Aliphatic hydrocarbon concentrations in short sediment cores from the southern Okinawa Trough: Implications for lipid deposition in a complex environment

Seven short sediment cores from the southern Okinawa Trough were collected and analyzed for the aliphatic hydrocarbon concentrations by capillary gas chromatography to explore the deposition of hydrocarbons to this area. For all cores studied, ratios of Σhydrocarbons/TOC, (nC27+nC29+nC31)/TOC, terrigenous/aquatic, and diploptene/ΣC25–33 n-alkanes fluctuated around a mean value with coefficients of variation ranging from 9.0% to 19.7%, 4.9% to 20.0%, 27.3% to 129%, and 3.8% to 163%, respectively. For the nC31/(nC27+nC29+nC31) ratio, only station 21 showed fluctuation. Moreover, the carbon preference indexes in the C25–C33 n-alkane range also exhibited fluctuating values with coefficients of variation of 1.9–14.4%. These results indicate that concentrations of hydrocarbon inputs to the sampling sites vary with time; this may result from complex current flow and sediment transport, leading to variable lipid deposition. In addition, significant correlation between diploptene (hop-22(29)-ene) and higher plant n-alkanes was found for cores 21, 42 and 46, indicating that diploptene was predominantly from higher plant sources. However, no correlation between diploptene and higher plant n-alkanes was found for cores 20, 36, 43 and 44; autochthonous sources of diploptene in these cores were quite probable.