Vertical fluxes of aromatic and aliphatic hydrocarbons in the Northwestern Mediterranean Sea

Aliphatic and aromatic hydrocarbon fluxes were measured in time series sediment trap samples at 200 m and at 1000 m depths in the open Northwestern Mediterranean Sea, from December 2000 to July 2002. Averaged fluxes of n-alkanes, UCM and T-PAH35 were 2.96 ± 2.60 μg m−2 d−1, 64 ± 60 μg m−2 d−1 and 0.68 ± 0.59 μg m−2 d−1, respectively. Molecular compositions of both hydrocarbon classes showed a contamination in petrogenic hydrocarbons well above the background levels of such an open site, whereas pyrolytic hydrocarbons stand in the range of other open Mediterranean locations. Fluxes displayed ample interannual and seasonal variabilities, mainly related to mass flux variation while concentration evolutions trigger secondary changes in pollutant fluxes. High lithogenic flux events exported particles with a larger pollutant load than biogenic particles formed during the spring bloom and during the summer. Sinking hydrocarbons were efficiently transported from 200 m to 1000 m.

► PAH composition, plots of diagnostic PAH ratios and the UCM abundance indicate that non aromatic and aromatic hydrocarbons in sinking particles in the Ligurian Sea were mainly of petrogenic origin. ► Fluxes of T-PAH35, n-alkanes and UCM transported downward at 200 m during the year 2001 were 269, 1218 and 26 910 mg m−2 yr−1, respectively. ► Vertical fluxes of aliphatic and aromatic hydrocarbons displayed ample seasonal and inter-annual variabilities, mainly related to mass flux variation. Concentration variation triggered smaller changes in pollutant fluxes. ► High fluxes of lithogenic particles occurring from early January to early March 2001 transported about 45% of the annual vertical export of contaminants. In April-May, high fluxes of biogenic particles also transported a significant fraction of pollutants, despite the dilution of petrogenic and pyrolytic PAHs by biogenic material.