Composition and distribution of organic matter in the surface sediments of the Changjiang River in Post-Three Gorges Dam period

n-alkanes and fatty acids (FA) were analyzed in the sediment samples collected in 2009 from the Changjiang River (main streams and tributaries) by gas chromatography. Combined with general geochemical parameters, e. g. grain size, organic carbon content, stable carbon isotopes etc. The characteristics of composition and distribution of organic matters were investigated. The variation of sources and control factors, especially the influence of Three Gorges Dam(TGD) were also evaluated initially. The result showed that the surface sediment matters were composed of silt (45.8%) and sand(41.4%); the content of OC% ranged between 0.08% to 1.99%; and δ¹³C of the OC varied -31.88‰ to -21.41‰ which were close to the signature of continental C3 vegetation. Total n-alkanes concentration varied from 0.71 to 13.69 μg/g(dry weight), or from 0.05 to 1.99 mg/gOC. Fatty acids (FA) concentration, from 2.77 to 32.52 μg/g (dry weight), or from 0.16 to 40.58 mg/gOC, the maximum was saturated FA and the minimum was branched FA compounds. Hydrodynamic sorting and grain size took an important role in the concentrations and distribution of organic matter in tributaries of the Changjiang. Compared to the upper reach of mainstream, middle and low reaches were rich in aquatic plants, and less degraded. TGD had a significant influence on the transportation and transformation of terrestrial OM in the Changjiang River system.