges as Asconema setubalense, Phakellia robusta, Hexactinellida undet). v) Bank Break Rock Southern slopes (BBRS): Located in the southern slope and in the Rucabado mount. With presence of colonies of cold water corals (Madrepora oculata, Lophelia pertusa), and a companion fauna similar to the latter groups, as gorgonians (Swiftia rosea), bamboo corals (Acanella arbuscula), black corals (Schizopathes ssp) and large sponges as Asconema setubalense or Aphrocallistes beatrix. There are clear differences in habitat complexity among assemblages, being BBRS the most complex assemblage in terms of biogenic structures, and the SPR the simplest. BBRS complexity can be explained in part for the higher presence of colonial scleractinia (mostly Lophelia and Madrepora). In the soft bottoms, main factors explaining clusters are depth, substrate type and water masses. Four aasemblages are: The 4 assemblages identified for sedimentary habitats are: i) Summit Sands (SS): shallowest samples (750-780 m), substrate of medium sands of low reflectivity, affected by the ENACW. Sands dwelled by a huge amount (9.5 kg/ha) of ophiuroids of the family Ophiacanthidae (Ophiacantha sp. and Ophiomyces grandis), and with lower densities the solitary coral Deltocyathus moseleyi and Flabellum chunii, and the bivalve Limopsis spp. (L. minuta and L. cristata). ii) Summit Sands with reef patches (SSrf) Substrate of medium sands of low reflectivity with depths ranged from 780 to 1000 m, also in the ENACW layer. Sands covered by patches of cold-water coral colonies (Lophelia pertusa and Madrepora oculata, 3.3 and 3.2 kg/ha of live coral). Together with these two scleractinian species, typifying species mostly are species living on coral as Desmophyllum cristagalli, Acanthogorgia spp., Lima marioni, Parantipathes sp., and as mobile fauna different species of Munidopsis and Uropthychus. iii) Bank Break Sands (BBS): substrate of medium sands of medium reflectivity located in depths ranged from 1000 to 1200 m, in the MOW layer. Sands dominated by the urchin Cidaris cidaris, and the sponge Thenea muricata. iv) Bank Flanks Sands (FS): fine and very fine sands of low reflectivity in depths ranged from 1400 to 1800 m, LSW mass. Clearly dominated by the holothurian Elasipodidae Benthogone rosea (6.3 kg/ha), companied by the leather urchin Araeosoma fenestratum, and several anthozoa as Umbellula sp., Acanella arbuscula and Swiftia rosea. Distribution of these 9 assemblages was correlated with environmental factors using binomial General Additive Models (GAMs). Finally, the distribution model of each assemblage were applied to produce continuous maps with the location of the main habitats located in the GB and pooled in a final map with the distribution of the main benthic habitats of GB. Results of this study was used in the Spanish proposal of inclusion in the Natura 2000 network due to the wide range of species and habitats of high conservation value found here, such as Lophelia and Madrepora communities and black and bamboo coral aggregations. The aim of present study is to establish a scientific basis for managing and protecting those environmental values.
... At that point, conservation of potential vorticity of the flow may already be dominated by variation of the Coriolis parameter, which requires a zonal direction of the flow. Since the depth limits of the MUC are 500/600 m and 1400/1500 m (Ambar and Howe, 1979; Ambar et al., 2008; Baringer and Price, 1997; Bashmachnikov et al., 2015a), the function of JEBAR can be integrated over the MW water layer, from 600 to 1500 m (or all the way to the bottom). The computed JEBAR field is quite patchy. ...
This work presents the quantitative study of climatological distributions of
mid-depth source water types in the northeast (NE) Atlantic using the optimum multiparameter analysis (OMP). It generalises a number of regional results
from particular synoptic sections. The cores of the Mediterranean Water (MW),
the modified Antarctic Intermediate Water (mAAIW) and the Subarctic
Intermediate Water (SAIW) are detected and spatial variations of their
depth/density are obtained: as expected, spreading of the water types is
predominantly isopycnic and follows the major mid-depth circulation patterns.
In some areas the turbulent transport also makes a considerable
contribution. MW in the Atlantic spreads in three cores of different density:
the upper MW core (northwest of the line 28° W,
35° N–14° W, 44° N) is found in the neutral
density range of 27.65–27.70 kg m−3 at depths of 900–1000 m;
the main MW core (between the line above and the line 35° W,
28° N–10° W, 37° N) has neutral density of around
27.75 kg m−3 and is found at 1000–1100 m; the lower MW core
(southeast of the line 35° W, 28° N–10° W,
37° N) has neutral density of around 27.80 kg m−3 and is found
at 1250–1350 m. The upper MW core has a comparatively low MW content
(below 30 %) and is speculated to be transported by the mean flow from
the northern Iberian Peninsula and the Bay of Biscay to the northern Azores.
The main MW core contains the majority of the outflowing MW. It primarily
originates from the Mediterranean undercurrent around Estremadura Promontory,
where the Joint Effect of Baroclinicity and Bottom Relief (JEBAR) overrides
the topographic β effect. It is transported west to the Azores Islands
mostly along 39° N. The lower MW core originates in the Gulf of
Cádiz, from where it is transported by the dominating flows southwestwards.
The SAIW core is detected between 27.70 and 27.75 kg m−3. It is found to
spread south along both slopes of the Mid-Atlantic Ridge (MAR). SAIW east of
the MAR mixes with the upper and the main MW cores and re-circulates in a
cyclonic gyre at 15–25° W and 34–39° N, penetrating as
far south as the Azores Current. The mAAIW core is detected between 27.60 and
27.65 kg m−3. Its northernmost spreading limit is between 25 and
29° N, but its influence can be observed along the African coast and
immediately west of the Canary Islands up to 32° N. The maximum
concentration of the mAAIW core is found south of the Canary Islands, from where
mAAIW is advected westwards, along with the westward spreading of the deep
fraction of MW.
... Formed in the Gulf of Cadiz by entrainment of Eastern North Atlantic Central water on the high-salinity outflow from the Mediterranean Sea, spreads at 800-1300 m, S > 36 and θ ~ 11-12 ºC. Zenk, 1975;Ambar and Howe, 1979;Castro el al., 1998;Álvarez-Salgado et al., 2013 Intermediate Carried into the South Atlantic by the Deep Western Boundary Current (DWBC). Characterized by salinity maximum and silicate minimum (4.6ºC) ; and θ-S discontinuity and oxygen maximum (2.0 ºC). ...
The transformation of biologically labile organic matter into refractory compounds by
prokaryotic activity has been termed the ‘microbial carbon pump’ (MCP) and may constitute an
effective mechanism to store reduced carbon in the dark ocean. Understanding its generation and its
role in carbon sequestration is crucial to assess its relevance in the context of the global carbon cycle.
The main aim of this PhD thesis is to test the significance of the chromophoric (CDOM) and fluorescent
(FDOM) fractions of dissolved organic matter (DOM) as tracers for the microbial production of
recalcitrant DOM in the global ocean. All its content is framed in the Malaspina 2010
circumnavigation, which allowed us to produce the first global inventory of the optical properties of
DOM in both the surface ocean (< 200 m), gathered by Longhurst’s biogeographic provinces, and the
dark ocean (> 200 m) by the main water masses.
In the dark ocean, ideal age and ageing (apparent oxygen utilization) of the main water masses
were tracked along the global thermohaline circulation, allowing the estimation of net
production/consumption rates of CDOM and FDOM and their respective turnover times. We found that
CDOM was generated in situ by microbial metabolism (at a global rate of 3.3 ± 0.5 x 10–5 m–1 yr–1),
with a turnover time of ca. 625 years and was accumulated in the dark ocean due to its recalcitrant
nature, with an increase in the degree of aromaticity and molecular weight along the thermohaline
circulation. We identified two distinct chromophores one was centred at 302 nm (UV chromophore)
and the other one at 415 nm (Visible chromophore). The UV chromophore was attributed partially to
nitrate and likely to the antioxidant gadusol and presented a turnover time of ca. 345 years. The
Visible chromophore! was related to the respiratory enzyme cytochrome c and presented a turnover
time of ca. 356 years.
The analysis of the fluorescent properties of DOM in both the surface and dark ocean allowed us
to identify four ubiquitous fluorophores. Two fluorophores were humic-like (C1, C2) components and
the other two were amino acid-like (tryptophan-like C3, tyrosine-like C4) components. The robustness
in the level of explanation for humic-like and amino acid-like components by biogeochemical variables
was much higher for the humic-like components than for the amino acid-like components both in the
surface and the dark ocean (~80% vs ~30%). The fluorescent humic-like material was explained by
water ageing and showed positive power functions both in the surface and in the dark ocean. In the
dark ocean, C1 showed a higher production rate than C2, with a net production rate of 2.3 ± 0.2 x 10–5
and 1.2 ± 0.1 x 10–5 RU yr–1 and turnover times of 530 and 740 years, respectively. However, in the
surface ocean both rates were similar. In the dark ocean, C1 and C2 showed higher conversion
efficiencies per unit of utilized oxygen than in the surface ocean, likely due to photobleaching. In the
dark ocean, tyrosine-like C4 presented an inverse power relationship with apparent oxygen utilization,
decreasing at a rate of –1.1 ± 10–5 RU yr–1. On the contrary,! the tryptophan-like component C3 did not
show a pattern with ageing. In the surface ocean, the amino acid-like components were apparently
more affected by physical processes, although the positive relationship of C4 with Chl a also implies a
microbial influence on this component.
The in situ production of the DOM fractions as by-products of microbial metabolism identified as
water masses turned older and the long turnover times of the humic-like components indicated the
relevant role of the MCP in the carbon sequestration in the dark ocean. Thus, the initial hypothesis of
this PhD thesis that was: “are the chromophoric and fluorescent fractions of DOM key components of
the recalcitrant DOM pool?” has been verified. Similarly, the fact that chromophores and fluorophores
can be used as tracers of the water mass mixing and biogeochemical processes operating at
centennial time scales will bring new insights into the ocean carbon storage.
... The continental margin is oriented from northnorthwest to south-southeast, with east-west sectors, as a result of recent tectonic movements (Baldy et al., 1977; Sanz de Galdeano, 1990 ) that have conditioned the coastal direction, as well as the location of river mouths. On the continental shelf, the hydrodynamic regime is controlled by the North Atlantic Surface Current (NASW), which sweeps the shelf from northwest to southeast, and by the Mediterranean Out-Flow (MOW), that pushes out over the continental slope, lying at a depth that varies from 300-1 500 m, towards the west and northwest, but without any substantial effect on the shelf itself (Heezen and Johnson, 1969; Madelian, 1970; Zenk, 1975; Ambar and Howe, 1979). The main coastal current is influenced mainly by the rough weather from the west and northwest, and flows towards the southeast; however, with an easterly wind (Levante), the current is observed to flow to the west, while southerly winds direct the current to the north and northwest. ...
... The Mediterranean outflow (MW) spreads over the Gulf of Cadiz basin at depths greater than 500 m (Zenk, 1975;Stevenson, 1977) showing two well differentiated cores (Ambar and Howe, 1979;Ambar, 1983): a shallow core (500∼600 m depth, 13.7 ºC, 37.1 ppt) flowing through the continental margin, and a deeper one (>750 m depth, 13.6 ºC, 37.4 ppt), which is split in different branches that circulate between submarine canyons at depths deeper than 1200 m (Madelain, 1970;Zenk, 1975) ( Protected from the north winds, the Algarve waters are characterised by an environment that is more benign than the Atlantic waters of the west coast, and are influenced by the Mediterranean Sea ( Figure AS-4). The wind regime (direction and frequency) is also different in the two areas of the Algarve. ...
The present study is a full study of the fisheries biology of the most important commercial cephalopod species in the Algarve (south of Portugal) and Gulf of Cadiz (Portuguese and Spanish waters). The main target species were the common octopus (Octopus vulgaris), curled octopus (Eledone cirrhosa), musky octopus (E. moschata), common cuttlefish (Sepia officinalis) and elegant cuttlefish (S. elegant). Some fisheries biological data are also presented of all cephalopod species discarded in Algarve in a total of 20 species.
Samples of each species were collected from the market (first sell) and on board commercial boats, during a twelve months period (1998-1999). However, in the case of Cadiz some biological material and raw data already existed from cruises or previous studies. The catches of two types of commercial fishery - the artisanal fishery (traps) and the trawl fishery - were sampled.
In order to understand the cephalopod fisheries and its future perspectives, official landings fishing data from the most important harbours in the study area was compiled and analysed.
In spite of the study area being a continues area with common waters and sometimes with common traditions, the commercial cephalopod species are caught with different fishing gears and sometimes with the same type but with slight modifications. Therefore, all fishing gears and métiers used by both countries in the study area are presented, with special attention on the artisanal fisheries.
Population parameters of the main cephalopod species studied are presented such as length frequency distributions, weight-length relationships, maturity, fecundity and growth. For the species common octopus (Octopus vulgaris) it was also studied the gametogenesis of the males and females. For the cephalopod discarded species the population parameters presented depends on the number of specimens caught but it was made an effort to present the weight-length relationship, the length frequency distribution and the maturity stages.
In the Chapter on assessment and management it is presented and discussed aspects of the fishing effort and the abundance, with special emphasis on the octopus fishery.
The NW Moroccan Margin has a complex geological evolution, being located close to the transition zone between the Azores – Gibraltar Fracture Zone and the western front of the Betic–Rif collisional orogen. The interaction between tectonic, halokinetic and fluid flow processes with bottom-current activity shapes the seafloor and influences the distribution of seafloor biological communities (such as the cold-water coral mounds) and deep-water sedimentation. The aims of this work are to study the interaction of the paleo-oceanographic and morpho-tectonic processes that generated the various seafloor features of the NW Moroccan Margin. To achieve this, high-resolution multibeam bathymetry and parasound data acquired in the “ALBOCA II” cruise have been used, complemented by high-resolution 2D seismic reflection data and the EMODnet bathymetric compilation.
Several morphological features were identified in the margin, which are related to different processes of sedimentary (contourites and sediment waves), structural (faults and diapirs), gravitational (slide scars, mass transport deposits), fluid migration (mud volcanoes and pockmarks) and biogenic (exposed and buried coral mounds) nature. The structural features (e.g., strike-slip faults and mud volcanoes) have a major control on the seafloor morphology and, consequently, on the development and evolution of the sedimentary systems in the study area.
The evolution of the NW Moroccan Margin during the late Quaternary has been controlled by climatic variations and tectonic activity. The action of these factors has been dominant in distinct parts of the study area where: i) contourite terraces developed when climatic and oceanographic changes were the prevalent factor, ii) mounded and confined contourite drifts and local mass transport deposits formed when the major control was tectonic activity.
The Gulf of Cádiz is home to the well-studied modern contourite depositional system, which was deposited through the influence of bottom currents sourced from the Mediterranean Outflow Water exiting the Straits of Gibraltar since Pliocene to present. However, the Late Miocene sedimentary evolution and Mediterranean-Atlantic water- mass exchange prior and during the Messinian salinity crisis are poorly understood. Some progress on the characterisation of an ancient Late Miocene contourite depositional system were established from field studies onshore in the Betic and Rifian corridors albeit limited due to outcrop availability, but their downstream continuation in the Gulf of Cádiz has yet to be identified. This is partially due to the complexity of the area resulting from tectonic deformation, whose effect on contourite deposition are ambiguous. Consequently, identification and characterisation of the Tortonian to Messinian interval in the Gulf of Cádiz could answer questions related to the relationship between the Mediterranean and Atlantic during that period, as well as increase our knowledge of contourite deposition in tectonically active settings. This thesis presents a regional-scale study on the Late Miocene evolution of the Gulf of Cádiz, focusing on the role of bottom currents in sedimentation. A detailed seismic stratigraphic analysis was carried out for the available seismic and borehole data acquired from scientific and industry sources, to characterise the sedimentary and paleoceanographic evolution during the later parts of the Late Miocene, assisted by chronostratigraphic correlation. An ancient contourite depositional system is identified consisting of three evolutionary stages: initial-drift, growth-drift, and maintenance-drift, prior to its burial in the latest Miocene. The formation of the Late Miocene contourite depositional system occurred following the main emplacement of the regional Gulf of Cádiz allochthonous unit and can be traced towards southern West Iberian margin. The results allowed us to reconstruct the evolution of the paleo-Mediterranean Outflow Water responsible for the bottom current activity depositing the ancient contourite depositional system until its severe weakening or cessation during the latest Miocene, which led to the Messinian salinity crisis, as well as to suggest its impact on North Atlantic paleoceanographic and climate. This is mainly controlled by continuous uplifting and subsequent closure of the Mediterranean-Atlantic paleo-gateways. These findings also allowed us to understand the influence of tectonic and orbital control on gateway evolution and gravitational processes, and thus on contourite deposition. By unravelling these control factors, they enable us to propose diagnostic criteria of contourite depositional system in tectonically active margins.
The differentiation of pure turbidites and contourites from mixed deposits —as the bottom current reworked sands (BCRS) — in sedimentary cores and outcrops from the modern or ancient records is still challenging. An accurate evaluation of facies associations calls for detailed understanding of processes controlling these deep-water systems and how they record interactions of along- and down-slope processes. Connecting features to processes is of particularly relevant in the study of contouritic drifts that include sandy deposits. This research describes and interprets BCRS within a Pleistocene contouritic drift located along the middle continental slope of the Gulf of Cadiz. Seismic, wireline, and sedimentological analyses of sediment samples were applied to core material representing a muddy drift that hosts sandy deposits. A sedimentary sequence, comprising five facies (F1 to F5) is defined here; it records background sedimentation influenced by the initial deposition of a gravity-driven flow followed by bottom current reworking. It is moderately bioturbated and contains distinctive trace fossil assemblages, often dominated by ichnofabrics of the Planolites and Thalassinoides. These assemblages represent intermittent deposition between down-slope and along-slope processes during relatively short time scales. The sequence is defined as a partial bi-gradational contourite sequence including BCRS reworked from underlying turbiditic deposits due to the interaction of down- and along- slope processes within the contouritic drift and its adjacent contouritic channel. Longer-term trends include an increase and subsequent decrease in both sediment supply and bottom currents. The sedimentary facies model presented here represents a new end member for a mixed turbiditic-contouritic system in which turbiditic flows influence a contourite drift. As such, it may support basic sedimentological interpretation and petroleum exploration strategies.
The stable isotope composition of dissolved silicon in seawater
(δ30SiDSi) was examined at 10 stations along the GEOVIDE
section (GEOTRACES GA-01), spanning the North Atlantic Ocean
(40–60° N) and Labrador Sea. Variations in
δ30SiDSi below 500 m were closely tied to the distribution
of water masses. Higher δ30SiDSi values are associated with
intermediate and deep water masses of northern Atlantic or Arctic Ocean
origin, whilst lower δ30SiDSi values are associated with
DSi-rich waters sourced ultimately from the Southern Ocean. Correspondingly,
the lowest δ30SiDSi values were observed in the deep and
abyssal eastern North Atlantic, where dense southern-sourced waters dominate.
The extent to which the spreading of water masses influences the
δ30SiDSi distribution is marked clearly by Labrador Sea
Water (LSW), whose high δ30SiDSi signature is visible not
only within its region of formation within the Labrador and Irminger seas,
but also throughout the mid-depth western and eastern North Atlantic Ocean.
Both δ30SiDSi and hydrographic parameters document the
circulation of LSW into the eastern North Atlantic, where it overlies
southern-sourced Lower Deep Water. The GEOVIDE δ30SiDSi
distribution thus provides a clear view of the direct interaction between
subpolar/polar water masses of northern and southern origin, and allow
examination of the extent to which these far-field signals influence the
local δ30SiDSi distribution.
Mediterranean–Atlantic exchange through the Strait of Gibraltar plays a significant role in the global ocean–climate dynamics in two ways. On one side, the injection of the saline and warm Mediterranean Outflow Water (MOW) contributes to North Atlantic deep-water formation. In return, the Atlantic inflow is considered a sink of less saline water for the North Atlantic Ocean. However, while the history of MOW is the focus of numerous studies, the Pliocene Atlantic inflow has received little attention so far. The present study provides an assessment of the Mediterranean–Atlantic exchange with a focus on the Atlantic inflow strength and its response to regional and global climate from 3.33 to 2.60 Ma. This time interval comprises the mid-Pliocene warm period (MPWP; 3.29–2.97 Ma) and the onset of the Northern Hemisphere glaciation (NHG). For this purpose, gradients in surface δ¹⁸O records of the planktonic foraminifer Globigerinoides ruber between the Integrated Ocean Drilling Program (IODP) Hole U1389E (Gulf of Cádiz) and Ocean Drilling Program (ODP) Hole 978A (Alboran Sea) have been evaluated. Interglacial stages and warm glacials of the MPWP revealed steep and reversed (relative to the present) W–E δ¹⁸O gradients suggesting a weakening of Mediterranean–Atlantic exchange likely caused by high levels of relative humidity in the Mediterranean region. In contrast, periods of stronger inflow are indicated by flat δ¹⁸O gradients due to more intense arid conditions during the severe glacial Marine Isotope Stage (MIS) M2 and the initiation of NHG (MIS G22, G14, G6–104). Intensified Mediterranean–Atlantic exchange in cold periods is linked to the occurrence of ice-rafted debris (IRD) at low latitudes and a weakening of the Atlantic Meridional Overturning Circulation (AMOC). Our results thus suggest the development of a negative feedback between AMOC and exchange rates at the Strait of Gibraltar in the latest Pliocene as it has been proposed for the late Quaternary.
Paleoceanographic reconstructions are fundamental for our understanding of past climate variabilities and have mainly focused on changes of circulation and environmental conditions in the surface or deep ocean. The thermocline (100 – 1000 m), acting as a link and buffer between the well-mixed warm surface and the slow and cold deep water, has been largely overlooked. In this study two cold-water coral (CWC) bearing cores from two depths in the thermocline of the southern Gulf of C´adiz (sGoC) were analysed in a mutli-proxy approach. U-series dated glacial CWCs were analysed for recorded Li/Mg temperatures, water mass 14C reservoir ages and radiogenic Nd isotope signatures (epsilonNd). For the first time, a CWC epsilonNd record was extended by two independent co-located hemipelagic sediment records. A seesaw pattern for the glacial sGoC intermediate waters alternating between predominant Eastern North Atlantic Central Water (ENACW) and Eastern Antarctic Intermediate Water (EAAIW) is proposed. Glacial ENACW and EAAIW both exhibited polar temperatures (~0°C) and more radiogenic epsilonNd signatures (~-9) than nowadays and were distinguishable by their reservoir ages, with better ventilated glacial ENACW than EAAIW. A compilation of existing CWC mound aggregation records allowed for a first estimate of initial CWC settlement in the ocean around 3.4Ma ago, coinciding with the mid-Pliocene warm period and the onset of Northern Hemisphere glaciation. Additionally, Ba isotopes (delta138/134Ba) recently introduced for seawater, was calibrated for the use in CWCs. This lays the foundation for the missing nutrient or surface biological productivity proxy in fossil CWCs.
We present the distribution of water masses along the GEOTRACES-GA01 section during the GEOVIDE cruise, which crossed the subpolar North Atlantic Ocean and the Labrador Sea in the summer of 2014. The water mass structure resulting from an extended Optimum MultiParameter (eOMP) analysis provides the framework for interpreting the observed distributions of trace elements and their isotopes. Central Waters and Subpolar Mode Waters (SPMW) dominated the upper part of the GEOTRACES-GA01 section in 2014. At intermediate depths, the dominant water mass was Labrador Sea Water, while the deep parts of the section were filled by Iceland–Scotland Overflow Water (ISOW) and North East Atlantic Deep Water. We also evaluate the water mass volume transports across the 2014 OVIDE line (Portugal to Greenland section) by combining the water mass fractions resulting from the eOMP analysis with the absolute geostrophic velocity field estimated through a box inverse model. This allowed us to assess the relative contribution of each water mass to the transport across the section. Finally, we discuss the changes in the distribution and transport of water masses between the 2014 OVIDE line and the 2002–2010 mean state. At the upper and intermediate water levels, colder end-member of the water masses replaced the warmer ones in 2014 with respect to 2002–2010, in agreement with the observed cooling of the surface and intermediate waters. Below 2000 dbar, ISOW increased its contribution in 2014 with respect to 2002–2010, increase related to the observed salinization since 2002. We also observed an increase in SPMW in the East Greenland Irminger Current in 2014 with respect to 2002–2010, which supports the recent deep convection events in the Irminger Sea. The assessment of the relative contribution of each water mass to the Atlantic Meridional Overturning Circulation (AMOC) across the OVIDE line allows identifying the water masses involved in the increase in the AMOC intensity from 2002–2010 to 2014. The increase in the AMOC intensity is related to the increase in the northward transport of the Central Waters in its upper limb, and to the increase in the southward flow of SPMW of the Irminger Basin and ISOW in its lower limb.
An optimum multiparameter analysis was applied to a data set for the eastern boundary of the North Atlantic subtropical gyre, gathered during November of two consecutive years and spanning from 16 to 36° N. This data set covers over 20° of latitude with good meridional and zonal resolution over the whole coastal transition zone. The contribution from six water types in the depth range between 100 and 2000 m is solved. In the 100 to 700 m depth range the central waters of southern and northern origin meet abruptly at the Cape Verde Frontal Zone. This front traditionally has been reported to stretch from Cape Blanc, at about 21.5° N, to the Cape Verde Islands, but in our case it penetrates as far as 24° N over the continental slope. South of 21° N latitude we actually find a less saline and more oxygenated variety of South Atlantic Central Water, which we ascribe to less diluted equatorial waters. In the 700 to 1500 m depth range the dominant water type is a diluted form of Antarctic Intermediate Water (AAIW), whose influence smoothly disappears north of the Canary Islands as it is replaced by Mediterranean Water (MW); at latitudes where both water masses coexist, we observe MW offshore while AAIW is found near-shore. North Atlantic Deep Water is the dominating water type below about 1300/1700 m depth south/north of the Canary Islands; this abrupt change in depth suggests the existence of different paths for the deep waters reaching both sides of the archipelago.
TSD observation of the Mediterranean outflow into the N. E. Atlantic were supplemented by measurements of nitrate, silicate, and phosphate. Apart from the normal salinity maximum at 1200 m, the occurrence of a T-S maximum at a depth of about 700 m in the Atlantic has often been observed. The chemical data suggest that this upper maximum is due to the release of water from a relatively shallow depth in the Straits of Gibraltar.
The St. Venant equations applying to flows of two layers have been used to develop a means of detecting steepening long internal waves and internal shock waves using time series data taken at a single point. The method is applied to the internal wave regime in the Straits of Gibraltar and the results help to explain the asymmetry of the internal tide cycle and the generation of short period internal waves east of the sill.
Mixing along neutral surfaces produces the small-scale structures in the deep ocean. New surfaces are defined for use on T, S curves for deep water spreading of Mediterranean Water in the N. E. Atlantic.
Typical results of continuous temperature-salinity measurements from stations west of Gibraltar are presented. Special features of the structure of the Mediterranean Water in this area in relation to the corresponding turbulent mixing and salt-fingering process are discussed: the two maxima in the vertical profiles, the variation in time of small-scale phenomena, and the step-like structure at the lower boundary of the Mediterranean Water.
The Mediterranean outflow is considered as two separate cores (upper and lower) which follow a well defined route between Gibraltar and Cabo São Vincente. Their physical and chemical properties are discussed in some detail and used as evidence for treating the upper core as a separate water mass. This core is also shown to suffer a 20% depletion in its concentration as it traverses the Gulf of Cadiz, whereas that of the lower core remains almost constant. In addition there was some indication of winter cooling and sinking of the shelf water which seems to be enhanced by the presence of the more prominent canyons in the area.
The 1958 survey provided the data for a preliminary synoptic study of the complicated dynamic phenomena of the Strait of Gibraltar. For this purpose, attention was focused near the boundary surface of the deep layer of pure Mediterranean water east of the sill. West of the sill this dense layer loses its character in a process of turbulent mixing with the lighter Atlantic water.The survey of the thermal microstructure of the sill area was made possible by the use of the new towed thermistor chain of the Woods Hole Oceanographic Institution (Richardson and Hubbard 1959). In 66 crossings of the Strait (north to south) above the sill and one passage (west to east) along the axis, the following were the most interesting findings: 1.(1) - The boundary between deep Mediterranean water and Atlantic water oscillates vertically with tidal periods.2.(2) - The boundary layer was found to slope down sharply at the centre of the channel at the time of internal high tide.3.(3) - Vertical motions of the isotherms of semi-diurnal period and in phase with the moon transits were dominat.4.(4) - Fluctuations of high amplitude and non-tidal period were superimposed on the dominant motions.5.(5) - Solitary rapid depth variations of some isotherms were recorded more frequently over the southern portion of the sill. The maximum depth change recorded was 80 m.6.(6) - The surface temperature is warmer on the Spanish than on the African side of the Strait. A four degree Centrigrade difference was found at one time along the four-mile long transverse above the sill.
From the Preface: Within the framework of the observational program in physical oceanography which was executed during the International Geophysical Year of 1957-58, the WOODS HOLE OCEANOGRAPHIC INSTITUTION, Woods Hole, Massachusetts, in partial association with the NATIONAL INSTITUTE OF OCEANOGRAPHY, England, performed a methodical oceanographic survey covering most of the Atlantic Ocean. Supplemented with similar observations made in the western North Atlantic just prior to the IGY, the data obtained constitute a comprehensive new body of information pertaining to the circulation of the entire Atlantic. To summarize many of the observations in a particularly useful and efficient manner, as well as to perform a fundamental analysis upon them, the following atlas of Atlantic thermohaline structure has been prepared. Although distributions of other properties measured on these cruises might also have been depicted in this volume, it was felt that a more coherent and serviceable atlas would be produced if attention were concentrated exclusively upon temperature and salinity; the measurements of these two variables, moreover, are of consistent quality, since they were all made by a single small research group employing the same techniques and equipment throughout the survey period. The cost of preparation and publication of this atlas was borne by a grant from the National Science Foundation, made in support of the International Geophysical Year Interdisciplinary Research Program.
Mixing in the deep stratified ocean Die Haufigkeitsverteilung von Wasserarten in Ausstrombereich yon Meeresstrassen The oceans, their physics, chemistry and general biology
- Pingree R D Siedler G
MAMAVEV O. 1. (1975) Temperature-salinity analysis o1 world ocean waters. Elsevier Scientific Publishing Co., 374 pp. PINGREE R. D. (1972) Mixing in the deep stratified ocean. Deep-Sea Research, 19, 549-561. SIEDLER G. (1968) Die Haufigkeitsverteilung von Wasserarten in Ausstrombereich yon Meeresstrassen. Kieler MeeresIorschungen, 24, 59-65. SVERDRUP H. U., M. W. JOHNSON and R. H. FLEMING (1942) The oceans, their physics, chemistry and general biology. Prentice-Hall, 1087 pp. ZEYK W. (1970)On temperature and salinity structure of the Mediterranean water in the N.E. Atlantic. Deep-Sea Research, 17, 627-632. ZENK W. (1975a) On the origin of the intermediate double maxima in T-S profiles from the North Atlantic. Meteor Forschungsergebnisse A, 16, 34-43.
A phy~cal and chemical description of the Mediterranean outflow in the Gulf of Cadiz Internal waves in the Straits of Gibraltar A preliminary survey of thermal microstructure in the Straits of Gibraltar
- M R Howe
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