Effects of sea‐ice and biogeochemical processes and storms on under‐ice water f CO 2 during the winter‐spring transition in the high A rctic O cean: Implications for sea‐air CO 2 fluxes

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 data set 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. Large f CO 2 undersaturation relative to atmospheric level throughout winter and spring Calcium carbonate dissolution acts as a major surface‐water f CO 2 loss and sustains undersaturation in Arctic Ocean surface water Winter CO 2 sink in leads at high winds exceeds the sink in spring despite larger f CO 2 undersaturation due to phytoplankton bloom