C/N ratios in a sediment core from Nakaumi Lagoon, southwest Japan—usefulness as an organic source indicator

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/Norg. 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/Norg. ratio and other proxies due to the effects of a constant level of inorganic nitrogen (Ninorg.: ca. 0.04%) in the sediments. When Corg. content is less than 1 wt%, the presence of Ninorg. lowers the C/N ratio considerably, giving an inaccurate indication of organic source. C-N and C/N ratio–Corg. plots are useful for evaluating the effect of Ninorg.; if the depth profiles of the C/N ratio and Corg. 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/Norg. 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.