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Journal of the Geological Society; 2005; v. 162; issue.5; p. 815-827;
DOI: 10.1144/0016-764904-133
© 2005 Geological Society of London

Original Article

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

A. Mazzini1,2, G. Aloisi3, G. G. Akhmanov4, J. Parnell1, B. T. Cronin1 & P. Murphy5

1 1Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King's College, Aberdeen AB24 3UE, UK
2 2Present address: Physics of Geological Processes, Department of Physics, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway (e-mail: adriano.mazzini@fys.uio.no)
3 3Laboratoire de Paléoenvironnements et Paléobiosphère, Université Claude-Bernard, UMR 5125 CNRS, 2, rue Dubois, 69622 Villeurbanne cedex, France
4 4UNESCO–MSU Centre for Marine Geology and Geophysics, Faculty of Geology, Moscow State University, Vorobjevy Gory, Moscow, 119992, Russia
5 5School of Earth Sciences and Geography, Kingston University, Kingston upon Thames KT1 2EE, UK

Authigenic carbonate crusts, nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the Vøring 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 13C (–31.6{per thousand} < {delta}13C < –52{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{per thousand} < {delta}18O < 4.9{per thousand}) suggests a precipitation temperature very close to the one recorded on the sea floor (between –1 and 2 °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 18O (4.6{per thousand} < {delta}18O < 6.2{per thousand}), which is interpreted as a result of precipitation from an 18O-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.


Keywords: Vøring Plateau, cold seeps, authigenic minerals, carbonate, methane.