Origin of remineralized organic matter in sediments from the Rhone River prodelta (NW Mediterranean) traced by Δ14C and δ13C signatures of pore water DIC

Rivers are important links between continents and oceans by transporting terrestrial particulate organic matter (POM) to continental shelf regions through estuaries or deltas and the Mediterranean basin is a good example of this strong linkage. In the vicinity of river mouths, an important fraction of the terrestrial POM settles on the seafloor, sometimes mixed with marine POM, where both can be recycled or buried. In the Rhone River prodelta, previous studies have investigated the origin of the POM in the sediments. They pointed at the large fraction of terrestrial POM in the sediments and the transition to older POM fractions in offshore direction. These studies suggested that terrestrial POM could be an important food source for benthic heterotrophic organisms in this area. In this study, the δ13C and Δ14C signatures of dissolved inorganic carbon (DIC) and DIC:NH4+ ratio in sediment pore water have been measured along a nearshore-offshore transect. The data were compared with available datasets concerning isotopic signature and C:N of the POM in bottom waters and suspended particles from the Rhone River and sediments from its delta, in order to determine which fraction of the POM is actually mineralized. Our results show that a fraction of terrestrial POM corresponding to riverine plankton is preferentially mineralized. Indeed, 13C-depleted riverine POM (δ13C = −25 to −27‰) which is enriched with 14C (Δ14C = +400 to +500‰) and shows a C:N < 8 gets mostly mineralized. This isotopic signature differs from that of sediment POM which highlights the selective mineralization already observed in other river deltas. In the Mediterranean context with large human influence in river watersheds such as dam building, our results highlight the importance of riverine primary production versus eroded organic matter in coastal carbon budgets.