Mediterranean Deep Water Research Articles

Toward an Understanding of Deep-Water Renewal in the Eastern Mediterranean

Abstract This paper presents a scenario for understanding the unexpectedly large changes of deep waters and thermohaline circulation in the Eastern Mediterranean during the past decade. It is demonstrated as a possible deep-water renewal mechanism in the Eastern Mediterranean in a numerical simulation with a high-resolution model, which has successfully reproduced the observed 1987 and 1995 regimes as shown by the two Meteor cruises. A budget study of the model simulation has shown that more than 75% of the salt added to the deep layers of the Eastern Mediterranean could have come from the top 1000 m by a salinity redistribution process triggered by intensive cooling over the Aegean Sea. A water transformation process analysis is carried out in the model simulation to reveal how colder and fresher dense deep water formed in the Aegean is turned into the Eastern Mediterranean Deep Water (EMDW) as observed. Surface heat and freshwater fluxes are diagnosed to show the roles of each component during the trans...

Interbasin deep water exchange in the western Mediterranean

Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean. The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provençal Basin was postulated in the depth range 700–1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero‐Provençal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero‐Provençal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600–1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv = 106 m3 s−1), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero‐Provençal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero‐Provençal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior.

137Caesium distribution in the eastern Mediterranean Sea: recent changes and future trends

A series of sampling campaigns were carried out in the eastern Mediterranean in the period 1995–1997, to examine the relationship between the distribution of 137Cs in the water column and water mass circulation. 137Cs concentration in the surface water ranges between 3.3 and 4.0 mBq/l all over the area, indicating that the input due to the Chernobyl accident has been transferred to deep water layers by convection processes. In fact, in the vertical profiles, relative maxima are observed in the intermediate (4 mBq/l) and deep waters (≈2.5 mBq/l) formed after the Chernobyl accident. A clear Chernobyl signal also traces the new deep waters formed in the Aegean Sea that, exiting from the Cretan Arc Straits, since 1991 are spreading in the bottom layer of the eastern Mediterranean. The changes in 137Cs vertical profiles related to the new thermohaline circulation of the intermediate and deep waters of the eastern Mediterranean are being monitored at a deep station in the western Ionian Sea. The 1997 profile shows a decrease in 137Cs concentration both in the Levantine intermediate water and in the eastern Mediterranean deep water with respect to 1996. The decrease in Levantine intermediate water is likely due to seasonal/interannual variability, while the changes in the deep layer are related to the spreading westward into the Ionian of the new Aegean dense water. Along the path towards the western Mediterranean, 137Cs content in the Levantine intermediate water decreases from ≈4 mBq/l in the W-Ionian Sea to ≈2 mBq/l at the western sill of the Sicily Straits, due to mixing with underlying water, with lower caesium content, near the Malta Sill. Time-series measurements at the western sill of the Sicily Straits show that, while 137Cs concentration in the surface water entering the eastern Mediterranean remained constant (≈3.5 mBq/l) in the period 1993–1997, its level in the Levantine intermediate water leaving the basin decreased from 3.5 to 2.0 mBq/l.

Indium and yttrium in North Atlantic and Mediterranean waters: comparison to the Pacific data

Vertical profiles of dissolved indium and yttrium were determined in the eastern North Atlantic and the Mediterranean Sea to compare with those of the North Pacific reported earlier. The Y concentrations in the surface waters are 120 pmol/kg in the North Atlantic and 205 pmol/kg in the Mediterranean Sea, which are significantly higher than 80 pmol/kg in the North Pacific. The difference may be attributable to the different strength of input of Y to the oceans from fluvial and aeolian sources. In contrast, the deep water concentration of Y increases in the order of North Atlantic < Mediterranean < North Pacific. This trend is similar to that of dissolved Si, suggesting that Y is involved in the biogeochemical regeneration cycle. The vertical profiles of In are far more complex than Y. The In profile shows a systematic increase from 0.6 pmol/kg at the surface to 1.7 pmol/kg at 2100 m in the North Atlantic, whereas it is almost featureless at a mean concentration of 3.8 ± 0.6 pmol/kg in the Mediterranean Sea. The North Atlantic and Mediterranean In concentrations are considerably higher than those observed in the North Pacific (∼0.1 pmol/kg), and such a large interoceanic variation has been reported before only for Al and Ga. Like Al, the deep water In concentration that decreases in the order of Mediterranean > North > North Pacific exhibits an inverse trend of Y and nutrients. Indium is highly particle-reactive (47% association in the Mediterranean Sea), and must have a short mean oceanic residence time. However, the featureless dissolved In profile in the Mediterranean Sea is clearly different from the profiles of dissolved Al, showing increase with depth (Hydes et al., 1988; Measures and Edmond, 1988), suggesting that significant fractionation of the two elements is taking place in the ocean. The interoceanic variations of dissolved In and Al may be ascribed to the different intensities of external input of which aeolian has been considered to be major rather than fluvial. However, the difference of In and Al concentrations in the deep waters of the above oceanic basins are significantly greater than those of other refractory elements, such as Ce, Ti, Hf, and Zr, whose major sources to the ocean are also considered to be aeolian. Furthermore, the In/Al ratios in seawater are about two orders of magnitude greater than the average crustal ratio. Thus, some additional sources, though not yet certain, may be required to explain the high concentrations of In in the Atlantic and the Mediterranean deep waters.

The Eastern Mediterranean in the 80s and in the 90s: the big transition in the intermediate and deep circulations

We present definitive observational evidence that the startling change of the Eastern Mediterranean deep circulation observed in winter 1995 and documented by [Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevich, V., Luchetta, A., 1996. Recent changes in the Eastern Mediterranean deep water. Science 271, 333–335.] actually started before October 1991. This change involved not only the deep water mass pathways but also the origin and pathways of the water mass spreading in the intermediate layer. We carry out the first unified analysis of the POEMBC-O91 data set, which shows that, differently from the previous decade of the 80s, the Cretan/Aegean Sea was in 1991 the `driving' engine of the intermediate, transitional and deep layer circulations, with Cretan Intermediate Water (CIW), transitional water and Cretan Deep Water (CDW) spreading out from the Cretan Sea into the basin interior. The most important new results are: (a) the Levantine Intermediate Water (LIW) formed inside or at the periphery of the Rhodes gyre is blocked in its traditional westbound route on its density horizons σθ=29.05 and 29.10 kg/m3 by a three-lobe strong anticyclonic structure in the Southern Levantine, which induces a substantial LIW recirculation in the Levantine basin itself; (b) the CIW exiting from the Western Cretan Arc Straits spreads into the Ionian interior on the σθ=29.05–29.10 kg/m3 isopycnal surfaces, thus replacing the LIW confined in the Levantine basin. A branch of CIW flows eastward in the Cretan passage and is entrained by the Ierapetra anticyclone to flow again into the Cretan Sea through the Eastern Cretan Arc Straits; (c) on the horizons σθ=29.15 and 29.18 kg/m3 a transitional water mass of Cretan origin, denser than CIW, and CDW are observed to spread out massively from the Cretan Arc Straits both into the Ionian and Levantine interiors. These isopycnal surfaces rise to much shallower depths in 1991 than in 1987, increasing the salt content of the intermediate, transitional and deep layers. This leads to a massive salt increase in the Ionian below 1200 m, clearly related to lateral advection of the new denser waters of Cretan/Aegean origin, thus contradicting the hypothesis of a vertical salt redistribution proposed by Roether et al.

The Eastern Mediterranean tritium distribution in 1987

We describe and evaluate the distribution of the transient tracer tritium in the Eastern Mediterranean, on the basis of approximately 700 data points obtained from cruise M5/6 of F/S METEOR, Aug.–Sept. 1987. The horizontal resolution provided by the dataset is limited, but the vertical resolution is sufficient to resolve most of the principal features of the tritium distribution, and the data quality is good. Previous results concerning the thermohaline circulation of the Eastern Mediterranean based on tracer data are confirmed. Up to 1987 Eastern Mediterranean Deep Water (EMDW) was replenished from the Adriatic Sea, a near-bottom replenishment from the Aegean Sea contributing at most about 10%. A comparison with tritium data from a 1978 METEOR cruise gives evidence of a transfer of near-surface waters into the EMDW during the period between the two surveys. Cretan Intermediate Water (CIW) with a core depth of approximately 700 m and replenished from the Aegean Sea is found to be a prominent water mass. CIW spreads primarily westward, and is bordered in the Levantine Sea by EMDW which in this region extends to about 500 m depth. We infer that CIW replenishment has been rather steady since at least some decades, with a turnover time on the order of 10 years, and we name evidence that suggests a possible contribution of CIW to the dense water formation in the Adriatic Sea. Specific differences between the distribution of tritium and that of the chlorofluorocarbon tracer CFC 12 are noted. Tritium concentrations in the near-surface waters are found to be somewhat variable. The explanation is that for this tracer redistribution within the water column is particularly relevant.

Dynamics and mixing of the Eastern Mediterranean outflow in the Tyrrhenian basin

Hydrographic measurements in the Sicily Channel and in the southern Tyrrhenian Sea show that the water outflowing from the Eastern Mediterranean Basin enters the Tyrrhenian Sea directly, close to the Sicily coast. Two water types have been distinguished: the upper part constituted by the Levantine Intermediate Water (LIW), and the transitional Eastern Mediterranean Deep Water (EMDW) below. Current measurements taken about 50 km after the entrance into the Tyrrhenian evidence a well-developed mean flow from a depth of 500 m to the bottom. While the LIW flows at the known depth for this type of water (namely between 500 and 800 m), a considerable volume of transitional EMDW was seen to sink from a depth of about 300 m in the Sicily Channel to a maximum depth of 1850 m in the Tyrrhenian Sea, where it floats over the Tyrrhenian Deep Water (TDW). The sinking is a consequence of the higher density of the incoming water ( σ θ >29.10) with respect to that of the intermediate Tyrrhenian waters ( σ θ ∼29.00–29.05). The progressive sinking of the transitional EMDW along the principal route from the Sicily Channel to the Tyrrhenian Sea, can be observed in the evolution of the isotherms, isohalines and isopycnals. The isopycnals, in particular, clearly tend to follow the bottom slope. The mean hydrographic characteristics of this vein of Eastern Mediterranean water progressively change along the route, and two regions of strong mixing are identified. The first occurs immediately after the Sicily Channel, while the second is found following the entrance of the vein into the Tyrrhenian Sea. An estimation of the entrainment stress shows that it largely dominates over the acceleration term. When the bottom slope does not influence the water motion anymore, the mixing is entirely ascribable to molecular diffusion via finger instability. The transport of transitional EMDW through the Sicily Channel is estimated to be about 0.2–0.3 Sv, implying a salinity anomaly export of 0.05–0.075 psu per Sv to the deep layers of the Tyrrhenian Sea.

Hydrology and currents observed in the channel of Sardinia during the PRIMO-1 experiment from November 1993 to October 1994

The channel of Sardinia is one of the less-known areas of the Western Mediterranean sea. The paths of the Levantine Intermediate Water (LIW), the Tyrrhenian Dense Water (TDW) and the Western Mediterranean Deep Water (WMDW) mainly, in this wide passage, are still debated. Two major aims of the PRIMO-1 (Programme de Recherche International en Mediterranée Occidentale) experiment were to specify the mean circulation paths of these intermediate and deep waters there and to verify hypotheses about the migration of mesoscale eddies from Algeria to Sardinia and their effect on the circulation paths of all the water masses. Despite an important loss of moored instruments, current time series have been collected from November 1993 to October 1994 at four different locations and two CTD cross-sections have been performed. Current meters on the Sardinian slope indicate, during 1 year, an almost permanent alongslope flow toward the northwest in the intermediate and deep layers. Currents recorded on the African slope are always directed alongslope eastward down to 2500 m. Coherently, in the mid-western part of the channel, the bottom flow is northward along the isobaths, while the 200-m flow is southward due to the influence of anticyclonic eddies. At the bottom in the central part of the channel, the current is low and changing: episodes of northeast current along the orientation of the deepest channel axis alternate with westward flows. No seasonal variations have been clearly detected, but the mesoscale variability is large and has a deep signature in the western part of the channel.

The large deep water transient in the Eastern Mediterranean

The recent changes in the thermohaline circulation of the Eastern Mediteranean caused by a transition from a system with a single source of deep water in the Adriatic to one with an additional source in the Aegean are described and assessed in detail. The name Cretan Sea Overflow Water (CSOW) is proposed for the new deep water mass. CSOW is warmer ( θ>13.6°C) and more saline ( S>38.80) than the previously dominating Eastern Mediterranean Deep Water (EMDW), causing temperatures and salinities to rise towards the bottom. All major water masses of the Eastern Mediterranean, including the Levantine Intermediate Water (LIW), have been strongly affected by the change. The stronger inflow into the bottom layer caused by the discharge of CSOW into the Ionian and Levantine Basins induced compensatory flows further up in the water column, affecting the circulation at intermediate depth. In the northeastern Ionian Sea the saline intermediate layer consisting of Levantine Intermediate Water and Cretan Intermediate Water (CIW) is found to be less pronounced. The layer thickness has been reduced by factor of about two, concurrently with a reduction of the maximum salinity, reducing advection of saline waters into the Adriatic. As a consequence, a salinity decrease is observed in the Adriatic Deep Water. Outside the Aegean the upwelling of mid-depth waters reaches depths shallow enough so that these waters are advected into the Aegean and form a mid-depth salinity-minimum layer. Notable changes have been found in the nutrient distributions. On the basin-scale the nutrient levels in the upper water column have been elevated by the uplifting of nutrient-rich deeper waters. Nutrient-rich water is now found closer to the euphotic zone than previously, which might induce enhanced biological activity. The observed salinity redistribution, i.e. decreasing values in the upper 500–1400 m and increasing values in the bottom layer, suggests that at least part of the transition is due to an internal redistribution of salt. An initiation of the event by a local enhancement of salinity in the Aegean through a strong change in the fresh water flux is conceivable and is supported by observations.

Property distributions and transient-tracer ages in Levantine Intermediate Water in the Eastern Mediterranean

We present distributions of chlorofluorocarbons (CFCs) and ages derived from them, of carbontetrachloride, and of hydrographic properties, in Levantine Intermediate Water (LIW) in the Eastern Mediterranean. The data originate from surveys of F/S METEOR in 1987 and 1995, which bracket the profound changes that have occurred in the Eastern Mediterranean deep waters, due to bottom water formation from Aegean Sea overflow and related enhanced upwelling (Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevich, V., Luchetta, A., 1996a. Recent changes in Eastern Mediterranean deep waters. Science, 271, pp. 333–335). As a framework for an interpretation, classical knowledge on LIW is summarized. A density horizon of σ θ =29.05 is selected to characterize LIW, for which salinities and temperatures in 1995 were still similar to classical values. A principal result derived from the CFC-age distributions is that the enhanced upwelling of deep waters has been continuous up into the LIW layer. Newly formed LIW in both surveys is found to be distributed over an extended region which includes the Cretan Sea. The lowest CFC ages in LIW, amounting to several years, are found in this region. Smaller but significant apparent CFC ages are present in the mixed layer in a winter situation (1995). The CFC data are compatible with a formation of LIW by open-ocean convection. Outcropping of the isopycnal typical of young LIW was observed in the Aegean Sea in 1995, while to the east and southeast of the Rhodes Gyre no evidence of a major recent LIW formation was found. The CFC age distributions give an upper limit for the apparent travel time of LIW up to the Strait of Sicily of about 8 years. CCl 4 is found to be chemically unstable in the Eastern Mediterranean (chemical lifetime in LIW <5 years), but this feature allows us to use this tracer as a low-life age marker. The present work can serve as a basis for future data evaluation by Mediterranean circulation models.

On the upper layer circulation in the Alboran Sea

We present new experimental results and assemble them with previous results in order to develop an improved picture of the upper layer circulation in the Alboran Sea. It is suggested that the key idea to understanding this upper layer circulation is the tendency of the Atlantic jet (AJ) to have negative curvature. Local interactions with the western Alboran gyre (WAG) or the African coast can, however, counterbalance this tendency and modify the anticyclonic path of the AJ. It is also proposed that the density gradients in the WAG can be maintained in part by means of an intermittent surface cross‐gyre current which results in an input, mixing, and renewal of Atlantic water. The static stability at the bottom of the gyre increases because of the mixing of Atlantic water with Western Mediterranean Deep Water which is uplifted close to the African coast. This mixing process thereby acts as a local source of potential vorticity. We also report the existence in the Alboran Sea of subsurface anticyclonic eddies (located between 100 and 400 m) of relatively cold water that appear to be detached from the Iberian shelf. Regarding the large‐scale variability of the AJ‐WAG system, we present evidence of an eastward migration of the WAG and the subsequent formation of a new anticyclonic gyre in the western Alboran basin on a timescale of 1 month. This eastward gyre migration process temporarily allows the simultaneous presence of three anticyclonic gyres in the Alboran Sea.

On the importance of the choice of wind stress forcing to the modeling of the Mediterranean Sea circulation

A 1/4° degree ocean general circulation model is used to examine the role that four different wind stress climatologies play on the circulation of the Mediterranean. The wind stress climatologies examined are those derived from numerical weather prediction models (National Meteorological Center (NMC) and European Centre for Medium‐Range Weather Forecasts (ECMWF)) and one based on observations (Southampton Oceanography Centre (SOC)). Significant differences exist between the wind climatologies over the Mediterranean and in the response of an ocean general circulation model forced by the different climatologies. Excessive coastal upwelling/downwelling is found to be associated with the extreme zonal nature of one of the climatologies. Surface circulation differences include the position and penetration of the Mid‐Mediterranean Jet into the Levantine, the Ionian, and the Tyrrhenian circulations. Significant differences exist in the pathways for dispersal of Levantine Intermediate Water. Under the SOC forcing, there is a reduction in Eastern Mediterranean Deep Water formation in the Southern Adriatic, compensated by the production of intermediate or deep water in the Aegean. The ECMWF climatology is found to be associated with much more cyclonic doming in the Gulf of Lions, leading to better formation of Western Mediterranean Deep Water.

Spreading of Labrador Sea Water in the eastern North Atlantic

The spreading of Labrador Sea Water (LSW) in the eastern North Atlantic south of 55°N is described on the basis of recent, high‐resolution hydrographic lines and using an inverse model of that basin general circulation. The isopycnic “salinity anomaly” relative to a standard θ/S curve is used to detect the main branches and limits of dominant influence of the intermediate water masses. The LSW crosses the Mid‐Atlantic Ridge at latitudes around 50°N with parts spreading farther eastward and southward. One southward pathway along the eastern side of the Mid‐Atlantic‐Ridge stands out as an eastern basin counterpart of the western Atlantic “Deep Western Boundary Current.” An entry into the Bay of Biscay in its northern part is also evidenced with striking similitudes between two data sets from 1974 and 1989–1990. At the southern limit of the LSW extension, in the vicinity of the Azores‐Biscay Rise, a marked front exists basinwide around 1800 m with the more saline Deep Mediterranean Water (DMW). The inverse model velocity field is consistent with most features of the tracer distributions and allows estimation of LSW transports: 6.3±1.2 Sv of LSW with salinities lower than 34.94 cross the 35°W meridian eastward between 44°N and 54°N, one fourth of which recirculates southwestward in the western basin, while another fourth turns northward across 54°N and the remaining half proceeds southward in the eastern basin. The model circulation suggests that DMW results from mixing between eastern basin LSW and the overlying Mediterranean Outflow Water, most probably through double diffusion. We finally describe several mesoscale structures of anomalously high LSW influence south of the LSW‐DMW front, notably one sampled by a RAFOS float. The velocity and potential vorticity fields suggest that these “LSW‐eddies” are formed through baroclinic instability at the LSW‐DMW front.

Warming and freshwater budget change in the Mediterranean since the 1940s, their possible relation to the greenhouse effect

Temperature and salinity of Mediterranean deep water have been increasing, based on observations extending back to 1959. Over the 1940–1995 period, the corresponding changes in heat and water budgets across the sea surface have been estimated to be 1.74 Wm−2 for the greenhouse effect change, 0.5°C and 0.4°C for mean air and sea‐surface temperature increases, respectively, and an increase of 0.1 m (or 11%) in the freshwater deficit, which is due partly to human activities and partly to climatic changes.

A tracer study of the formation, dispersal, and renewal of Levantine Intermediate Water

The formation and dispersal of Levantine Intermediate Water (LIW) are examined using a series of tracer experiments based on an eddy‐permitting general circulation model of the Mediterranean. LIW is formed in the region of the Rhodes gyre and disperses via both the time‐mean and time‐varying circulation. Tracer released in the Rhodes gyre is transferred westward by the time‐mean circulation but also southward and eastward by the time‐varying circulation. Off‐line tracer experiments suggest that the time‐varying circulation exhibits both advective and diffusive components, where the advective component opposes downgradient transfer in some regions. The rate of invasion of tracer into the main thermocline suggests that the renewal timescale is 10–20 years for LIW and of order 100 years for Eastern Mediterranean Deep Water. Assuming advection dominates the transfer of tracer, the implied annual subduction rate is 0.6±0.1 Sv in the Rhodes gyre and 0.2±0.1 Sv in the Adriatic Sea. The LIW mode is found to become cooler and fresher as a result of mixing with the Adriatic outflow, while its potential vorticity appears well conserved.

96/05775 Triboelectric separation of coal from mineral impurities

Dissolved gaseous mercury (DGM) was studied in surface and deep waters of the Mediterranean Sea over the last 15 years during several oceanographic cruises on board the Italian research vessels Urania and Minerva Uno, covering the Western and Eastern Mediterranean Basins as well as Adriatic Sea as its northernmost part. DGM was measured together with other mercury species (RHg - reactive Hg, THg - total Hg, MeHg - monomethyl Hg and DMeHg - dimethylmercury), and with water quality parameters in coastal and open sea deep water profiles. DGM represents a considerable portion of THg (on average 20%) in Mediterranean waters. Spatial and seasonal variations of measured DGM concentrations were observed in different identified water masses. DGM was the highest in the northern Adriatic, the most polluted part of the Mediterranean Sea as a consequence of Hg mining in Idrija and heavy industry in northern Italy, and near the Gulf of Lion. Generally, average DGM concentration was higher in the West and East Mediterranean Deep Waters (WMDW and EMDW) and Levantine Intermediate Water (LIW) than in overlaying Modified Atlantic Water (MAW); however, it was the highest in N Adriatic Surface waters (NAdSW) and consequently in outflowing Adriatic Deep Waters (AdDW). In deep water profiles the portion of DGM typically increased at depths with oxygen minimum and then towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its bacterial and/or geotectonic origin. During oceanographic cruises in 2011 and 2015 in the Tyrrhenian Sea, novel methods for continuous DGM determination in surface waters (Wangberg and Gardfeldt, 2011, Begu et al., 2016) were applied and compared to the standard method, and showed good agreement.

Active post-depositional oxidation of the most recent sapropel (S1) in sediments of the eastern Mediterranean Sea

Over a wide area of the eastern Mediterranean basin, two Mn-rich layers have been observed above the most recent sapropel (S1), one immediately above the sapropel top and one a few centimetres closer to the sediment surface. Different mechanisms have been proposed to explain the occurrence of these two Mn peaks: either both peaks have a diagenetic origin in which case the upper Mn peak is actively forming, or the lower peak is actively forming and the upper peak has a different formation mechanism. High-resolution porewater, including a gel sampler used for the first time in marine sediments, and solid phase data are now used to demonstrate that the oxidation front is located at the level of the lower Mn peak which, therefore, is presently being formed. A barium-organic carbon relationship is used to calculate the initial organic carbon profile of the S1 sapropel. The palaeoproductivity profiles generated by this method demonstrate that the original sapropel unit was bounded by the upper Mn peak. This implies that the interval between the two Mn peaks, where a low organic carbon content is now observed, was originally part of the sapropel. The initially deposited organic carbon has been oxidised by a progressive downwards-moving oxidation front. The penetration depth of this oxidation front, i.e., the distance between the two Mn peaks, is mainly determined by the organic carbon content, the sediment accumulation rate, and the bioturbation depth. The upper Mn peak appears to have formed as a result of either Mn precipitation upon oxygenation of previously anoxic eastern Mediterranean deep water, or preservation of a surficial Mn peak at the end of the high productivity episode. In either case the upper Mn peak marks the end of sapropel formation as indicated by the Ba profiles. This means that formation of the S1 sapropel ceased more recently than is indicated by radiocarbon dating of the visible top of S1.

Modeling the dispersal of Levantine Intermediate Water and its role in Mediterranean deep water formation

This paper demonstrates the importance of Levantine Intermediate Water (LIW) in the deep water formation process in the Mediterranean using the modular ocean general circulation model at 0.25° resolution, 19 vertical levels, over the entire Mediterranean with an open Gibraltar strait. LIW formation is strongly prescribed in the Rhodes Gyre region by Haney [1971] relaxation, while in other regions, surface salinity relaxation is much reduced by applying the ‘mixed’ thermohaline surface boundary conditions. Isopycnal diagnostics are used to trace water mass movements, and volume fluxes are monitored at straits. Low viscosity and diffusion are used to permit baroclinic eddies to play a role in water mass dispersal. The overall water budget is measured by an average flux at Gibraltar of 0.8 Sv, of which 0.7 Sv is exchanged with the eastern basin at Sicily. LIW (density around 28.95) spreads rapidly after formation throughout the entire Levantine due to baroclinic eddies. Toward the west, LIW accumulates in the northern and central Ionian, with some entering the Adriatic through Otranto and some mixing southward in eddies and exiting to the western Mediterranean through Sicily. LIW is converted to deep water in the south Adriatic at an average rate of 0.4 Sv. Water exchange through the Otranto strait appears to be buoyancy driven, with a strong bias to the end of winter (March–April), while at Sicily the exchange has a strong symmetric seasonal cycle, with maximum transport of 1.1 Sv in December indicating the effects of wind driving. LIW pathways in the west are complex and variable. In the Tyrrhenian, intermediate water becomes uniform on isopycnal surfaces due to eddy stirring. West of Sardinia, two LIW boundary currents are formed in the Balearic basin; one flows northward up the west coast of Sardinia and Corsica, and one westward along the northern African coast. The northward current is consistent with observations, while the westward current is intermittent for the first 10 years, often breaking up into eddies which enter the basin interior. Some observations of high‐salinity waters near the African coast may support this interpretation. LIW retains a subsurface salinity maximum of 38.4–38.5 practical salinity units (psu) when reaching the northwestern Mediterranean, contrasting with surface waters fresher than 38.0 psu. West Mediterranean deep water is formed below 1500 m depth with climatological characteristics, when it is mixed and cooled during winter convection in Lions Gyre.

Radium as internal tracer of Mediterranean Outflow Water

Radium‐226 and 228Ra data are presented for the Western Mediterranean Sea and the NE Atlantic. In the NW Mediterranean Sea, the near‐shore surface waters of the Ligurian Current contain much higher 228Ra concentrations than surface waters collected in the open Ligurian seawater. The gradient of 228Ra along a Villefranche ‐ Calvi transect has been used to calculate values of the horizontal eddy diffusion coefficient perpendicular to the flow direction of the Ligurian Current of about 106 cm2 s−1. The observed decay of 228Ra, as the Mediterranean Deep Water flows toward the Gibraltar Straits, is in good agreement with the assumption of a rapid turnover of the Mediterranean basin. Concentration profiles of 226Ra and 228Ra were measured in the Mediterranean Outflow Water (MOW) off the Gibraltar Straits in the NE Atlantic basin at two stations, 36°30′N ‐ 15°35′W and 36°27′N ‐ 10°35′W. These sites showed the hydrographic characteristics of Meddies (MOW which propagates as eddies in the NE Atlantic) at intermediate depth. At the center of the Meddies, around 1000 m depth, the activity of the short half‐life 228Ra increases up to 0.9 × 10−2 disintegrations per minute per liter, higher than usually found in North Atlantic Deep Water. This “young” 228Ra results from the contact, a few months earlier, of the MOW with the Spanish continental shelf in the vicinity of the Gibraltar Straits. The decay of 228Ra, used as an internal tracer of water mass motion, leads to a transit time of the Meddy Yseult of less than 1 year since its formation.

Recent Changes in Eastern Mediterranean Deep Waters

Results from a recent hydrographic survey show that an influx of Aegean Sea water has replaced 20 percent of the deep and bottom waters of the eastern Mediterranean. Previously, the only source of such waters was the Adriatic Sea, and the waters of the eastern Mediterranean were in near-steady state. The flux changed the water characteristics and displaced older waters upward. Its cause was increasing Aegean Sea salinity, resulting from changes in either the circulation pattern or the large-scale freshwater balance. Current deepwater studies may be affected by the intrusion, but effects might be found also at shallower depths and over a larger region.