Methane fluxes and turnover in permanent anoxia: In situ studies of the Dvurechenskii mud volcano (Black Sea)

The Dvurechenskii mud volcano (DMV) is located in permanently anoxic waters at 2060 m depth (Sorokin Trough, BlackSea). The DMV was studied during the RV Meteor expedition M72/2 as an example of an active mud volcano system, to inves-tigate the significance of submarine mud volcanism for the methane and sulfide budget of the anoxic Black Sea hydrosphere.Our studies included benthic fluxes of methane and sulfide, as well as the factors controlling transport, consumption and pro-duction of both compounds within the sediment. The pie-shaped mud volcano showed temperature anomalies as well as soluteand gas fluxes indicating high fluid flow at its summit north of the geographical center. The anaerobic oxidation of methane(AOM) coupled to sulfate reduction (SR) was repressed in this zone due to the upward flow of sulfate-depleted fluids throughrecently deposited subsurface muds, apparently limiting microbial methanotrophic activity. Consequently, the emission of dis-solved methane into the water column was high, with an estimated rate of 0.46 mol m2d1. On the wide plateau and edge ofthe mud volcano surrounding the summit, fluid flow and total methane flux were lower, allowing higher SR and AOM ratescorrelated with an increase in sulfate penetration into the sediment. Here, between 50% and 70% of the methane flux (0.07–0.1 mol m2d1) was consumed within the upper 10 cm of the sediment. The overall amount of dissolved methane releasedfrom the entire mud volcano structure into the water column was significant with a discharge of 1.3107mol yr1. TheDMV maintains also high areal rates of methane-fueled sulfide production and emission of on average 0.05 mol m2d1. Thisis a difference to mud volcanoes in oxic waters, which emit similar amounts of methane, but not sulfide. However, based on acomparison of this and other mud volcanoes of the Black Sea, we conclude that sulfide and methane emission into the hydro-sphere from deep-water mud volcanoes does not significantly contribute to the sulfide and methane inventory of the Black Sea.