Qualitative assessment of gas hydrate and gas concentrations from the AVO characteristics of the BSR in the Ulleung Basin, East Sea (Japan Sea)

The presence of gas hydrate in the Ulleung Basin, East Sea (Japan Sea), inferred by various seismic indicators, including the widespread bottom-simulating reflector (BSR), has been confirmed by coring and drilling. We applied the standard AVO technique to the BSRs in turbidite/hemipelagic sediments crosscutting the dipping beds and those in debris-flow deposits to qualitatively assess the gas hydrate and gas concentrations. These BSRs are not likely to be affected by thin-bed tuning which can significantly alter the AVO response of the BSR. The BSRs crosscutting the dipping beds in turbidite/hemipelagic sediments are of low-seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the gas-hydrate stability zone (GHSZ), which, in turn, suggests the presence of gas hydrate. The BSRs in debris-flow deposits are characterized by a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a few percent or greater gas saturations. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations, as predicted by AVO model studies based on rock physics. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ.     

Numerical modeling of the central Black Sea ecosystem functioning during the eutrophication phase

A one-dimensional coupled physical-biogeochemical model has been developed to simulate the ecosystem of the central Black Sea at the end of the 1980s when eutrophication and invasion by gelatinous organisms seriously affected the stability and dynamics of the system. The physical model is the General Ocean Turbulence Model (GOTM) and the biogeochemical model describes the foodweb from bacteria to gelatinous carnivores through 24 state variables including three groups of phytoplankton: diatoms, small phototrophic flagellates and dinoflagellates, two zooplankton groups: micro- and mesozooplankton, two groups of gelatinous zooplankton: the omnivorous and carnivorous forms, an explicit representation of the bacterial loop: bacteria, labile and semi-labile dissolved organic matter, particulate organic matter. The model simulates oxygen, nitrogen, silicate and carbon cycling. In addition, an innovation of this model is that it explicitly represents processes in the anoxic layer. Biogeochemical processes in anaerobic conditions have been represented using an approach similar to that used in the modeling of diagenetic processes in the sediments lumping together all the reduced substances in one state variable [Soetaert, K., Herman, P., 1996. A model of early diagenetic processes from the shelf to abyssal depths. Geochimica et Cosmochimica Acta 60 (6) 1019-1040]. In this way, processes in the upper oxygenated layer are fully coupled with anaerobic processes in the deep waters, allowing to perform longterm simulations.The mathematical modeling of phytoplankton and zooplankton dynamics, detritus and the microbial loop is based on the model developed by Van den Meersche et al. [Van den Meersche, K., Middelburg, J., Soetaert, K., van Rijswijk P.H.B., Heip, C., 2004. Carbon-nitrogen coupling and algal-bacterial interactions during an experimental bloom: Modeling a 13c tracer experiment. Limnology and Oceanography 49 (3), 862-878] and tested in the modeling of mesocosm experiments and of the Ligurian sea ecosystem [Raick, C., Delhez, E., Soetaert, K., Gregoire, M., 2005. Study of the seasonal cycle of the biogeochemical processes in the Ligurian sea using an 1D interdisciplinary model. Journal of Marine Systems 55 (3-4) 177-203]. This model has been extended to simulate the development of top predators, the aggregation of detritus as well as the degradation and chemical processes in suboxic/anoxic conditions (e.g. denitrification, anoxic remineralization, redox reactions).The coupled model extends down to the sediments ( depth) and is forced at the air-sea interface by the 6 hourly ERA-40 reanalysis of ECMWF data. The model has been calibrated and validated using a large set of data available in the Black Sea TU Ocean Base. The biogeochemical model involves some hundred parameters which are first calibrated by hand using published values. Then, an identifiability analysis has been performed in order to determine a subset of identifiable parameters (i.e. ensemble of parameters that can be together estimated from the amount of data we have at our disposal, see later in the text). Also a subset of 10 identifiable parameters was isolated and an automatic calibration subroutine (Levenberg Marquart) has been used to fine tune these parameters. Additionally, in order to assess the sensitivity of model results to the parameterization of the two gelatinous groups, Monte Carlo simulations were performed perturbing all the parameters governing their dynamics.In order to calibrate the particle dynamics and export, the chemical model was run off-line with the particle and microbial loop model in order to check its capacity of simulating anoxic waters. After a 10⁴ year run, the model simulated profiles similar to observations but steady state was not reached suggesting that the Black Sea deep waters are not at steady state. The fully coupled model was then used to simulate the period 1988-1992 of the Black Sea ecosystem. The model solution exhibits a complex dynamics with several years of transient adjustment. This complexity is imparted by the explicit modeling of top predators. The integrated chlorophyll and phytoplankton biomasses, the maximum concentration and depth of maximum, mesozooplankton biomass, depth of oxycline, primary production, bacterial production, surface concentrations of nutrients and plankton simulated by the model and obtained from available data analysis were compared and showed a satisfactory agreement. Also, as in the data, the model shows a continuous development of phytoplankton throughout the year, with an intense spring bloom dominated by diatoms and a fall bloom dominated by dinoflagellates. Dinoflagellates dominate from summer until late fall while small phototrophic flagellates are never dominant in terms of biomass, but are present almost throughout the year except in winter. The model simulates an intense silicate removal associated to increased diatoms blooms which were promoted by increased nutrient conditions, and by the presence of gelatinous zooplankton. This silicate pumping leads to silicate limitation of diatoms development in summer allowing the development of dinoflagellates.     

Seismic evidence and formation mechanism of gas hydrates in the Zhongjiannan Basin,Western margin of the South China Sea

An analysis of 3D seismic data from the Zhongjiannan Basin in the western margin of the South China Sea (SCS) reveals seismic evidence of gas hydrates and associated gases, including pockmarks, a bottom simulating reflector (BSR), enhanced reflection (ER), reverse polarity reflection (RPR), and a dim amplitude zone (DAZ). The BSR mainly surrounds Zhongjian Island, covering an area of 350 km² in this 3D survey area. The BSR area and pockmark area do not match each other; where there is a pockmark developed, there is no BSR. The gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m. A mature pockmark usually consists of an outside trough, a middle ridge, and one or more central pits, with a diameter of several kilometers and a depth of several hundreds of meters. The process of pockmark creation entails methane consumption. Dense faults in the study area efficiently transport fluid from large depths to the shallow layer, supporting the formation of gas hydrate and ultimately the pockmark.     

Summer larval fish assemblages in the Southern Tyrrhenian Sea (Western Mediterranean Sea)

The purpose of this paper was to study the community structure, in terms of species composition, abundance and spatial distribution, of fish larvae in a wide coastal area of Sicily facing the Southeastern Tyrrhenian Sea, extending for 2300 km² from Cape Cefalù to the west, to Cape Rasocolmo in the east. This study analyses how species are assembled in relation to an inshore–offshore gradient and also how environmental conditions, determined by surface circulation patterns occurring in the Central Mediterranean at the local scale, determine the distribution patterns. Samples from 39 stations were collected using a 60‐cm Bongo net during an ichthyoplanktonic survey carried out in June 2006. In all, 62 taxa, representing 32 families, were identified. Cyclothone braueri (59.6%), Engraulis encrasicolus (9.2%) Lampanyctus crocodilus (4.3%) and Lampanyctus pusillus (4.1%) were the most abundant species. The results showed that the highest abundance value (14830.6 fish larvae per 10 m² sea surface) was observed in the western part of the study area. MDS, SIMPER and CCA analyses revealed well defined groups of stations and assemblages of larvae in accordance with an inshore–offshore gradient. The results of this study could have implications for the management of marine resources because the investigated area has already been identified as a nursery area for many pelagic and coastal fishes and a natural habitat for many species of high commercial interest.     

Depositional environment and hydrocarbon source potential of the Oligocene Ruslar Formation (Kamchia Depression; Western Black Sea)

The Oligocene Ruslar Formation is a hydrocarbon source rock in the Kamchia Depression, located in the Western Black Sea area. Depositional environment and source potential of the predominantly pelitic rocks were investigated using core and cuttings samples from four offshore wells. In these wells the Ruslar Formation is up to 500 m thick. Based on lithology and well logs, the Ruslar Formation is subdivided from base to top into units I–VI. Dysoxic to anoxic conditions and mesohaline to euhaline salinities prevailed during deposition of the Ruslar Formation. Relatively high oxygen contents occurred during early Solenovian times (lower part of unit II), when brackish surface water favoured nannoplankton blooms and the deposition of bright marls (“Solenovian event”). Anoxic conditions with photic zone anoxia were established during late Oligocene times (units III and IV) and, probably, reflect a basin-wide anoxic event in the Eastern Paratethys during Kalmykian times. Organic carbon content in the Ruslar Formation is up to 3%. Autochthonous aquatic and allochthonous terrigenous biomass contribute to the organic matter. Relatively high amounts of aquatic organic matter occur in the lower part of the Ruslar Formation (units I and II) and in its upper part (unit VI). Diatoms are especially abundant in the lower part of unit VI. The kerogen is of type III and II with HI values ranging from 50 to 400 mgHC/gTOC. Units I and II (Pshekian, lower Solenovian) are characterized by a fair (to good) potential to produce gas and oil, but potential sources for gas and oil also occur in the Upper Oligocene units IV–VI.     

Analysis of the Black Sea sediments by evaluating DSDP Leg 42B drilling data for gas hydrate potential

Many gas seepages, temperature, pressure, salinity, anoxic environment and high source gas potential of the Black Sea indicates that the Black Sea might have huge potentials for biogenic and thermogenic gas hydrates. However, the last important parameter to consider gas hydrate as an energy source is the type of sediments. Coarse marine sands are considered as good hydrate reservoirs because of high porosity and high permeability. Only very limited data is available related to the types of lithology of the Black Sea sediments. Hence, in this study, the literature data (especially the drilling and coring data of DSDP Leg 42B program) about gas seepages, temperature gradient, pressure gradient, salinity, anoxic environment and high source gas potential, and the types of the sediments in the Black Sea were investigated and analyzed. Although gas seepages, temperature gradient, pressure gradient, salinity, anoxic environment and high source gas potential of the Black Sea are appropriate for producible gas hydrate reservoirs, the sediments of the Black Sea appear to be generally fine grained with high clay content. Sandy-silt and silty sand layers in turbidites of the Black Sea might be potential producible hydrate reservoirs but these sediments are fine. As well as turbidites, separate thin sand layers might be potential gas hydrate reservoirs as an energy source in the Black Sea.     

Evaluating coastal scenery using fuzzy logic: Application at selected sites in Western Black Sea coastal region of Turkey

Coastal areas of the world are under treat due to the conflicting requirements of functions such as habitation and/or recreation, which affect the strategic asset of coastal scenery itself. Coastal managers, together with planners, need coastal landscape inventories, where the quality of coastal scenery is a part of the inventory. In order to provide an evidence-based approach for sound coastal management decisions, [Ergin et al., 2004] and [Ergin et al., 2006] developed a novel technique ‘coastal scenic evaluation’ (CSE), which addresses the evaluation of coastal scenery. The CSE technique utilizes fuzzy logic to derive values obtained from a checklist of 26 physical and human parameters. The methodology enables the calculation of an evaluation index (D), which categorizes the scenic values of coastal sites into five distinct classes. Using this technique, coastal scenic evaluations were carried out at 34 selected sites on the Western Black Sea coast of Turkey. Based on the calculated D values, a five-class differentiation was obtained for the selected sites, to provide baseline information for any envisaged subsequent management plans for these areas.     

Spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the Yinggehai Basin,offshore South China Sea

About 120 gas seepage vents were documented along the west and southwest coast of the Hainan Island, South China Sea, in water depths usually less than 50 m. The principal seepage areas include the Lingtou Promontory, the Yinggehai Rivulet Mouth, Yazhou Bay, the Nanshan Promontory and the Tianya Promontory. They occur along three major zones, reflecting the control by faults and lateral conduits within the basement. It is estimated that the total gas emission from these seepage vents is 294–956 m³/year. The seepage gases are characterized by a high CH₄ content (76%), heavy δ¹³C₁ values (−38 to −33‰) and high C₁/C_1–5 ratios (0.95–1.0), resembling the thermogenic gases from the diapiric gas fields of the Yinggehai Basin. Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression. The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages. It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin. This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.     

Evidence of gas hydrate from downhole logging data in the Ulleung Basin, East Sea

The Gas Hydrate Research and Development Organization (GHDO) of Korea successfully accomplished both coring (hydraulic piston and pressure coring) and logging (logging-while-drilling, LWD, and wireline logging) to investigate the presence of gas hydrate during the first deep drilling expedition in the Ulleung Basin, East Sea of Korea (referred to as UBGH1) in 2007. The LWD data from two sites (UBGH1-9, UBGH1-10) showed elevated electrical resistivity (>80 Ω-m) and P-wave velocity (>2000 m/s) values indicating the presence of gas hydrate. During the coring period, the richest gas hydrate accumulation was discovered at these intervals. Based on log data, the occurrence of gas hydrate is primarily controlled by the presence of fractures. The gas hydrate saturation calculated using Archie’s relation shows greater than 60% (as high as ∼90%) of the pore space, although Archie’s equation typically overestimates gas hydrate saturation in near-vertical fractures. The saturation of gas hydrate is also estimated using the modified Biot-Gassmann theory (BGTL) by Lee and Collett (2006). The saturation values estimated rom BGTL are much lower than those calculated from Archie’s equation. Based on log data, the hydrate-bearing sediment section is approximately 70 m (UBGH1-9) to 130 m (UBGH1-10) in thickness at these two sites. This was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition. LWD data also strongly support the interpretation of the seismic gas hydrate indicators (e.g., vent or chimney structures and bottom-simulating reflectors), which imply the probability of widespread gas hydrate presence in the Ulleung Basin.     

Fate of the Black Sea Acartia clausi and Acartia tonsa (Copepoda) penetrating into the Marmara Sea through the Bosphorus

In October 2005 spatial distribution of live and dead Acartia clausi and Acartia tonsa was studied in the Black and Marmara Seas and near the Marmara Sea inlet of the Bosphorus, in order to understand their fate upon transportation between two seas. The morphometric characteristics in both species from all studied areas, and the decreased abundance of A. clausi and A. tonsa from the Black Sea towards the Marmara Sea indicate that the Marmara Sea Acartia populations are formed by recruitment from the Black Sea. We observed mass mortality of A. clausi in the Marmara Sea near the Prince Islands. The majority of carcasses (66% of total A. clausi numbers in the Marmara Sea) were found in the salinity gradient layer.     

Characteristics of two natural gas seepages in the North Sea

Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block , has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of 100 m. It is estimated to produce 24 m³ of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material.The second occurrence is located in U.K. block (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of 7.10⁵ cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.     

Vertical distribution of dissolved organic carbon (DOC) in the Western Mediterranean Sea in relation to the hydrological characteristics

Vertical profiles of dissolved organic carbon (DOC) from eight hydrological stations in the Tyrrhenian Sea, Sardinia Channel and Algerian Sea, are reported. DOC exhibits concentrations ranging from 58 to 88 μM in surface water, 43–57 μM in the intermediate layer and 49–63 μM in deep waters. The assessment of the hydrological characteristics allows different water masses in the study area to be identified; moreover, different hydrological processes are observed in the Tyrrhenian and Algerian basins. DOC exhibits different values in the different water masses. The lowest DOC concentrations (43–46 μM) were found in the Tyrrhenian Levantine Intermediate Water (LIW). Correlations between DOC and apparent oxygen utilization (AOU), investigated within each water mass, exhibit different behaviors in the intermediate and deep waters, suggesting the occurrence of different processes of oxygen consumption in the different water masses.     

Parameterization of iron and manganese cycling in the Black Sea suboxic and anoxic environment

New and published data on the distribution and speciation of manganese and iron in seawater are analyzed to identify and parameterize major biogeochemical processes of their cycling within the suboxic (15.6σ_t16.2) and anoxic layers (σ_t16.2) of the Black Sea. A steady-state transport-reaction model is applied to reveal layering and parameterize kinetics of redox and dissolution/precipitation processes. Previously published data on speciation of these elements in seawater are used to specify the nature of the transformations. Two particulate species of iron (Fe(III) hydroxide and Fe(II) sulfide) are necessary to adequately parameterize the vertical profile of suspended iron, while three particulate species (hydrous Mn(IV) oxide, Mn(II) sulfide, and Mn(II) carbonate) are necessary to describe the profile of suspended manganese. In addition to such processes as mixing and advection, precipitation, sinking, and dissolution of manganese carbonate are found to be essential in maintaining the observed vertical distribution of dissolved Mn(II). These results are used to interpret the observed difference in the form of vertical distribution for dissolved Mn(II) and Fe(II). Redox transformations of iron and manganese are coupled via oxidation of dissolved iron by sinking suspended manganese at σ_t16.2±0.2 kg m⁻³. The particulate manganese, necessary for this reaction, is supplied through oxidation of dissolved Mn(II). The best agreement with observations is achieved when nitrate, rather than oxygen, is set to oxidize dissolved Mn(II) in the lower part of the suboxic layer (15.90σ_t16.2). The results support the idea that, after sulfides of these metals are formed, they sink with particulate organic matter. The sinking rates of the particles and specific rates of individual redox and dissolved-particulate transformations have been estimated by fitting the vertical profile of the net rate.     

Methane gas Seeps Along the Oxic/Anoxic Gradient in the Black Sea: Manifestations, Biogenic Sediment Compounds and Preliminary Results on Benthic Ecology

Abstract. Sediment parameters (grain size, organic content, chloroplastic pigments, Adenylates, Potential hydrolytic activity and electron transport system activity) and benthic biota were studied in methane seep area south-west of the Crimean Peninsula in the Black Sea over a depth range from 60 to 260m. A control transect with similar depths was performed in an adjacent nonseep area. Methane seepage in this region occurs from 35 to ˜ 85m depth, passing the oxic anoxic interface zone at 130 to 180m. The methane seep areas were characterized by abundant carbonate precipitates which occurred in various shapes from small, flat structures to tall chimneys with increasing anoxia. The carbonates were associated with distinct bacterial mats. Most of the measured biochemical parameters in the sediment were quite similar in the seep and nonseep areas. The content of organic matter was higher and grain size was more uniform in the nonseep areas. However, the seep areas were charaterized by highter proportions of the larger fauna (size classes 0.5–1 mm and ≥ 1mm) as well as increased total numbers of benthic fauna in the suboxic and upper anoxic zone. The animals did not show any seep-specific adaptations. In addition to methane seepage, highly variable hydrochemical and sedimentary conditions on the lower shelf and upper slope may also play an important role in structuring the composition and distribution of the benthic fauna.     

Sulfate reduction below the sulfate-methane transition in Black Sea sediments

A sudden increase in salinity about 7000 years ago caused a shift in the deposited sediments of the Black Sea from limnic to brackish-marine. Due to the development of an anoxic deep water basin and a relatively high sulfate concentration, organic matter is mineralized primarily through sulfate reduction in modern Black Sea sediments. Earlier studies showed that sulfate-reducing bacteria are abundant within the limnic sub-surface sediment in spite of extremely low concentrations of sulfate and organic carbon. A main objective of the present study was therefore to understand the depth distribution of sulfate reduction across the different sediment units, even deep below the sulfate-methane transition. Our study combined experimental measurements of sulfate reduction using ³⁵S radiotracer with analyses of sulfur and iron geochemistry in pore water and sediment. Potential sulfate reduction rates were measured with ³⁵S in sediment samples that were amended with sulfate and organic substrates and incubated in time-series up to 48 h. Sulfate reduction could thereby be detected and quantified at depths where concentrations of sulfate were otherwise too low to enable calculation of the rates. The results demonstrate that sulfate-reducing bacteria are active several meters below the sulfate-methane transition in Black Sea sediments. The cryptic sulfate reduction below the sulfate-methane transition may be driven by sulfate produced from re-oxidation of reduced sulfur species with oxidized iron minerals buried in the deep limnic sediment.     

Meiobenthos from an active methane seepage area in the NW Black Sea

Meiobenthos densities and higher taxon composition were studied in an active gas seepage area at depths from 182 to 252 m in the submarine Dnieper Canyon located in the northwestern part of the Black Sea. The meiobenthos was represented by Ciliata, Foraminifera, Nematoda, Polychaeta, Bivalvia, Gastropoda, Amphipoda, and Acarina. Also present in the sediment samples were juvenile stages of Copepoda and Cladocera which may be of planktonic origin. Nematoda and Foraminifera were the dominant groups. The abundance of the meiobenthos varied between 2397 and 52,593 ind.·m⁻². Maximum densities of Nematoda and Foraminifera were recorded in the upper sediment layer of a permanent H₂S zone at depths from 220 to 250 m. This dense concentration of meiobenthos was found in an area where intense methane seeps were covered by methane‐oxidizing microbial mats. Results suggest that methane and its microbial oxidation products are the factors responsible for the presence of a highly sulfidic and biologically productive zone characterized by specially adapted benthic groups. At the same time, an inverse correlation was found between meiofauna densities and methane concentrations in the uppermost sediment layers. The hypothesis is that the concentration of Nematoda and Foraminifera within the areas enriched with methane is an ecological compromise between the food requirements of these organisms and their adaptations to the toxic H₂S.     

Spatial and temporal variation of Palaemon adspersus, Palaemon elegans, and Crangon crangon (Decapoda: Caridea) in the southern Black Sea

In order to characterize the seasonal, bathymetric, and spatial distribution of the species, Palaemon adspersus, Palaemon elegans, and Crangon crangon, shrimps were sampled with a beam trawl in four stations at depths between 1 and 30 m from February 2002 to January 2004 on the Sinop Peninsula coasts of the southern Black Sea. One-way analysis of similarity (ANOSIM) results demonstrated that the caridean composition was significantly different (p < 0.001) between seasons and between sampling areas. No significant relationship of the caridean composition was evident between depth zones ranging from 1 to 30 m. These three carideans occurred together in 28.5% of the sampling occasions. Different seasonal migration pattern was evident for all the three species. Palaemon adspersus migrated inshore during relatively higher water temperatures, whereas C. crangon density decreased in the shallow waters during the same period, and P. elegans population was mostly observed at depth zones of 5–10 m and was only observed in the 30 m depth zone in winter. The abiotic factors that characterize the coexistence of these three carideans were primarily determined by the habitat types and bottom structures.     

Temporal changes in the structure of the crustacean decapod assemblages associated with Cymodocea nodosa meadows from the Alboran Sea (Western Mediterranean Sea)

The decapod assemblages associated with two shallow meadows of Cymodocea nodosa, located in the same geographical area (Southern Spain) but on different substrates and with different patch size, have been analyzed. They display similar structure (diversity indices not significantly different), without a clear relation of richness and abundances to patch size, and with the same dominant species (the family Hippolytidae and, in particular, Hippolyte leptocerus are characteristic of this habitat). The composition of both crustacean assemblages is influenced by species that are common in neighbouring habitats. Therefore the connectivity among them is an important factor in the qualitative and quantitative structure of these decapod communities. Species richness appears to be higher than in Cymodocea meadows elsewhere in the Mediterranean and Atlantic at a similar depth, perhaps as a consequence of the biogeographical location and the high diversity and connectivity with surrounding biotopes. High evenness values are the result of the structure and location of these meadows, which are fragmented and interspersed with other biotopes (sandy and rocky bottoms), resulting in an ‘ecotone effect’. On the other hand, the structures of the decapod assemblages differ significantly according to sampling period. The abundance and species richness are both related to plant phenology and the dominant species present a positive correlation with the number of leaves per shoot. The maximum abundance of many species is coincident with the greatest seagrass development (spring – summer), which provides more resources (surface, biomass, protection, food). Therefore, seasonality is linked to plant life cycle, but also to the interrelationships and biology of the species, which are adapted and specialized to the environmental features of these shallow habitats.     

Structural-stratigraphic control on the Umitaka Spur gas hydrates of Joetsu Basin in the eastern margin of Japan Sea

Integrated geological, geochemical, and geophysical exploration since 2004 has identified massive accumulation of gas hydrate associated with active methane seeps on the Umitaka Spur, located in the Joetsu Basin on the eastern margin of Japan Sea. Umitaka Spur is an asymmetric anticline formed along an incipient subduction zone that extends throughout the western side of the Japanese island-arc system. Seismic surveys recognized chimney structures that seem strongly controlled by a complex anticlinal axial fault system, and exhibit high seismic amplitudes with apparent pull-up structures, probably due to massive and dense accumulation of gas hydrate. Bottom simulating reflectors are widely developed, in particular within gas chimneys and in the gently dipping eastern flank of the anticline, where debris can store gas hydrates that may represent a potential natural gas resource. The axial fault system, the shape of the anticline, and the carrier beds induce thermogenic gas migration to the top of the structure, and supply gas to the gas hydrate stability zone. Gas reaching the seafloor produces strong seepages and giant plumes in the sea water column.