Ikaite crystals in melting sea ice – implications for pCO₂ and pH levels in Arctic surface waters

Abstract. 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 CO₂ exchange. This has been complicated by the recent discoveries of ikaite (a polymorph of CaCO₃·6H₂O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO₂ and pH conditions in surface waters. Here, we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from a melting 1.7 km² (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 disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice floe thickness by 0.2 m per week and resulted in an estimated 3.8 ppm decrease of pCO₂ in the ocean surface mixed layer. This corresponds to an air–sea CO₂ uptake of 10.6 mmol m⁻² sea ice d⁻¹ or to 3.3 ton km⁻² ice floe week⁻¹. This is markedly higher than the estimated primary production within the ice floe of 0.3–1.3 mmol m⁻² sea ice d⁻¹. Finally, the presence of ikaite in sea ice and the dissolution of the mineral during melting of the sea ice and mixing of the melt water into the surface oceanic mixed layer accounted for half of the estimated pCO₂ uptake.