Origin of authigenic Mn-Fe carbonates and pore-water evolution in marine sediments: Evidence from Cenozoic strata of the Arctic Ocean and Norwegian-Geenland Sea (ODP Leg 151)

Early diagenetic Mn-Fe carbonates, occurring in thin layers, nodules, and b urrows, were recovered from Cenozoic silts and clays from the Arctic Ocean and the Norwegian-Greenland Sea during Ocean Drilling Program Leg 151, At S ites 909 (Fram Strait; Miocene-Quaternary) and 911 (Yermak Plateau; Pliocen e-Quaternary), fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate. Variable delta(13)C values (-21.8 to +7.9 parts per thousand PDB) suggest that dissolved carbon was derived from the Fe-reducti on suboxic zone, the oxidation of marine organic matter in the bacterial su lfate-reduction zone, and the early stages of methanogenesis. The range of delta(18)O values for siderite (-63 to +5.1 parts per thousand PDB) indicat es precipitation over a temperature range of 4-56 degrees C during successi ve burial. At Site 913 (East Greenland Margin; Eocene-Quaternary), concentrically zone d microspherules and rhombohedra of ferroan rhodochrosite and manganoan sid erite, 20600 pm in diameter, are the main authigenic carbonates. Although t hey have low delta(13)C values (-22.3 to -13.8 parts per thousand PDB), ele vated pore-water sulfate concentrations indicate law sulfate-reducing activ ity and suggest that dissolved carbon may have been partially derived from thermogenic methane. The variable delta(18)O values for rhodochrosite (-11. 9 to +1.5 parts per thousand PDB) and siderite (-11.4 to -10.3 parts per th ousand PDB) suggest precipitation at elevated temperatures (60-100 degrees C) during progressive burial. The close association with barite cement sugg ests that carbonate precipitation at Site 913 may have been influenced by h ydrothermal fluids. However, active circulation of hydrothermal fluids is p recluded by the occurrence of geochemical gradients in Site 913 pore waters . Pore waters at the three sites reflect various evolutionary pathways that a re partly due to variations in the extent of interaction with volcomic ash layers and the underlying basaltic crust, which led to an overall decrease in Mg2+, K+, and delta(18)O and an increase in Ca2+ with depth. However, po re waters at Site 913 have evolved more extensively to Na-Ca-Cl brines, The sources of freshwater that cause the downhole decrease in Cl- and Na+ conc entrations at Sites 909 and 913 are poorly constrained. The possible influe nce of meteoric-water incursion is suggested by the SD-delta(18)O relations hip of pore caters and the low delta(18)O values of Site 913 authigenic car bonates.