Relationships between magmatism and tectonics in a continental rift: The Pantelleria Island region (Sicily Channel, Italy)

Field geological data of the Pantelleria Island, a large Late Quaternary volcano located in the Sicily Channel rift zone, integrated with offshore geophysical information, are used to derive the structural setting of the Island and the surrounding region, and to analyse the relationships between tectonics and magmatism. Field work shows that the principal faults exposed on the Island fall into two systems trending NNE–SSW and NW–SE. Mapped faults from offshore multichannel seismic profiles show similar trends, and some of them represent the offshore extension of the Pantelleria Island structures. The NW–SE faults bound the Pantelleria Graben, one of the three main depressions formed since the Late Miocene–Early Pliocene within the African continental platform, which compose the Sicily Channel rift zone. A 3-D Moho depth geometry, derived from inversion of Bouguer gravity data, shows a significant uplift of the discontinuity up to 16–17 km beneath the westernmost part of the Pantelleria Graben and beneath the Pantelleria Island; it lows rapidly to 24–25 km away from the graben north-eastward and south-westward. The Moho uplift could explain the presence of a shallow magma chamber in the southern part of the Island, where processes of magmatic differentiation are documented. Geological and geophysical data suggest that the northwestern part of the Sicily Channel is presently dominated by a roughly E–W directed extensional regime. Crustal cracking feeding the Quaternary volcanism could be also related to this extensional field that would be further responsible for the development of the N–S trending volcanic belt that extends in the Sicily Channel from Lampedusa Island to the Graham Bank. This mode of deformation is confirmed also by geodetic data. This implies that in the northwestern part of the Sicily Channel, the E–W extension replaced the NE–SW crustal stretching that originated the NW-trending tectonic depressions constituting the rift zone.     

Seismic expressions of deep-shelf depositional and erosional morphologies,Miocene Utsira formation,North Sea Basin

The Middle–Late Miocene Utsira Formation of the North Sea Basin contains a fully preserved, regional marine sand deposit that records a stable paleogeographic setting of sand transport and accumulation within a deep, epeiric seaway which persisted for >8 Ma. The sediment dispersal system was defined by (1) input through a marginal prograding strandplain platform, coast-to-basin bypass, transport along a narrow strait, and accumulation in strait-mouth shoal complexes within a shelf sea; (2) a high-energy marine regime; (3) very low time-averaged rates of sediment supply and accumulation; and (4) consequent high sediment reworking ratio. Sand distribution and stratal architectures reflect regional along-strike sediment transport and local to sub-regional landward sediment transport. Plume-shaped, south-building, submarine sand shoals that formed along the recurved arc of the strandplain margin nourished the shoal system. Very low-angle sigmoid clinoforms and down-stepping, aggradational top sets are distinctive architectures of these strike-fed sand bodies. The combination of strong marine currents and slow but long-lived sand supply from the Shetland strandplain created regional, sandy shelf shoal depositional systems that individually covered 3,500 to 6,000 km² of the basin floor. Defining attributes of the shelf shoal systems include their location within the basin axis, abundance of autochthonous sediment, and sandy marine facies composition. Diagnostic depositional architectures include the along-strike-dipping sigmoidal clinoforms, poly-directional low-angle accretionary bedding at both regional and local scales, and mounded depositional topography. Erosional features include regional hummocky, low-relief shelf deflation surfaces, broad, elongate scours and sub-circular scour pits.     

Detailed seafloor geomorphology of the western region of the North Yellow Sea,China: The result of Holocene erosional and depositional processes sculpting the offshore continental shelf

High-resolution multi-beam/single-beam bathymetric data and seismic profiling data from the latest surveys are used to map and interpret the detailed seafloor geomorphology of the western region of the North Yellow Sea (NYS), China. The mapping area covers 156 410 km2, and incorporates a flat shelf plain, subaqueous accumulation shoals, tidal scouring troughs, and tidal sand ridge groups. Offshore areas with water depths less than 50 m in the western region of the NYS are mainly covered by thick, loose sediments, forming wide spread accumulation geomorphological features; these include the Liaodong Peninsula subaqueous accumulation system containing shoals and rugged scouring troughs, and the large mud wedge of the Shandong Peninsula. In the central part of the NYS, there is a relatively flat residual shelf plain with coarser sediment deposits. This flat shelf plain has a water depth larger than 50 m and a thin layer of sediment, on which there is a large pockmark field caused by seafloor seepage. These geomorphological structures indicate that modern sedimentary processes are the main driving force controlling the sculpture of the current seafloor surface landform. Extensive strong tidal current systems and abundant sediment sources provide the critical external forces and essential conditions for the formation of seafloor geomorphology. The tectonic basement controls the macroscopic morphological shape of the NYS, but is reflected very little in the seafloor geomorphic elements. Our results provide a detailed seafloor geomorphological map of the western region of the NYS, an area that has not previously mapped and also provide a scientific framework for further research into offshore seafloor geomorphology, shelf sedimentary processes, and submarine engineering construction in this region.     

Growth patterns of a proximal terrigenous margin offshore the Guadalfeo River, northern Alboran Sea (SW Mediterranean Sea): glacio-eustatic control and disturbing tectonic factors

The shelf-upper slope stratigraphy offshore and around the Guadalfeo River on the northern continental margin of the Alboran Sea, Western Mediterranean Basin, has been defined through the interpretation of a grid of Sparker seismic profiles. We tried to identify evolutionary trends in shelf growth, as well as to determine the regional/local factors that may modify the influence of glacio-eustatic fluctuations. Four major depositional sequences are identified in the sedimentary record by a detailed seismic interpretation, which defines three significant intervals of shelf-upper slope progradation, dominated by deposition of shelf-margin wedges, which resulted in uniform patterns of shelf-margin growth in response to significant sea-level falls. In contrast, the record of transgressive intervals is more variable, mainly as the result of distinct patterns of regressive-to-transgressive transitions. Major progradational wedges are internally composed of seaward-prograding, landward-thinning wedges, interpreted to represent shelf-margin deltaic deposits. In contrast, the last aggradational interval is composed of shelf-prograding wedges that show distinct characteristics, in terms of seismic facies, morphology and distribution when compared with previous shelf-margin wedges. These shelf wedges are thought to represent the particular case of Regressive Systems or Shelf Margin Systems Tracts, and their development seems to be controlled by a drastic change in main depocenter location, which moved from the upper slope to the shelf during the Pleistocene. The stacking pattern of seismic units, the shallowness of the acoustic basement and the migration of the shelf break are used to infer spatial and temporal changes in tectonic subsidence-uplift rates, which interact with low-order glacio-eustatic changes. For much of the Pliocene-Quaternary, uplifted sectors alternated laterally with sectors experiencing more subsidence. Subsequently, a significant change from lateral outgrowth to vertical accretion is recognised. This stratigraphic change could be related to the combined influence of increased subsidence rates on the shelf and the onset of higher-frequency glacio-eustatic cyclicity after the Mid Pleistocene Revolution that occurred around 1 Ma.     

Spatial variability of Posidonia oceanica (L.) Delile epiphytes around the mainland and the islands of Sicily (Mediterranean Sea)

This paper investigates patterns of variability in epiphytes of Posidonia oceanica leaves at various spatial scales around Sicily, including geographical differences among the Mediterranean basins, differences between the small islands and mainland, and the variability among and within replicated meadows in each of the previous conditions. Data on percentage cover of the most common epiphytic organisms were analysed by univariate techniques. Encrusting red algae, encrusting brown algae, filamentous algae, encrusting bryozoans, erect bryozoans, hydroids and Foraminifera were the most abundant taxa. Significant differences in the abundance of taxa were detected among geographical regions, while no significant differences were found between the islands and mainland. At smaller scales, variability was concentrated mostly among leaves 100s of centimetres apart and among meadows a few kilometres apart. These results suggest that both geographical and local processes are important in structuring the epiphytes of P. oceanica leaves in this area of the Mediterranean.     

Tectonic elements and crust of the eastern Mediterranean Sea

Seismic reflection and gravity data show the eastern Mediterranean Sea to be evolving into several basins as a result of differential vertical movements. The Levantine Basin and deeper Herodotus Basin are separated by a buried ridge (horst? or faulted geanticline?) lying west of Eratosthenes Seamount, which in turn is the more elevated part of a northeast-trending geanticline truncated along its eastern flank by a graben. To the east, gravity trends in the Levantine Basin are parallel to the graben. These features and trends are similar to those seen on land in Egypt and the Levant and imply continuity of structure offshore. Combined with other geological and geophysical information the observations suggest that the eastern Mediterranean crust is the marginal extension of the African continental crust. Although the character of the Florence Rise and Anaximander Mountains, the northward tilting and subsidence of the Antalya and Finike Basins, and the apparent continuation of the Strabo Trench south of the Florence Rise suggest underthrusting of the Turkish plate by Africa, there may be insufficient seismicity. There is no active volcanic arc, and the trench is too poorly developed to confirm active subduction as the sole manifestation of plate convergence. Normal subduction probably ended within the past 5 m.y. with the disappearance of all oceanic crust between Turkey and Africa. Plate convergence continues with only limited underthrusting of Africa along the Cypriot Arc, but with regional deformation along zones of weakness within a wide (300 km?) band stretching from the Herodotus Basin to the east along the northern edge of the African and Arabian plates.     

Tracing seafloor methane emissions with benthic foraminifera: Results from the Ana submarine landslide (Eivissa Channel,Western Mediterranean Sea)

The hypothesis that benthic foraminifera are useful proxies of local methane emissions from the seafloor has been verified on sediment core KS16 from the headwall of the Ana submarine landslide in the Eivissa Channel, Western Mediterranean Sea. The core MS312 from a nearby location with no known methane emissions is utilised as control. The core was analysed for biostratigraphy, benthic foraminiferal assemblages, Hyalinea balthica and Uvigerina peregrina carbon and oxygen stable isotope composition, and sedimentary structures. The upper part of the core records post-landslide deglacial and Holocene normal marine hemipelagic sediments with highly abundant benthic foraminifera species that are typical of outer neritic to upper bathyal environment. In this interval, the δ¹³C composition of benthic foraminifera indicates normal marine environment analogous to those found in the control core. Below the sedimentary hiatus caused by the emplacement of the slide, the foraminiferal assemblages are characterised by lower density and higher Shannon Index. Markedly negative δ¹³C shifts in benthic foraminifera are attributed to the release of methane through the seabed. The mean values of the ¹³C anomaly in U. peregrina are ? 0.951 ± 0.208 in the pre-landslide sediments, and ? 0.269 ± 0.152 in post-slide reworked sediments deposited immediately above the hiatus. The δ¹³C anomaly in Hyalinea balthica is ? 2.497 ± 0.080 and ? 2.153 ± 0.087, respectively. To discard the diagenetic effects on the δ¹³C anomaly, which could have been induced by Ca–Mg replacement and authigenic carbonate overgrowth on foraminifera tests, a benthic foraminifera subsample has been treated following an oxidative and reductive cleaning protocol. The cleaning has resulted, only in some cases, in a slight reduction of the anomaly by 0.95% for δ¹³C and < 0.80% for δ¹⁸O. Therefore, the first conclusion is that the diagenetic alteration is minor and it does not alter significantly the overall carbon isotopic anomaly in the core. Consequently, the pre-landslide sediments have been subject to pervasive methane emissions during a time interval of several thousand years. Methane emissions continued during and immediately after the occurrence of Ana Slide at about 61.5 ka. Subsequently, methane emissions decreased and definitely ceased during the last deglaciation and the Holocene.     

Seismic stratigraphy and structural setting of the Adventure Plateau (Sicily Channel)

Within the central Mediterranean, the northwestern sector of the Sicily Channel is the unique area where two independent tectonic processes can be analyzed: the building of the Sicilian–Maghrebian Chain occurred in Late Miocene and the continental lithospheric rifting of the northern African margin occurred since Early Pliocene. These two geodynamic processes generated a peculiar structural style that is largely recognizable in the Adventure Plateau. This plateau is the shallowest part of the Sicily Channel, where water depths do not generally exceed 150 m. It hosts several areas of geomorphic relief, which in some cases rise up to less than 20 m beneath sea-level. A series of submarine magmatic manifestations occur in this area, mainly associated with the extensional phase which produced the rift-related depressions of Pantelleria, Malta and Linosa. Seismic-stratigraphic and structural analyses, based on a large set of multichannel seismic reflection profiles and well information acquired mostly for commercial purposes in the 1970s and 1980s, have allowed us to reconstruct the Triassic-Quaternary sedimentary succession of the Adventure Plateau and define its structural setting. A broad lithological distinction can be made between the successions ranging from Triassic to Paleogene, predominantly carbonate, and the successions ranging from Miocene to Quaternary, predominantly siliciclastic. Three main structural belts have been identified within the Adventure Plateau: (1) the northern belt, affected during Late Miocene time by ESE-verging thrusts belonging to the External Thrust System orogenic domain, which represents the lowermost structural level of the Sicilian–Maghrebian Orogen; (2) the Apenninic–Maghrebian domain of the Sicilian–Maghrebian Orogen, which occupies the northwestern sector of the Adventure Plateau, and that is overthrusted on the External Thrust System orogenic domain during the Late Miocene; (3) the extensional belt of the southwestern sector of the Adventure Plateau, affected by broad NW-trending, high-angle normal faults associated with the Early Pliocene continental rifting phase. The eastern boundary of the Adventure Plateau corresponds to a broadly N–S trending lithospheric transfer zone separating two sectors of the Sicily Channel characterized by a different tectonic evolution.     

Long‐term sponge stranding along the shores of Sardinia Island (Western Mediterranean Sea)

The taxonomic richness and geographic distribution of sponges stranded along Sardinian shores were investigated in the long term by means of a revisited ancient sampling method in order to support a comprehensive species inventory. Almost all stranded species were Keratosa (n = 20), plus 6 species of other Demospongiae taxa. Dictyoceratida were dominant, with 6 genera and 19 species of the families Irciniidae, Spongiidae and Thorectidae: 9 Mediterranean endemics, 7 Atlanto‐Mediterranean and 3 widespread species. Regarding Dendroceratida, only the genus Spongionella was found. Some species (n = 8) were recorded for the first time in circum‐Sardinian seas. A high percentage (54.3%; 19 out of the 35 species) of the total Mediterranean Dictyoceratida fauna was recorded, including the most endangered Mediterranean species, that is of the genera Spongia and Hippospongia. Hippospongia communis, Ircinia variabilis, Spongia zimocca and Spongia officinalis were the most common species. Morphotraits of rare and/or poorly known species were investigated by scanning electron microscopy (SEM). Comparison among Sardinian sectors, and between Sardinian and adjacent seas, revealed a high similarity of stranded Dictyoceratida, despite the fact that the data from Tyrrhenian and Balearic–Catalan seas were collected mainly from SCUBA records. The cost‐effective sampling method used in the present study seems appropriate to the gross qualitative monitoring of coastal areas (e.g. large unexplored Northern African coasts), in order to assess the conservation status of Mediterranean Keratosa species. The wide distribution of all Mediterranean bath sponge species indicates their good conservation status in circum‐Sardinian seas, a datum that could usefully support the future management of this bioresource, particularly in protected areas.     

An Analysis of the Coastal Fish Assemblage of the Ustica Island Marine Reserve (Mediterranean Sea)

A study of the coastal fish assemblages in the marine park of Ustica Island (Southern Tyrrhenian Sea, Mediterranean) was conducted from June 1994 to September 1995 and from June 1996 to September 1997. The principal aims of the research were to: (1) define the faunistic features of local fish communities; (2) assess the effectiveness of the protection regime of the marine park on the fish assemblages ('reserve effect'); (3) provide information on the distribution of some Epinephelus and Diplodus species in the shallowest depth zone of the island. During seasonal surveys, underwater visual censuses were carried out at several sites located in three zones of the island, each with a different level of protection. Observations were made along 250 m² transects at 3–5, 10–15 and 25–30 m depth ranges. Additional surveys were made by SCUBA diving and snorkelling. A statistically significant effect of depth on the fish community parameters was observed, whereas differences linked to protection level, zones, sites and seasons were not significant. The abundance and frequency of occurrence of some species, particularly dusky grouper Epinephelus marginatus , were positively correlated with the degree of protection of the different zones of the marine park. Certain sites of the island are important as nursery areas for three species of the genus Diplodus ( i.e., D. puntazzo, D. sargus and D. vulgaris ).     

Assessing variations in suspensates,Ligurian Sea (northwestern Mediterranean)

High variability of suspensates in the surface water of the Ligurian Sea, an area of limited fluvial sources, sampled at a time of low terrestrial influx raises serious questions regarding conclusions about fluxes or budgets drawn from samples taken during a single cruise. The probability that concentrations found during a single cruise represent a long-term average is very small. On four cruises we found that concentrations of suspended material in the surface waters of the Ligurian Sea were highly variable, even at repeated stations. Variability is largely the result of changes in location of divergences and localized upwelling.     

渤海中南部沉积物中生源要素的分布特征

从化学结合形式出发,对渤海中南部海区沉积物中OC、N、P、BSi等生源要素的含量进行调查分析,讨论了各生源要素的分布及其影响因素。结果表明:受陆源输入的影响,表层沉积物中各形态氮、磷及有机碳均呈现"近岸高,远岸低"的分布趋势,生物硅的含量分布则与海区浮游硅藻的分布趋势相一致;可交换态氮是总氮中较活跃的部分,占总氮的比例约为3.7%;固定态铵是总氮的主要组成部分,约占总氮的38.4%,其主要与有机质含量及黏土矿物组成有关;渤海表层沉积物中OC/BSi、ON/BSi以及OP/BSi的比值分别为2.38、0.19、0.03,低于Redfield比值,说明有机质优先于生物硅分解;OC/ON平均值在6.0~14.0之间,判断渤海沉积物中有机质受陆源输入和海洋自生共同影响;其中,有机碳主要以陆源输入为主,而有机氮在沉积物表层主要以陆源输入为主,在沉积物下层则主要来源于海洋自生。     

Geochemistry of gas hydrate accumulation offshore NE Sakhalin Island (the Sea of Okhotsk): results from the KOMEX-2002 cruise

Geochemical properties of gas hydrate accumulation associated with an active gas vent on the continental slope offshore northeast Sakhalin Island in the Sea of Okhotsk have been investigated. The pore water chemistry data suggest that the gas hydrates (GHs) were formed in an environment of upward-migrating fluid combined with a mechanism of pore water segregation. The upward infiltration of water enriched mainly by Cl^– and K⁺ species appears to occur on the background of earlier diagenesis processes within the gas vent sediments. The GHs were formed from water with chlorinity ranging from 530 to 570 mM. The ¹⁸O and D of GH water varied from –1.4 to –1.8 and from –13 to –18, respectively, representing a mix of seawater and infiltrating fluid water. A complex interaction of pore water, water of ascending fluid and segregated pore water during hydrate formation is also supported by water content measurements and observed gas hydrate structure. The direction of segregated water is opposite to upward fluid migration. Decreasing activity of the gas vent is inferred by comparing the present top of the recovered hydrate layer with previous observations.     

A Deep-sea Channel in the Northwestern Mediterranean Sea: Morphology and seismic structure of the Valencia Channel and its surroundings

The 400 km long Valencia Channel occupies the axis of the Valencia Trough in the Northwestern Mediterranean. Four different types of seismic reflection profiles were used to analyze the morphology and structure of the Valencia Channel with regard to the role played by both margins, Balearic and Iberian, of the Valencia Trough. From a detailed morphoseismic analysis of the Valencia Channel, its upper, middle, and lower courses can be characterized as follows: (1) in the upper course, tributaries are short and only slightly incised, with recent mass-transport deposits occurring on the adjacent continental slopes; (2) in the middle course, the channel deepens, and tributary valleys merge into it; and (3) the lower course begins after a sudden change in the direction of the channel, has a meandering path, is flanked by levees, and is fed by some valleys.During the Pliocene and Quaternary, at least four erosional and filling phases are observed in seismic profiles of the lower course of the Valencia Channel. The varying intensity of mass-transport processes and associated retrogressive slumping, which are related with fluctuations in sediment supply and relative sea-level changes, have played a major role in the formation, maintenance and deepening of the Valencia Channel. In addition to these sedimentary processes, a basement tectonic control and some morphostructural features affect the direction of the Valencia Channel locally.     

Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C₁/_C2+ from all the seepage sites were in the range of 1,500–50,000, while δ¹³C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO₂. δ¹³C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ¹³C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.     

The coral assemblages of an off‐shore deep Mediterranean rocky bank (NW Sicily,Italy)

In this study we characterized the deep assemblages dwelling at 200–250 m depth on a large shoal off Capo St. Vito Promontory (Northwestern coast of Sicily, South Tyrrhenian Sea) by means of ROV‐imaging. Two assemblages of suspension feeders, dominated by the gorgonian Callogorgia verticillata and by the black coral Leiopathes glaberrima, together with a tanatocoenosis of the colonial yellow scleractinian coral Dendrophyllia cornigera, were examined. The three main species were significatively distributed into two areas corresponding to different habitat preferences: a more elevated hardground hosting black corals and a gently sloping, silted rocky bottom hosting the other coral species. The study area is subjected to a heavy pressure from the professional fishery, resulting in the mechanical damage of numerous colonies, some of which are then overgrown by various epibionts including a parasitic bioluminescent zoanthid, new for the Mediterranean fauna, and tentatively identified as Isozoanthus primoidus. In the Mediterranean Sea, these deep off‐shore rocky banks are widely known among recreational and professional fishermen due to their rich fish fauna. However, there has been still little effort into quantifying and characterizing the extent of the impact and its consequences on the benthic communities, which may represent, as in this case, only a partial picture of their original structure and extent.     

Zonation and Ecology of Epiphytic Hydroids in a Mediterranean Coastal Lagoon: The 'Stagnone' of Marsala (North-West Sicily)

Abstract. Distribution patterns of epiphytic hydroids of seagrass ( Posidonia occanica and Cymodocea nodosa ) beds were studied in the 'Stagnone', a moderately hypcrhaline lagoon in Sicily. The role of water movement in determining the spatial and temporal zonation of epiphytic hydroids and their morpho-physiological variations is evaluated. Remarks on systematics arc also included.     

Plankton Distribution Patterns in a Mediterranean Dilution Basin: Amvrakikos Gulf (Ionian Sea, Greece)

Abstract. The present work describes Amvrakikos Gulf (Ionian Sea, Greece) as a dilution basin and provides information concerning the relationships between the hydrology and plankton composition and distribution.
Amvrakikos Gulf is characterized by a differentiated planktonic system which is apparently directly influenced by the input of the rivers located at the northern part of the gulf.     

Eivissa slides, western Mediterranean Sea: morphology and processes

After obtaining full-coverage swath bathymetry data in 1995 and very high-resolution acoustic profiles in 2002, four slides at the Balearic Margin of the Eivissa Channel in the western Mediterranean Sea were revisited in 2004 when side-scan sonar data were collected using a MAK-1M deep-towed acoustic system. These new findings, higher in resolution than those for the swath bathymetry, show two main features previously undetected within these submarine landslides: (1) a series of step-forming inclined and detached slabs oriented perpendicular to the slide movement and located in the uppermost part of the slides, and (2) arcuate regular positive ridges oriented also normal to the slide movement and located in the depositional lobes of some of the slides. The former are interpreted as extensional ridges, suggesting a retrogressive post-failure evolution of the slides. The latter are interpreted as compression ridges, related to plastic deformation of the sediment before movement freezing. Moreover, the new data show that fluid escape features are even more widespread in the Eivissa Channel than previously thought, dozens of new pockmarks less than 20 m in diameter having been identified.