Carbon sources and cycle in the western Mediterranean—the use of molecular markers to determine the origin of organic matter

Abstract

A series of molecular organic markers were determined in surface sediment samples collected in the Rhone and Ebro prodeltas and the western Mediterranean Basin in order to map and estimate the flux of naturally-derived organic matter inputs to the area. Continental and marine inputs were characterized by the distributions of n-alkanes, n-fatty acids, sterols, and polycyclic aromatic hydrocarbons (PAHs) such as retene, perylene and tetrahydrochrysenes (ΣTHC). The distribution of terrestrial markers evidenced the influence of rivers as main vectors of continentally-derived inputs in the area. In the data set of the Rhone delta and slope, including stations from the Gulf of Lions, significant correlations were observed between the organic content of sediment and the ΣTHC and retene concentrations (R2 = 0.88 and 0.68, respectively) and between ETHC levels and the δ13C of the organic matter (R2 = 0.89). The transport and settling of naturally-derived terrigenous material in the Ebro prodelta were reflected by the distribution of vascular plant n-alkanes. These refractory materials were preserved in the deep basin. Additionally, the distributions of fatty acids and sterols in the Ebro prodelta clearly evidenced algal signatures resulting from the enrichment of deltaic waters by nutrients transported by the river. Based on biomarker data a criterion is proposed to assess the percentage of terrestrial organic carbon in sediments, considering the river mouth sediment as the continental end-member. Fluxes of terrigenous organic inputs were calculated using sediment accumulation rates. Depositional flux estimates for the deltaic area off the Rhone River (∼ 15 340 km2) were as follows: 216 kg/year for ΣTHC, 104 kg/year for retene, 11 t/year for terrestrial n-alkanes. The accumulation rates in the area of influence of the Ebro River (∼ 12 600 km2) were 69 kg/year for retene and 6 t/year for terrestrial n-alkanes. Comparison of these data with estimates of riverine inputs suggests that the bulk inputs of terrestrial biomarkers are trapped within the terminal part of the river bed and in the narrow prodeltaic area in front of the river mouths to a depth of approximately 70 m. Atmospheric deposition may also contribute to the accumulation of terrigenous materials in the deep basin.