DEPOSITIONAL PROCESSES OF ORGANIC MATTER IN THE RHONE RIVER DELTA (GULF OF LIONS, FRANCE) TRACED BY DENSITY FRACTIONATION COUPLED WITH Delta C-14 AND delta C-13

As a main source of freshwater and particles, the Rhone 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 Rhone River, we measured radiocarbon (Delta C-14) and stable carbon isotopes (delta C-13) 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 delta C-13 from -27.2% nearshore to -24.5 parts per thousand at offshore stations while Delta C-14 decreased from 59 parts per thousand to -320 parts per thousand. The distribution of delta C-13 with density displayed a convex pattern at all stations. Except for fraction >2.5 g cm(-3), delta C-13 increases by 2.5 parts per thousand between stations A and E, indicating a loss of terrestrial signature. The distribution of Delta C-14 versus density had a concave pattern at all stations: at a single station, it showed a large heterogeneity with a difference of 500-600 parts per thousand between the <1.6 and 2-2.5 g cm(-3) fractions. A decrease in Delta C-14 of -400 parts per thousand 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 delta C-13 of -24.6 +/- 0.4 parts per thousand and Delta C-14 of -370 +/- 115 parts per thousand. 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.