Origin and fate of dissolved inorganic carbon in interstitial waters of two freshwater intertidal areas: A case study of the Scheldt Estuary, Belgium

Processes affecting the concentration and isotopic composition of dissolved inorganic carbon (DIC) were investigated in pore waters of two freshwater intertidal areas of the Scheldt Estuary, Belgium. Pore water delta(13)C(DIC) values from marshes and mudflats varied from -27 to +13.4 parts per thousand, these very large variations reflect the contribution of different carbon sources to the DIC pool. In pore waters of the upper mudflat, river water DIC and dissolution of calcite contribute to a lesser extent (10% and 16% respectively) to the total DIC pool. Results indicate that inorganic carbon added to the pore water of the mudflats has a delta(13)C value of +20.3 parts per thousand in May 1998. These strongly enriched delta(13)C(DIC) values suggest that the major contribution (up to three-quarters) to total DIC is CO2 derived from methanogenesis. In pore waters of the marshes, CO2 derived from organic matter degradation (-27.5 parts per thousand) and river DIC (-11.5 to -16.1 parts per thousand) are the major sources of inorganic carbon contribution to the total DIC pool. In pore waters from a marsh site colonised by willow trees, the contribution from CO2 derived from organic matter degradation is larger than in pore waters from an area with only reed vegetation. In the latter case river water DIC is the major source of pore water DIC.