Unique siderite occurrence in Baltic Sea: a clue to siderite-water oxygen isotope fractionation at low temperatures

Stanisław Hałas, Roman Chlebowski


Recently formed siderite (between 1785 and 1996) at the bottom of South Baltic Sea was investigated by thin section petrography, XRD, electron microprobe (determination of major cation content) and mass spectrometry (d13 C and d18 O analysis). The siderite was interpreted as having formed from metallic iron partly protected from free oxygen access by wood tar with d13 C = -24 and from biogenic CO2 produced from this organic matter. Inasmuch as the siderite has highly positive d13 C values ranging from 8.05 to 15.41, we deduced that extremely isotopically heavy CO2 was generated in the process of biogenic decomposition of the organic matter. It was found that the d18 O values of siderite (26.58 to 27.74 vs. VSMOW) fit very well to the recalculated curve of Becker (1971) 10 3 059 10 4 53 3 6 2 ln . .a = T and to published extrapolated experimental data.


Baltic Sea; siderite; oxygen isotopes; isotope equilibrium; low temperatures; water

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