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Authigenic carbonates occur in thin layers, nodules, and burrows in fine-grained siliciclastic sediments of middle Eocene to Quatemary 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 euthigenic 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 radial-fibrous calcite and dolomite were also found at these three sites. Most of these carbonates are 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 12 C (δ 13 C = -22.9 to -13.8%) and show decreasing δ 13 C 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 δ 18 O 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.

Origin of Authigenic Carbonates in Eocene to Quaternary Sediments from the Arctic Ocean and Norwegian-Greenland Sea