Mediterranean Water Inflow Research Articles

Rare earth elements distribution in the Gulf of Cádiz (SW Spain): geogenic vs. anthropogenic influence

There is a current interest in the study of the geochemical behavior of the rare earth elements (REE) in aquatic systems since their identification as potential microcontaminants in natural waters. In this context, here we report the concentrations and patterns of dissolved REE (dREE) in the waters of the Gulf of Cádiz (GoC) and its major estuaries (Guadiana, Tinto-Odiel and Guadalquivir). Contamination in this area by metals has been extensively reported, linked to acid-mine inputs resulting from the mineralization of the Iberian Pyrite Belt, discharged mainly to the Tinto-Odiel river. However, the impact of these inputs on the concentrations and distribution of the dREE in the coastal waters of the GoC has not been addressed yet. Accordingly, elevated concentrations of dREE were obtained in the Tinto-Odiel estuary compared to the Guadiana and Guadalquivir, reflecting the impact of acid-mine drainage on this system. Only a significant fraction of anthropogenic gadolium (Gd) was observed in the Guadalquivir estuary, as a result of the input from major urban areas, where anthropogenic Gd fractions higher than 70% over the total dissolved pool were obtained for the freshwater end-member. Regarding the surface waters of the GoC, typical NE Atlantic distribution patterns of dREEs were obtained, but modified by two different end-members within this region. Accordingly, and despite the low water discharge of the Tinto-Odiel river, its elevated concentrations lead to an increase in the concentrations of dREEs in the nearshore waters of the GoC and producing a distinctive signature as observed for the MREE anomalies and HREE/LREE ratios. At the easternmost stations, close to the Strait of Gibraltar, these signals are also impacted by the inflow of Mediterranean waters and also by the discharge of the Guadalquivir river.

A holistic approach for determining the hydrology of the mar menor coastal lagoon by combining hydrological & hydrodynamic models

A combination of hydrological and hydrodynamic modelling can be applied to understand the hydrology and key water balance components of lakes and lagoons. In this research, the Soil and Water Assessment Tool (SWAT) model and the QGIS Water Ecosystems Tool (QWET) were applied for the Mar Menor coastal lagoon and its watershed known as Campo de Cartagena. First, the SWAT model was calibrated and validated based on remote sensing evapotranspiration data. Results showed an acceptable performance of the SWAT model in both calibration (R2 = 0.63, NSE = 0.62, PBIAS = 2.91%) and validation (R2 = 0.68, NSE = 0.68, PBIAS = 2.47%) periods on a monthly basis. The SWAT simulated streamflow was fed into the QWET model to simulate the water balance of the lagoon. The hydrodynamic model performance was evaluated based on a comparison between simulated and observed water temperatures and also the model estimated evaporation. Simulated daily temperatures showed a good agreement with observed data by capturing the timing and inter-annual variations, with an NSE of 0.98, and a BIAS of 2.7%. Our water balance estimation, using the reference period 2003–2019, yields a mean annual rainfall over the lake of 301 mm and a mean annual evaporation of 1325 mm. The average surface runoff and groundwater discharge to the lagoon are 49 hm3/year and 11 hm3/year, respectively. Extreme storm events cause annual surface runoff to vary between 8 hm3/year and 202 hm3/year. The water balance was closed with the water exchange with the Mediterranean Sea, resulting in an overall positive flow from the Mediterranean Sea of 82 hm3/year. Our study showed that during summer months, in particular, there is considerable inflow of Mediterranean water to the lagoon, whereas for some autumn and winter months (November, December and January) there is a net outflow from the lagoon to the Mediterranean. This novel approach by combining the SWAT hydrological model and QWET hydrodynamic model complex provides a useful tool for understanding the hydrology of the lagoon and may also play a role for decision makers when developing strategies for mitigating eutrophication.

Coupling of dissolved organic carbon, sulfur and iron cycling in Black Sea sediments over the Holocene and the late Pleistocene: Insights from an empirical dynamic model

An understanding of how the coupled cycles of carbon, iron and sulfur in sediments respond to environmental change throughout Earth history requires the reconstruction of biogeochemical processes over a range of spatial and temporal scales. In this study, sediment cores from the southwestern Black Sea were analyzed to gain insight into past changes in biogeochemical processes with particular focus on the cycling of dissolved organic carbon (DOC). The sediment consists of Late Pleistocene deposits of iron oxide-rich and organic-poor lacustrine sediments, a Holocene sapropel layer deposited after the inflow of saline Mediterranean seawater about 9300 yr BP, and overlying recent marine sediments. The porewaters displayed high concentrations of DOC, acetate, dissolved iron and an extended depth interval over which sulfate and methane were both present. The historical fluctuations of the fluxes of carbon, sulfur and iron species at the seafloor that led to these present-day geochemical profiles, and which cannot be easily interpreted from the measured data alone, were hindcasted with a reaction-transport model. The model suggests that the inflow of Mediterranean seawater impacted the rain rate and reactivity of organic matter reaching the sediments, which shifted the sedimentary redox regimes throughout the Holocene that now are reflected on different lithology units. Organic matter in the sapropel layer is apparently the main source of modern-day accumulations of DOC and acetate, both of which probably sustained subsurface microbial activity throughout the post-glacial period. The ratio between DOC and dissolved inorganic carbon (DIC) flux to the bottom water decreased from ∼40% before the inflow of Mediterranean water to ∼2% at the present day. We suggest that the coexistence of methanogenesis and sulfate reduction was associated with sulfate-reducing bacteria and methanogens sharing common substrates of acetate and lactate and utilizing non-competitive substrates such as methylated compounds in the sapropel layer and in the bottom of modern marine deposits. Intense sulfur and iron cycling mainly took place in the organic-poor freshwater deposits, today characterized by high concentrations of dissolved iron and methane. In contrast to previous studies in similar environments, anaerobic oxidation of methane coupled to the reduction of ferric iron was negligible. The results have broad implications for coastal environments that are currently experiencing deoxygenation and seawater intrusion and also for understanding the role of DOC in the sedimentary carbon cycle.

Assessing ultraphytoplankton and heterotrophic prokaryote composition by flow cytometry in a Mediterranean lagoon

In the eutrophic Ghar El Melh Lagoon (GML, Tunisia), the distribution of heterotrophic prokaryotes, pico- and nanophytoplankton was studied at five stations in November 2012 at the single cell level, along with environmental factors. Flow cytometry analysis of ultraplankton (<10μm) resolved (i) two heterotrophic prokaryote groups, low and high nucleic acid contents (LNA and HNA, respectively), and (ii) eight to nine ultraphytoplankton groups (cryptophyte-like cells, two nanoeukaryote subgroups, two picoeukaryote subgroups and three Synechococcus-like cells subgroups). Prochlorococcus was not detected. According to redundancy analysis (RDA), a significant difference was found in the distribution of the ultraplankton between stations (F=2.61, p<0.05); maximum proliferations of heterotrophic prokaryotes were observed in the inner parts of the lagoon at stations 3, 4 and 5 affected by urban, agricultural and industrial discharges. Ultraphytoplankton concentrations were the highest near the outlet of the lagoon at stations 1 and 2 influenced by freshwater outflow and oligotrophic Mediterranean water inflow, respectively. At station 1, the large ultraphytoplankton concentration derives from the high abundance of cryptophyte-like cells favoured by the freshwater outflow whereas at station 2, the input of oligotrophic Mediterranean water enhanced the abundance of Synechococcus and picoeukaryotes at the expense of nanoeukaryotes. Two trophic regimes were thus differentiated in GML.

Paleo-environmental controls on cold seep carbonate authigenesis in the Sea of Marmara

The factors controlling fluid emission dynamics at ocean margins are poorly understood. In particular, there are significant uncertainties on how fluid seepage at cold seeps may have responded to abrupt environmental changes in the geological past. This study reports on a detailed geochemical investigation of seafloor carbonate crusts sampled at cold seeps along the submerged part of the North Anatolian Fault system in the Sea of Marmara – an inland sea, which has experienced major paleo-environmental changes over the last deglaciation period. We also analyzed a series of authigenic carbonate concretions recovered from two sediment cores at the Western-High ridge, an active fluid venting area.The ages of seafloor carbonate crusts derived from isochron U–Th dating cover the last 7 kyr, suggesting that fluid activity along the fault system remained continuous over that time interval. In the sediment cores, carbonate concretions are concentrated at the lacustrine-to-marine transition, which corresponds to the period when Mediterranean waters flowed into the Marmara Basin about 12–14 kyr ago. U–Th isotopic data indicate that most of these concretions formed later during the Holocene, around 9–10 kyr ago, a period coinciding with an important anoxic event that led to the deposition of a sapropel layer in the Sea of Marmara.Based upon these results, we suggest that the absence of carbonate concretions in the lacustrine sediment unit indicates that dissolved sulfate concentrations in the Marmara lake pore waters during glacial time were too low to promote significant anaerobic methane oxidation, thereby preventing sedimentary carbonate authigenesis. In contrast, the progressive inflow of Mediterranean waters into the glacial Marmara lake after 15 ka provided a source of dissolved sulfate that allowed anaerobic oxidation of methane to proceed within the anoxic sediment. Importantly, the synchronism between the main phase of authigenic carbonate precipitation at the studied sites (average 9.4±1.8 ka, n=16) and the regional anoxic sapropel event support the idea that the drop in bottom water dissolved oxygen content was probably a key factor to enhance microbial activity and associated carbonate precipitation at that time. Overall, these results provide straightforward evidence that fluid emission dynamics and hydrocarbon oxidation at cold seeps can be directly related to changing environmental conditions through time.

Evolution and fluxes of 137Cs in the Black Sea/Turkish Straits System/North Aegean Sea

The vertical profiles of 137Cs were determined in the North Aegean, Marmara and Black Seas, to assess inventories and fluxes of the radionuclide in these basins. The inventory of 137Cs in the Western Black Sea integrated from the surface down to 400m water depth is 3.4±0.1kBqm−2, which is surprisingly close to the amount determined in 1988, decay corrected to 2007 (2.9±0.1kBqm−2). On the other hand, based on the comparison of profiles roughly 20years apart, it is estimated that about 1kBqm−2 has been transferred from above the halocline to depths below the halocline, emphasizing the effective redistribution of tracers within the same period.We estimate that about 12TBqy−1 of 137Cs presently leaves the Black Sea with the upper layer flow through the Bosphorus and only 2TBqy−1 is returned with the lower layer inflow of Mediterranean water from the Marmara Sea. Accounting for river fluxes, estimated on the order of 2TBqy−1 few years after the Chernobyl accident, and possibly decreased by now, we can thus estimate a net rate of loss of about 8–10TBqy−1.Investigating the effective redistribution in the upper water column, the supply by the inflowing Mediterranean water alone does not explain the increase of 137Cs concentration and inventory at intermediate depths in the Western Black Sea. The most important mechanism transferring 137Cs and dissolved contaminants from the surface water to the sub-pycnocline layer appears to be the turbulent entrainment of a larger quantity of Black Sea water into the inflowing plume of Mediterranean water through mixing processes on the southwestern shelf and continental slope following its exit from the Bosphorus. This process produces an extra export of some10TBqy−1 of 137Cs from the surface to the sub-pycnocline depths of the Black Sea, a quantity comparable in magnitude to the total export out from the basin. It is the entrainment flux resulting from the mixing, and the further advection and penetration of this water into the Black Sea deeper layer (200–600m) that seems to maintain the inventory with little change over time. Through these two processes the Black Sea surface layer (0–50m) loses every year about 4% of its total inventory of 137Cs.

Abrupt changes of temperature and water chemistry in the late Pleistocene and early Holocene Black Sea

New Mg/Ca, Sr/Ca, and published stable oxygen isotope and 87Sr/86Sr data obtained on ostracods from gravity cores located on the northwestern Black Sea slope were used to infer changes in the Black Sea hydrology and water chemistry for the period between 30 to 8 ka B.P. (calibrated radiocarbon years). The period prior to 16.5 ka B.P. was characterized by stable conditions in all records until a distinct drop in δ18O values combined with a sharp increase in 87Sr/86Sr occurred between 16.5 and 14.8 ka B.P. This event is attributed to an increased runoff from the northern drainage area of the Black Sea between Heinrich Event 1 and the onset of the Bølling warm period. While the Mg/Ca and Sr/Ca records remained rather unaffected by this inflow; they show an abrupt rise with the onset of the Bølling/Allerød warm period. This rise was caused by calcite precipitation in the surface water, which led to a sudden increase of the Sr/Ca and Mg/Ca ratios of the Black Sea water. The stable oxygen isotopes also start to increase around 15 ka B.P., although in a more gradual manner, due to isotopically enriched meteoric precipitation. While Sr/Ca remains constant during the following interval of the Younger Dryas cold period, a decrease in the Mg/Ca ratio implies that the intermediate water masses of the Black Sea temporarily cooled by 1–2°C during the Younger Dryas. The 87Sr/86Sr values drop after the cessation of the water inflow at 15 ka B.P. to a lower level until the Younger Dryas, where they reach values similar to those observed during the Last Glacial Maximum. This might point to a potential outflow to the Mediterranean Sea via the Sea of Marmara during this period. The inflow of Mediterranean water started around 9.3 ka B.P., which is clearly detectable in the abruptly increasing Mg/Ca, Sr/Ca, and 87Sr/86Sr values. The accompanying increase in the δ18O record is less pronounced and would fit to an inflow lasting ∼100 a.

Vertical distribution of mercury species at two sites in the Western Black Sea

The Black Sea is a unique natural laboratory for studying the biogeochemical cycling of mercury (Hg), a toxic metal present at trace levels in natural waters. The strong vertical density gradient in the Black Sea results in little ventilation of deep waters and therefore anoxic and sulfidic conditions at depth. In this way, the water column of the Black Sea resembles the geochemical transition that is observed in coastal sediments. This is of relevance to Hg studies, as coastal sediments are known to be locations of vigorous Hg methylation. Furthermore, the Black Sea supports a rich commercial fishery that could be at particular risk for accumulation of Hg as a result of its unique hydrography and anthropogenic inputs. The concentration of monomethylHg (CH 3Hg +) was highest at the top of the suboxic zone in the Black Sea, where dissolved oxygen and sulfide were very low. This is circumstantially consistent with the hypothesis that it is the chemical form of Hg which determines bioavailability and therefore methylation rate and not the activity of sulfate reducing bacteria. Hg concentrations below the suboxic zone increased in tandem with increasing sulfide, and appear to be approximately consistent with cinnabar solubility control. These concentrations (ca. 12 pM) are among the highest reported for an open ocean environment. A preliminary mass balance for Hg in the Black Sea, including source terms such as precipitation, river and Mediterranean water inflow as well as the evasional sink and Bosphorus outflow indicates that a large additional input term is required that is greater than 68 kmol year − 1. Potential candidates include tectonic-related sources and river-derived pollution. Scaling the total inputs to the Black Sea by area of the basin indicates that this system is comparably impacted by Hg as an urbanized embayment such as Long Island Sound or Chesapeake Bay.

Late Pleistocene uplift history along the southwestern Marmara Sea determined from raised coastal deposits and global sea-level variations

Pleistocene raised coastal deposits characterized by locally abundant shells, aragonite-cemented beachrock and associated nearshore deposits border the western Marmara Sea at elevations of 0–50 m. Field observations confirm that these deposits formed during a series of transgressive and regressive events. U/Th dates in 16 in situ shells from four localities show that the peak of the transgressions occurred during the highstands of oxygen isotopic stages 7 and 5, between ∼53 and ∼210 ka. The elevations of these dated deposits can be used to quantify their post-depositional uplift and indicate that the entire western Marmara shelf, including the Strait of Çanakkale (Dardanelles) has been rising at an average rate of ∼0.40 mm yr −1 since ∼225 ka. The primary cause of uplift is the local compression associated with a restraining bend in the western segment of the North Anatolian Fault. Paleogeographic maps constructed using the average rate of tectonic uplift and detailed topographic and bathymetric maps reveal that prior to glacial oxygen isotopic stage 8 the Marmara Sea was never isolated from the Aegean Sea, even when global sea level was low, because the floor of the Strait of Çanakkale was too deep early in its uplift history. The dominance of Mytilus edulis in raised coastal terraces dating from the lowstand of glacial oxygen isotopic stage 8 suggests that the degree of communication between the Aegean Sea and Marmara Sea was comparable to that of the present. During the peak of glacial oxygen isotopic stage 6, the floor of the strait was subaerially exposed, isolating the Marmara Sea for the first time in the Pleistocene from the higher salinity Mediterranean water inflow and possibly causing it to become a blackish-water lake. During interglacial isotopic stages 9, 7 and 5, the Strait of Çanakkale was very wide and deep (∼100–125 m), and there were two subsidiary channels (Bolayır and Eceabat channels), providing additional links between the Aegean Sea the Marmara Sea, further promoting significant water exchange between these basins. The dominance of Ostrea edulis in the raised terraces dating from isotopic stages 5 and 7 confirms an enhanced penetration of the Mediterranean water mass into the Marmara Sea. The history of communication between the Black Sea, fed by central and northern European drainage systems, and the low-latitude Mediterranean Sea is of fundamental importance in understanding the genesis of organic-rich sapropel deposits throughout the region. The results presented in this paper caution against the simple assumption that sea-level change alone controlled the degree of connection across this oceanographic gateway. Instead, the physiography and paleoceanography of the region were controlled both by variations in global sea level and the rate of uplift in an area of active transpression.

Mechanisms for Spreading of Mediterranean Water in Coarse-Resolution Numerical Models*

Different processes have been proposed to explain the large-scale spreading of Mediterranean Water (MW) in the North Atlantic, however, no systematic study comparing the efficiency of different processes is yet available. Here, the authors present a series of experiments in a unified framework that is designed to quantify the effects of several physical processes on the spreading of MW in an idealized model of the North Atlantic. The common technique of restoring temperature and salinity to an observed distribution near the Mediterranean inflow fails to produce an adequate amount of MW because the eastern boundary region near the MW inflow is rather quiescent in models. Diapycnal processes like double diffusion and cabbeling turn out too inefficient to alone account for the large-scale MW anomaly. However, with a preexisting anomaly, double diffusion leads to a considerable northward and zonal redistribution of MW. The density anomaly induced by cabbeling curtails the zonal spreading of MW while it increases the northward spreading. With isopycnal mixing and the weak mean flow that prevails in the outflow region, a spatial distribution of the MW anomaly is obtained that is inconsistent with observations. Unrealistically high diffusion coefficients would be necessary to reproduce the observed salt flux into the Atlantic. The most effective process in the experiments is the volume flux associated with the Atlantic–Mediterranean exchange. The current system that is established in response to the inflow of MW into the Atlantic carries the anomaly almost 30° of longitude into the basin and along the eastern margin up to the northeastern corner of the domain and farther along the northern boundary.

The intermediate Mediterranean water inflow along the northern slope of the Iberian Peninsula

During the SEFOS 95 cruise (25 March to 15 April 1995) ( figure 1) a number of CTD casts were made in order to study the intermediate Mediterranean water (IMW) off the west and north coasts of the Iberian Peninsula. One long-term mooring, for current measurement at four levels, was deployed at 43° 57′ N - 5° 51′ W from February to May. We establish a picture of the distribution and transformation of the IMW along the coast of the Iberian Peninsula based on the thermohaline characteristics of the water masses and the determination of the percentage content of the IMW by the use of the triangle of vertical mixing of the North Atlantic central water (NACW), IMW and Labrador water (LW) [3]. A generalised temperature/salinity (T/S) curve is plotted ( figure 6) on the basis of the T/S curves of the cruise. The results of 3 months' current measurements are shown.

Mediterranean Water in the Strait of Istanbul (Bosphorus) and the Black Sea Exit

Mediterranean water is always present in the Strait of Istanbul (Bosphorus) and it extends as a salt wedge into the Black Sea. Approximate average slopes of the surface (Black Sea) and bottom (Mediterranean) waters were determined along the Strait. Asymmetry observed in the interface slopes reflects the influence of hydraulic controls along the Strait and in the Black Sea exit. Bottom layer slopes are 1·6 and 1·4 m km −1in the southern part and 0·5 m km −1in the northern part. Results of the first moored current measurements and variability of the bottom-water salinity in the northern sill are given. Bathymetry and morphology of the sea-bed channel extension into the Black Sea, confinement of the Mediterranean water into the channel, and its slope were studied using recent hydrographic and seismic surveys. Based upon recent data on channel geometry and current measurements at the northern sill, Mediterranean water inflow was estimated to be 101·0 km 3year −1. Current and salinity measurements in the northern sill indicated short (∼15·0 h) and long (e.g. 10·6 days) period oscillations. Under the control of topography, Mediterranean water extends for about 7·5 km into the Black Sea with a slope of approximately 2·0 m km −1. It first runs northward and subsequently spreads westwards. The approximate spreading area of the Mediterranean water in the Black Sea far-field region was determined.

On the variability of Mediterranean water flow into the Black Sea

Mediterranean water inflow into the Black Sea has been investigated using current measurements at the northern sill of the Strait of Istanbul (Bosphorus). Variability of the inflow and its relation to sea level and atmospheric factors were studied. Current and salinity spectra show internal variability with periods of 3.5 days. This is correlated with N-S wind component and sea level variations. During southerly wind episodes the lower layer flow increases and relatively well-mixed Mediterranean water reaches the Black Sea. During northeasterlies the bottom layer flow decreases and relatively undiluted Mediterranean water flows into the Black Sea. Dominant energy input is in the low frequency (<0.6 cpd) range in salinity and current speed variations. Current speed also displays variability in short period range. Short period of oscillation (21.3) h is attributed to internal and Black Sea diurnal tidal oscillations and sea breeze.

The Adriatic sea hydrography and circulation in spring and autumn (1985–1987)

Abstract The Adriatic basin-wide circulation and its temporal variability are reviewed on the basis of results from the analysis of hydrographic data collected during four POEM cruises. Major well known features in the circulation are revealed in the data set which covers the period from October 1985 to April 1987. A prominent signal associated with the seasonal variability is identified in the water outflowing along the Italian coast. Differences between autumn and spring in the vein of cold and fresh water flowing along the Italian shelf manifest mainly in the temperature field. During the stratified season the fresh water spreads over the entire surface layer of the southern Adriatic. On the other hand, during spring, when the sea is vertically homogeneous, the fresh water remains confined to the surface longshore boundary layer over the entire length of the Italian coast. Layers below the seasonal thermocline at the eastern portion of the sea display very weak seasonal signals. A strong signal associated with the inter-annual variations also has been documented from the analysed data set; it mainly appears in the salinity field. It is shown that in spring 1986, the salinity averaged over the entire water column north of the Palagruza Sill is lower by 0.3 psu than in spring 1987. A similar, but less prominent difference is noted in the southern Adriatic. An attempt is made to associate these differences with variations in climatic conditions over the area, the river runoff and the Mediterranean water inflow.

A simple evolutionary model for water and salt in the Black Sea

This paper presents a time‐dependent variable‐size box model for the total fluid and salinity balances of the Black Sea. In addition to the advective flows due to riverine and Mediterranean inputs of water, the model accounts for entrainment flows which are parameterized using the theory of Ellison and Turner (1959) and employs a modified version of the Gargett (1984) formulation for eddy diffusivity in describing cross‐pycnocline mixing flows. The variable volume of the boxes permits explicit simulation of the replacement of the preexisting late Pleistocene lake water by the inflow of denser Mediterranean water. Our results indicate that the time needed to displace the old deep water with new saltier water is 2000 years or less. Because of the effects of mixing, the salinity of both the surface and deep waters requires a greater time interval to obtain present‐day values, i.e., ∼4000 years if the inflow of Mediterranean water is constant in time at its present value and ∼6000 years if this input changes with time in approximation to the rate of sea level rise. The occurrence of the marine coccolithophorid Emiliania huxleyi in the Holocene sapropel layer requires a minimum surface salinity of 11‰, which takes some 1600 to 3100 years to develop. This range is a result of the uncertainty in the history of the Mediterranean water input. The model is also employed to investigate the freshening of the Black Sea after cutoff of Mediterranean input. The salinity of both surface and deep waters is found to fall below 11‰ in less than 3000 years, and the freshening process is complete in ∼9000 years.