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Gas hydrates are a major component in the organic carbon cycle. Their stability is controlled by temperature, pressure, water chemistry, and gas composition. The bottom-simulating reflector (BSR) is the primary seismic indicator of the base of hydrate stability in continental margins. Here we use seismic, well log, and core data from the convergent margin offshore NW Borneo to demonstrate that the BSR does not always represent the base of hydrate stability and can instead approximate the boundary between structure I hydrates above and structure II hydrates below. At this location, gas hydrate saturation below the BSR is higher than above and a process of chemical fractionation of the migrating free gas is responsible for the structure I-II transition. This research shows that in geological settings dominated by thermogenic gas migration, the hydrate stability zone may extend much deeper than suggested by the BSR.

This study documents the tectono-stratigraphic setting and expulsion history of a major, previously undescribed mud volcano (MV) province in the Indus Submarine Fan, offshore Pakistan. A buried MV field of nine composite MVs has been recognized using two-dimensional (2D) and 3D seismic reflection data in a confined area of 50 x 65 km2. Conduits are recognized on each of these MVs connecting the pre-Eocene parent beds to the stacked mud cones. The buried MVs are up to 8.4 km wide (4.5 km average) with a central conduit of 1.23 km average diameter and an average mud cone thickness of 0.33 km. Three major phases of fluid and mud remobilization occurred in the Early to Middle Miocene, intra-Middle Miocene and in the Late Miocene to Plio-Pleistocene transition. Most of the mud source (parent beds) seems to be of pre-Eocene origin. Geometrical information from 21 mud cones allows an estimate of the volume required to build these fluid escape features. The calculated volume of remobilized sediments is 71.5 +/- 9 km3. The location of the MV field is limited to the pre-Eocene main depocentre, with major tectonic deformation occurring along the wrench system of the Indo-Arabian plate boundary, i.e. the southern edge of the Murray Ridge. The Indus MV field is, to our knowledge, the longest lived (similar to 22 Myr) remobilized, Cenozoic sedimentary system observed worldwide. No evidence of present-day mud flow activity is seen on the seabed seismic reflection in the study area.

Morphological parameters of offshore (Gulf of Cadiz, Mediterranean Sea, and Black Sea) and onshore (Azerbeidjan) mud volcanoes are compared with data from magmatic volcanoes. The offshore data series are compiled from multibeam bathymetry measurements (Gulf of Cadiz and Black Sea), and data from literature (Mediterrean Sea). The onshore data series (Azerbeidjan) is derived from Landsat 7 ETM+ and SRTMDEM data. The studied mud volcanoes range between 25 m and 285 m high and are up to 5.8 km in diameter. The main morphological aspects of the mud volcanoes, both onshore and offshore are, from the margin towards the centre, a subsidence rim or moat, the mud volcano slope, in some cases a deep crater, and a recent central mud dome at the top. The slope varies between concave, linear or convex and is characterised by radial outward sediment flow deposits or by a concentric pattern of terraces and steps. The sediment flow deposits can be divided into elongate outflows that accumulate at the base of the slope and short bulky outflow deposits that freeze on the steep slope. Morphological parameters distinguish two end-members: 'effusive' low rounded mud volcanoes and 'explosive' steep sloped mud volcanoes. Comparing with morphological data from magmatic volcanoes shows great similarity despite the large differences in size. It is demonstrated that the shape of a volcano is independent from its size but seems to be controlled by the flow behaviour of the erupted material. This study is the first to generate a representative data series about mud volcano morphology and to discuss its variability in terms of eruption mechanism. Comparison with magmatic mud volcanoes demonstrates that mud volcanoes are a good small scale analog to study big-scale volcanic processes.

Gas seepage and mud volcanism in a lacustrine environment was first discovered on the deep basin floors of Lake Baikal in 1999. Later on gas seeps were also detected in shallow parts of the lake on echosounder recordings or by visualization of gas bubbles at the lake surface. In this presentation we want to give an overview of the distribution of gas seepage and mud volcanism in relation to the geologic settings of the different seep areas.From the integration of the available data sets (echosounder bathymetric data, echosounder water-column data, side-scan sonar imagery and seismic subsurface data) we can observe different seeps locations, each within a different geological environment:shallow seeps located in deltaic environments (e.g. Babyshkin)shallow seeps located at fault scarps (e.g. Posolsky Bank)deep seeps located above mud volcanoesdeep seeps located near canyons (Kukuyu Canyon)The shallow seeps are commonly very active. Within the deltaic environments, the methane is most likely formed by the decomposition of organic matter in the shallow subsurface. Nevertheless deeper sources of methane cannot be excluded in the highly active tectonic setting of Lake Baikal. Other shallow seeps are located on a fault scarp, near Posolsky Bank. Seismic data suggest, however, that the fault does not act as a conduit for seepage itself, but rather cuts off a gas-bearing layer.For the deep seeps above the mud volcanoes, active faults and the presence of gas hydrates seem to be primordial. The venting sites are attributed to hydrate destabilization at the base of the GHSZ under the influence of a geothermal fluid pulse along nearby faults. Methane release is not continuous (probably tectonically controlled; most mud volcanoes are not active at present) and the source of methane is destabilizing gas hydrates at 200-300 m sub-bottom depth.Seepage in Kukuyu Canyon is linked to mud volcanoes and gas-hydrate destabilization in the deeper part. In the more shallow part seeps seem to originate from organic-rich sediments supplied by density currents sourced by the Selenga River.

Detailed geophysical, geochemical and petrological data over a cluster of large mud volcanoes at the Moroccan North Atlantic margin document the activity of sea floor mud volcanoes in relation to its morphology and structural setting. Mud volcanoes are often long-lived systems; their changing morphology bears witness of the evolution of fluid flow expulsion. The El Arraiche mud volcano field is a cluster of 9 mud volcanoes.It was discovered in 2002 at the Morrocan Atlantic margin in water depths from 200 m to 700 m. The largest mud volcano in the field is 255 m high and 5.4 km wide. Marine surveys between 2002 and 2005 yielded detailed geophysical, geochemical, sedimentological, and petrological data. The geophysical data include multibeam bathymetry, high-resolution seismics, deep-tow sub bottom profiles and side-scan sonar mosaics.Video imagery lines, video guided grab samples, dredge samples, gravity cores, and box cores were collected for groundtruthing purposes.Petrological and geochemical analysis of authigenic carbonates provided a record of hydrocarbon sources, fluid characteristics, processes of mixing and the mode of venting. The El Arraiche mud volcanoes cluster around two subparallel anticlines and are associated active extensional faults. Extruded rock clasts and regional seismic data locate the El Arraiche field over a Late Miocene to Pliocene extensional basin. The onset of mud volcanic activity is estimated at about 2.4 Ma and probably roots in the Cretacous to Miocene accretionary wedge. Stacked outflows are visible up to a depth of about 500 m below the sea floor. Stratigraphic correlation of the outflow lenses over the entire mud volcano field indicate that although large outflow events are not synchronized between the individual mud volcanoes, eruptions occurred more frequently during periods of active extensive tectonics. The morphology of the sea floor mud volcanoes is the result of a combination of extrusive and intrusive processes. Extrusive debris flow with low yield strength and high fluid content create long mud flows that accumulate at the base of the mud volcanic cone and tend to reduce the overall slope angle. Extrusive debris flows with high yield strength form short, irregular, and thick flow deposits that accumulate on the steep mud volcano slopes. The resulting slope profile is steep with a linear to convex profile. Smaller mud flows and fluidized sand deposits occur within the crater area. Besides the obvious extrusive processes, diapiric (buoyant) rise within earlier mud volcanic deposits is interpreted to be a main contribution in shaping large cone-shaped mud volcanoes. Large subsidence depressions at the base of the mud volcanoes are attributed to sediment removal whereas crater collapse is caused by dewatering and degassing of the intrusive sediment. The characteristics of mud volcanic activity are also reflected by geochemical, mineralogical and carbon isotope signatures of authigenic carbonates. These analyses allow distinguishing differences in the feeder system, the interference by gas hydrates, and differences in discharge rate at the surface. Other indications about the venting process are obtained from the type of mud breccia (size and abundance of clasts), pore water geochemistry, and the distribution of benthic fauna.

The north-western corner of Lake Le Bourget is situated along an active fault zone and accommodated a large sediment supply from the Rhone River until the end of the Late Glacial period. On the delta slope, the Holocene sheet drape that covers the largest buried mass wasting deposit (the HDU) shows undulations, small fractures and discontinuities that are attributed to downslope creep. Evidence for episodes of vigorous fluid expulsion is found in association with these discontinuities. All these features are rooted at the top of the HDU and occur along two specific isobaths. These observations indicate a close link between fractures and focused fluid flow. We suggest that focused fluid flow triggered by earthquakes facilitates the formation of small-scale faults that accommodate part of the downslope movement and eventually link up to form a head-scarp of a large slide (c. 107 m3).

After the discovery in 1999 of a series of mud volcanoes on the deep basin floors of Lake Baikal and of the presence of an anomalous thermally-mixed water layer attributed to methane venting, the lake floor was investigated in more detail in order to identify all possible sources of methane and to understand the processes leading to mud volcanism and methane release at the Baikal lake floor. New data show the presence of at least 4 mud volcano provinces, each consisting of several individual mud volcano structures, and of several areas of gas venting. All mud volcanoes occur in water depths of > 1000 m, within the GHSZ and in areas of abnormally shallow BSR, and are closely associated to large, active faults. They are attributed to hydrate destabilisation at the base of the GHSZ under the influence of a geothermal fluid pulse along the nearby fault. Methane release is not continuous (probably tectonically controlled; most mud volcanous are not active at present) and the source of methane is destabilising gas hydrates at 200-300 m subbottom depth. In addition, a whole series of methane vents (i.e. without distinct morphological expression) occur in shallower-water areas. These venting sites occur mostly in deltaic environments, but some are also associated with faults, and are always outside the GHSZ. Methane release appears to be more continuous (many are now active) and the source of the methane is probably shallow subsurface methane formed by the decomposition of organic matter, although deeper sources can not be excluded.Consequences of methane venting for the waters of Lake Baikal are the presence of a thermally-mixed water layer, wich could (if persisting and increasing in thickness) lead to a permanent stratification of the water column. This could influence the water mixing process and have major influences on the oxygenation of the lake and the benthic biota. In addition, increasing evidence is becoming available that some of these seeps may influence the water column (directly, or via associated temperature-driven circulation effects) up to the surface, causing localised melting or non-freezing of the winter-ice cover and even massive fish deaths. Measurements of surface-water and near-surface air methane concentrations are currently underway. The influence of the methane seeps (up to a few years not even suspected in the largest lake on Earth) on the entire lake system may thus be extremely important.

Evolutionary phases of deep water coral banks have been recognised by mapping the spatial distribution and the morphology of coral banks in three deep-water coral provinces in the Porcupine Seabight. In analogy with stages of tropical reefs references to the sealevel response, start-up, catch-up, keep-up and give-up can be distinguished in coldwater reefs. In case of deep water coral banks the critical factor in the development is the local current regime, controlling the sedimentation and the food supply. The start-up phase in the basin took place after a Late Pliocene erosional period, characterised by widespread coral patches. Once sedimentation starts the coral banks accrete in a vertical direction by active baffling and extension of the biological cap (Keep-up). During the Catch-up phase, the framework will progressively be clogged with sediment and the accommodation space is limited by clastic sediment supply. nevertheless the coral bank development occurs in pace with the clastic sedimentation. When sediment supply exceeds the growth capacity of the coral bank, the surface with living coral reduces and the coral bank gets buried and draped by sediment (Give-up). These development phases indicate climate changes reflecting in the change of watermasses and the oceanographic regime. As such, deep-water coral banks can act as deep water paleaoclimate proxies in analogy to their tropical counterparts.

The newly discovered Larache mud volcano field at the Moroccan Atlantic slope (200 m - 700m water depth), and a deep water mud volcano field (1000 m - 1200 m water depth) in the Portuguese sector of the Gulf of Cadiz, have been investigated using side scan sonar, multibeam, video imagery, TV-guided grab, dredge and coring. At five mud volcanoes (Al Idrissi mv, Mercator mv, Gemini mv, Aveiro mv, and Captain Arutnyov mv) biodiversity was studied in relation to environmental factors.This study illustrated that the occurrence of mud volcanoes on the continental slope create new habitats and add to biodiversity. The study yielded the following results:The thickness of hemipelagic drape overlying mud breccias showed that all mud volcanoes have been active in the last 1600 years.Video imagery observations indicate an eastbound current over the Al Arraiche mud volcano field. At greater water depths this current becomes more variable.Faunal assemblages change with the sedimentary facies of the sea floor at each mud volcanoCurrents influence the distribution of trophic groups over the mud volcanoes. Populations of filter feeders are more dense on the upcurrent slope. On the downcurrent slope they are replaced by deposit feeders and scavengers.Faunal diversity and bioturbation increases at seep sitesAbundances of megafauna decreases with increasing water depth.The disturbance of the fauna by mud eruptions is small as periods inactivity are much larger than the time needed for recolonization. Seepage of fluids has a longer-lasting influence, resulting in a decrease of sessile filter feeders and burrows towards the summit of most mud volcanoes.The data were acquired during 2 consecutive surveys by the R/V Belgica (CADIPOR cruise) and the R/V Prof Logachev (TTR 12 cruise).

Mud volcanoes are often regarded as a source of geological information about the subsurface, through the mineralogical and/or isotopic composition of expelled mud, clasts and fluids. Here, we propose that mud volcanoes might also be considered as a window into the deep biosphere. We present results obtained using different methods to investigate microbial abundance and community structure. The microbial community structure in mud volcano sediments from Captain Arutyunov mud volcano on the Moroccan Margin was studied by fluorescent in situ hybridation (FISH) using four general oligonucleotide probes, targeting all organisms, Eubacteria, Archea, and sulphate reducers. Also, TOC and TC analyses of these sediments were measured, and the carbon and oxygen isotopic composition of carbonate crusts from a fault-bounded cliff in a nearby region was analysed.The total bacterial count of the samples averaged 1.2 10^8 cells per g of sediment, with the highest abundance at 10 cm below the sediment surface. This high density indicates that the mud volcano sediments are a suitable environment for microbial growth. This is confirmed by a relatively high percentage of hybridization obtained with the general probe, which showed that about 50% of the cells were metabolically active. The highest relative activity (62% at 10 cm below the sediment surface) also coincides with the highest abundance.The isotopic analysis has been carried out on separate components of a crust sample. The lithified carbonate matrix shows a carbon isotopic value of about -25‰. The other components, nonlithified mud, corals and bioclasts, display values of about -5‰. These results indicate that carbonate crust formation may occur within the methanogenesissulphate reduction transition zone (MST), which is located at a very shallow depth. This implies an upward migration of methane, possibly driven by fluid flow.Our results suggest that some of the microorganisms, present in the shallow subsurface on and around mud volcanoes, are brought to the surface together with fluids and/or mud breccia from the deep source layers. With further phylogenetic analysis of microorganisms living in mud volcano sediments and involved in the carbonate precipitation, we propose to identify some of these deep-dwelling organisms which are apparently able to adapt to a shallow sedimentary environment.

The Gulf of Cadiz is situated in a tectonically complex and active region, close to a major plate boundary. The tectonic regime in the area is characterized by a combination of important strike-slip movement and compressional tectonics related to the Africa-Eurasia NW-directed convergence, responsible for the formation of the Gibraltar Arc. Extensive mud volcanism, pockmarks, mud diapirism and carbonate chimneys related to hydrocarbon rich fluid venting are observed throughout the area. There is an extensive coverage of seismic reflection profiles in the area that includes industry data, a few deep-multichannel lines (IAM, ARRIFANO and BIGSETS) and many single-channel lines (both Sparker and Airgun data). During the TTR-12 (July/2002) and the GAP (Nov-Dec/2003) cruises, several single and multi channel seismic lines were acquired in this area that complement the existing database collected during previous TTR Cruises. These lines have re-processed to enhance the deeper structure.A combined interpretation of the available side-scan sonar imaging obtained by the Naval Research Laboratory in 1992 and the available seismic lines (both single channel and multichannel) shows clear evidence of the structural control of the mud volcanism in the study area. In particular, it appears that some of the mud volcanoes are located at the intersection between NW-SE strike-slip faults and thrusts of variable orientation, reflecting the curvature of the Gibraltar Arc.