Gibraltar Sill Research Articles

One million years of Mediterranean outflow strength

An orbitally tuned age model is established, based principally on sortable silt mean grainsize (SS¯, proxy for flow speed), for sediments deposited under the influence of the Mediterranean Outflow (MO) at IODP Site U1389 in the Gulf of Cadiz. The possibility of tuning sortable silt grainsize to insolation was indicated by close agreement between these two parameters on an earlier isotopically-based age model for the upper 130 m of the core. This led to tuning of the whole 390 m-long record with addition of a magnetic susceptibility control at Marine Isotope Stage 16, and the last occurrence of R. asanoi datum at around 900 ka (now dated at 910 ka in this core).The strongest variability in SS¯ is precessional with speed maxima at insolation minima superimposed on an increase in flow speed from 620 ka to the present. Although the Gibraltar sill exercises a major control on the inflow and outflow to the Mediterranean, there is little change in flow depth over most 20 ka precessional cycles, and therefore minor hydraulic control. Accelerations in average flow speed are also seen in interglacial to glacial (IG-G) cycles, particularly after 550 ka, with strong speed maxima at maximum glacial lowering of sealevel. This has allowed calculation of the sensitivity of flow speed to sea-level (SL) change of ∼0.074 cm s−1/m. From 990 to 620 ka the average amplitude of precessional speed changes is 3.7 cm s−1, but this increases to 7.7 cm s−1 from 620 ka to present.Deep ocean flow speeds are mainly density driven. Previous work has demonstrated that the major drivers of this system's density on precessional timescales are the evaporation - precipitation - inflow balance in the eastern Mediterranean plus cooling in the Gulf of Lions. Published benthic δ18O for 125–235 ka (corrected for ice volume changes) which tracks the combination of seawater temperature and salinity that controls density, demonstrates that flow speed and density maxima are coincident for that period and probably for the whole record. Although precessional SL changes are too small to affect flow speed significantly, SL does control accelerations at glacial maxima because shallower flow depth reduces inflow and leads to longer evaporation time, resulting in increased outflow density and speed. The sensitivity of flow speed to sill depth changes has allowed estimation of the long-term shallowing of the Gibraltar sill of ∼68 m over the last 620 ka in agreement with dated terrace uplift.

Two new Eucalathis (Brachiopoda, Chlidonophoridae) species from the Pliocene of Italy and history of the genus in the Mediterranean

ABSTRACT Eucalathis, a member of the brachiopod superfamily Cancellothyridoidea, is common in Recent seas. The 15 known species have an almost cosmopolitan distribution, but no known occurrence in the Mediterranean Sea. Despite the rich Recent record, very little is known about the geological history of Eucalathis. In the Mediterranean, two fossil species are known from the Miocene of Italy. Two additional Eucalathis species from the Neogene of Italy are identified: E. davidi sp. nov. from NW Italy (Marmorito, Early Pliocene), and E. dorae sp. nov. from Sicily (Altavilla, Late Pliocene). It proves that Eucalathis did not disappear from the Mediterranean after the Messinian Salinity Crisis. Based on the four known species, Eucalathis (probably apart from the Messinian Crisis) was present in the Mediterranean from the Middle Miocene to the Late Pliocene. Its disappearance is possibly related to the Pleistocene cooling. Currently, three species of Eucalathis occur in the NE Atlantic, near the entrance to the Mediterranean. However, they are unable to enter because the distribution of all three species is restricted to deeper environments than the average 300 m deep Gibraltar sill. The circulation pattern of the Gibraltar area also prevents deeper water Atlantic species from entering the Mediterranean. https://zoobank.org/urn:lsid:zoobank.org:act:EA467DCC-301B-4C44-9B19-A9A3E8EF332B https://zoobank.org/urn:lsid:zoobank.org:act:6AB4E4A5-46F2-44F9-8BFD-907190039071

A new bathyal ophiacanthid brittle star (Ophiuroidea: Ophiacanthidae) with Caribbean affinities from the Plio-Pleistocene of the Mediterranean.

Identifiable remains of large deep-sea invertebrates are exceedingly rare in the fossil record. Thus, every new discovery adds to a better understanding of ancient deep-sea environments based on direct fossil evidence. Here we describe a collection of dissociated skeletal parts of ophiuroids (brittle stars) from the latest Pliocene to earliest Pleistocene of Sicily, Italy, preserved as microfossils in sediments deposited at shallow bathyal depths. The material belongs to a previously unknown species of ophiacanthid brittle star, Ophiacantha oceani sp. nov. On the basis of morphological comparison of skeletal microstructures, in particular spine articulations and vertebral articular structures of the lateral arm plates, we conclude that the new species shares closest ties with Ophiacantha stellata, a recent species living in the present-day Caribbean at bathyal depths. Since colonization of the deep Mediterranean following the Messinian crisis at the end of the Miocene was only possibly via the Gibraltar Sill, the presence of tropical western Atlantic clades in the Plio-Pleistocene of the Mediterranean suggests a major deep-sea faunal turnover yet to be explored.

The role of isostatic adjustment and gravitational effects on the dynamics of the Messinian salinity crisis

The Messinian Salinity Crisis (5.96 to 5.33 Ma) was one of the most dramatic periods of sea-level change in recent geologic history – a break in the longstanding connection between the Atlantic Ocean and Mediterranean Sea. Stratigraphic evidence throughout the Mediterranean basin records a series of sea-level oscillations during this period. We explore the contribution of isostatic adjustment and gravitational effects, in response to changing water loads in the Mediterranean, on sea-level change at the Gibraltar Strait using an ice-age sea-level model. During the filling of the Mediterranean basin, inflow is increased due to both the gravitational pull of the water in the basin and subsidence at the Gibraltar sill associated with the increased water load. These effects reverse as the basin empties. We show that sea-level changes at the sill depart dramatically from the average sea-level change in the global ocean, and that the relative contribution of deformational and gravitational effects to this departure varies with the period of the cyclicity. As a further illustration of the sea-level physics, we consider a model of the cyclical flooding and emptying of the Mediterranean driven by a competition between tectonic uplift and erosion at the Gibraltar Strait (with uplift leading to closure of the strait and erosion leading to opening of the strait). The positive feedback on both refilling and drying of the Mediterranean Sea associated with the sea-level physics significantly changes the uplift and erosion parameters that are required to generate the periodicity evident in the stratigraphy. We find that a tectonic uplift rate at the Gibraltar sill of 1.5 mmyr−1, three times smaller than previously published, yields a periodic sea-level curve when the feedback is included. This rate is consistent with uplift rates estimated by geodynamic models appropriate for the Gibraltar arc (<2.65 mmyr−1).

Tectonic and sedimentary conditions necessary for the deposition of the Messinian evaporite successions in the eastern Mediterranean: A simple 2D model

Abstract The existing literature shows that the only saline water source for the eastern Mediterranean during the Messinian was the Atlantic Ocean and that the Sicily sill created a physical barrier between the eastern and western Mediterranean, higher than the physical barrier at the Gibraltar sill between the western Mediterranean and the Atlantic Ocean. The presence of distinctive Messinian subunits in the eastern Mediterranean similar to those observed in the western Mediterranean, suggests the existence of a similar set of depositional processes across the entire Mediterranean during the Messinian Salinity Crisis. The above statements impose two critical conditions: the eastern Mediterranean massive salt deposits must be older than their counterparts in the western Mediterranean, and the Sicily Gateway must have remained within a “goldilocks” zone during the deposition of the massive eastern Mediterranean evaporites (predominantly halite), while the sea level in the western Mediterranean would remain at the level of the Sicily sill. A simple 2-D model is developed which satisfies these conditions. The model suggests that the eastern and western basin margins experienced a nearly synchronized gypsum deposition associated with the initial drawdown of the Mediterranean level, followed by re-sedimentation in the deep basins of the terrigenous and early evaporite deposits as the drawdown intensified. The synchroneity of evaporite deposition across the eastern and western basins broke down as the Sicily Gateway became largely subaerial during a period when the Calabrian Arc area experienced uplift associated with slab break-off: the Sicily sill must have remained within a “goldilocks” zone to allow the right amount of saline water inflow into the eastern Mediterranean so that evaporites (massive halite) could be deposited. During this time, the sea level in the western Mediterranean was at the breach-level of the Sicily sill, and still open to Atlantic water influx, thus no evaporite deposition took place there. The model suggests that further restriction of the inflow occurred across the Gibraltar region which became largely subaerial associated with the uplift of the Gibraltar Arc caused again by lithospheric slab break-off. Similar to the Sicily Gateway, the Gibraltar Gateway also remained within the “goldilocks” zone to allow the right amount of saline water inflow into the western Mediterranean so that massive halite could be deposited. The re-opening of the Gibraltar Gateway re-flooded the western Mediterranean first, then the eastern Mediterranean allowing the deposition of a mixed evaporite-siliciclastic unit, followed by the transgressive sediments with distinctive brackish water Lago Mare fauna.

Mediterranean outflow pump: An alternative mechanism for the Lago-mare and the end of the Messinian Salinity Crisis

Abstract The final stage of the Messinian salinity crisis (MSC) was characterized by brackish-water “Lago-mare” conditions in the intermediate and marginal basins of the Mediterranean Sea. The presence of Paratethyan (former Black Sea) fauna in these deposits has fueled long-lasting controversies over the connectivity between the Mediterranean and Paratethys and contemporary sea-level drops in both basins. Here, we use the results of sub-precessional climate simulations to calculate the freshwater budget of the Mediterranean and Paratethys in the Messinian. We show that, during the MSC, the freshwater budget of Paratethys was positive, while the Mediterranean was negative. Using these numerical constraints, we propose a Mediterranean outflow pump as an alternative scenario for the two most dramatic hydrological changes in the MSC: (1) the Halite–Lago-mare transition and (2) the Pliocene reestablishment of marine conditions. Following the maximum MSC lowstand during halite formation, progressive Mediterranean sea-level rise resulting from African river runoff and overspill from both the Atlantic and Paratethys eventually reached the level of the Paratethys sill. A density contrast at this gateway caused dense Mediterranean waters to flow into the Paratethys, driving a compensatory return flow. This “pump” mechanism significantly enhanced Paratethyan inflow to the Mediterranean, creating suitable conditions for the Lago-mare fauna to migrate and thrive. When the Mediterranean sea level finally reached the height of the Gibraltar sill, Mediterranean outflow restarted there and enhanced exchange with the Atlantic Ocean. During this reorganization of the circulation, brackish and hypersaline waters were pumped out of the Mediterranean, and open-marine conditions were reestablished without major flooding of the basin at the Miocene-Pliocene boundary.

The pre-Messinian deep-sea Neogene echinoid fauna of the Mediterranean: Surface productivity controls and biogeographical relationships

Autochthonous deep-water echinoid faunas are very rare in the fossil record. A well-preserved bathyal echinoid assemblage is described from the middle Miocene Pakhna Formation of southern Cyprus. This low-diversity fauna is dominated by just two genera of spatangoid ( Palaeobrissus and Heteropneustes), and implies a water depth of 200–1000 m. Echinoids occur in modest abundance but are restricted to a small number of intervals where sapropels are numerous and thick, although never in the sapropels themselves. This suggests that echinoids in the deep pre-Messinian Mediterranean were surviving at depth by exploiting raised levels of nutrient input driven by Milankovitch forcing, but being excluded at times of peak carbon settlement. In contrast to the modern depauperated deep-water fauna of the Mediterranean, which has Atlantic affinities, these two species show strongest phylogenetic affinities with bathyal Caribbean fauna. Caribbean and Mediterranean deep-sea macrobenthic faunas were thus closely connected until just before the Gibraltar Sill formed.

Depositional environments of the Mediterranean “Lower Evaporites” of the Messinian salinity crisis: Constraints from quantitative analyses

We use simple quantitative analyses to evaluate controversial water level scenarios for the Mediterranean “Lower Evaporites” of the Messinian salinity crisis. Our results indicate that a shallow-water scenario for the Lower Gypsum units – with Mediterranean water level lower than the sill at Gibraltar – would imply unrealistic salt thicknesses on the order of 3 km. Some outflow to the open ocean must have persisted, implying that the Mediterranean was a deep-water basin during Lower Gypsum formation. Since glacio-eustatic fluctuations do not seem to have had a major influence on Lower Gypsum deposits, Mediterranean water level was even substantially higher than the Gibraltar sill. Our analyses furthermore show that precessional changes in the freshwater budget may explain the observed cyclic lithological changes of gypsum and non-evaporitic sediments. Potential precipitation of gypsum in the deep Mediterranean basins would have critically depended on the availability of oxygen and thus on the stratification of the water column. Finally, our results indicate that the deep Mediterranean halite units could have been deposited under shallow conditions, assuming that they correspond to the ~ 70 kyr time interval between glacials TG12 and TG14, when Mediterranean outflow to the Atlantic was blocked.

Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. Radiocarbon ages verify that the inner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximum rate 234 cm/kyr) and surface sandy contourite layer of 0.2–1.2 m thickness on the Cadiz slope (1–12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.

Late glacial to recent paleoenvironmental changes in the Gulf of Cadiz and formation of sandy contourite layers

Planktonic foraminifera and coccoliths were analyzed from six gravity cores obtained on a deep terrace located on the upper continental slope of the Gulf of Cadiz, between 400 and 700 m water depth. The lithology of these cores consists mainly of muds with some interbedded sandy or silty foraminifer-rich layers that have been reported as contourites originated by the action of the Mediterranean Outflowing Waters (MOW) sweeping the sea-bottom today. After the Last Glacial, characterized by muddy sedimentation, sandy contourites start to deposit in the Gulf of Cadiz during the Bølling–Allerød (14–11 14C kyr BP) climatic optimum. This trend was broken during the Younger Dryas (11–10 14C kyr BP) and started again after the end of the Younger Dryas. This pattern is recorded in most of the cores by a single sandy contourite layer formed during the first deglaciation stage (Bølling–Allerød time) and a sandy contourite interval that initiates immediately after the end of the Younger Dryas, during the second stage of the deglaciation and continues during the Holocene. The stratigraphy and climatic reconstruction was based on the evolution of the calcareous plankton assemblages during the last climatic transition, from the Last Glacial to the recent Holocene. The sharp reduction of sinistral N. pachyderma along with the reduction of G. quinqueloba mark the base of Bølling–Allerød time. This is also related to a prominent peak of the subtropical species ( G. sacculifer and G. ruber). The Younger Dryas is identified by a reduction in abundance of the subtropical species, that again increase just after this period. The second step of deglaciation is marked by a sharp decrease in the relative abundance of dextrally coiled N. pachyderma. In previous papers the sequence of sandy contourites has been related to sea level rise, greater Gibraltar sill depth and enhanced energy of the MOW. In this work, we suggest that the individual sand layer episodes are condensed layers originating during times of rapid warming and relative sea-level rise within the last deglaciation. During these times the coastline migrated more rapidly landward and the terrigenous input decreased as it began to be trapped on land and on the shelf, resulting in a major drop in sedimentation rate on the upper continental slope. In times of low sedimentation rate the particles would have a longer residence time within the upper mixed layer of the near surface sediment and therefore the energy of currents would act longer, producing a more efficient winnowing of the sediment. At the same time more generations of benthic organisms would rework the mixed layer, favoring this winnowing.

Middle to Upper Miocene benthonic foraminiferal palaeoecology of the Tap Marls (Alicante Province, SE Spain) and its palaeoceanographic implications

The benthonic foraminiferal faunas of twenty sections have been analysed statistically (cluster analysis) and interpreted palaeoecologically. The analysis shows that oxygenation was the most important factor for the distribution of the faunas in the investigated area. The lifting and lowering of the sill to the Atlantic Ocean (Gibraltar sill), in conjunction with climatic changes and sea level fluctuations, led to changes in the current system and to two faunal turnovers at the transition from Langhian to Serravallian (∼15.2–15.0 Ma) and at the end of the Serravallian, respectively (∼11.7–11.4 Ma). The faunas indicate well oxygenated (Langhian), reduced oxygenated (Serravallian) and again well oxygenated (Tortonian) conditions. A simplified circulation model for the western Mediterranean, based on the author's interpretation and additional sources, suggests: (a) an estuarine circulation during the Langhian; (b) a `restricted' anti-estuarine pattern with sluggish circulation until the end of the Serravallian; and (c) an anti-estuarine circulation similar to today's situation during the Tortonian.

Mediterranean undercurrent sandy contourites, Gulf of Cadiz, Spain

The Pliocene—Quaternary pattern of contourite deposits on the eastern Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the Mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late Miocene opening of the Strait of Gibraltar. Coincident with the northwestward decrease in undercurrent speeds from the Strait there is the following northwestward gradation of sediment facies associations: (1) upper slope facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium-fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. Compared to this, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Further northwestward, sediment drift grades to biogenous silt near the Faro Drift at the Portuguese border. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean undercurrent, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. The bottom-current deposits of valleys and the contourites of the Cadiz slope intervalley areas are distinct from turbidite systems. The valley sequences are not aggradational like turbidite channel—levee complexes, but typically exhibit bedrock walls against ridges, extensive scour and fill into adjacent contourites, transverse bedform fields and bioclastic lag deposits. Both valley and contourite deposits exhibit reverse graded bedding and sharp upper bed contacts in coarse-grained layers, low deposition rates, and a regional pattern of bedform zones, textural variation, and compositional gradation. The surface sandy contourite layer of 0.2–1.2 m thickness that covers the Gulf of Cadiz slope has formed during the present Holocene high sea level because high sea level results in maximum water depth over the Gibraltar sill and full development of the Mediterranean undercurrent. The late Pleistocene age of the mud underlying the surface sand sheet correlates with the age of the last sea-level lowstand and apparent weak Mediterranean undercurrent development. Thus, the cyclic deposition of sand or mud layers and contourite or drape sequences appear to be related to late Pliocene and Quaternary sea-level changes and Mediterranean water circulation patterns. Since its Pliocene origin, the contourite sequence has had low deposition rates of < 5 cm/1000y on the upper slope and < 13 cm/1000y in the middle slope sediment drift.

The Mediterranean deep-sea fauna: pseudopopulations of Atlantic species?

The present deep-sea fauna of the Mediterranean is characterized by a very low degree of endemism and a low diversity compared to the fauna of the Northeast Atlantic. The Gibraltar sill, 280 m deep, has historically been regarded as the physical barrier to the potential colonization of the Mediterranean from the richer Atlantic fauna. We suggest instead that the larval ecology of individual species is the single most important factor governing the composition of the deep Mediterranean benthos. A number of deep-water species have planktotrophic larvae that undergo vertical ontogenetic migrations to surface layers and are thus potential colonizers of the Mediterranean via the inflowing surface current across the Gibraltar sill. The hydrological nature of the Mediterranean bottom water (high temperature, high salinity) is thought to act as a barrier to the successful establishment of species with such a dispersal capacity, firstly at the time of metamorphosis then at the time of reproduction. We also suggest that much of this Mediterranean deep-sea fauna consist of reproductively sterile pseudopopulations that are constantly derived through larval inflow from Atlantic mother populations. The same model applies, mutatus mutandis, to the deep benthos of other threshold basins, among which the Red Sea is probably the most remarkable example.

Zinc budget in the Mediterranean Sea: a hypothesis for non-steady-state behavior

The concentrations of Zn were determined in surface and deep-water samples at 11 sites in the Mediterranean Sea during the PHYCEMED 2 (October 1983) cruise. The Zn vertical profile is marked by a surface concentration (typically 3.0 nmol kg −1 in the Western Basin) higher than in the open ocean where the surface layer is Zn depleted. The subsurface layer (between 30 and 80 m) shows Zn and salinity minumum values along the path of the Atlantic inflow throughout the Western Basin. A weak intermediate maximum is present between 200- and 400-m depth, whereas Zn values are homogeneous in the deep waters from ∼300-m depth, as are T and S values. In deep water, the Zn/P ratios are similar to those of plankton; this is a good argument for a biological Zn cycle, which may explain the intermediate maximum caused by remineralization of organic matter settling from the euphotic layer. The deep layer is expected to be homogeneous as a result of the basin dynamics, whereas the biological cycle will induce a Zn depletion in the surface layer. The relatively high values observed in the surface waters of the Western and Eastern basins are explained by the Zn cycle's non steady state, linked to recent increases in Zn supplied to the sea, which can be estimated as ∼4 × 10 8 mol year −1 in 1983, i.e. about twice the Mediterranean deep Zn outflow over the Gibraltar Sill. Owing to the Zn and salinity minimum values in the subsurface Atlantic inflow, we can conclude that most of the Zn originates from atmospheric and terrestrial inputs around the Mediterranean Sea.

Relationships between deep-sea tunicate populations west and east of the Straits of Gibraltar

Twenty-four species of tunicates were collected from deep bottoms on each side of the Gibraltar sill, in the adjacent Mediterranean Sea and Atlantic Ocean. In the Atlantic, stations bathed by Atlantic and Mediterranean waters were both sampled. No transport of ascidian taxa by the outflow of Mediterranean water into the Atlantic is apparent. The alternative hypothesis of an Atlantic origin of bathyal ascidian species in the Mediterranean Sea is proposed.

Direct measurements of the flow of western Mediterranean deep water over the Gibraltar sill

The Gibraltar Pilot Experiment was carried out in June 1984 in order to obtain preliminary moored current measurements in the vicinity of the sill in the Strait of Gibraltar. Bottom‐mounted Doppler current measurements and moored temperature measurements from this experiment show directly the westward transport of Western Mediterranean Deep Water (WMDW) over the sill. The net westward transport of WMDW over the sill is roughly estimated to be 0.2×106 m3 s−1.

Yes, some of the Mediterranean outflow does come from great depth

In 1973 Stommel et al. predicted that the Western Mediterranean Deep Water (WMDW) contributes directly to the Mediterranean outflow through the Strait of Gibraltar (Strommel et al., 1973). Hydrographic data obtained in November 1985 showed unambiguous evidence of WMDW west of the major Gibraltar sill, thus confirming the prediction.

Water masses and circulation in the Western Alboran sea and in the Straits of Gibraltar

Some new elements of Alboran Sea circulation and water masses in close connection with flows through the Straits of Gibraltar are presented. A general description is given of water masses found in 1981 during MEDIPROD IV cruise from the Gulf of Cadiz to the Gulf of Lions, followed by a brief review of the conditions found in the Straits of Gibraltar based on 1981 and 1961 observations. The hydrological structure (vertical and horizontal) and surface and bottom currents typical of the western Alboran Sea are analysed. We present some detailed observations about vertical currents in the deep waters near the Straits and finally we analyse the characteristics of the Mediterranean overflow through the Straits. Our conclusions in part confirm those of Bryden and Stommel (1982) and in part shed light on the general pattern of the Mediterranean currents entering the Alboran Sea from the east. However, considering the suggestion of Stommel, Bryden and Mangelsdorf (1973) that the deep currents in the interior of the western Mediterranean basin are basically controlled by a Bernoulli flow induced by the intense currents in the Straits and the weak vertical stratification in the Mediterranean, we suggest that an interaction of a large scale circulation pattern and the topography of the Alboran basin, which is much shallower than the western Mediterranean basin, may play a major role in controlling, hydraulically, the deep Mediterranean waters overflow. A detailed water mass analysis of 1981 data and comparison with previous data revealed a time and space variability in the Mediterranean waters. The scales of this variability are unknown but the θ- S variations make it difficult to make assumptions as to the depth of origin of the deep waters found in the Alboran sea which flow into the Atlantic Ocean over the Gibraltar sill.

No current reversal at 10,000 B.P. in the strait of Gibraltar — A discussion

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. Radiocarbon ages verify that the inner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximum rate 234 cm/kyr) and surface sandy contourite layer of 0.2–1.2 m thickness on the Cadiz slope (1–12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.