Sea-ice dynamics can affect carbon cycling in polar oceans, with sea-ice ikaite acting as a potentially important carbon pump. However, there is no large-scale direct field evidence to support this. Here we used a unique data set that combined continuous measurements of atmospheric and water CO2 concentrations with water chemistry data collected over 1,200 km along the East Siberian Sea, the widest Arctic shelf sea. Our results reveal large spatial heterogeneity of sea-ice ikaite contents, which directly interact with carbonate chemistry in the water column. Our findings demonstrate that the CO2 drawdown by sea-ice ikaite dissolution could be as important as that by primary production. We suggest that the role of ikaite in regulating the seasonal carbon cycle on a regional scale could be more important than we previously thought. Effects of the warmer climate on sea ice loss might also play a role in the ikaite inventory.