Origin of Authigenic Mn-Fe Carbonates and Pore-Water Evolution in Marine Sediments:
Evidence from Cenozoic Strata of the Arctic Ocan and Norwegian-Greenland Sea
(ODP Leg 151)
Nancy Chow (University of Manitoba), Sadoon Morad (Uppsala University)
and Ihsan S. Al-Aasm (University of Windsor)
Journal of Sedimentary Research (2000), v. 70, p. 682-699.
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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 13C values (-21.8 to +7.9‰
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 18O values for siderite (-6.3 to +5.1‰ PDB) indicates
precipitation over a temperature range of 4-56°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 µm in diameter, are
the main authigenic carbonates. Although they have low 13C values (-22.3 to -13.8‰ 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 18O
values for rhodochrosite (-11.9 to +1.5‰ PDB) and siderite (-11.4 to -10.3‰ PDB) suggest
precipitation at elevated temperatures (60-100°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 Mg2+, K+, and 18O and an increase in Ca2+ 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- and Na+ concentrations at Sites
909 and 913 are poorly constrained. The possible influence of meteoric-water incursion is
suggested by the D-18O relationship of pore waters and the low 18O values of Site 913
authigenic carbonates.