Contribution of river-borne soil organic carbon to the Gulf of Lions (NW Mediterranean)

We determined the spatial distribution patterns of the Branched and Isoprenoid Tetraether (BIT) index, a specific proxy of riverborne soil organic carbon (OC) input from land to the ocean, by analyzing soils from the Tet watershed (France) and surface sediments from the Gulf of Lions (NW Mediterranean) into which the Tet and Rhone Rivers flow. The BIT index pattern in the Tet watershed is heterogeneously distributed, varying between 0.2 and 1.0, with an average BIT value of 0.84 +/- 0.14 (n = 58). Soil pH is the major factor controlling soil BIT values among the environmental factors considered on the basis of the statistical analysis of environmental parameters and glycerol dialkyl glycerol tetraether concentrations. In the Gulf of Lions, BIT values range from 0.01 to 0.85, with higher values along the coast compared with those offshore. The soil OC contributions to the total organic carbon (TOC), estimated on the basis of the BIT index, are > 10% along the coast, whereas those from the outer shelf and the continental slope are < 10%.