Methane in Sediments of the Deep Marmara Sea and Its Relation to Local Tectonic Structures

Abstract

The central and western part of the Sea of Marmara consists of two deep pull-apart-basins and an intermediate push-up structure. The bathymétric high between the Tekirdag and the Central Marmara Basin is cut by a deep E-W-trending furrow which is the eastern branch of the Ganos Fault which in turn is part of the active North Anatolian Fracture Zone. This area was mapped during the Meteor cruise M 44/1 in February 1999. During the cruise seafloor observations and sediment coring within this prominent furrow structure have shown that methane is emanating at several sites into the water column and that the sediments are also methane-enriched in their deeper parts. Within the porewater regime of hemipelagic clay sediments the typical process of bacterially mediated anoxic methane oxidation is taking place. The resulting geochemical profiles indicate a reaction-zone in a depth of several meters where methane is oxidized to H₂ and CO₂; the H₂ reduces the sulfate that finally is converted to H₂S. This H₂S forms iron-sulfide precipitates in the presence of reduced iron. However, our geochemical concentration profiles are not typical for steady-state conditions, but are rather characterized by a very steep drop in sulfate-concentrations just above the reaction zone. We suggest that this specific gradient is caused by a situation that is superimposed by an advective upward-migration of a methane-enriched fluid. This advective fluid flow is controlled by the tectonic strain of the Ganos Fault system.