Discrete seawater samples for dissolved inorganic carbon (DIC) and total alkalinity (TA) were collected from the Polarstern (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_321) and Ocean City (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_935) CTD/Rosette systems during the Multidisciplinary drifting observatory for the study of Arctic Climate (MOSAiC) expedition, 20 September 2019 – 14 October 2020. Following Dickson et al. (2007), seawater samples were collected between 30 October 2019 and 28 September 2020 in borosilicate (3.3) bottles (250 ml), fixed with saturated mercuric chloride (HgCl₂) solution (100 µl), capped with greased (Apiezon® L) ground glass stoppers secured by insulating tape, and stored dark +4°C until post-cruise analysis at the Institute of Marine Research (IMR, Norway), University of East Anglia (UEA, UK), and Hokkaido University (Japan). DIC was determined by coulometric titration (Johnson et al., 1985) using a Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA 3C, Marianda, Germany) at UEA, a VINDTA 3D at IMR, and a custom-built extraction system at Hokkaido University (Ono et al., 1998). TA was determined by potentiometric titration using a VINDTA 3C at UEA, a Versatile Instrument for the Determination of Titration Alkalinity (VINDTA 3S, Marianda, Germany) at IMR, and a TA analyzer ATT-05 (Kimoto Electric Co., Ltd., Japan) at Hokkaido University. The accuracy for both DIC and TA was set at IMR and UEA by routine analysis of certified reference material (CRM batch #182, #191) distributed by A. G. Dickson (Scripps Institution of Oceanography, La Jolla, CA, USA). Reference materials (Batch AR and AU; KANSO Technos Co., Ltd., Osaka, Japan), traceable to the Scripps CO₂ CRM, were used at Hokkaido University. Analytical precision for DIC and TA content is < ±2 μmol/kg and < ±2 μmol/kg, respectively, based on CRM replicates. Based on secondary quality control (Jutterström et al., 2010; Tanhua et al., 2010), DIC and TA were adjusted by -8 µmol/kg and -23 µmol/kg, respectively, for events PS122/1_7-49, PS122/1_8-46, PS122/1_9-50, and PS122/1_10-45. Data quality is indicated by flags following the consolidated WOCE system of Jiang et al. (2022).

This dataset was produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020. We thank all persons involved in the expedition of the Research Vessel Polarstern during MOSAiC in 2019-2020 (AWI_PS122_00) as listed in Nixdorf et al. (2021). This work was supported by the Swedish Polar Research Secretariat as part of the MOSAiC 2019-2020 expedition.