A major issue of Arctic marine science is to understand whether the Arctic Ocean is, or will be, a source or sink for air-sea CO<sub>2</sub> exchange. This has been complicated by the recent discoveries of ikaite (CaCO<sub>3</sub>·6H<sub>2</sub>O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO<sub>2</sub> and pH conditions in surface waters. Here we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from an actively melting 1.7 km<sup>2</sup> (0.5–1 m thick) drifting ice floe in the Fram Strait during summer. Our findings show that ikaite crystals are present throughout the sea ice but with larger crystals appearing in the upper ice layers. Ikaite crystals placed at elevated temperatures gradually disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice flow thickness by ca. 0.2 m per week and resulted in an estimated 1.6 ppm decrease of <i>p</i>CO<sub>2</sub> in the ocean surface mixed layer. This corresponds to an air-sea CO<sub>2</sub> uptake of 11 mmol m<sup>−2</sup> sea ice d<sup>−1</sup> or to 3.5 ton km<sup>−2</sup> ice floe week<sup>−1</sup>
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