Biogenic aromatic hydrocarbon geochemistry in the Rhone river delta and in surface sediments from the open North-Western Mediterranean sea

Abstract

The origin, evolution and transport of biogenic polycyclic aromatic hydrocarbons (PAH) were studied in the Rhone river delta and the Gulf of Lions (North-western Mediterranean). Sediments and riverborne suspended particulate matter were collected and analysed for PAH using a combination of high performance liquid chromatography (HPLC), gas chromatography (GC) and GC/mass spectrometry (GC/MS). Special attention was given to aromatic hydrocarbons from natural sources: retene and its precursors related to abietic acid found in the resins of conifers, and pentacyclic triterpenoids derived from α - and β -amyrins, constituents of terrestrial plants. Retene concentrations varied in the following ranges: 15–59 ng g −1 in river-borne particulate material, 20–70 ng g −1 in deltaic sediments, 3–16 ng g −1 in open-sea sediments. Retene precursor concentrations were generally higher than those of retene in the delta, whereas the inverse was observed in open-sea sediments. In these sediments the low concentrations or the absence of precursors suggested a predominant transport of pre-formed retene by aerosols and fine particles issued from the Rhone river. In the deltaic area, both retene and its precursors are transported by river waters from the forest runoff of the drainage basin. Transformation of precursors to retene occurred during transport by river waters and/or fast after deposition in surface deltaic sediments. Pentacyclic triterpenoid concentrations varied from 54 to 296 ng g −1 in deltaic sediments and from 0 to 21 ng g −1 in open-sea sediments. Concentrations of tetrahydrochrysenes (3,3,7-THC and 3,4,7-THC) decreased with increasing distance from the Rhone river mouth. The presence of THC in the riverine particulate matter (10–105 ng g −1 ) indicated that the degradation of terrestrial organic matter by microbial activity had begun before the deposition of particulate matter in the surface sediment. A good correlation was observed between ΣTHC and δ 13 C ( r 2 = 0·89; n = 9), which suggests that the THC tracer information was representative of natural continental organic carbon inputs.