Carbon recycling in deltaic sediments: variations at different timescales in the Rhone River delta

Jens Rassmann1, Christophe Rabouille1, Flora Toussaint1, Cecile Cathalot2, Nadine Tisnerat-Laborde1 and Bruno Lansard1, (1)Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS-UVSQ et IPSL, Gif sur Yvette, France, (2)IFREMER, REM/GM/LCG, Plouzané, France
Abstract:
Terrestrial aquatic environments and the land-sea connection are a major contributors to the Earth carbon cycle, as these act as a source to the atmosphere of about 1 Gt/yr for rivers and up to 0.4 Gt/yr for estuarine/detaic systems. These estimates are largely uncertain because of the large temporal variability of these systems.

In estuarine and deltaic sediments, variability combines hydrological variation from the river (floods and drought) and the hydrology of the coastal seas (storms, current surge, wind induced circulation, upwelling) wich are both influencial on biogeochemistry. This hydrological variability interacts with the seasonal variation of in situ production and with thermal activation of bacterial recycling due to seasonal warming of shallow waters. The combination of these processes gives rise to a range of timescales: typically hours to days for resuspension during storms to interannual variations for particulate discharge modulations. Using in situ oxygen microprofiling devices, we have collected a new dataset on organic matter recycling in the Rhone delta and shelf sediments (Northwestern Mediterranean Sea) which covers a wide range of timescales: from hours to a decade. The hourly variation is collected using a new benthic station deployed on the sediments and specially adapted to monitor short-term variations of sediment oxygen micro-profiles during flood or storms. The seasonal to decadal timescale is constituted by a set of oxygen micro-profiles measured on an array of stations in the Rhône prodelta and shelf by an in situ microprofiler during seasonal cruises over 10 year. The results show that Diffusive Oxygen Fluxes which are related to organic matter recycling vary at all timescales, driven mostly by deposition of river material during floods. Resuspension during storms plays a role over short time scales, and its long-term effect could raise the overall oxygen demand of the sediment by 20-30%.