Effects of sea-ice and biogeochemical processes and storms on under-ice water fCO_2 during the winter-spring transition in the high Arctic Ocean: Implications for sea-air CO_2 fluxes

摘要

We performed measurements of carbon dioxide fugacity (fCO_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 fCO_2 observations in this region. The observed under-ice fCO_2 ranged between 315 matm in winter and 153 matm in spring, hence was undersaturated relative to the atmospheric fCO_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 fCO_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 fCO_2 during the entire study. The relative effects of the total fCO_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.