As a main source of freshwater and particles, the Rhône River plays a major role in the biogeochemical cycle of organic carbon (OC) in the Mediterranean Sea. To better understand the origin of organic matter and the processes leading to its export to the coastal sea near the Rhône River, we measured radiocarbon (Δ¹⁴C) and stable carbon isotopes (δ¹³C) in the sediments of the delta, after density fractionation. In April 2007, 3 sites located along an offshore transect (A, C, and E) were sampled for surface sediments, and bulk sediment was separated into 4 fractions of different densities (<1.6, 1.6–2, 2–2.5, and >2.5 g cm⁻³). In order to better understand the evolution of the OC along the transect, we investigated the OC sources and their evolution for each density fraction. Bulk OC shows a large increase in δ¹³C from −27.2′ nearshore to −24.5′ at offshore stations while Δ¹⁴C decreased from 59′ to −320′. The distribution of δ¹³C with density displayed a convex pattern at all stations. Except for fraction >2.5 g cm⁻³, δ¹³C increases by 2.5′ between stations A and E, indicating a loss of terrestrial signature. The distribution of Δ¹⁴C versus density had a concave pattern at all stations: at a single station, it showed a large heterogeneity with a difference of 500–600′ between the <1.6 and 2–2.5 g cm⁻³ fractions. A decrease in Δ¹⁴C of −400′ among the different density fractions was observed along the offshore transect. The density fraction >2.5 g cm⁻³ had less variability, with an average δ¹³C of −24.6 ± 0.4′ and Δ¹⁴C of −370 ± 115′. Several processes may explain this distribution: retention in the prodelta of large particles; mineralization of all fractions during the transport and deposition in the delta and shelf sediments; and dilution of terrestrial particles in continental shelf pool.