Molecular characterization of the organic fraction of suspended matter in the surface waters and bottom nepheloid layer of the Rhone delta using analytical pyrolysis

Abstract

Curie Point-pyrolysis-gas chromatography (CuPy-GC) and Curie Point-pyrolusis-gas chromatography-mass spectrometry (CuPy-GC-MS) were applied to characterize the macromolecular content of the suspended particles in the surface waters and benthic nepheloid layer of the Rhone delta. The chromatogram of the pyrolysate of the Rhone River particles revealed a low pyrolysis yield from the riverine material in which polysaccharides and lipid-derived substances prevailed. The absence of levoglucosan and other pyrolysis products related to cellulose suggested that no intact polysaccharides were present. Lignin-derived products were virtually absent. In the salinity gradient, a wide variety of products, including saturated and monounsaturated acids, phytadienes, n-alkylnitriles and pyrolysis products from proteins were determined, indicating a major contribution from freshly produced autochthonous material. A suite of dipeptides of bacterial origin was also identified. Lignin-derived products from terrigenous sources were minor. Further offshore qualitative differences, with respect to the previous samples were apparent. Polysaccharides were less pronounced, possibly due to the dilution of the suspended load of the waters, and/or the microbial consumption of these readily degradable compounds. In contrast, the relative abundances of autochthonously derived compounds increased as a result of nutrient inputs from the Rhone River which fertilize coastal waters. The occurrence of 1,1,3,3,5,5, hexamethylcyclotrioxane as well as styrene provided indications of anthropogenic inputs to the site. The macromolecular constituents of suspended solids in the benthic nepheloid layer strikingly resembled those of the riverine material. Polysaccharides together with phytadienes and C 14, C 16 and C 18 acids accounted for the major pyrolysis products. The persistence of this fingerprint in the benthic layer was observed from the mouth to stations ZD1 and ZA7. Beyond this point, due to the influence of the Liguro-Provencal current flowing westwards, the composition of the pyrolysates changed towards a marine signature. Flocculation of suspended matter in which polysaccharides would make particles stick together or salt flocculation were proposed as an alternative scenarios to explain the formation of the nepheloid layer.