Sulfur and organic carbon relationship in sediments from coastal brackish lakes in the Shimane peninsula district, southwest Japan

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This paper presents the characteristics of sulfur and organic carbon concentrations, and the general factors controlling them in the brackish coastal lake sediments. Three sediment cores taken from the Lakes Nakaumi and Shinji, southwest Japan, were analyzed for total sulfur (TS), total organic carbon (TOC) and total nitrogen (TN) concentrations in dry weight. The cores' lengths were 12-30 m and the ages of the bottom layers in each core were 7000-10000 yr.B.P. The TS, TOC and TN profiles in the cores were similar, i.e., they were low in the lower portion of the cores, increased upward in the middle portion and decreased upward in the upper portion. Although there were variations in the TS, TOC and TN, the relationship between TS and TOC, and that between TN and TOC showed good correlations. The slopes of TS-TOC and TN-TOC regression lines depended on the depositional environments. The slope of the TS-TOC regression line generally correlated with the mean C/N ratio. The high value of the slope corresponded to the low C/N ratio. This suggests that the effective sulfate reduction depends on the organic matter type. Primarily the degree of preservation of freshly deposited planktonic organic matter is important. The TS-TOC regression line was: TS = 1.13TOC + 0.084 (r = 0.777, n = 278), which slope was apparently different from the slope of normal marine sediments (0.36). This is indirectly due to the primary productivity, the sedimentation rate and the water depth. In addition, the “initial minimum” amount of consumed organic carbon in the process of sulfate reduction was estimated on the basis of mass balance between consumed TOC and fixed TS.