Mediterranean Margin Research Articles

210Pb sedimentation rates from the Northwestern Mediterranean margin

A total of 83 cores were collected in the Gulf of Lions continental margins and analysed for 210Pb xs (excess 210Pb) in order to understand sedimentation patterns. Apparent sedimentation rates (ASR) range from 0.65 cm year −1 in the vicinity of the Rhône River mouth to 0.01 cm year −1 in the deep basin. Except for the prodelta area, rates decrease with depth linearly with the water depth. On the slope, ASR do not differ between canyons and open slope, except for the western area where the rates are slightly higher in the Lacaze–Duthiers canyon compared to its adjacent, open slope. In the canyon and open slope areas, mass accumulation rates determined from 210Pb xs profiles (0.10 and 0.08 g cm −2 year −1, respectively) are in good agreement with particulate fluxes calculated from 5 years of near-bottom sediment trap data, even when the trap particle fluxes and the apparent accumulation rates are overestimated in response to resuspension and bioturbation effects. However, differences in sediment trap data, between west and east portion of the slope, are not observed in the sedimentation rates calculated with 210Pb xs. The outer shelf area may have an important role in trapping sediment but it is not sufficiently documented. Sediment surface mixed layer depths decrease with water depth, with a mean value for the whole margin of 8±6 cm. 210Pb xs inventories in the sediment are systematically higher than the net 210Pb export flux estimated from the above water column. Over the margin, the ratio between accumulated 210Pb and available 210Pb is about 3, suggesting boundary scavenging effects and advective transport.

The Impact of Humans on Strata Formation Along Mediterranean Margins

The Impact of Humans on Strata Formation Along Mediterranean Margins

Delay times and shear wave splitting in the Mediterranean region

SUMMARY We measure SKS splitting parameters and P and S delay times in the Mediterranean region using broad-band recordings from the temporary MIDSEA array and permanent networks. The MIDSEA seismic array has substantially improved data coverage along the European‐ Mediterranean margins and in northern Africa. Shear wave splitting is observed for 95 per cent of the stations, with splitting times up to 2 s. We propose five anisotropic domains, based on the characteristics of the observations. In the western part of the region we observe an alignment with plate motion, suggesting an asthenospheric origin for anisotropy. In the central and near eastern part, the fast directions are less coherent laterally and influenced by extension and subduction processes. Along the Dead Sea fault the fast directions are parallel to the relative motion between Arabia and Africa, which we attribute in part to coherent vertical deformation. Our measurements of teleseismic, relative P- and S-wave delay times within the Mediterranean region reach peak-to-peak values of 2.5 and 8 s, respectively. Consistent with International Seismological Centre (ISC) delay times, we observe the earliest arrivals along the Hellenic subduction zone and some of the latest along the Dead Sea fault. We find that S-wave delays are roughly three times as large as those of P waves. From a slight frequency dependence of our cross-correlation based delay time measurements, we estimate Mediterranean lateral heterogeneity to have a dominant size of one or more hundred kilometres. We further determine that the anisotropy implied by our SKS splitting measurements does not significantly bias the pattern of teleseismic delay times.

The volcanism-related multistage hydrothermal system of El Jaroso (SE Spain): Implications for the exploration of Mars

The SE Mediterranean margin of Spain is an extremely interesting area of synchronous interaction of tectonic, volcanic, evaporitic and mineralizing hydrothermal processes. This works tackles the multiple relations among these processes by the study of a specific and representative case: the ‘Jaroso Hydrothermal System’. The hydrothermal fluids were genetically linked with the late episodes of the Upper Miocene calc-alkaline and shoshonitic volcanism of the area. The ascent of the fluids was mainly controlled by the Palomares fault in Sierra Almagrera. In the shallow-marine basin of Las Herrerias, the movement of the acid solutions was controlled by both NNE-SSW and N150E normal faults and WNW-ESE wrench reverse faults. At least three mineralising stages were identified, although the particular formation of jarosite could be associated with both hypogenic and supergenic processes. We suggest that the multistage hydrothermal system of El Jaroso (Sierra Almagrera, Almeria province, SE Spain), which is responsible for both the Jaroso ores (especially rich in jarosite) and the Las Herrerias sulfate-rich, shallowmarine laminites, could be exploited as a potential model with important implications for the exploration of Mars.

Th- and U-bearing minerals in the SE Mediterranean margin of Spain

Th- and U-bearing minerals in the SE Mediterranean margin of Spain

Isotopic compositions of diagenetic dolomites in the Tortonian marls of the western Mediterranean margins: evidence of past gas hydrate formation and dissociation

Previous studies in the Lorca basin, S.E. Spain (Sediment. Geol., 2002) have shown that the diagenetic dolomitic concretions from the Tortonian marls are related to the past occurrence of gas hydrates which were rapidly dissociated when the sea level dropped during the Messinian salinity crisis. These conclusions were based on the oxygen and carbon isotopic compositions of these dolomites which indicated that the carbonate diagenesis occurred either in 13C-rich/ 18O-poor solutions as it is typical in gas hydrate systems or in 13C-poor/ 18O-rich solutions generated during gas hydrate decomposition. We have recently discovered similar dolomitic nodules in other Tortonian marly successions from S.E. Spain and Northern Morocco. The oxygen and carbon isotopic compositions of the diagenetic dolomites from these areas in Spain and Morocco are very similar to those from the Lorca basin. This demonstrates that the dolomitic nodules from the Tortonian marls of the Betic and Rifian basins were all related to the same diagenetic environments where pore fluids were modified by gas hydrate formation–dissociation processes. These overall observations in S.E. Spain and Northern Morocco suggest that the dissociation of gas hydrates during the Messinian salinity crisis was not a local phenomenon but corresponded to a major event which affected both sides of the Western Mediterranean margins; it might have been initiated during the first major restriction step that started around 6.7 Ma in the Mediterranean Sea, and probably climaxed at 5.9 Ma when the Mediterranean sea level dropped by more than 1000 m during the salt deposition stage.

Onshore and offshore compressional tectonics in the eastern Betic Cordillera (SE Spain)

This study concerns the northern terminal splay of the Eastern Betic Shear Zone (Betic Cordillera, SE Spain), where the Bajo Segura Basin lies, and its offshore prolongation along the western Mediterranean margin. We have integrated seismic reflection profiles with gravimetry, seismicity, wells and outcrop data from the Bajo Segura Basin and the Alicante shelf in order to determine the current geodynamic setting of the area. The results indicate that the same compressional structures in Upper Miocene–Quaternary rocks are observed both on- and offshore. In the onshore Bajo Segura Basin, there are ENE–WSW growth folds related to reverse faults in the basement. In the Alicante shelf, the main structure is an ENE–WSW anticlinorium which deforms Upper Miocene–Quaternary syntectonic deposits. These compressional structures are still active at present, as shown by the offshore seismicity. From the structural analysis and focal mechanisms of the earthquakes we conclude that the Bajo Segura Basin and its adjacent shelf have been subject to NNW–SSE compression since Late Miocene until the Present. Folding and reverse faulting of the Upper Miocene–Quaternary sedimentary cover and of its basement have accommodated this compression. The main active structures onshore, located in the Bajo Segura Basin, extend eastwards into the Mediterranean Sea. This fact is interpreted as the Eastern Betic Shear Zone continues offshore to the east.

Neutral monosaccharides in surface sediments of the northwestern Mediterranean Sea

This paper is focused on carbohydrates, an abundant chemical class in marine organic matter, mostly found as polysaccharides. An experiment showed that the use of H 2SO 4 1 M (95 °C, 4 h) yielded a good compromise between recovery and degradation of all neutral monosaccharides from two kinds of organic material: surface sediments and trapped particles. Twenty samples of surface sediments (0–0.5 cm) were collected in various physiographic and bathymetric locations on the Northwestern Mediterranean margin. Neutral monosaccharides were determined by an ionic chromatography system with electrochemical detection (HPAEC–PAD). Differences between total neutral monosaccharides (NM TOT) and total carbohydrates, determined by a spectrophotometric method on the same samples, brought further assumptions on the nature of the carbohydrate pool. Total neutral monosaccharides contents varied from 0.46 to 1.04 mg g −1 and their proportions donated to the organic carbon pool (NM TOT–C/OC) barely reached 5%. The spatial distribution of NM TOT and organic carbon (OC) clearly showed that organic inputs, originating from the shallower epicontinental area, spread along canyon axis while undergoing dilution processes. In spite of the high physiographic and bathymetric diversity, surface sediments from most stations were characterized markedly by the homogeneity in the neutral monosaccharide compositions. Two stations, E21 (Ebro prodelta) and EC36 (Balearic slope) showed, however, distinct signatures with high relative abundances of glucose and galactose, respectively. Glucose was found to trace the continental inputs, whereas galactose characterized the marine inputs. The relative “freshness” of the organic material at station EC36, assumed by the high proportion of hexose neutral monosaccharides, confirms the dominance of phytoplankton remains at this station. This work showed that the use of monosaccharides as tracers of the biological source and fate of organic compounds is deeply linked to the quality of the organic material. The monosaccharide signature is only preserved in samples of surface sediments where inputs of fresh organic material are higher than the refractory one.

Distributions of Pu, Am and Cs in margin sediments from the western Mediterranean (Spanish coast)

Continental margins are important areas to be considered when studying the distributions and depositions of pollutants, both conventional and radioactive. Coastal sediments accumulate most of those contaminants which can be introduced following atmospheric and/or fluvial pathways. Moreover, their residence times within the water column are usually shortened due to their affinity to associate with the downward falling particulate matter, more abundant at shallower depths. In this paper, the distribution profiles and inventories of plutonium, americium and cesium are detailed, providing useful information about recent sedimentation phenomena such as sediment mixing, slumping processes and bioturbation. Unsupported 210Pb data are used as reliable indicators of enhanced/reduced deposition events. Also, the calculated inventories have enabled the estimation of the radiological contribution of the Spanish Mediterranean margin to the total radioactivity deposited onto the Mediterranean sea floor.

Biotracers and geotracers of depositional events in NW Mediterranean margin over the past two centuries

A detailed sedimentological and geochemical investigation was performed on slope sediments collected from the Gulf of Lions and Catalonian margins and from the northern Balearic margin. The study was based on cores, about 50 cm long. Various physiographic sites (canyons, open slopes) were selected according to the available sediment accumulation rates obtained with the 210Pb geochronological method. At a short time scale, various parts of the slope were characterized by muddy hemipelagic sediment. Sedimentological data suggested that deposition was continuous. Total organic carbon and its hydrolysable fraction decreased downcore. Significant increase of C/N ratio downcore resulted from an early diagenetic process in response to the burial effect. However in other parts, the values of the C/N ratio were found to be irregular and maxima could frequently be related to both higher sand content and relict coastal remains derived from erosion and reworking of outcropping deposits of the last glacial maximum (lowstand system tract). Such tracts crop out over much of the shelf slope break seafloor. They provide various biological and lithological indicators: relatively well-preserved Miliolidea, coastal or lagoonal foraminifera ( Ammonia beccarii, Elphidium crispum), Characea oogons, oxidized beach-rock debris, gypsum, previously mature glauconite (with cracks) and coarse vegetal debris. Throughout the sections, the degree of diagenetic overprinting of the organic matter frequently showed a positive correlation with the abundance of some coastal indicators supporting the hypothesis of sedimentation induced predominantly by gravity These downslope accumulation may correspond to 10–40 % of the total deposit. The gravity induced transfers have been shown to be negligible in the sediment record of the northern Balearic margin. It is suggested that a reduced sedimentation rate (half that observed in the Gulf of Lions) would be expressed by a higher stability of the slope. The consequence of low sedimentation would be early diagenetic solution/reprecipitation of carbonate near the redox transition layer.

Tertiary compressional deformation of the Iberian plate

The Tertiary deformation of the Iberian plate is here interpreted as the result of changes in the coupling between the Iberian–African plates. During the early stages of the Africa/Iberia subduction (Palaeocene), deformation was confined at the Betic plate boundary. From the Eocene, during the collision in the southern plate margin, compressional deformation delocalized and distributed throughout the Iberian plate. First, in the Pyrenees, where the main stage of thrusting occurred during the Late Eocene – Early Oligocene. Then (mainly Oligocene – Late Miocene), in the inner part of the Iberian plate, forming basement uplifts in the Iberian Chain and the Central System, in correspondence of pre‐existing (Mesozoic and Variscan) structures. Finally, during the decay of compression inside the Iberian plate, extension took place the Mediterranean margin and the Alboran Sea.

ABSTRACT: The Sequence Stratigraphic Evolution of the Late Mesozoic Southeastern Mediterranean Continental Margin

ABSTRACT: The Sequence Stratigraphic Evolution of the Late Mesozoic Southeastern Mediterranean Continental Margin

Phanerozoic geodynamic evolution of northeastern Africa and the northwestern Arabian platform

The Northeast African–Arabian plate formed a segment of the passive margins of, successively, the Paleotethys and the Neotethys. During the Paleozoic, rifting events were relatively minor, but epicontinental sagging focused deposition within the N–S-trending Ghazalat–Tehenu basin of the Western Desert, the Jafr basin of southern Jordan, and along a proto-Gulf of Suez trend. Gentle arching occurred at the end Silurian, end Devonian, end Serpukhovian and Late Carboniferous. The opening of the Neotethys in the Permian marked the onset of a new geodynamic episode. Extensional faulting was pronounced along the present Mediterranean margin region from the Permian to the Jurassic. Early Cretaceous intra-continental rifting occurred in the E–W-trending arm of the Sirt basin, and in the Abu Gharadiq and the Shushan–Matruh basins of the Western Desert. In the Santonian, a major shift in tectonics swept the Tethyan realm, producing dextral transpression across NE Africa and marking the onset of the main Syrian arc folding and reverse faulting. By the Campanian–Maastrichtian, extension resumed, with subsidence focused in the Sirt, Abu Gharadiq and Azraq–Sirhan basins. A compressive pulse marked the Cretaceous–Paleocene transition, followed by renewed rifting during the Paleocene–Eocene in the Sirt basins and El Gindi basin of northern Egypt. At approximately the Bartonian–Priabonian boundary, strong dextral transpression again affected northern Egypt and the E–W Aswan fault swarm. In the Late Oligocene, the Gulf of Suez–Red Sea rift initiated, often reactivating older tectonic trends. During the Middle Miocene, the Red Sea divergent plate boundary abandoned the Gulf of Suez and broke through the Arabian plate via the Gulf of Aqaba–Dead Sea transform fault, establishing the present-day plate kinematic framework. A rapid rotation of stress fields occurred in the Late Pleistocene, with active faulting continuing to the Recent along the central Sinai–Cairo fault zone and the Gulf of Aqaba–Red Sea plate boundary.

Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay)

During the ECOFER experiment (French ECOMARGE program), surficial sediments were sampled on the Aquitanian margin with box corers and analyzed to determine the quantity and quality of organic matter. Sediments from the margin are enriched in organic carbon (mean value 1.35%) in comparison to deep-sea and shelf sediments, due to a fine grain-size sedimentation. As sedimentation rates are high, the margin appears to be an organic depocenter. Some preferential organic enrichment zones were identified in the Cap-Ferret Canyon. There is a supply of continental material from the Gironde estuary, but marine contribution seems more possible than Adour or spanish rivers. No seasonal variations of organic matter were observed at the surface of sediments, suggesting mineralization processes of labile organic matter: average organic carbon consumption was evaluated to 9.0 g C m −2 yr −1. Rapid biological mineralization processes are lower than on the Mediterranean margin, mainly related to significant differences in water temperature. The great width of the canyon, its distance from the continent, and the current circulation pattern prevent any precise recording of the variable organic inputs to the sediment and favor nepheloı̈d transport, resuspension and shelf break processes, which wipe out any print of fresh material input. An organic carbon budget indicates that an equilibrium between organic inputs and organic mineralization+accumulation is not obtainable. The supply of suspended matter could have been minor during the year in question, and sedimentation rates are still imprecise.

Early diagenesis in rapidly accumulating sediments on the Alboran slope, ODP site 976

The oxidation and reduction that occur during early diagenesis of sediments has been studied in the interstitial waters of a rapidly accumulating sedimentary sequence from the Mediterranean margin of Spain. A series of reactions that are mediated by progressively lower free energy derived from oxidation of organic matter is evident in the sedimentary sequence. Iron and manganese are rapidly reduced. Phosphate and alkalinity maxima at a subbottom depth of 15 m indicate maximal organic matter degradation. Methane first appears at ∼20 m subbottom after sulfate is depleted, and its concentrations quickly climb.

Heat flow, heat production, and lithospheric thermal regime in the Iberian Peninsula

The first heat-flow and heat-production maps of the Iberian Peninsula and its margins using previously acquired data are presented. The surface heat-flow map includes 553 determinations carried out on water and mining exploration wells, oil exploration wells and on the seafloor. The surface heat flow varies noticeably from the Iberian mainland (65 ± 10 mW m −2) to the Atlantic and the Mediterranean margins, where the heat flow reaches values of about 40–50 mW m −2 and 80–100 mW m −2, respectively. The heat-production map consists of 664 determinations carried out on rock samples from the Variscan Iberian Massif and the Betics. The higher values are obtained for granitic rocks (2.5–3.5 μW m −3), whereas metasediments and basic rocks reach values of 1–2.5 μW m −3 and nearly zero, respectively. The lithospheric structure deduced for the Iberian Peninsula by combining heat-flow, heat-production and elevation data indicates that most of the Iberian mainland is characterized by a lithospheric thickness of 110 ± 5 km. This value is maintained across the West Atlantic margin, whereas towards the Mediterranean margin, the lithospheric thickness decreases down to 60-40 km. This study also suggests that the heat production in the southern Variscan Iberian Massif must be noticeably higher (3.7 ± 0.5 μW m −3) than in the rest of the areas in order to fit the measured heat-flow, crustal thickness and elevation data.

Modeling nitrate fluxes in an open coastal environment (Gulf of Lions): Transport versus biogeochemical processes

A 13‐compartment model of primary production and degradation of dissolved organic matter has been coupled with a general circulation model in an open coastal environment (Gulf of Lions, Mediterranean) so as to quantify exchanges with the open sea. The biogeochemical model had been previously calibrated with a one‐dimensional vertical version on a 1‐year data set, and this simulation provides boundary conditions for the three‐dimensional model. After a 1‐year spin‐up simulation, quasi‐equilibrium is obtained for all the compartments, and the results of an additional annual simulation are compared with coastal zone color scanner images and other data collected in the area: the simulation of chlorophyll a, nutrient concentrations, and primary production is quite satisfactory, although the spring bloom starts slightly earlier than usual. Total gross community production is estimated at 76 g C m−2 y−1, and New Gross Community Production at 45 g Carbon m−2 y−1 (f = 0.37). The model was used to understand the nitrate annual cycle in the coastal zone of the Gulf of Lions. Nitrate input from the Rhône river, from the sediment, and from marine advection are compared in terms of potential fertilization. Model results indicate that, regarding the open sea, the margin acts most of the time as a sink for nitrate. However, during winter, when phytoplankton growth is reduced and cascading of dense waters is active along the shelf, the margin is shown to export nitrate toward the open sea. These results may qualitatively apply to other open Mediterranean margins, although the intensity of the fluxes would be modulated by local features.

Stratal architecture of western Mediterranean margins as the result of the stacking of Quaternary lowstand deposits below ‘glacio-eustatic fluctuation base-level’

Stratal architecture of western Mediterranean margins as the result of the stacking of Quaternary lowstand deposits below ‘glacio-eustatic fluctuation base-level’

Late Permian to Recent magmatic activity on the African-Arabian margin of Tethys

Abstract Magmatic activity on the African-Arabian margin of Tethys has fluctuated significantly during the past 250 Ma in response to major phases of mantle plume activity and to extensional stresses within the African plate related to periods of continental break-up. A series of maps have been compiled, based upon stratigraphic constraints and K-Ar and 40 Ar- 39 Ar age determinations, to illustrate the changing patterns of igneous activity since the Late Permian. These are divided into five stages: I, Late Permian (256 Ma)-Latest Triassic (208 Ma); II, Early Jurassic (208 Ma, Liassic)-Mid Jurassic (157 Ma, Dogger); III, Late Jurassic (157 Ma)-Earliest Aptian (120 Ma); IV, Mid Aptian (119 Ma)-Mid Eocene (42 Ma); V, Late Eocene (42 Ma)-Recent. Extensive tholeiitic basaltic magmatism at c. 200 Ma, preceding continental break-up in the Central Atlantic, is attributed to upwelling of the Cape Verde mantle plume beneath the West African craton. Contemporaneous magmatic activity in the Levant is related to the break-up of the Eastern Mediterranean margin. Early Cretaceous alkaline magmatism around the margins of the Equatorial Atlantic and within the Levant may also be plume related. During Neogene-Recent times there was a major change in the convection pattern within the upper mantle, generating extensive magmatism across the entire margin. Magmatism appears to be extremely long lived in a number of areas (e.g. Sudan, Air), with volcanic centres located along major basement fault zones, such as the Guinean-Nubian lineaments.

Mesozoic rifting and basin inversion along the northern African Tethyan margin: an overview

Abstract The northern African Tethyan margin registered three major rifting episodes from the latest Palaeozoic-earliest Mesozoic to the earliest Cenozoic. Break-up of Gondwana was initiated in the late Carboniferous. Along the northern African-Arabian plate margin rifting propagated westward from the northeastern Arabian margin to Morocco during the Permian and Triassic, and was accompanied by Mid-Late Triassic-earliest Liassic extensive alkaline flow basalts. Rifting continued during the Liassic, e.g. in the Moghrebian Atlas troughs. A second stage of rifting occurred in the Late Jurassic and continued into, or was rejuvenated during, the Early Cretaceous. Along the east Mediterranean margin, some large E-W trending rifts formed, often with associated volcanism, e.g. southern Sirt and Abu Gharadig. Most researchers believe the oceanization of the eastern Mediterranean basin occurred at this time. Flexural subsidence affected the Moghrebian Atlas, including the Riffian-Tellian domain, where a thick flysch series was deposited. Two short-lived, but widespread and tectonically important, compressional or wrench-dominated events occurred during the Santonian and the latest Maastrichtian. From Morocco to Oman, most sedimentary basins were folded and inverted at these times. The Santonian event resulted from onset of the collision between the African-Arabian and Eurasian plates. New stress-fields favoured NE-SW extension along the African plate margin, generating or rejuvenating some rifts, e.g. in the Euphrates trough and in the Libya-Tunisia region (northwestern Sirt, Pelagian Sea). Rifting and magmatism continued in these regions into the Paleocene. During the Mesozoic, therefore, the northern margin of the African-Arabian plate registered both rifting resulting in the oceanization of the Tethys and rifting resulting from the initiation of the closure of the Tethys. The intraplate domain exhibited echoes of the tectonic events affecting the margin.