Origin of remineralized organic matter in sediments from the Rhone River prodelta (NW Mediterranean) traced by Δ 14 C and δ 13 C signatures of pore water DIC

•Mineralized organic matter in delta sediments is traced by 13C/14C in porewater DIC.•In Rhone River delta (NW Med Sea), porewater DIC shows low δ13C and positive Δ14C.•Riverine plankton may be the source of mineralized OM.•This contrasts with preserved OM mostly originating from soil (flood origin).•Changes linked to anthropogenic activities may alter the CO2 budget in deltas.