We performed measurements of carbon dioxide fugacity (fCO2) in the surface water under 8

Arctic sea ice from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition. Over 9

this period, the ship drifted with four different ice floes and covered the deep Nansen Basin, the slopes 10

north of Svalbard, and the Yermak Plateau. This unique winter-to-spring data set includes the first 11

winter-time under-ice water fCO2 observations in this region. The observed under-ice fCO2 ranged between 12

315 matm in winter and 153 matm in spring, hence was undersaturated relative to the atmospheric fCO2. 13

Although the sea ice partly prevented direct CO2 exchange between ocean and atmosphere, frequently 14

occurring leads and breakup of the ice sheet promoted sea-air CO2 fluxes. The CO2 sink varied between 0.3 15

and 86 mmol C m22 d21, depending strongly on the open-water fractions (OW) and storm events. The 16

maximum sea-air CO2 fluxes occurred during storm events in February and June. In winter, the main drivers 17

of the change in under-ice water fCO2 were dissolution of CaCO3 (ikaite) and vertical mixing. In June, in 18

addition to these processes, primary production and sea-air CO2 fluxes were important. The cumulative loss 19

due to CaCO3 dissolution of 0.7 mol C m22 in the upper 10 m played a major role in sustaining the 20

undersaturation of fCO2 during the entire study. The relative effects of the total fCO2 change due to CaCO3 21

dissolution was 38%, primary production 26%, vertical mixing 16%, sea-air CO2 fluxes 16%, and temperature 22

and salinity insignificant.