Hydrocarbons in the Mediterranean Sea: Their Occurrence and Fate in the Sediment and in the Water Column, as Dissolved and Associated with Small and Large Size Particulates

Abstract

Transfer and transformation processes of hydrocarbons between the surficial water and the sediment have been studied in the open Western Mediterranean Sea, by determining concentrations and distributions of non-volatile n-alkanes, isoprenoids, non-aromatic and aromatic hydrocarbons in the surficial sediment and in three fractions of the water column: dissolved matter, small-size and large-size particulates (> 63 μm). Hydrocarbons have been analysed by capillary gas liquid chromatography and HPLC/UV spectrophotometry. Hydrocarbons are present essentially in a dissolved form in the water column (for more than 90% for samples collected below 1,000m) and associated with small-size particles. The stock of hydrocarbons present in large-size, rapidly sinking particles accounts for less than 0.3%. Concentrations found in the water column are low, except for the surficial water in the Ligurian Sea (>20μg1⁻¹). They decrease with depth to reach values less than 0.2 μg 1⁻¹ below 1,000 m. The sediments are characterized by low concentrations (3.9–7.1 μgg⁻¹). The nature of hydrocarbons is very different with respect to each pool: essentially petrogenic for the water column, derived from marine productivity and terrestrial higher plants waxes for large-size particles and sediments. Estimations of vertical flux of hydrocarbons are discussed in terms of mode of settling of hydrocarbons from the sea surface.