A geochemical investigation of carboxylic acids released via sequential treatments of two surficial sediments from the Changjiang delta and East China Sea

Two surface sediments from the delta of the Changjiang River (the fourth largest river in the world in terms of water discharge and the seventh in terms of sediment discharge) and an adjacent area of the East China Sea were subjected to sequential treatments in order to obtain complete, quantitative and qualitative, information on carboxylic acid moieties. Five successive treatments allowed for the release of all acid moieties occurring in each sample: (i) the “unbound” acids isolated from extracts, (ii) the OH−-labile acids released by classical saponification, (iii) the OH−(PTC)-labile acids obtained via an additional saponification with a phase transfer catalyst, (iv) the H+-labile acids released by a subsequent acid hydrolysis and (v) the tightly bound acids only released by thermolysis. These five fractions correspond to distinct pools whose occurrence reflects differences in the mode of linkage of the acid moieties and/or in the protection provided by the structures to which they are linked. Such analysis of acid moieties provided information on (i) the relative contributions from microalgae, bacteria and higher plants in the “delta” and “seaward” stations, (ii) the degree of early diagenetic alteration for the acids derived from these different sources and (iii) the types of microalgal and bacterial species implicated. These features were also compared with the results of the only previous study where the sequential treatments were applied, on surficial sediments from a sharply different environment (the Lena River delta, Arctic).