THE ROLE OF ORGANIC SULFUR DURING EARLY DIAGENETIC PYRITE FORMATION IN ESTUARINE SEDIMENTS OF TOYAMA BAY, JAPAN

Anoxic organic-rich estuarine sediments were collected from four stations in Toyama Bay, Japan to disclose the early diagenetic formations of organic to inorganic sulfur . Abundance and sulfur isotopic compositions were examined for acid-volatile sulfide (AVS), pyrite, fulvic acid-sulfur (FA-S), and humic acid-sulfur (HU-S) in sediments. Sedimentary pyrite represents the largest sulfur fraction in four stations whereas HU-S is only a minor component of total sulfur. Relative proportions of inorganic sulfur (pyrite and AVS) are higher than those of organic sulfur (FA-S and HU-S). Horizontal and vertical distribution profiles of inorganic and organic sulfur indicate rapid consumption of organic matter by bacterial sulfate reduction in estuarine sediments. Precipitations of sedimentary sulfides take place largely within the uppermost several cm although sulfate reduction continues to greater depth. All of AVS (amorphous iron monosulfides, mackinawite, and greigite) have been proposed as possible precursor for pyrite. Actually, although the decrease of AVS may be explained by transformation of these phases to form pyrite, quantitatively, transformation of AVS to pyrite contributes less than several % to the observed concentrations of pyrite-sulfur. Positive correlation between incorporating iron and sulfur into fulvic acid suggests that formation of pyrite sulfur is also dependent of the progressive sulfidation and conversion of FA-S to pyrite. That is, formation of FA-S can coincide with an even precede the formation of pyrite. Thus it appears that any of reactions precede at rates enough to complete formation of pyrite within several cm of burial.