Expanding Blue Carbon horizons: Alkalinity as a key carbon sequestration pathway in Arcachon salt marshes

Coastal intertidal wetlands are widespread along the French Atlantic coast. As blue carbon ecosystems, they can capture and sequester carbon in a sustainable manner. Local carbon density, net CO₂ sequestration, CH₄ emission, alkalinity production and its transfer to the ocean are regulated by a variety of factors, including salinity, nutrient loads, tides and seasonality. This study focuses on carbon sequestration as alkalinity in the intertidal zone of the Arcachon lagoon. Carbon dioxide capture through alkalinity production represents durable (>1,000 years) CO₂ removal. Specifically, we studied the hydrology and chemical composition of waters draining from salt marshes at low tide by sampling these waters along a tidal creek, which is mainly fed by seepage of pore water from the salt marsh sediments. We also measured discharge rates. These measurements were taken in different seasons and at different tidal amplitudes. Our results show that brackish waters are systematically enriched in alkalinity and depleted in sulfate compared to the mixture between local seawater and freshwater endmembers. This suggests that sulfate reduction in sediments and sulfide precipitation in sediments are important processes in alkalinity generation. Measured fluxes show that for a drained salt marsh area of 30 ha, about 2,000 moles of C are sequestered as alkalinity (560 kgC/ha/yr), and transferred to the open Arcachon lagoon per tide. This indicates that salt marshes are efficient at sequestering carbon, not only as "blue carbon" but also as alkalinity. Therefore, tidal pumping is an important and generally overlooked process in this sequestration.