How can geochemistry and isotope hydrology tools tell us about alluvial groundwater-wetlands interactions and subsequent carbon balance? Case of the Moltifau Mediterranean peatland (Corsica Island, France)

Wetlands are essential for humankind as they provide climate change mitigation through ecosystem services. Peatlands are wetlands also known as carbon sinks through peat accumulation. In the Mediterranean, peatlands represent up to 42% of the total wetland surface but remain under-investigated. The existing knowledge suggests they are shaped by their local environment, making them small, scattered, and groundwater-dependent. One of the most important knowledge gaps lies in the characterization and quantification of the water balance components necessary for biodiversity conservation and peat accumulation. Thus, additional information on peatlands could help delineate their hydrological vulnerability and resilience capacity to climate change. Approaches combining water level monitoring, geochemistry, and isotope hydrology tools have been widely employed to characterize the origin, periodicity, and recharge mechanisms of very large and well-developed wetlands mainly in cold humid climates. The same approach towards the investigation of more seasonal and inhomogeneous recharge mechanisms of Mediterranean peatlands is largely missing and has never been attempted to investigate the subsequent impacts on their carbon balance. To fill this gap, the alluvial peatland of Moltifau has been selected for its location in the western Mediterranean region at low altitude worsening seasonal drought conditions and vulnerability to climate change. An innovative multi-tracing experiment including geochemical and isotope hydrology tools such as physic-chemical parameters, major ions, δ¹⁸O, δ²H, ³H, ⁸⁷Sr/⁸⁶Sr and δ¹³C_DIC has been conducted benefiting from strong isotope contrasts due to varied lithology in a high elevation catchment. Five quarterly field campaigns in contrasting hydrological conditions were carried out between May-2018 and Jun-2019. The tracers highlighted the peatland’s rain and river dependency during fall-winter, as well as its river and multiple groundwater origin during spring-summer. Furthermore, our results show the impacts of such a recharge seasonality on hydrochemical processes and explain most of the variation in carbon fluxes, except in summer. In the future, the analysis and interpretation of peat chemical properties and isotopic signatures of CO₂ emissions will provide further information on the carbon storage dynamics of Mediterranean peatlands. This work illustrates the strong potential of isotopes in the study of water and carbon cycle interactions at the hydro-ecosystem scale.