Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area ("Batumi seep area") located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups. X-ray diffractometry revealed the presence of two different authigenic carbonate phases, i.e. pure aragonitic rims associated with vital microbial mats and high-Mg calcite cementing the hemipelagic sediment. As indicated by the CT images, the initial sediment has been strongly deformed, first plastic then brittle, leading to brecciation of the progressively cemented sediment. The aragonitic rims on the other hand, represent a presumably recent carbonate growth phase since they cover the already deformed sediment. The stable oxygen isotope signature indicates that the high-Mg calcite cement incorporated pore water mixed with substantial hydrate water amounts. This points at a dominant role of high gas/fluid flux from decomposing gashydrates leadingtothe deformationandcementationof the overlyingsediment. Incontrast, the aragonitic rims do not showan influence of d¹⁸O-enriched hydratewater. The differences in d¹⁸O between the presumably recent aragonite precipitates and the older high-Mgcements suggest that periods of hydrate dissociation and vigorous fluid discharge alternated with times of hydrate stability and moderate fluid flow. These results indicate that shallow-buried gas hydrates are prone to episodic decompositionwith associated vigorous fluid flow. This might have a profound impact on the seafloor morphology resulting e.g. in the formation of carbonate pavements and pockmark-like structures but might also affect the local carbon cycle.

Within the last decade a large dataset of high-resolution bathymetric and sidescan sonar data has been acquired for the deeper parts of the Black Sea. They show many manifestations of gas seepage that is accompanied in several cases by mud expulsion. The sidescan sonar data allow imaging the seafloor at metres scale resolution and the detail shown has modified our understanding of gas seepage in the Black Sea. Unexpectedly, the presence of gas bubbles in the water column has been detected even in great water depth, where gas hydrates should form. Some of these gas flares are intermittent at various timescales, as shown by pulses of high backscatter on sidescan sonar records. Sidescan sonar data show a variety of mound structures that are not related to mud volcanism, but are formed mainly by authigenic carbonate precipitation. Finally, several mud volcanoes have been mapped in the Sorokin Trough, where they show strong variations in morphology and backscatter distribution on even small lateral distances. Several, previously unknown mud volcanoes and the areal extent of individual mud flows are also mapped and underline the strong variability in probably both space and time of the mud expulsion phenomena. These images will be shown and their distribution discussed in the light of the great availability of microbial gas in Black Sea sediments. In the deep Black Sea available gas should form gas hydrates, but where are do these accumulate? Bottom simulating reflectors that indicate the base of the hydrate stability zone are rather rare in the Black Sea.

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

Multidisciplinary study of seep-related structures on Southern Voring Plateau has been performed during several UNESCO/IOC TTR cruises on R/V Professor Logachev. High-resolution sidescan sonar and subbottom profiler data suggest that most of the studied fluid discharge structures have a positive relief at their central part surrounded by depression. Our data shows that the present day fluid activity is concentrated on the top of these "seep mounds". Number of high hydrocarbon (HC) gas saturated sediment cores and 5 cores with gas hydrate presence have been recovered from these structures. d₁₃C of methane (between _68 and _94.6& VPDB) and dry composition of the gas points to its biogenic origin. The sulfate depletion generally occurs within the upper 30-200 cm bsf and usually coincides with an increase of methane concentration. Pore water d₁₈O ranges from 0.29 to 1.14& showing an overall gradual increase from bottom water values (d¹⁸O w 0.35&). Although no obvious evidence of fluid seepage was observed during the TV surveys, coring data revealed a broad distribution of living Pogonophora and bacterial colonies on sea bottom inside seep structures. These evidences point to ongoing fluid activity (continuous seepage of methane) through these structures. From other side, considerable number and variety of chemosynthetic macro fauna with complete absence of living species suggest that present day level of fluid activity is significantly lower than it was in past. Dead and subfossil fauna recovered from various seep sites consist of solemyid (Acharax sp.), thyasirid and vesicomyid (cf. Calyptogena sp.) bivalves belonging to chemosymbiotic families. Significant variations in d¹³C (-31.6‰ to -59.2‰) and d₁₈O (0.42‰ and 6.4‰) of methane-derived carbonates collected from these structures most probably related to changes in gas composition and bottom water temperature between periods of their precipitation. This led us to ideas that: (1) seep activity on the Southern Voring Plateau was started with large input of the deep thermogenic gas and gradually decries in time with increasing of biogenic constituent; (2) authigenic carbonate precipitation started at the near normal deep sea environments with bottom water temperature around +5^oC and continues with gradual cooling up to negative temperatures recording at present time.

Computer microtomography (microCT) is one among the most dynamically developed methods used for studying oil and gas reservoirs structure. Combined with classical complex of lithological and petrophysical researches the microCT allows to conduct simultaneous study of rock composition and structure of pore space. Along with constructing volumetric 3D models, application of microCT provides a possibility of evaluating individual structural characteristics of rock in formation of residual water saturation