DISTRIBUTIONS OF INORGANIC AND ORGANIC SULFUR IN ESTUARINE SURFACE SEDIMENTS FROM TOYAMA BAY, JAPAN

Reaction rates for formation of organic sulfur species during early diagenisis from surface sediments are slower than these of inorganic sulfur such as pyrite. Delayed organic sulfur species are suggested by enriched 34S of fulvic and humic acid-S relative to co-exisiting pyrite. In this study we examine the abundance and isotopic ratio of dissolved sulfate and sulfide, inorganic and organic sulfur from Zinzu and Oyabe estuarine surface sediments in Toyama Bay, Japan.

The concentration of dissolved sulfate at Zinzu shows a near than that of seawater sulfate (28 mM), whereas the value of Oyabe is lower than that of seawater sulfate. In Zinzu samples, porewater sulfate show a slight enrichment in 34S relative to seawater sulfate (+20.3 permil), whereas the isotopic composition of Oyabe samples range from +25.0 to +40 permil, indicating that the rate of sulfate reduction and the removal of sulfur from porewater are fast. The contents of acid-volatile sulfur (AVS) in Zinzu and Oyabe sediment samples.are lower than that of pyrite sulfur The inorganic sulfur (AVS and pyrite) content in Oyabe is higher than that of Zinzu. The sulfur isotope ratios of pyrite sulfur range from -21 to -7.7 permilin Zinzu and -18.7 to +2.0 permil in Oyabe, respectively, and decrease with increasing pyrite content, indicating the proceeding process of sulfate reduction in sediments. The values of sulfur isotope for fulvic-acid sulfur are -19.2 to -9.4 permil in Zinzu and -12.3 to +0.5 permil in Oyabe, respectively, and in general are higher than those of pyrite sulfur. The concentration of humic-acid sulfur represents only a small fraction of total sulfur. The kinetic isotope effect during sulfate reduction obtained from sulfur isotope ratios of pyrite is about 40 permil in Zinzu and about 30 permil in Oyabe. Since industrial contamination is strongest in Oyabe, active sulfate reduction occurred in enriched organic matter in river water. Thus, it would be expected that isotopic fractionation during sulfate reduction is small.