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1.
Three sectors of the south Barbados prism between 1000 and 2000 m depth were explored by the French submersible Nautile. Chemosynthesis-based benthic communities were discovered on several structures affected by diapirism, including mud volcanoes, domes and an anticlinal ridge. The communities are associated with the expulsion of methane-rich fluids which is a wide-spread process in the area. These communities are dominated by large bivalves and vestimentiferans which harbour chemoautotrophic symbiotic bacteria. The symbiotic bivalves include two species of Mytilidae and one of Vesicomyidae, with dominance of a methanotrophic mussel. Cartography of the benthic communities, interpretation of thermal measurements and observation of sedimentary patterns have been used to define the life habits of each of the three species of symbiotic bivalves. Each species has a characteristic preference for different conditions of edaphic and fluid flow: the dominant methanotrophic mussel appears to require high velocity vents and hard substratum. The vesicomyids and the other species of mussel are able to take up sulfide from the sediments, and so are associated with low seepages, but also require soft sediment. The three bivalve species are assumed successively to colonize the top of a diapiric ridge, in a succession related to the temporal evolution of fluid flow and sedimentation. The composition of the bivalve assemblages, their densities and biomasses all differ between the several mud volcanoes and domes studied, and these parameters are thought to be related to the spatial and temporal variations of fluid expulsion through the structures, and the lithification processes linked to fluid expulsion. One very active dome is at present colonized by an exceptionally large and dense population of the methanotrophic mussel. In contrast, communities in another area, on the domes and volcanoes that are currently inactive, were colonized by only a few living vesicomyids and mussels, both associated with sulfur-oxydizing bacteria, and there were numerous empty shells. The densities and biomasses of symbiotic bivalves were far greater in the area studied than in a deeper mud volcano field on the same prism that had been studied previously. This is consistent with a report that methane production is greater in the southern region of this accretionary prism than in the northern. Numerous non-symbiotic organisms were observed in and around the areas of the seeps, some are endemic to the seep communities, including some gastropods and shrimps, others are either colonists or vagrants from the surrounding deep-sea floor. Filter feeders were very abundant, and some of these, like the serpulids and large sponges, may also be dependent on the chemosynthetic production. Faunistic composition of both symbiotic and non-symbiotic taxa, of the assemblages around these cold seeps, is closely related to that reported for communities living on hydrocarbon seeps in the Gulf of Mexico.  相似文献   

2.
Detailed multibeam, sedimentological, and geophysical surveys provide ample new data to confirm that the Anaximander Mountains (Eastern Mediterranean) are an important area for active mud volcanism and gas hydrate formation. More than 3000 km of multibeam track length was acquired during two recent missions and 80 gravity and box cores were recovered. Morphology and backscatter data of the study area have better resolution than previous surveys, and very detailed morphology maps have been made of the known targeted mud volcanoes (Amsterdam, Kazan and Kula), especially the Amsterdam “crater” and the related mud breccia flows. Gas hydrates collected repeatedly from a large area of Amsterdam mud volcano at a sub-bottom depth of around 0.3–1.5 m resemble compacted snow and have a rather flaky form. New gas hydrate sites were found at Amsterdam mud volcano, including the mud flow sloping off to the south. Gas hydrates sampled for the first time at Kazan mud volcano are dispersed throughout the core samples deeper than 0.3 m and display a ‘rice’-like appearance. Relative chronology and AMS dating of interbedded pelagic sediments (Late Holocene hemipelagic, sapropel layer S1 and ash layers) within the mud flows indicate that successive eruptions of Kula mud volcano have a periodicity of about 5–10 kyrs. New mud volcanoes identified on the basis of multibeam backscatter intensity were sampled, documented as active and named “Athina” and “Thessaloniki”. Gas hydrates were sampled also in Thessaloniki mud volcano, the shallowest (1264 m) among all the active Mediterranean sites, at the boundary of the gas hydrate stability zone. Biostratigraphical analyses of mud breccia clasts indicated that the source of the subsurface sedimentary sequences consists of Late Cretaceous limestones, Paleocene siliciclastic rocks, Eocene biogenic limestones and Miocene mudstones. Rough estimations of the total capacity of the Anaximander mud volcanoes in methane gas are 2.56–6.40 km3.  相似文献   

3.
《Marine Geology》2006,225(1-4):191-208
The pore water chemistry of mud volcanoes from the Olimpi Mud Volcano Field and the Anaximander Mountains in the eastern Mediterranean Sea have been studied for three major purposes: (1) modes and velocities of fluid transport were derived to assess the role of (upward) advection, and bioirrigation for benthic fluxes. (2) Differences in the fluid chemistry at sites of Milano mud volcano (Olimpi area) were compiled in a map to illustrate the spatial heterogeneity reflecting differences in fluid origin and transport in discrete conduits in near proximity. (3) Formation water temperatures of seeping fluids were calculated from theoretical geothermometers to predict the depth of fluid origin and geochemical reactions in the deeper subsurface.No indications for downward advection as required for convection cells have been found. Instead, measured pore water profiles have been simulated successfully by accounting for upward advection and bioirrigation. Advective flow velocities are found to be generally moderate (3–50 cm y− 1) compared to other cold seep areas. Depth-integrated rates of bioirrigation are 1–2 orders of magnitude higher than advective flow velocities documenting the importance of bioirrigation for flux considerations in surface sediments. Calculated formation water temperatures from the Anaximander Mountains are in the range of 80 to 145 °C suggesting a fluid origin from a depth zone associated with the seismic decollement. It is proposed that at that depth clay mineral dehydration leads to the formation and advection of fluids reduced in salinity relative to sea water. This explains the ubiquitous pore water freshening observed in surface sediments of the Anaximander Mountain area. Multiple fluid sources and formation water temperatures of 55 to 80 °C were derived for expelled fluids of the Olimpi area.  相似文献   

4.
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.  相似文献   

5.
Four mud volcanoes of several kilometres diameter named Amon, Osiris, Isis, and North Alex and located above gas chimneys on the Central Nile Deep Sea Fan, were investigated for the first time with the submersible Nautile. One of the objectives was to characterize the seafloor morphology and the seepage activity across the mud volcanoes. The seepage activity was dominated by emissions of methane and heavier hydrocarbons associated with a major thermal contribution. The most active parts of the mud volcanoes were highly gas-saturated (methane concentrations in the water and in the sediments, respectively, of several hundreds of nmol/L and several mmol/L of wet sediment) and associated with significantly high thermal gradients (at 10 m below the seafloor, the recorded temperatures reached more than 40 °C). Patches of highly reduced blackish sediments, mats of sulphide-oxidizing bacteria, and precipitates of authigenic carbonate were detected, indicative of anaerobic methane consumption. The chemosynthetic fauna was, however, not very abundant, inhibited most likely by the high and vigorous fluxes, and was associated mainly with carbonate-crust-covered seafloor encountered on the southwestern flank of Amon. Mud expulsions are not very common at present and were found limited to the most active emission centres of two mud volcanoes, where slow extrusion of mud occurs. Each of the mud volcanoes is fed principally by a main narrow channel located below the most elevated areas, most commonly in the centres of the structures. The distribution, shape, and seafloor morphology of the mud volcanoes and associated seeps over the Central Nile Deep Sea Fan are clearly tectonically controlled.  相似文献   

6.
We present high-resolution isotopic records and cathodoluminescence studies of recently dead and live bivalve specimens from cold seeps, in an attempt to reconstruct environmental conditions during organism growth, and thereby the possible variability of fluid-venting activity at the seafloor. Shells of the burrowing lucinid Myrtea aff. amorpha were collected at three localities near actively venting methane seeps in the Eastern Mediterranean deep sea, using the Nautile submersible during two French oceanographic cruises: from the Kazan mud volcano, in the vicinity of the Anaximander mounts (MEDINAUT cruise, 1998), and from the central pockmark province and the Amon mud volcano of the Nile deep-sea fan (NAUTINIL cruise, 2003). The oxygen and carbon isotope compositions of 18 shells from the various localities, and also from different sites at the same locality show a rather strong scatter (1.8 < δ 18O‰ < 3.4; −10.2 < δ 13C‰ < 2.2), and values lower than those expected for carbonate precipitated at equilibrium with present-day bottom waters. This means that warm methane-rich fluids were mixed with bottom seawater during precipitation of shell carbonates. We have tried to determine ontogenetic age of two shells by using cathodoluminescence as a sclerochronological proxy, because the direct counting of carbonate increments was not possible in these specimens. There is a relatively good correspondence between cathodoluminescence trends and oxygen isotope profiles that might support the link between manganese incorporation during growth and temperature. Eight specimens of lucinid shells were selected for high-resolution isotopic profiling. A few shells exhibit decreasing δ 18O and δ 13C values from the umbo to the actively growing ventral shell margin, which can be attributed to the commonly observed physiologically controlled deceleration of growth with increasing organism age, this metabolic effect corresponding to the increase of incorporation of respiratory CO2. A few shells exhibit high-frequency δ 18O variations with an amplitude of about 1.5‰ that might be related to temperature variations controlled by fluid-venting activity. One shell from the pockmark province of the Nile deep-sea fan records a strong, sharp δ 13C decrease of about 9‰, and extending over a 5-mm interval in the shell that can be related to a major methane release event. Another shell from the Kazan mud volcano exhibits a progressive increase of δ 13C values from −10‰ to 0‰ with age, which might indicate decreasing methane flow throughout the organism’s life. This study has demonstrated that bivalve shells from deep-sea cold seeps represent good indicators of variability in seepage activity of methane-rich fluids, at various scales in both space and time. Although the precise chronology of the observed events was not established, because shell growth rate is not known in this case, this remains a priority for future studies in such environments.  相似文献   

7.
Thirteen new species of gastropods are described from the Zairov 1-2 and Biozaire 1-3 cruises to the methane seeps off the Congo River: Patellogastropoda: Paralepetopsis sasakii sp. nov. (Neolepetopsidae); Cocculiniformia: Pyropelta oluae sp. nov. and P. sibuetae sp. nov. (Pyropeltidae); Tentaoculus granulatus sp. nov. (Pseudococculinidae); Neomphalina: Leptogyra costellata sp. nov. (Family uncertain); Vetigastropoda: Puncturella similis sp. nov. (Fissurellidae); Lepetodrilus shannonae sp. nov. (Lepetodrilidae); Caenogastropoda: Provanna reticulata sp. nov. and P. chevalieri sp. nov., Cordesia provannoides gen. et sp. nov. (Provannidae); Phymorhynchus coseli sp. nov. and P. cingulata sp. nov. (Conidae); Heterobranchia: Hyalogyrina rissoella sp. nov. (Hyalogyrinidae). All species except T. granulatus (from a settlement trap) belong to groups known from cold seeps and the entire seep fauna here is new to science. Biogeographical affinity of this gastropod fauna is to the West Atlantic seeps, not to the Mediterranean seeps or Mid-Atlantic vents. Fragments of the autecology of the species are presented. The evolution of the seep gastropod fauna is briefly discussed and a continuous immigration of taxa is supported. The oldest verified occurrences of modern taxa in the seeps date back to Cenomanian (Cretaceous) time, while some taxa seem not to appear until very late Tertiary.  相似文献   

8.
Deep faunal assemblages were observed on cold seeps, between 1000 and 2000m depth, along the southern termination of the Barbados accretionary prism, during the Caracolante II and Diapicar cruises. Faunal composition and microdistribution of a cold seep community were analyzed at one site using deep-sea photographs and seismic (3.5 Kz) profiles. The community is dominated by large mussels up to 20cm long (possibly related to the genus Bathymodiolus), tiny vesicomyid clams, and vestimentiferan clusters in which animals are up to 2m long. There is a high density of sponges (three species), bryozoans and gorgonians. Other taxa occasionally associated with this assemblage include galatheid crabs, anemones, holothurians and possibly pennatulaceans which are numerous at boundaries of the site. A typical deep-sea fauna of fish, asteroids and ophiuroids is also scattered about the vicinity. This seep community is distributed as a band of discontinuous colonies, 40–50m in width and more than 10km in length. Living colonies alternate with accumulations shells and bare areas. The substratum is hard mud covered by a ferrugineous crust and cemented by carbonates. The early diagenetic character of the bottom and the occurrence of biological assemblages indicate that seepages of sulphur and/or methane, which have been revealed along the decollement zone of the Barbados prism, must diffuse through the sea floor. These biological and geological features are located on the crests of NE-SW anticlinal ridges and mud volcanoes which have been built by diaprism.  相似文献   

9.
The worlds >1500 mud volcanoes are normally in a dormant stage due to the short duration of eruptions. Their dormant stage activity is often characterized by vigorous seepage of water, gas, and petroleum. However, the source of the fluids and the fluid–rock interactions within the mud volcano conduit remain poorly understood. In order to investigate this type of activity, we have combined satellite images with fieldwork and extensive sampling of water and gas at seeping gryphons, pools and salsa lakes at the Dashgil mud volcano in Azerbaijan. We find that caldera collapse faults and E–W oriented faults determine the location of the seeps. The seeping gas is dominated by methane (94.9–99.6%), with a δ13C (‰ V-PDB) in the −43.9 to −40.4‰ range, consistent throughout the 12 analysed seeps. Ethane and carbon dioxide occur in minor amounts. Seventeen samples of seeping water show a wide range in solute content and isotopic composition. Pools and salsa lakes have the highest salinities (up to 101,043 ppm Cl) and the lowest δ18O (‰ V-SMOW) values (1–4‰). The mud-rich gryphons have low salinities (<18,000 ppm Cl) and are enriched in 18O (δ18O = 4–6‰).  相似文献   

10.
A global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from 12 countries and 60 seeps independent from mud volcanism from eight countries, was compiled and examined in order to provide the first worldwide statistics on the origin of methane seeping at the earth's surface. Sixteen seep data were coupled with their associated subsurface reservoirs.  相似文献   

11.
A shallow water benthic sample from off the east coast of the Coromandel Peninsula contained compact shelly sand overlain by mud. The fauna, confined to the sand layer, consisted of infaunal bivalves dominated by Tawera spissa (Deshayes). The presence of dead or decaying bivalves only, indicates a heavy mortality in the community attributed to burial by the mud layer.  相似文献   

12.
Macro- and meiofaunal communities were examined at four geomorphologically distinct sites with different gas- and oil-bearing fluid characteristics in the northern, central and southern basins of Lake Baikal. All sites had elevated concentrations of bicarbonate, nitrate, sulphate and chloride ions in pore fluids, with highest values at the Frolikha vent. Elevated levels of iron ions were found in pore waters of the St. Petersburg methane seep and the Gorevoy Utes oil seep. The chemical composition of pore waters at the Malenky mud volcano was similar to that reported in earlier work. Consistent with published data, the Frolikha vent (northern basin) and the St. Petersburg methane seep (central basin) were characterised by methane of mixed genesis (thermogenic + biogenic), whereas the methane source was mainly thermogenic at the Gorevoy Utes oil seep (central basin) and biogenic at the Malenky mud volcano (southern basin). In contrast to marine seep ecosystems, the macrofauna was dominated only by amphipods, giant planarians and oligochaetes, whereas bivalves were absent; the meiofauna was similar to its marine counterpart, being dominated by nematodes, cyclops, harpacticoids and ostracods. A statistically significant positive relationship was revealed between faunal abundance and the availability of bacterial mats on seep sediments. Moreover, ANOVA tests showed significant increases in both meiozoobenthic and macrozoobenthic densities at “hot spot” vent/seep sites relative to discharge-free reference sites. The isotopic composition of carbon and nitrogen at various trophic levels of these benthic vent/seep communities was found to differ markedly from that reported by earlier studies for the pelagic and other benthic food webs in Lake Baikal. As in marine seeps, the macrofauna had variable isotopic signatures. Light δ13C and δ15N values suggest the utilization of chemosynthetically fixed and/or methane-derived organic matter. By contrast, the heavy δ13C signatures of some mobile amphipods likely reflect consumption of photosynthetically derived carbon. These findings would at least partly explain why Lake Baikal is a notable outlier in global temperature–biodiversity patterns, exhibiting the highest biodiversity of any lake worldwide but at an extremely cold average temperature.  相似文献   

13.
Infaunal bivalve molluscs on the flood tidal delta of Tauranga Harbour, north‐eastern New Zealand, were surveyed twice over a 6‐month period, with the aim of assessing shellfish resources, assessing the magnitudes of temporal and spatial variability in abundances, and identifying potentially important processes. The surveys identified a total of 31 bivalve taxa from 27 sites. Species richness per site ranged between 0 and 9, and varied greatly in space and time, as did two other measures of diversity. Extremely high densities of bivalves (>20 000 m‐2) were encountered at several sites. Densities of several abundant species varied greatly in time, as a result of recruitment pulses. The abundances of the three species that dominated the fauna (Paphies australis, Tawera spissa, and Ruditapes largillierti) had characteristic spatial scales on the order of 200 m. Comparisons of size frequency distributions for individual sites suggested that mortality or emigration in the first year of benthic life was high. Substantial populations of bivalves occur on Centre Bank, and the most abundant of these are greatly influenced by events early in their lives.  相似文献   

14.
The Håkon Mosby Mud Volcano (HMMV) is a methane seep that is densely populated by the bacteria Beggiatoa spp. as well as by tubeworms of the Family Siboglinidae. Analyses of samples from seven stations located in five different habitats (bare sediments, bacterial mats, siboglinid fields, sediments with small patches of siboglinids and areas of ‘normal’ sea floor) revealed differences in the density and species‐level diversity of nematode communities. Total densities increased from stations inside the mud volcano and on its rim towards the ‘normal’ areas outside the volcano. Nematode species diversity was similar in samples from the siboglinid fields and the bare HMMV sediments. However, the sediment with patches of siboglinids collected outside the volcano was characterised by a diverse nematode community comprising 47 species, whereas the control sediments without siboglinids yielded only 27 species. The nematode assemblage at the bacterial mat site included only two identified species, Halomonhystera disjuncta and Sabatieria ornata, with H. disjuncta being strongly dominant. Terschellingia distlamphida, S. ornata and Aponema ninae dominated nematode communities at the siboglinid fields and in bare HMMV sediments. Species dominating at stations inside the caldera were found in comparable low densities in the sediment with siboglinid patches collected outside the volcano, but were missing in the background samples, where species of Endeolophos, Acantholaimus and Desmoscolex were dominant. Species diversity generally decreased with sediment depth. A subsurface‐dwelling nematode community was observed in the siboglinid fields and the bare sediments. Background sediments showed a sharp decline with 18–20 species at 0–2 cm versus three species at 3–5 cm sediment depth. The trophic composition of the nematode fauna at the HMMV showed a prevalence of deposit feeders at almost all stations, whereas background sediments exhibited a uniform distribution of non‐selective and selective deposit feeders and epistrate feeders. The high percentage of deposit feeders inside the mud volcano could be related to the grain‐size distribution and the significantly higher bacterial biomass compared to the control sediments.  相似文献   

15.
The relative contribution of chemosynthesis in heterotrophic fauna at seeps is known to be influenced by depth and by habitat. Using stable isotopes of carbon and nitrogen, we investigated macro‐ and megafaunal nutritional patterns in Norwegian margin cold seeps by comparing food webs both among habitats within a seep site and between different sites. The very active Håkon Mosby mud volcano (HMMV) is characterized by geochemical gradients, microbial activity and faunal zonation from the centre to the periphery. The Storegga Slide (600–900 m depth) has pockmarks with patchy less active seeps, and also shows concentric zonation of habitats but at much smaller spatial scale. The dominant carbon source for macrofaunal nutrition in both areas was chemosynthetically fixed and the bulk of organic carbon was derived from sulphur‐oxidizing bacteria. In HMMV, food chains were clearly separated according to habitats, with significantly lighter δ13C signatures on microbial mats and adjacent sediment (?33.06 to ?50.62‰) than in siboglinid fields (?19.83 to ?35.03‰). Mixing model outputs revealed that the contribution of methane‐derived carbon was small in siboglinid fields (0–17%) but significant (39–61%) in the microbial mats. Moreover, the variability of macrofauna signatures within this later habitat suggests the co‐occurrence of two food chains, one based on primary production via methanotrophy and the other via sulphide oxidation. The length of the food chains also varied among habitats, with at least one more trophic level in the siboglinid fields located at the periphery of the volcano. Conversely, in Storrega pockmarks, faunal δ13C signatures did not vary among habitats but among species, although separate food chains seem to co‐occur. The small size of the seepage areas and their lower fluxes compared to HMMV allow more background species to penetrate the seep area, increasing the range of δ15N and the trophic level number. Probably due to the higher flux of photosynthetic particulate organic carbon, the overall chemosynthesis‐based carbon contribution in invertebrate nutrition was lower than that in HMMV.  相似文献   

16.
This paper documents and describes through the use of 3D seismic data a prolific mud volcano province within the Eastern Mediterranean. As many as 386 mud volcanoes were mapped within the post-salt succession of the western slope of the Nile Cone, offshore Egypt, using high resolution 3D seismic data. The mud volcanoes within this field display significant geometrical variability in diameter (c. 550 m to c. 5660 m), height (c. 25 m to c. 510 m) and volume (c. 0.1 km3 to c. 3.3 km3) and lie at depths ranging from c. > 6000 m subsea to c. 3100 m at the seafloor. A close spatial relationship between mud volcanoes and base-salt depressions and regions of anomalous thinning within the immediate pre-salt succession, combined with documented core samples taken from mud volcanoes within this region present a powerful argument for a pre-salt source of mud. 3D seismic interpretation and volumetric analysis of these mud volcanoes and their source region permit the definition and quantification of their depletion zones. A conceptual model for a dynamic liquefaction and sediment withdrawal process is proposed whereby mud is fed into a central conduit as the depletion zone propagates radially and episodically outwards resulting in a the formation of a concentric depletion zones. Prolonged mud volcanism within this region over the last ∼5.3 Ma implies the potential for long lived pre-salt overpressure and continued mud volcanism following the catastrophic hydrodynamic impact of the Messinian Salinity Crisis. It is suggested that the scale of mud volcanism means that this region should be considered as among the largest mud volcano provinces in the world.  相似文献   

17.
The West Alboran Basin was previously classified as a mud volcanic province consisting of two mud volcano (MV) fields that are inactive at the present day: the Northern (Spanish) and the Southern (Moroccan) fields. The discovery of the first active mud volcano (Carmen; cruise TTR-17) in 2008, along with several pockmarks at the central part of the basin, motivates more careful geological and geochemical analysis of previous data and comparison to new observations.Gas bubbling from the crater of Carmen MV was observed and recorded using an underwater TV-system and a large TV-grab sample. The gas mainly consisted of methane with less than 1% wetness. However, all sets of homologues up to pentane were detected in the mud breccia of Carmen MV. Both molecular and stable carbon isotopic compositions, and their distribution along the core length, suggest a deep thermogenic source of hydrocarbons (HCs). Composition of the pore water from Carmen MV also points to a deep source of mud volcanic water. The isotopic results indicate that the source of mud volcanic water is the dehydration of clay minerals in the thermal zone of the smectite-to-illite transformation. Our observations allow us to infer the presence of structure II gas hydrates in mud breccia on the top of Carmen MV.High HC gas saturation in sediments in some pockmarks accompanied with live chemosynthetic fauna directly indicates the strong seepage activity of these structures. For the first time, authigenic carbonate crusts and chimneys with associated living chemosynthetic bivalves and tubeworms were sampled from a seep site in the West Alboran Sea. Authigenic carbonates consist of aragonite and calcite, and are characterized by a light carbon isotopic signature, up to −37.2‰ PDB, which points to their methane-derived origin.  相似文献   

18.
Meteor cruise M52/1 documented the presence of gas hydrates in sediments from mud volcanoes in the Sorokin Trough of the Black Sea. In a mud flow on the Odessa mud volcano, a carbonate crust currently forms in association with anaerobic methane oxidation. Dvurechenskii mud volcano (DMV), a flat-topped mud pie-type structure, appeared to be very active. Pore water in sediments of DMV is enriched in several constituents, such as ammonium and chloride, which seem to originate at depth. High sediment temperatures of up to 16.5 °C in close contact to the ambient bottom water of 9 °C also suggest strong advective transport of material from greater depth. Steep temperature gradients indicate a high fluid and/or mud flux within DMV, which is confirmed by the shape of the pore water profiles. Active fluid expulsion sites are evidenced by direct seafloor observation, and a potential flux of methane from the sediment to the bottom water is indicated by water-column methane measurements.  相似文献   

19.
Mud volcanoes are cold seeps, in which the escape of gas and fluids associated with mud creates 3-D bottom structures that enhance the spatial heterogeneity and potentially alter the functioning of the benthic ecosystems. We investigated a complex system of mud volcanoes of the Mediterranean Sea characterised by the presence of different structures (i.e., isolated domes, domes surrounded by moats, clustered domes, and ridges) displaying different levels of seepage. We hypothesize that the combined effects of seafloor heterogeneity (i.e., different 3-D structures as revealed by detailed topographic analysis), fluid emissions and trophic characteristics of these systems can influence the structural and functional biodiversity of meiofauna (with special focus on the nematodes). We found that sediments affected by intensive seepage displayed the lowest faunal abundances and number of higher taxa. However, mud volcanoes without emissions, but characterized by a high structural complexity (such as the ridges), were associated with the highest meiofaunal abundances and number of higher taxa. Mud volcanoes hosted also a remarkable abundance of rare taxa specifically associated with these structures (e.g., acarians, cumaceans, tanaids, cladocerans and hydroids) and absent in slope sediments (used as a control). Each mud-volcano area displayed a different nematode species composition. Overall 76 nematode species (from a total of 235) were exclusively associated with mud-volcano structures, whilst 29 were exclusively encountered in slope sediments. We conclude that the presence of mud volcanoes, for their contribution to increase spatial heterogeneity and for the extreme conditions associated with gas emissions, promotes higher levels of beta diversity, thus enhancing the regional (gamma) benthic diversity. These findings provide new insights on the factors controlling meiobenthic biodiversity in mud volcanoes and clues for future action of conservation of the biodiversity specifically associated with these habitats.  相似文献   

20.
Cold seeps have recently been discovered in the Nile deep‐sea fan (Eastern Mediterranean), and data regarding associated fauna are still scarce. In this study, two bivalve species associated with carbonate crusts and reduced sediment are identified based on sequence analysis of their 18S and 28S rRNA‐encoding genes, and associated bacterial symbioses are investigated using 16S rRNA gene sequencing and microscopy‐based approaches. The specimens are closely related to Myrtea spinifera and Thyasira flexuosa, two species previously documented at various depths from other regions but not yet reported from the Eastern Mediterranean. Both species harbour abundant gammaproteobacterial endosymbionts in specialized gill epithelial cells. The Myrtea‐associated bacterium is closely related to lucinid symbionts from both deep‐sea and coastal species, whereas the Thyasira‐associated bacterium is closely related to the symbiont of a T. flexuosa from coastal waters off the U.K. An epsilonproteobacterial sequence has also been identified in Thyasira which could correspond to a helicoid‐shaped morphotype observed by electron microscopy, but this was not confirmed using fluorescent in situ hybridization. Virus‐like particles were observed within some symbionts in Thyasira, mostly in bacteriocytes localized close to the ciliated zone of the gill filament. Overall, results indicate that very close relatives of shallow species M. spinifera and T. flexuosa occur at cold seeps in the Eastern Mediterranean and harbour chemoautotrophic symbioses similar to those found in their coastal relatives.  相似文献   

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