Integrated petrographic and geochemical record of hydrocarbon seepage on the Voring Plateau

Authigenic carbonate crusts, nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the Voting Plateau during the TTR-8 and TTR-10 marine expeditions. A petrographic and geochemical study was carried out to investigate their possible relationship with the seepage of hydrocarbon fluids. All authigenic carbonates are depleted in C-13 (-31.6 parts per thousand. < delta C-13 < -52 parts per thousand) indicating that methane is the primary source of the carbonate carbon. Furthermore, pyrite framboids are often associated with these samples, indicating that sulphate reduction is spatially coupled with methane oxidation and implying that the carbonates are formed through the anaerobic oxidation of methane. The oxygen stable isotope composition of the near-subsurface carbonates (3.1%. < delta O-18 < 4.9 parts per thousand) suggests a precipitation temperature very close to the one recorded on the sea floor (between -1 and 2 degrees C), which is consistent with their stratigraphic position, and a recent (Holocene?) age of formation. Carbonates sampled from greater depths (up to 5.5 m below the sea floor) are richer in O-18 (4.6 parts per thousand < delta O-18 < 6.2 parts per thousand), which is interpreted as a result of precipitation from an O-18-rich fluid. The occurrence of different carbonate mineral phases (aragonite, calcite, dolomite) is possibly related to varying dissolved sulphate concentrations in the diagenetic environment. Fluid inclusion microthermometry and Raman spectroscopy indicate the presence of an aqueous + hydrocarbon mixture inside the inclusions. This seepage mixture was almost certainly immiscible, resulting in heterogeneous trapping.