During extreme flood events, coastal deltas experience large sediment deposition within a short time period. The biogeochemical consequences of these deposition-recycling-burial processes on carbon and nutrient cycles are not fully understood. Using a coupled data model approach, we explore the early diagenetic responses of deltaic sediments influenced by two intense floods (in spring and fall) on the Rhône River in 2008. The data set shows that sediment porewater composition responded abruptly to this almost instantaneous change in deposition. The model calculated that these flood-related depositions increased organic carbon mineralization by a factor of 2–4 compared to preflood levels, and were dominated by sulfate reduction (68%), and methanogenesis (16%). The two floods (organic-poor in spring and organic-rich in fall) cause different diagenetic effects in terms of dissolved inorganic carbon (DIC) fluxes—the 30-cm organic-poor flood deposition induced a large storage of DIC in porewaters, which largely decreased its flux to the water column, whereas the 10-cm organic-rich sediment induced a large efflux of DIC. The model reveals the absence of dissolved sulfide in porewaters after flood deposition due to iron bound precipitation. The sequential flood depositions caused a temporary memory effect (i.e., interaction between two successive floods), with stronger effect for methane (38%), whose longer relaxation timescale limits complete recovery before the next event separated by 6 months. Increasing the frequency and intensity of these events in the future could lead to memory accumulation of flood biogeochemical signatures. |
