Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (δ13COC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O2 uptake (5.2 to 29.3 mmol m−2 d−1) and NH4+ release (−0.1 to −3.5 mmol m−2 d−1) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ13COC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (∼52%) or remineralized (∼8%), mostly on the prodelta area. The remaining fraction (∼40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.