Isotopic evidence of methane-related diagenesis in the mud volcanic sediments of the Barbados Accretionary Prism

Extensive sampling of chosen sectors of the Barbados Accretionary Prism during four French oceanographic cruises (1985-1993) evidenced the presence of abundant diagenetic carbonate deposits associated mostly with zones of active or ancient fluid venting and mud expulsion processes (mud volcanoes, mud diapirs, diapiric ridges). Diagenetic carbonates (low- and high-Mg calcite, aragonite and low-Mg dolomite) are found to bind together dead benthic communities associated to fluid venting and to cover extensive areas of the sea floor where complete lithification of both autochtonous deposits and mud volcanic products occurs. Extreme cases of early diagenesis occur on mud dome structures on the summit of the ridge where the expelled sediments have been entirely lithified for a thickness of several metres. Low delta(13)C values of most dolomites (down to -60.2parts per thousand.) and of most calcites and aragonites (down to -53.07parts per thousand.) show the major contribution of oxidized methane to the dissolved inorganic carbon in the water from which these minerals precipitated. In contrast, a number of authigenic carbonates have delta(13)C values close to 0parts per thousand and possibly precipitated from bottom waters. Unusually high delta(18)O values of the methane-related authigenic carbonates (up to 7.66parts per thousand in dolomites and up to 6.98parts per thousand in calcites and aragonites) are interpreted as due to precipitation from O-18-rich diagenetic fluids. (C) 2002 Elsevier Science Ltd. All rights reserved.