A multi-proxy study of sedimentary humic substances in the salt marsh of the Changjiang Estuary, China

To better understand the origin, composition, and reactivity of sedimentary humic substances (HSs) in salt marshes in the Changjiang Estuary, HS samples were isolated from a sediment core that was collected from the Eastern Chongming salt marsh. Chemical and spectroscopic methods were used to analyze the features of these HSs. The results indicate that the studied HSs in the salt marsh sediments are mainly terrestrial-derived and that the sedimentary organic matter (SOM) in the top layer may contain more organic matter from marine sources and/or autochthonous materials due to the dramatic decreasing of the sediment supply as a result of damming. The degradation of labile carbohydrates and proteins and the preservation of refractory lignin components dominate the early diagenetic reactions of SOM in the salt marsh area. The average contents of the carboxylic groups in FAs and HAs are 11.64 ± 1.08 and 7.13 ± 0.16 meq/gC, and those of phenolic groups are 1.95 ± 0.13 and 2.40 ± 0.44 meq/gC, respectively. The content of carboxylic groups increased with increasing depth, while there were no obvious changes in the content of phenolic groups. The average concentration of total proton-binding sites is approximately 12.5 μmol/g sediment for the studied HSs. These values may provide insight into the migration and fate of HS-bound contaminants in sediments and the overlying sea water in the salt marsh areas of the Changjiang Estuary.