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

Abstract : 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 δ¹³C values of most dolomites (down to -60.2‰) and of most calcites and aragonites (down to -53.07‰) 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 δ¹³C values close to 0‰ and possibly precipitated from bottom waters. Unusually high δ¹⁸O values of the methane-related authigenic carbonates (up to 7.66‰ in dolomites and up to 6.98‰ in calcites and aragonites) are interpreted as due to precipitation from ¹⁸O-rich diagenetic fluids.