Composition, distribution and origination of n-alkanes in surface sediments from shallow sea wetlands in the Liaohe River estuary

Fifty-three surface sediment samples were collected in the shallow sea wetland of Liaohe River estuary, Liaodong Bay, Bohai Sea, NW China. The composition and content of the sediment-borne n-alkanes, the composition distribution, and molecular combination characteristics of the n-alkanes were systematically analyzed. Results show that the n-alkanes in the study area displayed bimodal distributions, indicating that they were influenced by a mixture of terrestrial and marine sources. The first peak reflected obviously short-chain alkanes, indicating organic origination from marine planktonic algae and bacteria; and the second peak suggested odd-carbon n-alkanes, indicating organic source from terrestrial higher plants, of which woody plants accounted for about 49% and herbaceous plants for about 39.6%. Both the ratio of the total hydrocarbon contents from terrestrial and marine sources and the ratio of the total contents of dominant alkanes from land and sea sources agreed with the organic origination of the n-alkanes. Except for a few stations where the two ratios were less than 1, all other areas had the two ratios greater than 1, indicating that land-sourced organic matter was greater than marine-sourced one in this study area. In particular, the high-ratio areas were concentrated in the coastal areas of the Liaohe River estuary, and showed a decreasing trend from the coast to the outside, indicating that the terrestrial input was the strongest in the estuary. In-depth analysis of the characteristic index of the molecular composition of n-alkane showed that the sedimentary organic matter in the shallow marine wetland of the Liaohe River estuary mainly came from terrestrial input. The quantitative estimation results show that terrestrial plants contributed the largest proportion, averaging 66.41% ± 10.74%. It can be seen that the estuarine wetland ecosystem contributes greatly to the carbon sink in the coastal sea area.