Methane and sulfide fluxes in permanent anoxia: In situ studies at the Dvurechenskii mud volcano (Sorokin Trough, Black Sea)

摘要

The Dvurechenskii mud volcano (DMV) is located in permanently anoxic waters at 2060m depth (Sorokin Trough, Black Sea). The DMV was studied during the RV Meteor expedition M72/2 as an example of an active mud volcano system, to investigate 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 production of both compounds within the sediment. The pie-shaped mud volcano showed temperature anomalies as well as solute and 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 through recently deposited subsurface muds, apparently limiting microbial methanotrophic activity. Consequently, the emission of dissolved methane into the water column was high, with an estimated rate of 0.46molm~(-2)d~(-1). On the wide plateau and edge of the mud volcano surrounding the summit, fluid flow and total methane flux were lower, allowing higher SR and AOM rates correlated with an increase in sulfate penetration into the sediment. Here, between 50% and 70% of the methane flux (0.07-0.1molm~(-2)d~(-1)) was consumed within the upper 10cm of the sediment. The overall amount of dissolved methane released from the entire mud volcano structure into the water column was significant with a discharge of 1.3×10~7molyr~(-1). The DMV maintains also high areal rates of methane-fueled sulfide production and emission of on average 0.05molm~(-2)d~(-1). This is a difference to mud volcanoes in oxic waters, which emit similar amounts of methane, but not sulfide. However, based on a comparison of this and other mud volcanoes of the Black Sea, we conclude that sulfide and methane emission into the hydrosphere from deep-water mud volcanoes does not significantly contribute to the sulfide and methane inventory of the Black Sea.