Measurements of dissolved inorganic carbon (DIC) concentration and its isotopic composition (δ13CDIC) are essential to study chemical and biological processes involved in the ocean carbon cycle, including photosynthesis, respiration, and air-sea CO2 fluxes. Anthropogenic CO2 emissions from fossil fuel combustion have caused an increase in DIC accompanied by a decline in δ13CDIC (called the Suess effect). δ13CDIC is thus a useful tracer to assess the oceanic uptake of anthropogenic CO2. Annual assessments of the Global Carbon Budget (e.g. Friedlingstein et al., 2023) have revealed a growing deviation over the last 10 to 15 years between the estimates of the ocean carbon sink based on observations and models, with the growth of the observation-based ocean CO2 sink being larger compared to the models. Discrepancies in the multi-decadal trend originate from all latitudes but are greatest in the Southern Ocean. Here, we present DIC and δ13CDIC measurements from surface and water column samples collected in the South-West Indian Ocean during repeated summer cruises over the last two decades (1998-2021) conducted on board the RV Marion Dufresne within the French monitoring program OISO (Océan Indien Service d’Observation). We compare these measurements with the DIC and δ13CDIC simulated over the same period by the δ13C-enabled version of the NEMO-PISCES ocean-biogeochemical model. We use different methods to separate the natural and anthropogenic signals over the last 20 years. Our analysis reveals some inconsistencies between simulated and observed DIC and δ13CDIC, as well as between other simulated and observed biogeochemical parameters, whereas physical parameters are generally well reproduced by the model. Identifying the cause for this mismatch bears the potential to explain all or part of the divergence between the observation-based and model-based estimates of oceanic carbon uptake.

We investigated fluid seepage within the Nyegga pockmark field (600–900 m water depths) off mid-Norway from Remotely Operated Vehicle dives at the so-called CNE sites (CNE01 to CNE17). The seafloor morphology of some of these features corresponds to pockmarks and adjacent ridges, with the latter being the focus of present seepage activity. These structures are underlain by chimneys above a gas-charged zone with, in some cases, a substantial body of hydrate-invaded sediment (down to 1.3 s in two-way travel time at CNE03). Present-day methane-rich fluid seepage through the seabed is indicated by chemosynthetic fauna, in particular Siboglinidae polychaetes (Oligobrachia haakonmobiensis webbi and Sclerolinum contortum), microbial mats and associated Rissoidae gastropod (Alvania sp.) grazers, and confirmed by measured in situ bottom-water methane anomalies, up to 2,130 nL/L. No free-gas bubble emissions were observed or acoustically identified. The presence of authigenic carbonates reveals past seepage with very low δ13C values (down to −58‰) indicating that the major source of carbon was methane carried by the venting fluids. The ages of major periods of methane venting are provided by vesicomyid bivalve shells (Isorropodon nyeggaensis) present in two sedimentary layers, 14,930 and 15,500 14C yr BP (ca. 17,238 and 17,952 cal yr BP), respectively, corresponding to the time of Melt Water Pulse IA. The seafloor morphology and pattern of seepage -chemosynthetic fauna and microbial mat distribution and dissolved methane concentration-are remarkably heterogeneous. Pore-water chemistry profiles in a gravity core taken only 40 m from major seepage sites indicate no seepage and anaerobic methane oxidation at a sub-bottom depth of about 2 m. Present-day seepage from the studied pockmark-chimney fluid-flow system charged with gas hydrate is dominated by the advection of methane solution in pore water. Some of this methane could result from the dissolution of hydrate in the chimney, most of which would have formed during an earlier period (post-LGM times) of history of the chimney, when it was venting free gas. However, the presence of free gas beneath this chimney is probably why the water entering the chimney is already saturated with methane and the process of hydrate formation in the chimney continues today.

Abstract. The characteristics of the CISE-LOCEAN sea water isotope data set (δ18O, δ2H, later designed as δD) are presented. This data set covers the time period from 1998 to 2021 and currently includes close to 8000 data entries, all with δ18O, three quarters of them also with δD, associated with a time and space stamp and usually a salinity measurement. Until 2010, samples were analysed by isotopic ratio mass spectrometry, and since then mostly by cavity ring-down spectroscopy (CRDS). Instrumental uncertainty on individual data in this dataset is usually with a standard deviation as low as 0.03 / 0.15 ‰ for δ18O and δD. An additional uncertainty is related to uncertain isotopic composition of the in-house standards that are used to convert daily data into the VSMOW scale. Different comparisons suggest that since 2010 the latter have remained within at most 0.03 / 0.20 ‰ for δ18O and δD. Therefore, combining the two suggests a standard deviation of at most 0.05 / 0.25 ‰ for δ18O / δD. Finally, for some samples, we find that there has been evaporation during collection and storage, requiring adjustment of the isotopic data produced by CRDS, based on d-excess. This adds an uncertainty on the adjusted data of roughly 0.05 / 0.10 ‰ on δ18O and δD. This issue of conservation of samples is certainly a strong source of quality loss for parts of the database, and ‘small’ effects may have remained undetected. The internal consistency of the database can be tested for subsets of the dataset, when time series can be obtained (such as in the southern Indian Ocean or North Atlantic subpolar gyre). These comparisons suggest that the overall uncertainty of the spatially (for a cruise) or temporally (over a year) averaged data is on the order of or less than 0.03 / 0.15 ‰ for δ18O / δD. On the other hand, 17 comparisons with duplicate sea water data analysed in other laboratories or with other data sets in deep regions suggest a larger scatter. When averaging the 17 comparisons done for δ18O, we find a difference close to the adjustment applied at LOCEAN to convert salty water data from the activity to the concentration scale. Such a difference is expected, but the scatter found suggests that care is needed when merging datasets from different laboratories. Examples of time series in the surface North Atlantic subpolar gyre illustrate the temporal changes in water isotope composition that can be detected with a carefully validated dataset.

<p>   During the International Ocean Discovery Program (IODP) Expedition 385, diagenetic carbonates were recovered at different depths (from 10 to 534 mbsf) in six drilling sites (U1545, U1546, U1547, U1548, U1550 and U1552) distributed from the northern spreading segment of the Guaymas Basin (Gulf of California) to ~52 km away from the axial graben. As this basin is a site of active sea floor spreading, hemipelagic diatomaceous and organic-rich sediments rapidly accumulating at all these drilling sites are influenced by magmatic intrusions, at depths greater than 90 mbsf. The geothermal gradient observed ranges from 135 °C/km to 682 °C/km. Except in sites U1547 and U1548, both located close to a circular hydrothermal mound (called Ringvent), sills are in thermal equilibrium with the surrounding sediments.</p><p>   Carbonate samples (over fifty) collected during this expedition are composed of fine-grained micritic sediments and weakly to strongly lithified carbonate concretions. The petrographic observations and XRD analyses show that stoichiometric dolomite is the dominant authigenic carbonate phase, with minor contribution of Fe-rich dolomite, and low- to high-Mg calcite. Centimeter-sized concretions composed of ankerite and Mg-calcite also occur in the sediments collected from the northern axial graben (site U1550) and a site with subsurface gas hydrates (U1552), respectively. Other authigenic minerals are often associated with these carbonates including pyrite, barite, anhydrite, zeolites and crystalline opal (opal-CT). The carbon isotopic compositions of the bulk carbonate from these samples exhibit large variations ranging from -28.9 to +12.0‰ VPDB. Very low d<sup> 13</sup>C values, only measured in a centimetre-seized concretion associated with cold methane seepage (site U1552), indicate that the bicarbonate used for carbonate precipitation is derived from anaerobic oxidation of methane (AOM) coupled with bacterial sulfate reduction (BSR). The very high d<sup>13</sup>C values of diagenetic carbonates indicate the use of biogenic <sup>13</sup>C-rich CO<sub>2</sub> reservoirs related to active methanogenesis or other autotrophic microbial pathways. The oxygen isotopic compositions of these carbonates cover a large range from -5.3 to +4.0‰ VPDB. The decreasing d<sup>18</sup>O values with increasing depth reflect the effect of high geothermal gradients due to the close proximity of magmatic sills.  </p>

Ice-sheet and sea-level fluctuations during the Early and Middle Pleistocene are as yet poorly understood. A stalactite from a karst cave in North West Sicily (Italy) provides the first evidence of four marine inundations that correspond to relative sea-level highstands at the time of the Middle Pleistocene Transition. The speleothem is located similar to 97 m above mean sea level as result of Quaternary uplift. Its section reveals three marine hiatuses and a coral overgrowth that fixes the age of final marine ingression at 1.124 +/- 0.2, thus making this speleothem the oldest stalactite with marine hiatuses ever studied to date. Scleractinian coral species witness light-limited conditions and water depth of 20-50 m. Integrating the coral-constrained depth with the geologically constrained uplift rate and an ensemble of RSL scenarios, we find that the age of the last marine ingression most likely coincides with Marine Isotope Stage 35 on the basis of a probabilistic assessment. Our findings are consistent with a significant Antarctic ice-sheet retreat. (C) 2017 Elsevier Ltd. All rights reserved.

The New Jersey continental shelf extends 150 km off the shoreline. During IODP Expedition 313, siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631, 669 and 755 m below seafloor at sites 27A, 28A and 29A respectively in very shallow waters (33.5 to 36 m depth). Pore water salinities display multilayered brackish-salty-brine units 10 to 170 m thick, where low-salinity water is preferentially stored in fine-grained sediments. The sharp boundaries of these buried aquifers are often marked by cemented layers a few centimetres thick. The mineralogy and scanning electron microscope observations of these layers show two phases of cementation by authigenic minerals: (1) the early carbonate cement is frequently associated with pyrite, and (2) the late silicate cement infills the residual porosity. The isotopic compositions of the carbonate cements vary widely: −2.4 < δ 18 O ‰ VPDB < +2.8; −15.1 < δ 13 C ‰ VPDB < +15.6. The δ 18 O values indicate that the carbonate cements precipitated with pore waters comprising variable mixtures of seawater and 18 O-depleted fresh water originating from submarine groundwater discharge. The δ 13 C values of the carbonate cements are related to organic matter diagenesis, providing 13 C-depleted dissolved inorganic carbon during bacterial sulphate reduction and anaerobic oxidation of methane, and 13 C-rich dissolved inorganic carbon during methanogenesis. The diagenetic cementation processes included chemical weathering of reactive silicate minerals by the CO 2 -rich pore waters issued from organic matter diagenesis that released bicarbonate, cations and dissolved silica, which were further precipitated as carbonate and silicate cements. The estimated range of temperature (18±4 °C) during carbonate precipitation is consistent with carbonate cementation at moderate burial depths; however, silicate cementation occurred later during diagenesis at deeper burial depths.

Dolomite nodules are widespread within the Tortonian marls of Fortuna and Lorca basins in southern Spain. They occur as large bodies of various forms (round, ovoid, tabular) that are parallel or secant relative to the stratification. They are massive and present sometimes internal conduits that are considered as drains used for the migration of fluids.

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ13C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ13C (up to –5.2‰) and low δ18O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by 13C-rich CO2 and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.

A new fossil diatom with a well-developed raphe-bearing keel and continuous junction line in the strongly bilobate wings is described as Entomoneis calixasini Pailles, Blanc-Valleron & Poulin sp. nov. This species was found at numerous levels within a 4.6-m-long core recovered from the north-western slope of the Central High of the Marmara Sea, Turkey. Detailed light and scanning electron microscopy were used to characterize this species. Entomoneis calixasini is unique by a wing resembling interconnecting dunces' caps. This species, sometimes the most abundant, is present between 44 and 395 cm of MET09-GR02 core sediments, corresponding to the Pleistocene last glacial period (brackish lacustrine sediments). It was not observed in the marine sediments above, or in other core sediments from the Marmara Sea or elsewhere in Pleistocene or Holocene sequences. It is absent in modern collections and from the literature, and is considered extinct.

This Introduction presents an overview of selected contributions from the 11th Gas in Marine Sediments International Conference held on the 4–7 September 2012 in Nice, France, and published in this special issue of Geo-Marine Letters under the guest editorship of Catherine Pierre, Patrice Imbert and Jean Mascle. These cover fluid seepage dynamics at widely varying spatiotemporal scales in a giant buried caldera of the Caspian Sea, mud volcanoes and pockmarks in the Mediterranean and adjoining Gulf of Cadiz, as well as Lake Baikal, pockmarks of shallower waters along the Atlantic French coast and in Baltic Sea lagoons, deepwater pockmarks and cold seeps on the Norwegian margin and the Hikurangi Margin of New Zealand, asphalt seepage sites offshore southern California, and the tectonically controlled southern Chile forearc. We look forward to meeting all again at the 12th Gas in Marine Sediments conference scheduled for 1–6 September 2014 in Taipei, Taiwan.

Submarine seeps release substantial amounts of methane into the overlying water column at continental margins, leading to the formation of calcium carbonate deposits. Analyses of methane-derived carbonate build-ups on the Nile Delta suggest that their formation coincided with the development of deep-water anoxic or suboxic conditions. Marine sediments at ocean margins vent substantial amounts of methane1,2. Microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3,4. The factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained. Here, we characterize the petrology and geochemistry of rocks sampled from metre-size build-ups of methane-derived carbonate chimneys located at the Amon mud volcano on the Nile deep-sea fan. We find that these carbonates comprise porous structures composed of aggregated spherules of aragonite, and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the Black Sea5. Using U-series dating, we show that the Amon carbonate build-ups formed between 12 and 7 thousand years ago, contemporaneous with the deposition of organic-rich sediments in the eastern Mediterranean, the so-called sapropel layer S1. We propose that the onset of deep-water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor, and thus the formation of carbonate chimneys.

The factors controlling fluid emission dynamics at ocean margins are poorly understood. In particular, there are significant uncertainties on how fluid seepage at cold seeps may have responded to abrupt environmental changes in the geological past. This study reports on a detailed geochemical investigation of seafloor carbonate crusts sampled at cold seeps along the submerged part of the North Anatolian Fault system in the Sea of Marmara - an inland sea, which has experienced major paleo-environmental changes over the last deglaciation period. We also analyzed a series of authigenic carbonate concretions recovered from two sediment cores at the Western-High ridge, an active fluid venting area.The ages of seafloor carbonate crusts derived from isochron U-Th dating cover the last 7 kyr, suggesting that fluid activity along the fault system remained continuous over that time interval. In the sediment cores, carbonate concretions are concentrated at the lacustrine-to-marine transition, which corresponds to the period when Mediterranean waters flowed into the Marmara Basin about 12-14 kyr ago. U-Th isotopic data indicate that most of these concretions formed later during the Holocene, around 9-10 kyr ago, a period coinciding with an important anoxic event that led to the deposition of a sapropel layer in the Sea of Marmara.Based upon these results, we suggest that the absence of carbonate concretions in the lacustrine sediment unit indicates that dissolved sulfate concentrations in the Marmara lake pore waters during glacial time were too low to promote significant anaerobic methane oxidation, thereby preventing sedimentary carbonate authigenesis. In contrast, the progressive inflow of Mediterranean waters into the glacial Marmara lake after 15 ka provided a source of dissolved sulfate that allowed anaerobic oxidation of methane to proceed within the anoxic sediment. Importantly, the synchronism between the main phase of authigenic carbonate precipitation at the studied sites (average 9.4 +/- 1.8 ka, n = 16) and the regional anoxic sapropel event support the idea that the drop in bottom water dissolved oxygen content was probably a key factor to enhance microbial activity and associated carbonate precipitation at that time. Overall, these results provide straightforward evidence that fluid emission dynamics and hydrocarbon oxidation at cold seeps can be directly related to changing environmental conditions through time. (C) 2013 Elsevier B.V. All rights reserved.

Ricerche su una stalattite a livelli marini e continentali piu antica di un milione di anni, Custonaci, Sicilia NW.Questo lavoro si basa sul ritrovamento, in una grotta carsica, a 100 metri di quota a Custonaci in provincia di Trapani, di una stalattite di notevole interesse paleo climatico, ricoperta da coralli marini, la cui sezione rivela la presenza di 3 iatus che denotano una interruzione della deposizione di carbonato continentale presumibilmente in seguito a trasgressioni marine. Il carbonato, datato attraverso la metodologia 230Th/U MC-ICPMS (NeptunePlus), ha fornito una eta piu antica del limite temporale massimo di applicabilita del metodo (~ 600 ka). I coralli, che ricoprono il livello continentale piu giovane, analizzati usando il rapporto 87Sr/86Sr, hanno fornito una eta di 1.1±0.2 Myrs. Alcune considerazioni sui risultati delle analisi δ18O sui livelli continentali dello speleotema indicano una deposizione durante il MIS 27-31.

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5 m length indicates a maximum age of about 60,000–80,000 years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0 < δ13C ‰ V-PDB < –40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4 < δ18O ‰ V-PDB < +6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ18O values might reflect the contribution of 18O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca2+ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ18O–δ13C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.

The Marnaut cruise (May–June 2007) investigated the submerged part of the North Anatolian fault system, an active tectonic area in the Sea of Marmara. Already known and new fluid venting sites along the fault system were visited by submersible diving. Cold seeps present a considerable diversity of geochemical background associated with occurrences of authigenic carbonate crusts outcropping at the seafloor. Buried carbonate concretions were also recovered by coring within the sediments of the Tekirdağ Basin and of the Western-High ridge that separates the Tekirdağ and Central Basins. Interestingly, numerous of these early diagenetic carbonates were found within the transitional sediments from lacustrine to marine environment deposited after the late glacial maximum. The authigenic carbonates are mainly composed of aragonite, Mg-calcite and minor amounts of dolomite, and are often associated with pyrite and barite. The carbon isotopic compositions of carbonates present a wide range of values from −50.6‰ to +14.2‰ V-PDB indicating different diagenetic settings and complex mixtures of dissolved inorganic carbon from different sources. The low δ13C values of the seafloor crusts and of most buried concretions indicate that the carbon source was a mixture of microbial and thermogenic methane and possibly other hydrocarbons that were oxidized by anaerobic microbial processes. The positive δ13C values of a few buried concretions from the Western-High ridge reflect the mineralization of heavy CO2, which is thought to represent the residual by-product of oil biodegradation in a subsurface petroleum reservoir that migrated up with brines. Most of the oxygen isotopic compositions of seafloor carbonates are close to the isotopic equilibrium with the present-day bottom water conditions but a few values as low as −1.9‰ V-PDB indicate precipitation from brackish waters. In buried carbonate concretions, δ18O values as high as +4.9‰ V-PDB reflect the contribution of water enriched in 18O. The results support the hypothesis that after the late glacial/Holocene transition, precipitation of authigenic carbonates, now buried within the sediments of the Western-High mound structures, was promoted due to enhancement of anaerobic oxidation of methane, possibly from massive methane release by gas hydrate dissociation, and by sulfate rich Mediterranean water incursion.

An integrated high resolution study based both on a seismostratigraphic approach and on a sedimentary core (VIB 10), collected in the outer shelf (127 m depth) from the southern Tyrrhenian Sea (Gulf of Termini, Sicily), provides new data about climatic. eustatic and paleoenvironmental changes during the last similar to 41,000 years. The results based on the interpretation of a seismic profile, on benthic foraminifera assemblages and on delta O-18 records, allowed recognition of two drastic sea-level falls during the Last Glacial Maximum (LGM) and the Younger Dryas (YD). The short deglacial event, between LGM and YD, known as Bolling/Allerod, played an important role in the sea-level rise that produced changes in benthic foraminiferal assemblages, favoring the proliferation of shallow water species of the inner shelf. After the Younger Dryas, warmer climatic conditions were rapidly established (Climatic Optimum) as indicated by the decrease of delta O-18 values. The rapid sea-level rise due to the input of fresh water from ice caps melting following the increase of Earth's mean temperature is also indicated by the aggradational geometries of sedimentary layers observed in the seismic profile and by the increase of benthic foraminiferal species typical of the outer shelf. (c) 2010 Elsevier Ltd and INQUA. All rights reserved.

Active gas venting was observed along the submerged part of the North-Anatolian fault system (Sea of Marmara) both by acoustic methods (EK60 echosounder) and Nautile submersible dives during the MARNAUT cruise (2007). On the Western-High ridge, thermogenic gas and crude oil seep at the seafloor with deep-sourced brines. During submersible dives at this site, authigenic carbonate crusts outcropping at the seabed were sampled. Coring in the sediments revealed the occurrence of cemented carbonate levels and gas hydrates. Porous carbonate crusts are composed of aragonite-cemented pelagic deposits. The C depletion (-44 < C ‰ V-PDB < -12.8) attests that methane and possibly other hydrocarbons were the main source of carbon and implies that microbial oxidation processes have transformed these reduced forms of carbon into dissolved inorganic carbon. The O values of seafloor carbonate crusts (+2.5 < O ‰ VPDB < +3.1) suggest precipitation in equilibrium with the present-day bottom water. Abundant diagenetic carbonate concretions found in the younger marine deposits are made of high Mg-calcite and aragonite. Concretions have a wide range of carbon isotope compositions (-22 < C ‰ V-PDB < +14.2) suggesting multiple sources or diagenesis. In particular, extreme C-enrichment indicates the migration of C-rich CO2 produced by microbial activity in the methanogenic zone. The C-depleted, methane-derived, concretions are most enriched in O, suggesting that gas hydrates provided a source of O-rich water  Corresponding author: Phone +33 (0)1 44 27 84 79 Fax +44 (0)1 44 27 71 59 E-mail: antoine.cremiere@locean-ipsl.upmc.fr Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

The Nile Deep-Sea Fan (NDSF) is located on the passive continental margin off Egypt and is characterized by the occurrence of active fluid seepage such as brine lakes, pockmarks and mud volcanoes. This study characterizes the structure of faunal assemblages of such active seepage systems of the NDSF. Benthic communities associated with reduced, sulphidic microhabitats such as sediments and carbonate crusts were sampled by remotely operated vehicles during two cruises in 2006 (BIONIL) and 2007 (MEDECO). Environmental conditions and biological factors including family-level faunal composition, density and diversity were measured at local and regional scales. Significant differences were detected at different spatial scales: (1) the fauna of reduced habitats differed substantially in activity, diversity and biomass from the non-seep environment at similar water depth, (2) cold seep microhabitats showed differences in community structure and composition related to substratum type as well as to the intensity and location of fluid emissions.