Coastal sediments play an important role in the global carbon cycle as long-term carbon sinks. River deltas are major sediment deposition sites, receiving large amounts of sediment, especially during periods of high flooding.
Biogeochemical Implication of Massive Episodic Flood Deposition: Model‐Data Integration. Published in Journal of Geophysical Research: Oceans, 130, e2025JC022414.
https://doi.org/10.1029/2025JC022414
Coastal sediments play an important role in the global carbon cycle as long-term carbon sinks. River deltas are major sediment deposition sites, receiving large amounts of sediment, particularly during periods of high flooding. These deposits can trigger biogeochemical changes in sediments. However, the biogeochemical consequences of these deposition, recycling, and burial processes on carbon and nutrient cycles are not yet fully understood. To address this question, a numerical model and datasets from two floods that occurred in 2008 due to high flows in the Rhône River were used to study the magnitude of the biogeochemical response following these events. Our results suggest that these floods could lead to different responses, largely determined by the nature of the deposited sediment layer.
The data set shows that the composition of sediment pore water reacted abruptly to this almost instantaneous change in the deposits. Using the model, we observed a 2- to 4-fold increase in overall bacterial recycling rates of organic matter compared to pre-flood conditions. These were dominated by sulfate reduction (68%) and methanogenesis (16%). The two floods (poor in organic matter in spring and rich in organic matter in fall) had opposite diagenetic effects in terms of dissolved inorganic carbon (DIC) flux: the 30 cm organic-poor flood deposit induced significant DIC storage in interstitial waters, which considerably reduced its flux to the water column. In contrast, the 10 cm of organic-rich sediments deposited in the fall induced a significant DIC flux to the water column. Successive deposits caused by floods caused a temporary memory effect (i.e., an interaction between two successive floods), with a more pronounced effect for methane (38%), whose longer relaxation time limits complete recovery before the next event, six months later. The increase in the frequency and intensity of these events in the future as part of climate change could lead to an accumulation of the memory of the biogeochemical signatures of floods.
