Inorganic carbon transported into the Gulf of Trieste by rivers draining karstic areas

Rivers act as important sources of carbon dioxide (CO2) to the atmosphere, playing an important role in changing the estuarine carbonate system. Rivers and underwater springs draining carbonate watersheds deliver freshwater into the Gulf of Trieste (GoT) and contribute significantly to increasing the alkalinity in coastal waters. The CO2 system of the Isonzo (at the mouth) and the subterranean Timavo (at the mouth and at one underwater spring) Rivers flowing into the GoT was sampled monthly between September 2021 and December 2022 to understand the dynamics of the carbonate system (total alkalinity, pH, calcium, magnesium, strontium and δ¹³C - DIC) at the end of the catchment and to estimate the input of alkalinity and bicarbonate into the marine system. Both rivers were characterized by high mean alkalinities (3056-4632 µmol kg-1), with higher concentrations in the Timavo system, which also showed greater weathering of calcium carbonates than the Isonzo, where the dissolution of dolomites increases the Mg:Ca molar ratio (Timavo spring 0.08±0.02, Timavo River: 0.17±0.04, Isonzo River: 0.29±0.03). Timavo spring and River were highly enriched in pCO2 (24182±7800 µatm and 12297±5433 µatm, respectively) and δ¹³C-DIC depleted (-13.2±0.4‰ and -11.5±0.8 ‰, respectively) due to limited degassing and degradative processes occurring in the subterranean course. Conversely lower pCO2 (2299±1624 µatm) and higher δ¹³C-DIC (-8.6±0.8‰) values were observed in the Isonzo River. Both rivers act as a CO2 source to the atmosphere, with a higher contribution of the Isonzo River compared to the Timavo subterranean system.