This dataset comprises key carbonate chemistry parameters measured and calculated in incubation experiments under different experimental conditions. pH, water temperature, and salinity were measured with a WTW multimeter (MultiLine® Multi 3630 IDS). Total alkalinity was determined by open-cell titration with an 888 Titrando (Metrohm). Saturation state of calcite and aragonite were calculated using phreeqpython, a Python wrapper of the PhreeqC engine (Vitens 2021) with pH, water temperature, total alkalinity, and major ions as major input, and phreeqc.dat as database for the thermodynamic data (Parkhurst and Appelo 2013). As the original Elbe water was supersaturated with carbon dioxide (CO2) with respect to the atmosphere, its partial pressure of CO2 (pCO2) level decreased during the incubation period with open flasks, which caused an adjustment of calcite saturation state (ΩC) for ambient air conditions. To adapt for the impact of pCO2 variations during the experiment, saturation state of calcite and aragonite was calculated assuming an equilibrium with an atmospheric pCO2 of 415 ppm (normalized ΩC and normalized aragonite sautration state ΩA). Since ion concentrations were measured for only a small number of samples, the ion concentrations of the remaining samples were reconstructed using stoichiometry based on the initial solution composition and total alkalinity. The concentrations of conservative ions (Na+, K+, Cl-, SO42-) were assumed remain constant, while ions related to carbonate precipitation (Ca2+, Mg2+) were calculated based on changes in measured alkalinity (see Figure 5 of the associated paper). Detailed analysis and calculation procedures are described in the Method section of the associated paper.

The study was additionally supported by the Ocean Alkalinity Enhancement (OAE) R&D Program (https://www.carbontosea.org/).---Details regarding the experimental setup:The experiments at Geomatikum were conducted from 2023-03-27 to 2023-12-14.In total, five incubation experiments were conducted to determine the stability ranges of riverine alkalinity (see Table 1 in associated paper). To account for seasonal variability of the carbonate chemistry due to biogeochemical matter cycling (Amann et al 2015), water samples were collected in the freshwater part of the Elbe estuary (Wedel: 53.56835 N, 9.70007 E) in late March (pH: 7.926, TA: 1747 μmol/kgw, and salinity ∼0.3) and August 2023 (pH: 7.695, TA: 1704 μmol/kgw, and salinity ∼0.3). Seawater samples from the North Sea (German Bight, 54.93 N 6.43 E; pH ∼8.06, TA: 2275 μmol/kgw, and salinity ∼33 for summer; and pH ∼8.18, TA ∼2291 μmol/kgw, and salinity ∼33 for winter) were mixed with river water to simulate an estuarine salinity gradient with three salinity levels (0, 4, and 16). Pure river water accounted for salinity 0 setups, while the salinity 4 and 16 setups containing approximately 10% and 43% of seawater, respectively. To explore the impact of suspended particles, two control groups were established: unfiltered (consisting of both unfiltered river and seawater) and filtered (river and seawater filtered through a 0.2 μm polycarbonate filter).In total, two experimental setups were conducted using winter and summer waters. Each setup involved filtered or unfiltered water at three salinity levels, resulting in 12 gradient approaches.