Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate

Cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre-evaporitic Messinian in a marginal basin of the western Mediterranean. The fluctuations coincide with a dominantly precession-controlled sedimentary cyclicity (sapropels). During sapropel deposition, high planktic foraminiferal diversities are indicative of relatively stable marine conditions, while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable, more saline surface water conditions. The dominance of a precession-related signal indicates that regional climate oscillations rather than (obliquity-related) glacio-eustatically controlled influxes of Atlantic and/or Mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity. Our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run-off along the western Mediterranean at times that perihelion occurred in Northern Hemisphere summer. Less favourable, highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes. The cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins.     

On the influence of Mediterranean Water on the Central Waters of the North Atlantic Ocean

Within the Central waters of the North Atlantic Ocean there is a significant east–west difference in salinity, similar to that caused by Mediterranean Water at deeper levels. In this paper we hypothesize that the salinity of the Central Water is influenced by the saline Mediterranean Outflow Water, despite physical separation of the two water masses by a salinity minimum over most of the ocean basin. It is suggested that there occurs a cross-isopycnal flux of salinity from the Mediterranean Outflow Water towards the low-density Central Water (detrainment) in the eastern Gulf of Cadiz, not far from the Strait of Gibraltar, where the two water masses are in physical contact. Laboratory experiments, inverse modeling and direct current observations are applied to support the hypothesis.     

Neogene sapropels in the Mediterranean: a review

For 50 years the existence of sapropels (organic-carbon-rich sediments) deposited within Plio–Pleistocene sediments of the Mediterranean Sea has been known. Initially, research concentrated on material recovered in relatively short gravity/piston cores taken from the eastern basins where sequences were found to be well developed/preserved and had extensive spatial coverage. In the main, previous studies concentrated upon establishing a workable stratigraphy, spatial correlation of individual layers and determining the probable depositional mechanisms. However, despite a plethora of research papers, some issues still remain unresolved. This is in part due to a lack of agreement between investigators; sampling and analytical short comings, restricted sample size and the fact that, in many instances, like was not being compared with like. Recently, the limit of sapropels in the western basin has been further extended. As a result, the palaeoceanographic/palaeoclimate models which had previously been developed for deposition of sapropels in the eastern basin have been modified. Most recently, strong links have been established between astronomical cyclicity and sapropel formation. This review paper provides a summary of sapropel research to date, and ongoing sapropel research in the Mediterranean, some of which appears in this thematic issue of Marine Geology. It is fitting that this thematic issue of Marine Geology be dedicated to the memory of Colette Vergnaud-Grazzini and Rob Kidd who in many ways helped to initiate the resurgence in sapropel studies in the 1970s in the Mediterranean —perhaps in 50 more years we will know all of the answers!     

Seasonal variability of water transport through the Straits of Gibraltar, Sicily and Corsica, derived from a high-resolution model of the Mediterranean circulation

The variability of the water transport through three major straits of the Mediterranean Sea (Gibraltar, Sicily and Corsica) was investigated using a high-resolution model. This model of the Mediterranean circulation was developed in the context of the Mercator project.The region of interest is the western Mediterranean between the Strait of Gibraltar and the Strait of Sicily. The major water masses and the winter convection in the Gulf of Lions were simulated. The model reproduced the meso-scale and large-scale patterns of the circulation in very good agreement with recent observations. The western and the eastern gyres of the Alboran Sea were observed but high interannual variability was noticed. The Algerian Current splits into several branches at the longitude of the Strait of Sicily level, forming the Tyrrhenian branch, and, the Atlantic Ionian Stream and the Atlantic Tunisian Current in the eastern Mediterranean. The North Current retroflexed north of the Balearic Islands and a dome structure was observed in the Gulf of Lions. The cyclonic barotropic Algerian gyre, which was recently observed during the MATER and ELISA experiment, was evidenced in the simulation.From time-series of 10-day mean transport, the three straits presented a high variability at short time-scales. The transport was generally maximum, in April for the Strait of Gibraltar, in November for the Strait of Sicily, and in January for the Strait of Corsica. The amplitudes of the transport through the Straits of Gibraltar (0.11 Sv) and Sicily (0.30 Sv) presented a weaker seasonal variability than that of the Strait of Corsica (0.70 Sv).The study of the relation between transport and wind forcing showed that the transport through the Strait of Gibraltar is dependent on local zonal wind over short time-scales (70%), which was not the case for the other straits (less than 30%). The maximum (minimum) of the transport occurred for an eastward (westward) wind stress in the strait. An interannual event was noticed in November–December 2001, which corresponded to a very low transport (0.3 Sv), which was characterised by a cyclonic circulation in the western Alboran Sea. That circulation was also reproduced by the model for other periods than winter during the interannual simulation.The transport through the Strait of Sicily is not influenced by local wind.The wind stress curl of the northwestern Mediterranean influenced the transport through the Strait of Corsica.     

Origin and transformation of organic matter in Pliocene–Pleistocene Mediterranean sapropels: organic geochemical evidence reviewed

Drilling of deep-water post-Messinian sedimentary sequences by ODP Legs 160 and 161 in the Mediterranean Sea has shown that occurrence of organic-carbon-rich sapropels and sapropel-like sediments extends from the Levantine Basin westward into the Alboran Basin. In the eastern Mediterranean, sapropel deposition started in the Early Pliocene, whereas in the Western Basin the onset of sapropel formation occurred later, in the Early Pleistocene. Precessional cycles are apparently the primary external forcing for sapropel formation. Nevertheless, the pattern of sapropel occurrence suggests that the precessional influence is modulated by the glaciation cycles. Large differences were observed in the organic carbon contents of sapropels recovered in the eastern and western Mediterranean. Correspondence between organic carbon contents, Rock-Eval hydrogen index values and elemental C/N ratios indicate that both variations in the production and preservation of marine organic matter have led to the accumulation of high amounts of organic matter in sapropels. Molecular organic geochemical compositions of sapropels from the eastern Mediterranean further confirm that the major fraction of organic matter in sapropels is derived from marine algal sources and has undergone variable oxidation. Enhanced marine productivity and improved preservation of organic matter is central to sapropel formation. Accumulation of increased amounts of land-derived material at times of sapropel formation is also evidenced, supporting the hypothesis of significant periodic freshwater discharges.     

A synthetic pollen record of the eastern Mediterranean sapropels of the last 1 Ma: implications for the time-scale and formation of sapropels

The Quaternary climate of southern Europe (south Italy and Greece) is investigated by pollen analysis of the sapropels which were deposited in the deep eastern Mediterranean Sea during the last 1 million year (Ma). The time-scale of core KC01b in the Ionian Sea has been established by tuning its oxygen isotopic record to the ice volume model of Imbrie and Imbrie (1980). For the last 250,000 year (250 ka), the previous pollen studies and astronomical tuning have been confirmed. Sapropels were deposited under a large range of Mediterranean climates: fully interglacial, fully glacial, and intermediary, as revealed mainly by the balance between the respective pollen abundances of oak (Quercus) and sage-brush (Artemisia). The high value of the oak reveals the warm and wet climate of an Interglacial, and the high value of the sage-brush, the dry and cold climate of a Glacial. Whereas the Mediterranean climate is directly related to the variation of the high-latitude ice sheets, the deposition of sapropels is not so. In contrast with the wide climatic range, sapropels were deposited only when summer insolation in the low latitudes reached its highest peaks. However, between 250 ka and 1 Ma, that stable pattern is not yet established. Only six sapropels are observed, many expected ones do not appear, even as ghosts signalled by peaks of barium abundance, that remain after the post-deposition oxidation of organic matter. The pattern of sapropel formation in stable and direct relationship to highest insolation does not seem to apply. For five of those sapropels, neither climate extremes are observed; they mainly formed during intermediary types of Mediterranean climate. In contrast, one sapropel (and one ghost) relates to a relatively low peak of insolation, and its climate is of a unique, composite type not seen later. This might suggest an unsuspected, more complex pattern linking the formation of Mediterranean sapropels to the astronomical configuration.     

First record of a Vestimentifera (Polychaeta: Siboglinidae) from chemosynthetic habitats in the western Mediterranean Sea—Biogeographical implications and future exploration

A new population of vestimentiferan tubeworms was discovered during a recent expedition to a mud volcano field in the Alboran Sea, western Mediterranean Sea. Morphological data and mitochondrial cytochrome-c-oxidase subunit 1 (COI) sequences show that the Alboran tubeworm is essentially identical to Lamellibrachia sp. found in the eastern Mediterranean. This is the first record of a vestimentiferan species in the western basin of the Mediterranean, an area with direct connection to the Atlantic via the Strait of Gibraltar and therefore of great importance to the study of distributional patterns and evolution of Mediterranean species. We examine the current hypotheses on the biogeographic distribution of vestimentiferan species in the eastern Atlantic and Mediterranean Sea and conclude that independently of when Lamellibrachia colonized the Mediterranean, neither the present hydrological settings of both Mediterranean Sea and Atlantic Ocean, nor vestimentiferans reproductive biology are impeditive to the presence of the Mediterranean species of Lamellibrachia in the NE Atlantic. The West African and Lusitanian margins are the most likely places to find living populations of this species in the NE Atlantic.     

Diagenetic magnetic enhancement of sapropels from the eastern Mediterranean Sea

Diagenetic dissolution of magnetic minerals has been widely observed in organic-rich sediments from many environments. Organic-rich sediments from the eastern Mediterranean Sea (sapropels), recovered during Leg 160 of the Ocean Drilling Program, reveal a surprising catalogue of magnetic properties. Sapropels, from all sites studied across the eastern Mediterranean Sea, are strongly magnetic and the magnetization is directly proportional to the organic carbon content. The magnetization of the sapropels is dominated by a low-coercivity, probably single domain magnetic mineral (with an inverse magnetic fabric) that exhibits a clear decay in magnetic properties when exposed to air. During heating, the magnetic particles irreversibly break down between 360 and 400°C. The contrast between the magnetic properties of sapropels and surrounding sediments is marked, with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments. The contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non-steady-state diagenesis: sulphate-reducing conditions dominated during sapropel deposition, while overlying sediments were deposited under oxic conditions. The mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate-reducing conditions that existed during times of sapropel formation. Attempts to identify this mineral have been unsuccessful, but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase. The influence of diagenesis on the magnetic properties of cyclically-deposited eastern Mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments.     

Large warming and salinification of the Mediterranean outflow due to changes in its composition

The Mediterranean Sea transforms surface Atlantic Water (AW) into a set of cooler and saltier typical Mediterranean Waters (tMWs) that are formed in different subbasins within the sea and thus have distinct hydrological characteristics. Depending on the mixing conditions along their route and on their relative amounts, the tMWs are more or less differentiated at any given place, and some mix together up to forming new water masses. We emphasise the fact that any of these Mediterranean Waters (MWs) must outflow from the sea, even if more or less identifiable and/or in a more or less continuous way. Historical data from the 1960s–1980s showed that the densest MW outflowing through the Strait of Gibraltar at Camarinal Sill South (CSS) was a relatively cool and fresh tMW formed in the western basin, namely the Western Mediterranean Deep Water (WMDW). At these times, the sole other tMW identified in the strait was the Levantine Intermediate Water (LIW); no mention was made there of, in particular, the two densest tMWs formed in the eastern basin (in the Aegean and the Adriatic) that are now named Eastern Overflow Water (EOW) when they reach the Channel of Sicily (where they cannot be differentiated). A fortiori, no mention was made of the Tyrrhenian Dense Water (TDW) that results from the mixing of EOW with waters resident in the western basin (in particular WMDW) when it cascades down to ∼2000 m from the channel of Sicily. New measurements (essentially temperature and salinity time series) collected at CSS since the mid-1990s indicate that the densest MWs outflowing through the strait have been continuously changing; temperature and salinity there have been increasing, being actually (early 2000s) much warmer (∼0.3 °C) and saltier (0.06) than ∼20 years ago. These changes are one order of magnitude larger than the decadal trends shown for WMDW in particular. We thus demonstrate that, in the early 2000s, (i) the densest MW outflowing at Gibraltar is TDW and (ii) TDW is mainly composed of EOW (the percentage of MWs from the western basin, in particular WMDW, is lower): the densest part of the outflow is thus “more eastern than western”. This Mediterranean Sea Transient (a shift from the western basin to the eastern one) could be linked to the Eastern Mediterranean Transient (a shift from the Adriatic subbasin to the Aegean one). Whatever the case, we demonstrate that the proper functioning of the Mediterranean Sea leads to a variability in its outflow's composition that can have consequences for the mid-depth water characteristics in the North-Atlantic much more dramatic than previously thought.     

Metal fluxes through the Strait of Gibraltar: the influence of the Tinto and Odiel rivers (SW Spain)

A large set of new data concerning dissolved metal concentrations has been acquired in the Gulf of Cadiz and in the Strait of Gibraltar from 1996 to 1999. These data, associated with models (hydrodynamic, tracer advection–dispersion and mixing), have been used to assess the influence of rivers draining the South Iberian Pyrite Belt on the Gulf of Cadiz and on the Atlantic inflow in the Strait of Gibraltar.Metal concentrations in surface waters from the Gulf of Cadiz are maximal near the mouth of the Tinto/Odiel rivers with values exceeding 50 nmol/kg (Mn), 5 nmol/kg (Ni), 30 nmol/kg (Cu), 100 nmol/kg (Zn), 0.9 nmol/kg (Cd) and 45 nmol/kg (As). From the Tinto/Odiel river, a plume of contamination follows the coast in the direction of the Strait of Gibraltar. The computation of a tracer advection–dispersion model confirms that the coastal currents carry the metals discharged from the Tinto and Odiel to the Strait of Gibraltar.From temperature–salinity and metal–salinity plots, four water masses can be recognised in the Gulf of Cadiz and in the Strait of Gibraltar: North Atlantic Surface Water (NASW), North Atlantic Central Water (NACW) and metal-enriched Spanish Shelf Waters from the Gulf of Cadiz (SSW). The Mediterranean Outflow Water (MOW) is also clearly seen at depths greater than 300 m.The chemical characteristics of these various water masses have been used in a mixing model to evaluate their relative contribution to the Atlantic inflow through the Strait of Gibraltar. These contributions are seasonally variable. In June 1997, the contribution was: 80±20%, 5±5% and 15±10% for NASW, NACW and SSW, respectively. In September, the SSW contribution was apparently negligible.Finally, these relative contributions allow the evaluation of the metal fluxes in the Strait of Gibraltar. The presence of SSW in the Strait increases the metal flux to the Mediterranean Sea by a factor of 2.3 (Cu), 2.4 (Cd), 3 (Zn) and 7 (Mn). It does not modify significantly As and Ni fluxes.     

The oxygen and carbon isotope distribution in the Mediterranean water masses

The oxygen and carbon stable isotope compositions of the present-day Mediterranean waters have been measured in order to evaluate their variability, which is related to the specific climatic and hydrological conditions within the basin. The experimental equation between the δ¹⁸O value and the salinity of water, based on 300 measurements on surface, intermediate, and deep waters sampled during the VICOMED 2 and 3 cruises in the western, central and eastern Mediterranean, has a slope of 0.27, a value which is significantly lower than the slope of 0.45, as defined in the northeast Atlantic Ocean. This difference in the δ¹⁸O–salinity relationship, which occurs immediately in the Alboran basin, is basically a characteristic of the climatic regime of the Mediterranean, i.e., of an excess evaporation over fresh water input. The largest variations of these two parameters, δ¹⁸O of water and δ¹³C of ∑CO₂, are observed in the surface waters, mostly in the western Mediterranean. This evolution mirrors the progressive eastward restriction, which separates the less-evaporated and more-productive western basins from the more-evaporated and less-productive eastern basins. The intermediate waters constitute a homogeneous layer. However, their δ¹⁸O values decrease eastward by 0.35‰ at maximum, due to progressive dilution by mixing with overlying and underlying water masses; their δ¹³C values decrease also eastward by 0.35‰ at maximum, due to an increasing input of nutrients issued from the regeneration of sinking organic particles. The deep waters have similar δ¹⁸O values but slightly higher δ¹³C values (often by less than 0.1‰) than the overlying intermediate waters, indicating generally well ventilated conditions due to active winter convection.     

Total organic carbon distribution and budget through the Strait of Gibraltar in April 1998

In order to investigate total organic carbon (TOC) exchange through the Strait of Gibraltar, samples were taken along two sections from the western (Gulf of Cádiz) and eastern (Western Alboran Sea) entrances of the Strait and at the middle of the Strait in April 1998. TOC was measured by using a high-temperature catalytic oxidation method. The results referenced here are based on a three-layer model of water mass exchange through the Strait, which includes the Atlantic inflow, Mediterranean outflow and an interface layer in between. All layers were characterised by a decrease of TOC concentrations from the Gulf of Cádiz to the Western Alboran Sea: from 60–79 to 59–66 μM C in the Atlantic inflow and from 40–60 to 38–52 μM C in the Mediterranean waters, respectively. TOC concentrations in the modified North Atlantic Central Water varied from 43 to 55 μM C. Intermediate TOC values were measured in the interface layer (43–60 μM C). TOC concentrations increased from the middle of the Strait towards continents indicating a contribution of organic carbon of photosynthetic origin along Spain and Morocco coasts or TOC accumulation due to upwelling in the northeastern part of the Strait. Our results indicate that the short-term variability caused by the tide greatly impacts the TOC distribution, particularly in the Gulf of Cádiz. The TOC input from the Atlantic Ocean to the Mediterranean Sea through the Strait of Gibraltar varies from 0.9×10⁴ to 1.0×10⁴ mol C s⁻¹ (or 0.28×10¹² to 0.35×10¹² mol C year⁻¹, respectively). This estimate suggests that the TOC inflow and outflow through the Strait of Gibraltar are two and three orders of magnitude higher than reported via the Turkish Straits and Mediterranean River inputs.     

Middle Miocene (Langhian) sapropel formation in the easternmost Mediterranean deep-water basin: Evidence from northern Cyprus

Mid-Miocene (Langhian; ∼15.4 Ma) sapropels formed within the easternmost Mediterranean basin, now uplifted in northern Cyprus. These sapropels represent the oldest known sapropels in a predominantly marl succession. Six well-developed sapropels were studied. Strontium isotope dating of twelve samples gave a preferred age of ∼15.4 Ma (Langhian); i.e. during the final phases of the Middle Miocene Climatic Optimum (MCO). The age of the best-preserved nannofossil assemblage (Langhian) is close to the strontium ages. The Langhian strontium ages are preferred over an alternative early Serravallian age for less well-preserved nannofossil assemblages. Total organic carbon contents in the sapropels reach maximum values of 3.9 wt.%. Relative to the host marls, the sapropels show enrichments in terrigenous-derived minerals and related major and trace elements. Sedimentological evidence indicates that the terrigenous sediments were eroded from the northern borderlands of the deep-water basin under warm, humid conditions. High fresh-water run-off from surrounding landmasses is likely to have promoted a low-salinity lid to the eastern Mediterranean deep-water basin. This, in turn, would have restricted deep-water ventilation and promoted widespread anoxia. Exceptionally high concentrations of chalcophile elements (e.g. Cu, Ni and Zn) are consistent with anoxic conditions. Abundant nutrient-rich fresh-water input is also likely to have stimulated siliceous productivity (although any siliceous microfossils did not survive diagenesis). A significant role for diagenesis in sapropel formation is indicated by the mobilisation of Ba from sapropels to marl directly beneath. Orbitally induced dry–wet oscillation, the mechanism invoked to explain the Pliocene to Holocene sapropels, apparently was already in place during the latest stages of the MCO when the Langhian sapropels accumulated. These sapropels accumulated immediately after the Middle Miocene closure of the Southern Neotethys when the Eastern Mediterranean Sea apparently became more sensitive to orbital cyclicity. The development of a semi-enclosed deep-water basin was, therefore, a prerequisite for sapropel formation.     

Distribution and transfer of trace elements in the Western Mediterranean

Results concerning the concentration of cadmium and lead in Mediterranean waters collected during the 2nd PHYCEMED cruise (Oct. 1983) are discussed. Sampling has been performed at seven stations in the Western Mediterranean Basin, two in the Strait of Gibraltar and the near Atlantic, two in the Sicily Strait and the Eastern Basin.In the Western Basin the observations are in fair agreement with those of PHYCEMED 1. Cadmium has a fairly homogeneous distribution vertically as well as from one station to another, with an average concentration of 8 ng l⁻¹; while lead shows a slight but continuous decrease in concentrations with depth (from at least 50 ng l⁻¹ in surface waters to 20 or 25 ng l⁻¹ at depth). On the other hand, at the basin boundaries, where waters from different origins are present, vertical distributions appear very different. On the basis of calculated water budgets it can be estimated that the Mediterranean Sea discharges about 200 t y⁻¹ of cadmium and about 250 t y⁻¹ of lead into the Atlantic Ocean while 1000 t y⁻¹ of lead are transferred from the Western to the Eastern Basin.     

The occurrence and significance of Pleistocene and Upper Pliocene sapropels in the Tyrrhenian Sea

Numerous sapropels and sapropelic strata from Upper Pliocene and Pleistocene hemipelagic sediments of the Tyrrhenian Sea show that intermittent anoxia, possibly related to strongly increased biological productivity, was not restricted to the eastern Mediterranean basins and may be a basin-wide result of Late Pliocene-Pleistocene climatic variability. Even though the sapropel assemblage of the Tyrrhenian Sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic-rich suspensates, many “pelagic sapropels” have been recognized. Stratigraphic ages calculated for the organic-rich strata recovered during ODP Leg 107 indicate that the frequency of sapropel formation increased from the lowermost Pleistocene to the base of the Jaramillo magnetic event, coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the Mediterranean. A second, very pronounced peak in sapropel formation occurred in the Middle to Late Pleistocene (0.73-0.26 Ma). Formainifers studied in three high-resolution sample sets suggest that changes in surface-water temperature may have been responsible for establishing anoxic conditions, while salinity differences were not noted in the faunal assemblage. However, comparison of sapropel occurrence at Site 653 with the oxygen isotopic record of planktonic foraminifers established by Thunell et al. (Proc. ODP, Sci. Results 107, 1990) indicates that sapropel occurrences coincide with negative δ¹⁸O excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in Hole 653A. A variant of the meltwater hypothesis accepted for sapropel formation in the Late Pleistocene eastern Mediterranean may thus be the cause of several “anoxic events” in the Tyrrhenian as well. Model calculations indicate that the amount of oxygen advection from Western Mediterranean Deep Water exerts the dominant control on the oxygen content in deep water of the Tyrrhenian Sea. Inhibition of deep-water formation in the northern Adriatic and the Balearic Basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the Tyrrhenian Sea.     

Topographic control on the nascent Mediterranean outflow

Data collected during a 12-day cruise in July 2009 served to examine the structure of the nascent Mediterranean Outflow Water (MOW) immediately west of the Espartel Sill, the westernmost sill in the Strait of Gibraltar. The MOW is characterized by high salinities (>37.0 and reaching 38.3) and high velocities (exceeding 1?m s^?1 at 100?m above the seafloor), and follows a submerged valley along a 30?km stretch, the natural western extension of the strait. It is approx. 150?m thick and 10?km wide, and experiences a substantial drop from 420 to 530?m over a distance of some 3?km between two relatively flat regions. Measurements indicate that the nascent MOW behaves as a gravity current with nearly maximal traveling speed; if this condition is maintained, then the maximum MOW velocity would decrease slowly with distance from the Espartel Sill, remaining significantly high until the gravity current excess density is only a small fraction of its original value. The sharp pycnocline between the Mediterranean and the overlying North Atlantic Central waters is dynamically unstable, particularly where the flow interacts with the 100?m decrease in bottom depth. Here, subcritical gradient Richardson numbers coincide with the development of large interfacial undulations and billows. The very energetic downslope flow is likely responsible for the development of a narrow V-shaped channel downstream of the seafloor drop along the axis of the submerged valley, this probably being the very first erosional scour produced by the nascent MOW. The coincidence of subcritical gradient Richardson numbers with relatively high turbidity values above the channel flanks suggests it may be undergoing upstream erosion.     

Suspended-sediment concentration and mineralogy in the central and western Mediterranean and mineralogic comparison with bottom sediment

Concentrations and mineralogy of suspensates in the central and western Mediterranean are vertically and laterally variable. This variability is related to input by resuspension of bottom sediments and from local terrigenous sources. Bottom currents flowing through constrictions at the straits of Sicily and Gibraltar and the eastern entrance of the Alboran Sea resuspend bottom sediments, giving rise to increased concentrations of suspensates in near-bottom waters and limited inputs to higher levels. There is no evidence of a suspensate-rich bottom water in the Balearic Sea.Terrigenous sources are believed to be the cause of increasing relative amounts of montmorillonite in surface waters as they flow eastward within the Mediterranean. Montmorillomite is relatively more important in suspended sediments than in bottom sediments where kaolinite—chlorite is dominant.     

Biogeochemical evolution of the outflow of the Mediterranean deep-lying particulate organic matter into the northeastern Atlantic

Small- and large-size particles were collected in January and August 1989 throughout the water column (50–3000 m) in a northeastern Atlantic area where deep Mediterranean waters outflowing through the Strait of Gibraltar are incorporated at mid-depth into Atlantic waters. Particles collected by water filtration (0.7 μm pore size) and by vertical hauls of a plankton net (50 μm mesh size) were analysed for their organic carbon and lipid composition, namely fatty acids, hydrocarbons, aliphatic and alicyclic alcohols and ketones, and pigments. Small-size particles exhibited POC concentrations two to three orders of magnitude higher (5–50 μg/l) than large particles (0.01–0.32 μg/l). Strong spatial and temporal variations were also observed. Surface small particles collected in January accounted for 15–50 μg/l of POC, whereas in August, these represented only 5–15 μg/l following the seasonal variability of primary production. Concurrently, the variety of lipid components was larger in January.The lipid components of large-size particles were dominated by zooplankton markers, whereas small particles showed evidence of a mixed algal composition (mainly of haptophytes and moderately of diatoms, prasinophytes, chrysophytes and dinoflagellates) with a bacterial contribution. The vertical profiles of the different lipid classes showed a general decrease with depth, particularly significant in the upper 200 m, consistently with the POC contents. Compositional changes were more evident in small particles and included the loss of unsaturated compounds and the increase of diagenetic and bacterial markers. Unusual increases were observed at mid-depths indicating additional particle inputs, either by in situ formation or by advective transport from the Mediterranean. The latter was recognized because small particles in Mediterranean waters entrained large quantities of continental detritic materials that were also found in the Mediterranean water lenses in the Atlantic. From these observations, it was inferred that the outflow of the Mediterranean deep-lying particulate organic matter might still preserve its signature in the mid-depth northeastern Atlantic waters.     

Sedimentology and geochemistry of an exceptionally preserved last interglacial sapropel S5 in the Levantine Basin (Mediterranean Sea)

A combined study of lithological, geochemical and physical sediment properties is reported from a completely laminated S5 sapropel, recovered in three gravity cores (M40-4 SL67, M51-3 SL103, M51-3 SL104) from the Pliny Trench region of the eastern Mediterranean. The thickness of the studied sapropel S5 varies between 85 and 91 cm and tops most S5-sapropels in the Mediterranean. Based on optical features like color and thickness of laminae, the sapropels were subdivided into thirteen distinct lithostratigraphic zones. These zones, as well as the finer layering pattern within them, could be followed exactly among the three cores, indicating that the processes responsible for this variation acted at least on a regional scale. The sapropel sediment is characterized by exceptionally high porosity, which is strongly correlated with Si/Ca. This relationship implies that the sapropel is in essence an organic-matter rich diatomite and its exceptional thickness can be explained by preservation of diatoms forming a loosely packed sediment fabric. Compared to other S5 sapropels, the preservation of diatoms has apparently led to a twofold increase in the thickness of the sapropel layer. Relative abundances of 10 elements were determined at ultra-high resolution (0.2 mm) by XRF-scanner over the complete length of each sapropel including several cm of enclosing marl. An analysis of the chemical data indicates that the lowermost 13 cm of the sapropel is chemically more similar to the underlying marl and that the sediment chemistry shows different signals at different scales. The strongest pattern is the contrast between the sapropel and the surrounding marl, which is accentuated in elements indicative for redox conditions as well as terrigenous sediment input and productivity. Within the sapropel, a mm- to cm-scale layering is observed. The abundances of many elements are systematically linked to the pattern of these layers, indicating a common origin, related to productivity and/or terrigenous sediment and/or redox conditions. This pattern indicates a link to a regional climatic process, making the S5 sapropel horizon in M40-4 SL67, M51-3 SL103 and M51-3 SL104 a potential high-resolution archive of climatic variability during the last interglacial in the Mediterranean Sea and its adjacent landmasses.     

Geochemistry of eastern Mediterranean sediments: Primary sediment composition and diagenetic alterations

Two cores recovered in the eastern Mediterranean were analysed for major, minor and trace elements. The primary chemical composition of the sediment is different at each location, probably because the lithological sources and the relative biogenic contributions differ.

Carbonates are important for the concentration of Ca, Mg and Sr, whereas aluminosilicates determine the concentration of Si, Al, K, Li, Y and Be, and to a lesser extent that of Fe, Cr, Ti, Mg, Zn and Zr. In sapropels, organic carbon and sulphur seem to be closely related. Bromine, Mo, P, Fe, V, Cu, Zn, Co, Ni and Cr are closely associated with organic and sulphidic compounds. The concentration versus depth profile for organic carbon in two sapropels points to a rapid establishment of conditions that gave rise to sapropel formation, followed by a gradual transition back to “normal” conditions.

The primary composition is overprinted by diagenetic processes. Sulphate-reducing conditions occurred during and just after sapropel deposition. A progressive oxidation front mechanism, which became active after sapropel deposition, is responsible for additional major geochemical changes. This diagenetic phenomenon has strong implications for the chemistry of Fe, Mn, Ni, Co, Zn, Cu, Cr, V, U, As and Sb.