The results obtained during AMOR-BFLUX (Figure 1) showed well marked alkalinity and dissolved inorganic carbon (DIC) fluxes in the proximal zone (near the river mouth) which decreased sharply on the continental shelf, which are consistent with the results already found on pore water during previous campaigns (Rassmann et al., 2016).

Figure 1 : Alcalinity and DIC flux from the sediment to the water at stations near the river mouth (A and Z) and on the continental shelf (E)

An explanation has been put forward for the genesis of these alkalinity fluxes in the thesis of Jens Rassmann (2016) and in his article on this question (Rassmann et al., 2020). This is the possibility for the two reduced metabolites, Fe2 + and HS-, to recombine in the form of a precipitate of FeS or FeS2. This precipitation remove reduced elements produced at the same time as alkalinity during anaerobic recycling from re-oxidation with oxygen (brown-black limit in Figure 2). This re-oxidation generally consumes a large part of the alkalinity produced in depth and does generally not allow alkalinity fluxes towards the water column in a system without precipitation. In the case of the Rhône prodelta, the precipitation of FeS or FeS2 therefore generates a strong alkalinity flux as measured in Figure 1. For more details, refer to Rassmann et al. (Biogeoscience, 2020).

Figure 2 : Explanation of the generation of Alkalinity fluxes by the precipitation and burial (red arrow) of FeS in sediments.