Abstract:
Early diagenetic Mn-Fe carbonates, occurring in thin layers, nodules, and burrows, were recovered from Cenozoic silts and clays from the Arctic Ocean and the Norwegian-Greenland Sea during Ocean Drilling Program Leg 151. At Sites 909 (Fram Strait; Miocene-Quaternary) and 911 (Yermak Plateau; Pliocene-Quaternary), fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate. Variable delta (super 13) C values (-21.8 to +7.9 per mil PDB) suggest that dissolved carbon was derived from the Fe-reduction suboxic zone, the oxidation of marine organic matter in the bacterial sulfate-reduction zone, and the early stages of methanogenesis. The range of delta (super 18) O values for siderite (-6.3 to +5.1 per mil PDB) indicates precipitation over a temperature range of 4-56 degrees C during successive burial. At Site 913 (East Greenland Margin; Eocene-Quaternary), concentrically zoned microspherules and rhombohedra of ferroan rhodochrosite and manganoan siderite, 20-600 micrometers in diameter, are the main authigenic carbonates. Although they have low delta (super 13) C values (-22.3 to -13.8 per mil PDB), elevated pore-water sulfate concentrations indicate low sulfate-reducing activity and suggest that dissolved carbon may have been partially derived from thermogenic methane. The variable delta (super 18) O values for rhodochrosite (-11.9 to +1.5 per mil PDB) and siderite (-11.4 to -10.3 per mil PDB) suggest precipitation at elevated temperatures (60-100 degrees C) during progressive burial. The close association with barite cement suggests that carbonate precipitation at Site 913 may have been influenced by hydrothermal fluids. However, active circulation of hydrothermal fluids is precluded by the occurrence of geochemical gradients in Site 913 pore waters. Pore waters at the three sites reflect various evolutionary pathways that are partly due to variations in the extent of interaction with volcanic ash layers and the underlying basaltic crust, which led to an overall decrease in Mg (super 2+) , K (super +) , and delta (super 18) O and an increase in Ca (super 2+) with depth. However, pore waters at Site 913 have evolved more extensively to Na-Ca-Cl brines. The sources of freshwater that cause the downhole decrease in Cl (super -) and Na (super +) concentrations at Sites 909 and 913 are poorly constrained. The possible influence of meteoric-water incursion is suggested by the delta D-delta (super 18) O relationship of pore waters and the low delta (super 18) O values of Site 913 authigenic carbonates.