Nancy Chow, Sadoon Morad, and Ihsan S. Al-Aasm


  Authigenic carbonates occur in thin layers, nodules, and burrows in fine-grained siliciclastic sediments of middle Eocene to Quaternary age that were recovered from the Arctic Ocean and Norwegian-Greenland Sea during Leg 151. At Site 913 (East Greenland Margin), concentrically zoned microspherules and rhombohedrons of ferroan rhodochrosite and manganoan siderite, 20-600 µm in diameter, are the main authigenic carbonates in the sedimentary succession. They grew displacively within the host clays and silts, and have locally coalesced to form aggregates. At Sites 909 (Fram Strait) and 911 (Yermak Plateau), very fine to fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate in the sedimentary succession. It occurs as interparticle cement, disseminated crystals, and rounded intraclasts in host clays, silts, and muds. Minor micritic or radialfibrous calcite and dolomite were also found at these three sites. Most of these carbonatesare interpreted to have precipitated at shallow-burial depths during early diagenesis.
  All authigenic carbonates (rhodochrosite, siderite, and calcite) from Site 913 are strongly enriched in 12C(delta13C = -22.9 to -13.8‰) and show decreasing delta13C values with depth below the seafloor. This evidence, along with authigenic pyrite found in microspherule cores, suggests that dissolved carbon was derived from the oxidation of marine organic matter in the bacterial sulfate-reduction zone and during the early stages of methanogenesis. However, low organic matter content and headspace methane concentrations suggest additional derivation of dissolved carbon from thermogenic methane. The very low delta18O values (-11.8‰) for rhodochrosite and the close association with barite in voids and fractures suggest rhodochrosite precipitation from Mn-charged, hydrothermal fluids, possibly related to volcanic activity associated with seafloor spreading.
  The delta13C values of most siderites from Site 909 are close to -6‰, which suggests an origin in the Fe-reduction, suboxic zone, possibly only centimeters or decimeters below the seafloor. However, minor occurrences of siderite with very low delta13C values (-21.8‰), along with their close association with authigenic pyrite, indicate additional derivation of dissolved carbon from bacterial sulfate-reduction and the early stages of methanogenesis. The importance of anaerobic microbial processes is further corroborated by the moderate to high organic matter content and headspace methane concentration at Site 909. The range of delta18O values for siderite from Sites 909 and 911 (-10.3 to +5.0‰) appears to be related to both increasing burial temperatures and interaction between labile volcaniclastic sediment and marine pore waters.

Date of initial receipt: 1 July 1995
Date of acceptance: 30 December 1995

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