Description: Four mud extrusions were investigated along the erosive subduction zone off Costa Rica. Active fluid seepage from these structures is indicated by chemosynthetic communities, authigenic carbonates and methane plumes in the water column. We estimate the methane output from the individual mud extrusions using two independent approaches. The first is based on the amount of CH4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities, which ranges from 104 to 105 mol yr− 1. The remaining portion of CH4, which is released into the ocean, has been estimated to be 102–104 mol yr− 1 per mud extrusion. The second approach estimates the amount of CH4 discharging into the water column based on measurements of the near-bottom methane distribution and current velocities. This approach yields estimates between 104–105 mol yr−1. The discrepancy of the amount of CH4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth, e.g. focused fluid emission through channels in sediments and fractures in carbonates. Extrapolated over the 48 mud extrusions discovered off Costa Rica, we estimate a CH4 output of 20·106 mol yr− 1 from mud extrusions along this 350 km long section of the continental margin. These estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins. Almost half of the continental margins are described as non-accretionary. Assuming that the moderate emission of methane at the mud extrusions off Costa Rica are typical for this kind of setting, then global estimates of methane emissions from submarine mud extrusions, which are based on data of mud extrusions located at accretionary and passive continental margins, appear to be significantly too high.

Description: Salt tectonics and mud volcanism in the Latakia and Cyprus Basin, eastern Mediterranean, is investigated by means of swath sounding, reflection seismics and side-scan data as well as by camera and video sledge observations. Both basins are located east of Cyprus and are associated with the collision front between the African and Anatolian plate. The Pliocene–Quaternary sediment succession is underlain by up to 1 km thick Messinian evaporites. Both thick-skinned plate tectonic and thin-skinned salt tectonic control fluid dynamics and associated mud volcanism in the Latakia and Cyprus Basin as well as at the Troodos Latacia Culmination, which separates both basins. An end-member model is proposed which explains the presence of elongated topographic highs and trenches along the Troodos Larnaca Culmination and south of it by gravity gliding of the Messinian evaporites and associated fluid migration. Thin-skinned extension in the Troodos Larnaca Culmination and boudinage, respectively, facilitate fluid flow through and out of the evaporites. The fluid or mud flow dissolutes the salt layer and creates elongated trenches. Mud intrudes into the Pliocene–Quaternary sediments above the trenches. Consequently, the overburden is thickened and forms morphological ridges. South of the culmination the evaporites and overburden are folded due to thin-skinned shortening of the evaporites. In one instance fluid extrusion out of the evaporites is inferred from seismic data interpretation. The outflow caused a volume reduction and collapse of the evaporites. Mud volcanoes and fold anticlines align above deep-rooted transpressional fault systems which are associated with the African–Anatolian collision zone. The faults may act as conduits for rising fluids. In the western part of the survey area, where the Cyprus Arc strikes almost West–East and the collision occurred more frontal and stress was highest, mud volcanoes emerged. Further to the east, where the Cyprus Arc runs SW–NE and sinistral strike-slip has been proposed, fold anticlines evolved. Particular mud volcanoes and folds emerged prior to the deposition of the Messinian evaporites. The undisturbed upper Pleistocene sequences as well as the absence of significant mud outflow on the seafloor strongly suggest that the main fluid dynamic ceased.

Description: Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to similar to 650 mmol L-1 at similar to 150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle QUEST revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at similar to 60 cm sediment depth that were 0.9 degrees C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (delta D-CH4 = -170.8 parts per thousand (SMOW), delta C-13-CH4 = -61.0 parts per thousand (V-PDB), delta C-13-C2H6 = -44.0 parts per thousand (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 +/- 0.5 x 106 mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column. (C) 2009 Elsevier Ltd. All rights reserved.

Description: Three pockmarks named "Hydrate Hole", "Black Hole", and "Worm Hole" were studied in the northern Congo Fan area at water depths around 3100 m. The cross-disciplinary investigations include seafloor observations by TV-sled, sampling by TV-guided grab and multicorer as well as gravity coring, in addition to hydroacoustic mapping by a swath system, a parametric sediment echosounder and a deep-towed sidescan sonar. The pockmarks are morphologically complex features consisting of one or more up to 1000 m wide and 10-15 m deep depressions revealed by swath-mapping. High reflection amplitudes in the sediment echosounder records indicate the presence of a 25-30 m thick shallow sediment section with gas hydrates, which have been recovered by gravity corer. Hydrates, chemosynthetic communities, and authigenic carbonates clearly indicate fluid flow from depths, which we propose to be mainly in the form of ascending gas bubbles rather than advection of methane-rich porewater. Evidence for seepage at the seafloor is confined to small areas within the seafloor depressions and was revealed by characteristic backscatter facies. Small meter-scale sized depressions signified as "pits" exist in or close to the pockmarks but seafloor observations did not reveal evidence for the presence of typical seep organisms or authigenic carbonates. Areas of intermediate back-scatter were inhabited by vesicomyid clams in soft sediments. High backscatter was associated with vestimentiferan tubeworms (Siboglinidae) and authigenic carbonates. We discuss the three different environments "pits", "vesicomyid clams", "vestimentifera/carbonate" in the light of differences in the geochemical setting. Pits are probably formed by escaping gas bubbles but seepage is too transient to sustain chemosynthetic life. Vesicomyid clams are present in sediments with gas hydrate deposits. However, the hydrates occur several meters below the surface indicating a lower flux compared to the vestimentifera/carbonate environment. In the latter environment, accumulated carbonates and clam shells indicate that fine grained particles have been eroded away. Gas hydrates were found in this environment at depths below about 50 cm suggesting the highest supply with methane compared to the other environments. (C) 2007 Elsevier B.V. All rights reserved.

Description: Several gas seeps and near-surface gas hydrate deposits have been identified in 850–900-m water depth on the continental slope offshore Batumi, Georgia (eastern Black Sea) using deep-towed high-resolution sidescan sonar data. The seeps are located on a ridge named Kobuleti Ridge separating two canyons: the Supsa canyon north of the ridge and the deeply incised central canyon south of it. The southern wall of this canyon shows signs for additional gas seeps. Gas seeps are shown by acoustic anomalies in the water column on raw sonar records and as high backscatter intensity areas on processed data. The seeps on Kobuleti Ridge are characterised by carbonate deposits at the centre and a much wider area where finely disseminated gas hydrates are present. Fractures of a NW–SE direction are present at the seep sites and are probably related to the formation and decomposition of gas. Individual sites of gas emission apparently exert their influence for a circular area of up to 40 m in diameter. Gas geochemistry from gravity cores shows high gas content and a mixture of biogenic and thermogenic gases together with the presence of gas hydrates. The seeps offshore Georgia are different from other known cold seeps in the Black Sea such as shallow water seeps of biogenic gas and deep water mud volcanoes. They are located in deep water within the zone of gas hydrate stability, lack significant relief and are characterised by active gas emission and the absence of mud volcanism.