The weight ratio of total organic carbon to total nitrogen (C/N ratio) in 20 meters of sediment core from Nakaumi Lagoon (coastal brackish lake), Japan is examined for its suitability as an organic source indicator. The C/N ratio depth profile is compared to the depth profiles of other proxies, i.e., weight ratio of total organic carbon to total organic nitrogen (C/N org. ratio), stanol compositions, n-alkane compositions and the hydrogen-index by Rock-Eval pyrolysis.The C/N ratios of the lagoonal sediments, which have been deposited over the last ca. 8,000 years, have a different profile from that of the C/N org. ratio and other proxies due to the effects of a constant level of inorganic nitrogen (N inorg. : ca. 0.04%) in the sediments. When C org. content is less than 1 wt%, the presence of N inorg. lowers the C/N ratio considerably, giving an inaccurate indication of organic source. C-N and C/N ratio-C org. plots are useful for evaluating the effect of N inorg. ; if the depth profiles of the C/N ratio and C org. content exhibit similar trends, this is taken as a warning that the apparent variation in the C/N ratio should be carefully examined in conjunction with the other plots and proxies. It is also found that the C/N org. ratio itself also tends to increase due to the preferential decomposition of nitrogen compounds, and that in sediments containing high proportions of planktonic organic matter (OM), the sensitivity of the C/N ratio to changes in the proportions of planktonic-terrestrial OM is relatively low. lignin phenols, are not easily analyzed due to chemical complexity. Hence, the C/N ratio, having recently become easier to obtain through the popularization of rapid elemental analysis techniques, appears to be an effective and simple indicator of organic source, particularly in lacustrine depositional environments, including estuaries.The C/N ratio in aquatic systems is governed by the mixing of terrestrial and autochthonous OM
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 ap parently 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.
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