ALBERT

Description: We performed measurements of carbon dioxide fugacity ( f CO 2 ) in the surface water under Arctic sea ice from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition. Over this period, the ship drifted with four different ice floes and covered the deep Nansen Basin, the slopes north of Svalbard and the Yermak Plateau. This unique winter-to-spring dataset includes the first winter-time under-ice water f CO 2 observations in this region. The observed under-ice f CO 2 ranged between 315 µatm in winter and 153 µatm in spring, hence was undersaturated relative to the atmospheric f CO 2 . Although the sea ice partly prevented direct CO 2 exchange between ocean and atmosphere, frequently occurring leads and breakup of the ice sheet promoted sea-air CO 2 fluxes. The CO 2 sink varied between 0.3 and 86 mmol C m −2 d −1 , depending strongly on the open-water fractions (OW) and storm events. The maximum sea-air CO 2 fluxes occurred during storm events in February and June. In winter, the main drivers of the change in under-ice water f CO 2 were dissolution of CaCO 3 (ikaite) and vertical mixing. In June, in addition to these processes, primary production and sea-air CO 2 fluxes were important. The cumulative loss due to CaCO 3 dissolution of 0.7 mol C m −2 in the upper 10 m played a major role in sustaining the undersaturation of f CO 2 during the entire study. The relative effects of the total f CO 2 change due to CaCO 3 dissolution was 38%, primary production 26%, vertical mixing 16%, sea-air CO 2 fluxes 16%, and temperature and salinity insignificant. This article is protected by copyright. All rights reserved.