The exceptional winter flood of Loire river 2021: an unexpected source of methane in the inner estuary

Particles generating Maximum Turbidity Zones (MTZ), in estuaries undergo several cycles of deposition/resuspension cycles before definitive burial or expelling towards the continental shelf. Positioning and spatial coverage depend on its morphology, riverine discharge and tidal dynamics. In the Loire estuary, the decadal flood of February 2021 displaced a lot of material from the upper to the lower estuary. Consequently, cores sampled at four sites upstream Paimboeuf (15 km from estuarine mouth) showed very dark cohesive sediments while those taken at two stations downstream, within the MTZ, showed a thick (10-50 cm), unconsolidated layer of a light-brown sediment over a darker-cohesive one. More striking, these four upstream stations, covering a river line of some 40 km, showed gas ebullition generating numerous cracks on the first decimetres of interface cores.

A few months later (June 2021), interface and long cores were sampled and methane analysed at three stations along the salinity gradient (two upstream and one downstream of Paimboeuf). Upstream, the entire interface and long cores showed methane saturated samples (about 2 mmol L^-1) and cracks remained ebullitive. Near Paimboeuf, the cores were no longer ebullitive and a clear sulphate-methane transition zone (SMTZ) was observed at 50 cm depth. Downstream, the SMTZ was located at 30 cm depth. These results suggest that the important currents induced by the 2021 winter flood eroded a significant layer of sediment to generate depressurization and allow methane to escape in the upper estuary while the lower estuary remained capped by enough sediment to maintain the SMTZ. They also showed that the decrease of riverine discharge allowed the MTZ to migrate upstream stopping methane to escape in the mid estuary while the upper estuary continued to release methane through ebullition four months after the erosion event. Crack formation and methane release also affected benthic fluxes, increasing total oxygen uptake by a factor of ten (94 mmol m^-2 d^-1) compared to diffusive flux, for example.

The rare opportunity to document such processes because of extreme navigation conditions, particularly for the deployment of the corer, allows us to emphasize that winter flooding can be an important source of methane that is immediately transferred to the atmosphere due to shallow depth of estuaries and must be taken into consideration for budgets and fluxes between reservoirs. This process must also be taken into account for a better understanding of other estuarine biogeochemical cycles, such as oxygen and nutrient cycles, as crack formation and methane release significantly increase their benthic fluxes.