Methane-related authigenic carbonates of eastern Mediterranean Sea mud volcanoes and their possible relation to gas hydrate destabilisation

Nautile submersible investigations of mud volcanoes and brine seep areas of the eastern Mediterranean Sea during the MEDINAUT cruise in November 1998 discovered extensive areas of authigenic carbonate crusts associated with m ethane emissions. Carbonate crusts form pavements, round slabs and circular mounds on the central, most active parts of mud volcanoes and in a fault-r elated valley where brines have accumulated to form a submarine brine lake. Authigenic carbonate nodules have been recovered from the same areas durin g the MEDINETH cruise in July 1999. Large C-13 depletions of authigenic cal cite, aragonite and dolomite indicate methane as a major carbon source for the carbonate. Crust pavements are formed when methane from a freshly empla ced, methane-charged mud flow is oxidised at the seafloor. In this environm ent, where bottom waters provide the sulphate and magnesium, aragonite is f avoured versus calcite and accounts for the majority of the methane-related authigenic carbonates. Calcite, when present, contains significant amounts of Mg2+ (high-Mg calcite), and possibly other divalent ions in its crystal lattice. In areas of brine seep and accumulation, dolomitic nodules are pr esent at shallow depth in the sediment. The O-18 enrichment of the authigen ic carbonates (up to 4 parts per thousand greater than calculated values fo r carbonates precipitating from modern eastern Mediterranean bottom waters) is interpreted as due to precipitation from O-18-rich fluids rather than a s a temperature effect. The source of the O-18-rich fluids may be multiple and possibly includes the destabilisation of gas hydrates present at shallo w subbottom depth, and the seepage of relic Messinian brines. (C) 2000 Else vier Science B.V. All rights reserved.