The Composition And Distribution Of N-alkanes In Surface Sediments From The South Yellow Sea And Their Potential As Organic Matter Source Indicators

Estimating sources of organic matters in marginal seas is an important aspect of the research of carbon cycle. The Yellow Sea is an important carbon sink, but its sediment organic matters are multi-sourced, and they play different role in carbon cycle. n-alkanes are typical biomarkers, they have been used to indicate the sources of organic matters. At present, however, the distribution of n-alkanes in the South Yellow Sea (SYS) is still unclear, and it is uncertain whether the composition of n-alkanes can be used as an indicator to identify the sources of organic matters in the SYS. In this study, the distribution and composition of n-alkanes in 63 surface sediments from the SYS were studied and compared with other biomarker proxies (the content ratio of terrestrial n-alkanes to sterols), in order to evaluate the use of n-alkanes as organic matter source indicators.From the carbon number distribution of n-alkanes, the contents of C₁₅^C₃₅ n-alkanes in surface sediments from the SYS are higher than others. n-alkanes mostly display bimodal distributions. The shorter chain n-alkanes (C₁₅^C₂₁) have no obvious odd-even predominance, and their main peaks are C₁₈ or C₁₉. They mainly derived from marine phytoplankton and microbes, but the impact of oil polution could not be ruled out. The longer chain n-alkanes (C₂₅^C₃₅) have a strong odd-over-even predominance, which are mainly produced by terrestrial higher plants, and their main peak is C₃₁, the second peak is C₂₉. The total contents of n-alkanes are lower in coastal region, a wide and obvious high value region is located in the central part of the SYS. Both marine and terrestrial n-alkanes have high contents in the central part of the SYS, due to stronger adsorption of organic matter on fine-grained sediments and lower sedimentation rates. The differences of the distribution between marine and terrestrial n-alkanes near the northern Jiangsu coast and the estuary of the Old Yellow River are due to different sources. The contents of marine n-alkanes also show a similar distribution to the total contents of n-alkanes.The indicators of n-alkanes: CPI (Carbon Preference Index), ACL (Average Chain Length), P_mar-aq (the content ratio of C₂₃+C₂₅ to C₂₃+C₂₅+ C₂₉+C₃₁),ΣT/ΣM (the content ratio of terrestrial to marine n-alkanes), TAR (the content ratio of terrestrial to marine dominant n-alkanes) andΣT/1.5ΣB (the content ratio of terrestrial dominant n-alkanes to sterols). They indicate higher terrestrial inputs occur in the northern region of the SYS near the Shangdong Peninsula, and in the regions near northern Jiangsu coast and the estuary of the Old Yellow River. The lower terrestrial inputs (or higher marine inputs) occur in the middle, east and south parts of the SYS. The high value region ofΣT/1.5ΣB in northern part near the North Yellow Sea is not obvious as CPI, ACL,ΣT/ΣM, TAR, could be ascribed to stronger degradation of shorter chain n-alkanes owing to intense bacterial activity and greater water depths in this region. The distribution patterns of terrestrial and marine sources of organic matters indicated by CPI, ACL, P_mar-aq,ΣT/ΣM, TAR andΣT/1.5ΣB in surface sediments are similar, so these indicators can be used to indicate relative trend of the sources of sedimentary organic matters in the SYS.