The Arctic Ocean is subject to high rates of ocean warming and acidification, with critical implications for marine organisms, ecosystems and the services they provide. We report here on the first high-frequency (1 h), multi-year (5 years) dataset of the carbonate system at a coastal site in a high-Arctic fjord (Kongsfjorden, Svalbard). This site is a significant sink for CO₂ every month of the year (9 to 17 mol m^-2 yr^-1). The saturation state of aragonite can be as low as 1.3.