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The article cities a research study that explores a submarine mud volcano of the Nankai accretionary complex for the production of deep-biosphere methane with the help of geofluids. Scientists found that 90% of methane is microbially produced at 16 degree to 30 degree celsius and 300 to 900 m below seafloor, which was suggested by stable isotopic compositions of water and carbon compounds.

Abstract: We investigated gas bubble emissions at the Don–Kuban paleo-fan in the northeastern Black Sea regarding their geological setting, quantities as well as spatial and temporal variabilities during three ship expeditions between 2007 and 2011. About 600 bubble-induced hydroacoustic anomalies in the water column (flares) originating from the seafloor above the gas hydrate stability zone (GHSZ) at ~700m water depth were found. At about 890m water depth a hydrocarbon seep area named “Kerch seep area” was newly discovered within the GHSZ. We propose locally domed sediments (“mounds”) discovered during ultra-high resolution bathymetric mapping with an autonomous underwater vehicle (AUV) to result from gas hydrate accumulation at shallow depths. In situ measurements indicated spatially limited temperature elevations in the shallow sediment likely induced by upward fluid flow which may confine the local GHSZ to a few meters below the seafloor. As a result, gas bubbles are suspected to migrate into near-surface sediments and to escape the seafloor through small-scale faults. Hydroacoustic surveys revealed that several flares originated from a seafloor area of about 1km2 in size. The highest flare disappeared in about 350m water depth, suggesting that the released methane remains in the water column. A methane flux estimate, combining data from visual quantifications during dives with a remotely operated vehicle (ROV) with results from ship-based hydroacoustic surveys and gas analysis revealed that between 2 and 87×106 mol CH4 yr−1 escaped into the water column above the Kerch seep area. Our results show that the finding of the Kerch seep area represents a so far underestimated type of hydrocarbon seep, which has to be considered in methane budget calculations. [Copyright &y& Elsevier]

Abstract: The Isis mud volcano is an active camembert-like mud expulsion structure in the eastern Nile deep-sea fan area. It has been investigated during the NAUTINIL (2003) and MIMES (2004) cruises carried out within the MEDIFLUX project. Active, gas-rich fluid venting occurs at the Isis mud volcano as indicated by enhanced gas content in the sediments, having a measured 7.5 mmol of methane per liter of wet sediment in the central area of the mud volcano. In addition, gas bubble expulsions have resulted in the formation of multiple hydrocarbon plumes in the water column, with methane concentrations as high as 1550 nmol/L, compared to ∼1 nmol/L for background levels in deep water. In view of the stable carbon and hydrogen isotopic composition of methane (δ 13C=−55.1‰ VPDB, δD=−203‰ VSMOW) and of higher hydrocarbons (δ 13C of −30.3‰, −26.1‰, −28.4‰, and −25.0‰ VPDB, for ethane, propane, i-butane and n-butane, respectively), their origin is mainly thermogenic. These isotopic data also show that the wet compounds are co-generated with oil and we suggest they predominantly originate from type II kerogen source rocks of potentially variable maturity levels. At the centre of the mud volcano, the distinct kink observed in the pore water isotopic and dissolved species composition and its depth level change between subsequent cruises indicates a high frequency of fluid/gas expulsion. The seawater signature of the pore fluid in the upper part of the sediment column suggests that each expulsion is followed by a downward migration of overlying seawater. Such downward advection of bottom seawater may not only be related to the replacement of the ejected gas but also to the replacement of lower density pore fluid by bottom seawater. This mechanism is thought to be a common feature at mud volcanoes and gas seepage sites. In the peripheral zone, pore fluid is at seawater concentration and isotopic composition, down to a coring depth of 430 cmbsf. We suggest that the deeper downward advection of bottom seawater is related to the less frequent fluid/gas expulsion occurring at these sites compared to the centre sites. In this paper we show that a combination of geochemical tools used for hydrocarbons and pore water geochemistry is a powerful approach to detect sources of ascending fluid and to reveal post-eruptional fluidodynamic processes within mud volcanoes. [Copyright &y& Elsevier]