Alkalinity (AT) and dissolved inorganic carbon (CT) data of fjord stations during MSM110

Sampling was carried out from 13 to 23 August 2022 on board RV Maria S. Merian. Water column samples for alkalinity (AT) and dissolved inorganic carbon (CT) were collected at eight stations using a conductivity–temperature–depth (CTD) rosette system with non-metallic water samplers (10 L, Ocean Test Equipment; Fort Lauderdale, USA). In addition, discrete surface water samples were obtained at 38 stations accessed by zodiac to improve spatial coverage within each fjord. Samples were gathered using non-metallic water samplers (2.5 L, Hydro-Bios; Altenholz, Germany), triggered by a plastic-coated drop weight. Depth estimation was performed using a nylon rope marked at 1 m intervals. A multi-parameter portable meter (Multi 340i IDS, WTW; Weilheim, Germany) was used to measure temperature and salinity.  Samples were collected in 300 mL BOD (biological oxygen demand) bottles (Environmental Express, Charleston, USA) with an addition of 300 μL saturated mercury chloride solution (Carl Roth, Karlsruhe, Germany). The bottles were sealed with ground-glass stoppers (Wheaton Science Products, New Jersey, USA), silicone- and halogen-free grease (type M, Apiezon, Manchester, UK), and plastic caps (Wheaton Science Products, New Jersey, USA), leaving no headspace. The samples were stored in darkness at ambient temperature until analysis. Measurements were performed using a VINDTA 3 C system (Marianda; Kiel, Germany). AT was quantified by potentiometric titration via an 800 Dosino system equipped with an Aquatrode Plus electrode (Metrohm, Filderstadt, Germany), whereas CT was measured by coulometric titration using a CM5017O coulometer (UIC, Illinois, USA). Both analytical systems were calibrated against certified reference materials (CRMs) from the Scripps Institution of Oceanography (San Diego, USA) to ensure a precision of ± 1 µmol kg-1 for each parameter. Data is provided for stations in Ardencaple Fjord, Kong Oscar Fjord and Dove Bugt.