Study of the biogeochemical processes of iron and ET in sediments, their remobilization/sequestration, and particle transformation in the Rhône River.
As a major deposition site for organic and inorganic particles transported from the continents, the benthic compartment plays a central role in the biogeochemical cycles of continental margins. As these particles are buried, they undergo remineralization, leading to the remobilization of numerous chemical elements, such as nitrogen, phosphorus, iron, and trace metals (TM) and metalloids. These elements—many of which are essential to the functioning of coastal ecosystems, while others are contaminants—can be transferred into the water column, thereby influencing the productivity and quality of coastal and ocean waters.
The example of iron is quite telling: This element, which is a major limiting factor for primary production in the ocean, plays a key role in the remineralization of organic matter. Indeed, iron oxides are major oxidizing agents in sediments, and their reduction releases dissolved iron into the interstitial waters. The immobilization of iron through reoxidation in oxygenated surface sediments limits its transfer to the water column, but certain conditions (deoxygenation, presence of organic ligands) can intensify this transfer. Trace elements, often associated with organic matter or iron oxides, undergo similar cycles related to the diagenesis of iron. The fluxes of these elements from the sediment are very poorly quantified due to a lack of data and tools to measure them in situ.
The BentFeET campaign, which took place from July 17 to 30, 2026, aboard the research vessel “Téthys 2,” focused on:
- Identification and localization of the biogeochemical processes involved in the remobilization and sequestration of iron and ETs in surface sediments.
- Characterization of general diagenetic processes to understand the biogeochemical functioning of the benthic compartment and the transformation of particles transported by the Rhône along a coast-to-shore transect, from the Rhône prodelta to the continental slope.
- Conduct an initial assessment of the importance of benthic Fe and ET fluxes on the biogeochemical functioning of the coastal zone.
This campaign was carried out in collaboration with the LSCE laboratories (CEA-CNRS-UVSQ), CHROME (University of Nîmes), Georgia Tech (USA), and GEOPS in Orsay, and with funding from the Graduate School of Paris-Saclay University.
During the campaign, in situ measurements were taken using landers (instrumented underwater vehicles), and sediment samples were collected using a multitube corer, while water samples were collected using a CTDO2 rosette. The sediment cores collected allowed for the sampling of interstitial water and sediment, as well as the creation of polarographic profiles to analyze all associated diagenetic processes and benthic fluxes for iron and trace elements and to assess their contribution to the water column of the Gulf of Lion. This work was conducted along a coast-to-shore transect extending from the Rhône prodelta (see map above)—where there are high annual sediment inputs—to the continental slope, where the sediments are older and finer-grained, with the presence of nepheloid layers. This transect allows for the study of contrasting physicochemical conditions and for accounting for the effects of the age of particulate deposits and their accumulation history.
The collaboration with Georgia Tech will allow us to compare the Gulf of Lion with results obtained from the Atlantic margins of the U.S. East Coast (Cape Lookout, Currituck Slide, and Long Island Slope), which are characterized by significant particulate organic matter inputs and the presence of benthic nepheloid layers.